JPH08229142A - Automatic threshold evaluation method in implantable pacemaker - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a capture threshold automatically assessable by giving a twined sequence of stimulus to a heart tissue, measuring a repolarizing time to re-polarize a heart tissue and determing the capture threshold approximately equal to a prior stimulating energy content. SOLUTION: A pacemaker 10 is connected to the heart 12 by way of leads 14, 16. Each of the lead 14, 16 carries an electric stimulating pulse from an atrium pulse generator 18 and an atrium pulse generator 20 to electrodes 15, 17. And a succeeding stimulus of each pair of twined stimulus always has more energy content than a capture threshold and provide the twined stimulating sequence to the heart tissue with adjusting a prior stimulating energy content to the capture threshold. Then, a repolarizing time is measured as a required time for the heart tissue to repolarize subsequently to each twined pairs. And, the capture threshold is determined approximately equal to a stimulating energy content of the each twined pair that precedes a change in the repolarizing time.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to implantable medical devices and methods, and more particularly to a simple and straightforward method for automatically assessing the capture threshold ("automatic threshold") of an implantable pacemaker. An implantable pacemaker or pacemaker system that provides easy technology. Once the capture threshold is known, the stimulation energy can be automatically adjusted as needed to ensure that the output stimulus achieves capture ("auto-capture").

[0002]

2. Description of the Related Art In the near future, a pacemaker allows the capture threshold of a given patient to be determined on a regular basis so that the output energy of the pacer stimulus is higher than the capture threshold but not too high. It is expected to incorporate the features of auto-capture and auto-threshold that allow the values to be optimally set. Ensuring that the output energy is above the threshold ensures that capture occurs, while keeping the output energy below the threshold is a limited availability of pacemaker batteries. Save electricity.

Reliable determination of capture within implantable pacers has heretofore been a daunting task. "Capture" occurs when the energy of the electrical stimulation generated and delivered by the pacemaker is sufficient to stimulate or depolarize the heart muscle tissue, thereby causing the heart to contract. Capture does not occur when the energy of the delivered stimulation is insufficient to stimulate or depolarize heart tissue. Needless to say, for a cardiac pacemaker to properly perform its intended function, it is critical that the electrical stimulus it emits has sufficient energy to capture the heart, ie depolarize the heart tissue. Importantly.

The classical approach to determining capture is to apply a ventricular stimulation (V pulse) to the ventricles of the heart and look for the evoked R wave response by each V pulse so applied. The required R-wave response is usually monitored between the tip of the bipolar lead and the ring connected to the pacemaker's detection circuitry. The evoked R-wave response can also be monitored between the ring electrode and the pacemaker case. In either case, bipolar pacing leads are generally needed to detect the evoked response.

The evoked R-wave response is the result of the electrode / when the polarization voltage or electrical stimulus is applied to the tissue via the electrode.
Immediately (within 5-20 ms) pacing pulse (V pulse) when the voltage appearing at the tissue interface is relatively high
Being monitored for. In order to avoid the detection of the polarization voltage and its classification as a evoked R-wave response, it is necessary to use a low polarization material for the electrodes. Moreover, computational algorithms are typically needed to set the evoked R-wave response threshold voltage, since the evoked R-wave response and the polarization voltage occur at the same time. Thus it would not be possible to reliably detect the evoked R-wave response if the polarization voltage is very high.

Assuming that a evoked R-wave response can be detected, a high-power backup pulse is generated when no evoked R-wave response from the original pacer pulse is detected (ie, when loss of capture occurs). Can be given. Given that two consecutive loss-of-capture events occurred, capture occurred as determined by Pacer's automatic capture routine by detecting the evoked R-wave response from the original stimulus of new increased energy. Until the original stimulation amplitude is incrementally increased.

The automatic threshold algorithm may be invoked periodically, for example once or twice a day, during which the pacer changes its output stimulus amplitude until capture is lost (no evoked response). Reduce. The output is then incremented incrementally until the capture is restored. With each loss of capture, to maintain the patient's heart rhythm,
The original stimulation is continued by the high-power backup stimulation.

All conventional auto-capture and auto-threshold schemes require a bipolar pacing system, or at least a bipolar detection configuration. Thus, it has heretofore not been possible to reliably provide the features of automatic capture and automatic threshold when only monopolar pacing is used. Moreover, autocapture and / or autothreshold cannot work with all pacing leads. (Ie cannot work with leads that exhibit high polarization potential). Additionally, demonstrating capture on every pacing pulse may require significant additional expense to the pacing circuit to ensure that the polarization voltage is not falsely detected as a evoked R-wave response.

[0009]

Accordingly, there is a need for an improved method for reliably assessing capture thresholds.
In particular, capture that does not require detection of the evoked R-wave response, and thus eliminates the problems associated with distinguishing the evoked R-wave response from the polarization voltage, and avoids the need to use low polarization materials for the electrodes. Evaluation technology is required.

[0010]

According to the present invention, to meet the above and other needs, stimulation pulses separated by a time less than the refractory period of the heart tissue, eg, about 60-100 milliseconds. Is provided to provide an implantable pacemaker whose capture is assessed. The second pulse of the stimulus pair is always set to, and remains at, the energy level with a high probability of causing capture, eg the maximum energy level. In a preferred embodiment, the first pulse of the stimulation pair is also initially set to an energy level that has a high probability of causing capture, eg the same energy level as the second pulse of the pair, and that energy level is then Systematically reduced. For example, after initially generating two or three pairs of pulses with a fixed amplitude, the amplitude of the first pulse of that pair is reduced by a set magnitude each time a pair of stimulation pulses is emitted. Can be struck. One of the stimulation pulses of each stimulation pair always causes capture. This is (1) the first pulse when its energy level is greater than the capture threshold (in this case the second pulse is emitted into refractory heart tissue but has no effect).
And (2) the second pulse when the energy of the first pulse drops below the capture threshold. Whether the capture pulse is the first pulse or the second pulse, a T-wave follows the capture pulse and represents repolarization of the cardiac tissue.

Advantageously, the T-wave occurs at the point following the capture pulse when the polarization voltage is no longer present at the electrode / tissue interface. The capture threshold is thus evaluated by measuring the fixed reference point associated with the stimulus pair, eg the time interval between the first pulse and the T-wave. A significant change in this time interval, for example a change of at least 20% or approximately 60-100 ms, indicates that the first pulse has dropped below the capture threshold.

Thus, the present invention advantageously provides capture realization or non-capture realization by simply measuring the time period between the stimulus and the T wave, rather than looking for the evoked response immediately after the stimulus is applied. decide. Advantageously, by measuring the time period between the stimulus and the T wave rather than looking for the evoked response immediately following the applied stimulus, the problems associated with the relatively high polarization voltage at the electrode / tissue interface are eliminated.

The T-wave is preferably detected using an intracardiac electrogram (IEGM) amplifier which is separated from the standard detection amplifier. Such IEGM amplifiers detect activity between the tip electrode and the pacer case. Thus
Bipolar or monopolar leads or pacing configurations can be used for this purpose.

The present invention can be broadly considered as a method for automatically assessing the capture threshold of an implantable pacemaker electrically connected to heart tissue. To implement this method, the pacemaker must include means for generating a sequence of electrical stimuli each having a controlled energy content, and means for measuring time intervals. The method includes (a) generating a sequence of paired stimuli at a predetermined pacing rate, the preceding stimulus of each pair of paired stimuli being on one side of the capture threshold. Has a tunable energy content beginning with the energy content of, and subsequent stimuli of each pair of paired stimuli always have an energy content above the capture threshold, (b) The process of imparting a sequence of paired stimuli to the heart tissue while adjusting the energy content of the preceding stimulus towards the capture threshold; and (c) each pair of paired stimuli following heart tissue Measuring the repolarization time as the length of time it takes to repolarize, and
(D) defining a capture threshold approximately equal to the energy content of the preceding stimulus of each pair of paired stimuli immediately preceding a substantial change in repolarization time.

In such a method, the leading stimulus of each pair is of fixed magnitude from the trailing stimulus of each pair, typically 60-.
Must be separated by 100 ms. It is this isolation time in each pair of stimuli that causes the "substantial change" that occurs within the measured time to the occurrence of the T wave, which can be referred to as the "repolarization time." However, "repolarization time" as used herein is not necessarily "physiological repolarization time", but rather a paired stimulus, such as the first
It should be noted that it is a fixed reference point associated with the pulse and the time measured from the occurrence of the T wave. Given that the fixed reference point is the first pulse,
Also, if the first pulse is the pulse that causes capture, the measured "repolarization time" is the true physiological repolarization time. Physiological repolarization time (time between stimulus capture and T-wave) is typically 200-300 ms, and "substantial changes" in the measured repolarization time are usually at least 20
% Change.

As can be seen from the above broad description of the method according to the invention, the invention is not limited to being performed in the ventricle. As a practical matter, it is usually performed intraventricularly because the T wave provides a convenient signal that allows measurement of ventricular repolarization time. However, if methods were developed to measure atrial repolarization time, the present invention could be used to easily assess the atrial capture threshold.

As can be seen from the broad description above, the preceding stimulus of a stimulus pair may be below or above the initial capture threshold. As a practical matter, it is usually, but need not be, set to an initial value above the capture threshold, eg equal to the subsequent stimulus. All that is important for the purposes of the present invention is that the energy of the preceding stimulus is systematically regulated, e.g. in the direction that whenever a stimulus pair is generated it may cross the capture threshold. It is to be adjusted incrementally. When such a crossover occurs, there is a substantial change in the measured "repolarization time".

The present invention may also be characterized as a capture threshold evaluation system for use in an implantable pacemaker. This system includes (a) pacing interval timer, (b) pulse generator responsive to the pacing interval timer, (c) detection amplifier for detecting T-wave, (d) stop timer for measuring time interval, and (e) time. It includes a comparator circuit and (f) a loss of capture signal generator responsive to the time comparator circuit.

In operation, the pacing interval timer defines the pacing interval. When the capture threshold is to be evaluated, the pulse generator produces a pair of stimulation pulses in synchronization with the pacing interval.
The first pulse of the pair has a first programmed amplitude and the second pulse of the pair has a second programmed amplitude, the first programmed amplitude first known. Is set to a value that causes capture, the second programmed amplitude is set to a value known to cause capture, and the first pulse amplitude is then systematically changed. To be done. The first and second pulses are separated by a time period t _S , where t _S is less than the refractory period of heart tissue. The time period t _S is typically about 6
0 to 100 milliseconds. The sense amplifier detects the T-wave that occurs as a result of the pair of stimulation pulses and one of the stimulation pulses of the pair always causes capture. The stop timer measures at each pacing interval the time interval that starts when the pulse generator generates one of the stimulation pulses (fixed reference point) and stops when the sense amplifier detects the T wave.
The time interval thus measured gives a measure of the time it takes for the T wave to occur at each pacing interval following the fixed reference point of the pulse pair. The time comparison circuit simply compares the time interval measured by the stop timer with the reference time interval. The reference time interval here defines the rated delay between the stimulation pulse that causes capture and the T-wave that occurs as a result of such capture. The loss of capture signal generator then generates a loss of capture signal whenever the time interval measured by the stop timer differs from the reference time interval by an amount approximately equal to t _S.

Implantable, where capture can be evaluated without having to detect a evoked response immediately following a given stimulus, and at the potentially absence of interfering polarization voltages at the electrode / tissue interface It is a feature and advantage of the present invention to provide a flexible pacemaker.

It is another feature and advantage of the present invention to provide a simple capture determination technique that does not require bipolar pacing leads or special low polarization electrode materials.

The automatic threshold feature that reliably detects the capture threshold is periodically and automatically invoked, and the automatic capture feature stimulates to a higher level than the determined capture threshold by a predetermined safety margin. It is yet another feature and advantage of the present invention to provide a pacing system that is then selectively triggered to set energy.

[0023]

The above and other aspects, features and advantages of the present invention will become apparent from the more detailed description below in conjunction with the accompanying drawings.

The following description is of the best presently contemplated mode of carrying out the invention. This description is not intended to limit the scope of the invention, but merely to illustrate the general principles of the invention. The scope of the invention should be determined with reference to the claims.

The present invention is intended for use in an implantable pacemaker. A functional block diagram of a typical pacemaker in which the present invention may be used is shown in FIG. As seen in FIG. 1, pacemaker 1
0 is connected to the heart 12 via leads 14 and 16. (In the subsequent figures, eg, FIG. 6, leads 14 and 16 are referred to as lead system 19.) Lead 14 has an electrode 15 in contact with one of the atria of the heart, and lead 16 is of the heart. It has an electrode 17 in contact with one of the ventricles. Leads 14 and 16 provide electrical stimulation pulses (stimulation) to the atrial pulse generator (AP), respectively.
G) 18 and ventricular pulse generator (V-PG) 20 to electrodes 15 and 17. Further, an electrical signal from the atrium is transmitted through the lead 14 to the atrial channel detection amplifier (P-
The electrical signal from the ventricle is transmitted to the input terminal of the AMP) 22 through the lead 16 and the ventricular channel detection amplifier (R).
-AMP) 24 input terminal. In addition, in accordance with the present invention, a T-wave amplifier (T-AMP) 110 is used to detect T-waves in the ventricles, as described in more detail below.

Controlling the dual chamber pacer 10 is a control circuit or control system 26. Control system 26 receives the output signal from atrial amplifier 22 via signal line 28. Similarly, control system 26 uses signal line 30.
To receive the output signal from the ventricular amplifier 24, and to receive the output signal of the T-wave amplifier 110 via the signal line 111. The output signals on the signal lines 28 and 30 are P waves or R waves.
A wave is generated each time a wave is detected in the heart 12. Similarly,
As for the output signal on the signal line 111, the P wave or the R wave is the heart 12
It is generated each time it is detected in the ventricle. The control circuit or system 26 also includes atrial pulse generator 18 and ventricular pulse generator 20 via signal lines 32 and 34, respectively.
Generate a trigger signal that is sent to. These trigger signals are generated whenever a stimulation pulse is to be generated by the respective pulse generator 18 or 20. A-PG1
The stimulation pulse generated by 8 is called "A pulse", and the stimulation pulse generated by V-PG20 is called "V pulse". For the purposes of the present invention, as will be explained later, whenever the pacemaker 20 is operating in an automatic threshold mode, i.e. the mode in which the capture threshold of the heart 12 is to be evaluated, the V-PG 20 is used.
Generate a pair of V pulses. During the time that the A or V pulse is being delivered to the heart, the corresponding amplifier PA
MP22 and / or R-AMP24 and T-AM
P114 typically connects each of these amplifiers with signal line 3
It is disabled by a blanking signal provided by the control system via 6, 38 and 113. This blanking action prevents the amplifiers 22, 24 and 110 from becoming saturated by the relatively large A or V pulses respectively present at the input terminals of such amplifiers during this time. Such a blanking effect helps prevent residual electrical signals present in muscle tissue as a result of pacer stimulation from being interpreted as P or R waves.

Continuing to refer to FIG. 1, pacer 10 controls system 26 via a suitable data / address bus 42.
A memory circuit 40 connected to. The memory circuit 40 is programmable in that it stores a number of control parameters used to control the operation of the pacemaker, and optionally as necessary to customize the operation of the pacer to suit the needs of a particular patient. Allowed to be modified. Such data includes the basic timing intervals used during pacemaker operation, such as the programmed atrial escape interval (AEI). For the purposes of the present invention, such data includes energy (amplitude) data used to initially set and / or systematically reduce V-pulses during automatic thresholding, and And a safety factor that specifies how above the measured capture threshold the stimulation energy should be set, as described more fully in. Further, data detected during operation of the pacer, such as measured capture thresholds, can be stored in memory 40 for later retrieval and analysis.

Further included in the pacer 10 is a telemetry circuit 44. The telemetry circuit 44 is connected to the control system 26 via a suitable command / data bus 46. The telemetry circuit 44 contained within the implantable pacer 10 may be selectively connected to an external programmer 48 by a suitable communication link 50. Communication link 50
May be any suitable electromagnetic link, such as an RF (radio frequency) channel. Advantageously, the external programmer 48
And desired commands may be sent to the control system 26 via the communication link 50. Similarly, data may be received remotely from the pacer 10 through the communication link 50 and the external programmer 48 (data held in the control system 26, such as data held in a data latch or stored in the memory 40). In this way, non-invasive communication with the pacer 10 being implanted from a remote, non-implanted location may sometimes be established. Telemetry circuit 4
Many telemetry circuits that can be used according to the invention as 4 are known in the art. For example, U.S. Pat.
See 7,617.

The pacer 10 in FIG. 1 interfaces with both the atrium and the ventricle and is therefore referred to as a dual chamber pacemaker. The portion of pacer 10 that interfaces with the atrium, such as lead 14, P-wave detector amplifier 2
2, the A pulse generator 18 and corresponding portions of the control system 26 are commonly referred to as the atrial channel. Similarly, the portion of pacer 10 that interfaces with the ventricles, such as lead 16, R-wave detection amplifier 24, T-wave amplifier 110, V
Corresponding portions of pulse generator 20 and control system 26 are commonly referred to as ventricular channels. However, it should be appreciated that the automatic threshold evaluation capability of the present invention is not limited to use with dual chamber pacemakers only, it senses and paces within a single chamber pacemaker, ie, only one chamber of the heart. It can also be used with a pacemaker. In fact, it is one of the advantages of the present invention that it can be used with single chamber or dual chamber pacemakers.

In some pacemakers embodying the present invention, pacemaker 10 includes one or more physiological sensors 52 connected to pacer control system 26 via suitable connection lines 54. Although the sensor 52 is shown in FIG. 1 as being contained within the pacer 10, the sensor may be external to the pacer 10 and may be further implanted inside the patient or carried by the patient. May be. A common type of sensor is an activity sensor such as a piezoelectric crystal that is attached to the pacemaker case. Other types of sensors, such as sensors that detect oxygen content of blood, respiration rate, pH of blood, physical activity, etc., may be used instead of or in addition to activity sensors. If a sensor is used, the type of sensor used is not critical to the invention. Any sensor or combination thereof that can detect physiological parameters related to the rate at which the heart should beat can be used. Pacemakers that use such sensors are commonly referred to as "rate response" (PR) pacemakers. This is because such pacemakers regulate pacer rates (basic pacing or escape intervals) in a manner that tracks the patient's physiological needs.

The pacemaker control system 26 may take a variety of forms, any of which are suitable for the purposes of the present invention. The details of the control system 26, whether it be a microprocessor, a state machine or other type of controller or a simulated controller, are not critical to the understanding or practice of the present invention and are therefore not Not shown in. Such details, if desired,
It can be found in the literature. For example, U.S. Pat. No. 4,712,555 describing the operation of a state machine type pacemaker.
U.S. Pat. No. 4,788,980, which more fully describes the various timing intervals used within a pacemaker and their interrelationships, U.S. Pat. No. 4,788,980, which describes an atrial rate based programmable pacemaker. See No. 4,944,298. The contents of these patent specifications are incorporated herein by reference. All that is important for the purposes of the present invention is that the pacemaker's control system, in conjunction with other pacemaker circuits, will generate a pair of V-pulses (at least one of which is at least 1) issued during an automatic threshold mode, as will be described below. One is to always measure the time interval between one of the V-pulses of the heart) and thus the T-wave generated by the V-pulse pair.

Referring now to FIG. 2, there is shown a timing waveform diagram that conceptually illustrates how the present invention automatically evaluates or determines a capture threshold. The waveform diagram of FIG. 2 shows the major cardiac events that occur during the ventricular pacing cycle. Figure 2
The cardiac events shown in Figure 2 appear over time along the horizontal axis as if such events appeared in an intracardiac electrogram (IEGM). Thus, the pacing cycle is, for the purpose of FIG. 2, a time interval T between ventricular stimulations.
It is defined as P1, that is, the time interval between the first V pulse 60 and the second V pulse 62. 60
For a heart that is paced to beat at a rate of beats per minute (bpm), the time interval TP1 is thus 1000 milliseconds (1 second). (Note: atrial events such as P-waves and / or A-pulses are not shown in Figure 2.)

As shown in FIG. 2, each V pulse 60 and 62 has sufficient energy to capture cardiac tissue. Therefore, each V pulse 60 and 62 is immediately followed by a evoked R wave 64 and 66, respectively. The R wave represents the depolarization of ventricular muscle tissue. This depolarization of muscle tissue physically contracts ventricular tissue, thereby pumping blood (which is of course a function of the heart). Thus, when the V-pulse captures the heart, it causes the heart's ventricles to contract, thereby forcing the heart to pump blood at a set rate.

Also shown in FIG. 2 is a time period or "time window" called the electrode / tissue polarization region. It is during this region that the polarization voltage is likely to be present at the electrode / tissue interface, for example between region 68 following V pulse 60 and region 70 following V pulse 62. Such a polarization voltage causes a evoked response, such as an R wave 64, as described above.
Or complicate the process of trying to detect the R wave 66. The polarization voltage is thus an obstacle to the accurate assessment of whether capture has taken place. Advantageously, the present invention avoids the problems associated with polarization voltage by not detecting an evoked response during the polarization region.

Further, as shown in FIG. 2, a T wave 72 or 74 occurs, respectively, following the application of the capturing V pulse 60 or 62. The T wave always follows the depolarization of the heart tissue. The T wave indicates repolarization of ventricular tissue. (note:
Although atrial activity is not included in FIG. 2, it also repolarizes when atrial tissue is being depolarized, eg, by an atrial stimulation pulse, eg, an A pulse. There are usually no detectable waves in the IEGM that indicate such atrial repolarization. Because it is masked out by the R wave. 3.) Ventricular tissue is refractory during the time period following depolarization until the T-wave occurs within the cardiac pacing cycle. That is, it cannot contract. Because it is just recovering from depolarization / contraction. The time interval from the evoked response R wave 64 or 66 to the occurrence of the T wave 68 or 70 thus includes the repolarization time. Assuming a given V pulse captures the ventricles, this repolarization time interval is thus V
It is approximately equal to the time interval between the pulse and the resulting T-wave. Such a time interval is referred to as time T _W in FIG.
Shown inside.

According to the present invention, an automatic threshold mode is used to evaluate or determine the capture threshold. In FIG. 2, the autothreshold mode provides a sequence of decreasing energy V _i pulses each followed by a high energy backup stimulus V _B , and also the time interval T between the V _i pulse and the subsequent T wave. Start by measuring _i . Each pair of V pulses in the sequence is thus the original V pulse V followed by the backup pulse V _B.
contains _i . The automatic threshold mode is thus 76
Then, it starts with the first original V pulse V ₁ . The backup pulse V _B follows the original stimulation V _i with a delay of time t _S. Here, t _S is typically 60 to 100 milliseconds (time period smaller than the natural refractory period of the heart). The backup stimulus V _B has sufficient energy, eg sufficient amplitude, to ensure capture. The original stimulus V _i also begins with sufficient energy to cause capture. For example, the original stimulus V _i may initially be set to the same amplitude as the backup stimulus V _B. When the original stimulus V _i captures the heart, a backup stimulus V _B is delivered to the heart, which is refractory and does no harm. If the original stimulus V _i did not have enough energy to accomplish the capture, the backup stimulus V _B captures the heart. In each case, a T-wave occurs with a delay of time T _W following the capture stimulus. Therefore, V _i-
By providing the heart with a sequence of V _B pulse pairs (each V _i pulse has energy reduced from V _{i -1} of the previous pulse pair), and also between the V _i pulse and the subsequent T wave. Time interval T _i (time interval T _i is approximately equal to T _W if the V _i pulse caused capture, but
If the V _i pulse did not cause capture, then the capture threshold can be easily determined by measuring (which is substantially longer than T _W ).

As shown in FIG. 2, the rated (mean) stimulation-T interwave time interval T _W first monitors a predetermined number of pacing heart cycles, eg 3-10, For each cycle, T _W (the time interval from the V pulse to the T wave) is measured and the T thus measured
It can be determined by taking the average of the values of _W. This T _W
Is preferably made while the patient is at rest and the heart rhythm is stable.

Once the rating or average T _W is known, the automatic threshold routine begins at time 76 the first of the stimulation pulse.
It starts by issuing a backup pulse V _B which follows the original V ₁ and its 60 to 100 milliseconds. The time interval from the original V ₁ of the pulse pair to the resulting T-wave (shown as T _{1 in} FIG. 2) is measured. First to the situation shown in FIG.
The first pulse V ₁ of the stimulus pair of D ₁ achieves capture, and the measured time interval T ₁ is thus approximately equal to the predetermined value of T _W. A second pair of stimulation pulses, V ₂ , followed by a backup pulse V _B is then emitted. V ₂ has a smaller energy (amplitude) than V ₁ . The time interval between the original V pulse and the T wave was measured again and found to be equal to T ₂ (= T _W ), with pulse V ₂ being the pulse pair ventricular stimulation that caused capture. Instruct that. V ₃ is then emitted, followed by the third pair of stimulation pulses, the backup pulse V _B. V ₃ has smaller energy (amplitude) than V ₂ . The time interval between the V pulse and the T wave was measured again and found not to be equal to T ₃ (≠ T _W ), which caused V _{3 to} have insufficient energy to achieve capture. Also, backup pulse V _B
Indicates that he played the role of capture.

Thus, the present invention evaluates capture by looking for significant changes in the time interval T _i between the original V and T waves for a series of ventricular pulse pairs. In doing so, the first pulse of each pair has energy that is reduced from the energy of the original or first pulse of the previous pair, and the backup pulse of each pair always has sufficient energy to achieve capture. Have. The T wave always follows a pulse pair.
Because the first stimulation pulse or backup pulse causes capture, ie depolarizes the heart, a T-wave occurs as a result of repolarization of the heart.

The automatic threshold process shown in FIG. 2 may advantageously be implemented in an implantable pacemaker using the method as shown in the flow chart of FIG. Such a method not only determines the threshold, but also sets the energy of the pacing stimulus to a level just above the determined threshold, thereby providing the pacemaker with a capture feature. (In the flow chart of FIG. 3, each major step of the method is shown in a numbered "box" or "block" for reference purposes.)

Referring to FIG. 3, the pacemaker is operating in the desired programmed mode and all of the pacing parameters required by the pacemaker to operate in the desired program have been set. Are assumed (block 80). In addition to the usual pacing parameters, such parameters are, for the purposes of the present invention, the size of the step of reducing the energy of the original V pulse in each pulse pair (V pulse decrease step size), an automatic threshold evaluation process. Contains the frequency (eg, four times a day) and the safety factor SF that should be requested. Safety factor SF
Defines how above the determined capture threshold the ventricular stimulation should be set. Also, the frequency with which the automatic threshold mode should be requested can be specified (eg, 1 per day).
Times, 4 times a day, etc.).

Then, assuming that all of the pacing parameters have been set (block 80), an initial decision is made as to whether it is time to start the automatic threshold evaluation process (block 81). ). Typically,
The automatic threshold is, for example, once every 8 to 12 hours or 2 times.
Starts automatically once or at a specified frequency of 3-6 times per day. Also, an automatic threshold routine may be triggered by an instruction from an external programmer or upon the occurrence of a particular event within the pacemaker. If it is not time to start the automatic threshold (No branch of block 82), the pacemaker simply continues to pace in its programmed mode (block 83).

If it is time to start the automatic threshold routine (yes branch of block 82), the first step is to establish that there are appropriate conditions for the correct execution of the automatic threshold evaluation routine. . Generally, such conditions require the pacemaker to be engaged in ventricular pacing (V-pulse generation) at a steady rate (eg, when the patient is at rest). To establish that such a condition exists, a determination is made as to whether ventricular output is inhibited by the naturally occurring R wave (block 85). If the naturally occurring R-wave inhibits the generation of V-pulses (yes branch of block 85), some action will ensure ventricular pacing, ie force the pacemaker to generate V-pulses. It must be done. Typically, this is done by increasing the pacing rate if the pacemaker is a single chamber pacemaker, and by shortening the AV delay if the pacemaker is a dual chamber pacemaker (block 87). . Such action should force ventricular pacing (no branch of block 85). Other effects may be performed to achieve the same result, ventricular pacing.

Once ventricular pacing is achieved, a determination is made as to whether there is detected patient activity (block 89). Detected patient activity can be readily determined, if the pacemaker is a rate-responsive pacemaker, by simply checking the sensor-indicated rate (SIR) generated by the physiological sensor of the pacemaker. If the pacemaker is a dual chamber pacemaker capable of detecting naturally occurring P-waves, patient activity can be detected by a simple focus on the P-wave rate. If the P-wave rate is high, eg, greater than 90-100 pulses per minute (ppm), it is likely that the patient is exercising (or engaging in some other non-resting activity). Instruct. If patient activity is detected (Yes branch of block 89), it indicates that the patient is not resting. Thus, the automatic threshold evaluation procedure is delayed by x minutes. Here x can be 5 to 10 minutes and the pacemaker continues to operate in its programmed mode during this delay. At the end of the x minute delay, the pacer again determines that the naturally occurring R wave inhibits the V pulse (block 85) and the process repeats. If no patient activity is detected (No branch of block 89), the correct conditions for performing an automatic threshold assessment have been established and the automatic threshold assessment procedure begins.

The first step in the automatic threshold evaluation procedure is that once the correct conditions for its use have been established,
It typically includes determining the nominal V pulse-T wave time period (block 84). This time period is called T _W. Such a rating T _W may be determined using any suitable technique. To determine T _W , the pacemaker needs to issue a V pulse every cardiac cycle. It is also preferred that T _W be determined when the heart rhythm is relatively stable. Thus, it is preferred that T _W be determined while the patient is at rest. Such conditions have already been assured from the preceding proof stage. For example,
The AV delay of the pacemaker is shortened by a predetermined amount, eg ─────milliseconds, and S
The IR rate or P-wave rate is checked to establish that the patient is resting. Other techniques for ensuring ventricular pacing are described below. Once ventricular pacing is guaranteed, the T _W time period is a predetermined number,
For example, it is measured for 3 to 10 pacing cycles. The average T _W can then be calculated. This average or rated T _W is then used as a reference code time interval and the V pulse-T measured during it and the automatic threshold routine.
The interwave time intervals can be compared.

After the rating T _W is determined, the index pointer i is initialized (block 86), for example i is set equal to one. Then, the i-th ventricular stimulation pair, that is, the time t _S selected to be smaller than the refractory period of the heart
The actual pulse V _i is emitted, which is followed by the backup stimuli V _B, which are separated in time only (block 88). It is important that the backup stimulus V _B has sufficient energy to cause capture, and that the original stimulus V _i for the first or first pulse pair also has sufficient energy to cause capture. An easy way to achieve this goal is simply to use the first of the first pulse pair.
Is to set the original pulse V _{1 of the same} as the backup pulse V _B. Then, after the generation of the i-th pulse pair, the time period T _Wi from the original pulse V _i of the pulse pair to the resulting T-wave is measured (block 9).
0). This measured time T _Wi is then compared to the previously determined reference value of T _W at block 84 (block 9).
2). If the two times are approximately equal, ie T _W ≈T _Wi (which corresponds to the yes branch of block 92), it means that the original pulse V _i of the pulse pair is ventricular tissue. Indicates a captured pulse.
Therefore, the pointer is incremented (block 9
4) The energy of the V _i pulse is reduced by the programmed step size (block 96) and another pulse pair is emitted (block 88) at the appropriate time, ie at the beginning of the next pacing cycle.

In the manner described above, a series of ventricular pulse pairs is then emitted, each pulse of the original or first pulse starting with sufficient energy to capture the ventricle, but each time a new pulse pair is emitted. It has decreasing energy, and each pair of backup pulses has a fixed energy sufficient to capture the ventricle (blocks 88, 9).
0, 92, 94 and 96). In some cases, the energy of the preceding pulse of each pulse pair drops below the trap threshold. When this happens, T _Wi is the reference time interval T _W
But substantially greater than T _W (no branch of block 92). This is because the pulse V that precedes the capture is V
_This is because it is generated not by _i but by the backup pulse V _B. Therefore, T _Wi is approximately equal to t _S + T _W. Where t _S is the time interval between V _i and V _B.
If t _S is equal to 60-100 ms, the measured time T _Wi from the preceding pulse V _i to the T wave is the reference V
60-100 ms longer than the pulse-T wave interval. The capture threshold is thus determined just above the energy of the current V _i pulse. Accordingly, the energy of the V pulse may be reset to a value equal to the sum of the energy of the current V _i pulse and the safety factor (block 93). Once the energy of the V pulse has been reset, pacer operation may then continue according to the programmed mode (block 83).

In many cases, the present invention simplifies by looking for significant changes in ventricular stimulation-T interwave time interval T _Wi as the energy of the first pulse of a series of pulse pairs is systematically reduced. Can be carried out. When so implemented, it is not necessary to first measure the rated (or average) T _W. All that is needed is the first of the pulse
Make sure that the leading pulse of the pair is above the capture threshold, measure T _WI for the first pair of pulses, and then add an additional pair of pulses with the leading pulse being systematically deenergized. Keep on going and also easily look for significant changes in T _Wi . Here significant change is approximately the time interval t _S separating from the preceding or the original stimulating backup stimulation, for example 60 to 100 milliseconds.

Since the present invention relies on changes in the stimulus-T interwave time interval to signal that the capture threshold has been reached, the time interval measurement is always T from the preceding V pulse of each pulse pair. It doesn't have to be done to the waves. All that is required is that such time measurements are always taken from the same reference point within the pacing cycle. For example, the time interval from the backup pulse V _B to the T wave could easily be monitored after each pulse pair. As soon as the preceding pulse is the pulse causing the capture, V _B −
The time interval between T waves is (approximately approximately shorter than T _W by t _S )
Takes one value. As soon as the preceding pulse no longer causes capture, the V _B -T inter-wave time interval is at another value (≈
T _W ), thereby signaling that the capture threshold has been exceeded.

Referring now to FIGS. 4 and 5, a functional block diagram (FIG. 4) showing the components of a pacing system used to implement the present invention according to one embodiment, and such a pacing system. And a timing waveform diagram (FIG. 5) showing the operation of FIG. Note that the block diagram of FIG. 4 does not include all of the implantable pacemaker components. It contains only the components needed to practice the present invention in addition to the conventional components of the pacemaker. For a more complete view of all of the implantable pacemaker components, reference should be made, for example, to FIG. 1 above or FIGS. 6-8 below. It should be further pointed out that the block diagram in FIG. 4 is a functional block diagram, which means that the various blocks shown are indicative of the functions that need to be performed. A variety of different types of circuits, including dedicated hardware circuits and program controlled processor circuits (eg, microprocessors), can be configured by one of ordinary skill in the art to perform the desired functions of the invention taught in FIG. See.

As shown in FIG. 4, to practice the present invention, the pacemaker includes a first timer 102 that sets or defines the pacemaker's basic pacing interval T _PI . This first timer 102 is _marked as " _TPI timer" in FIG. As can be seen in the timing waveform diagram of FIG. 5, the T _PI timer is T _PI every T _PI seconds.
_The pacing interval is determined by issuing a _P pulse. Here, T _PI may be, for example, 800 milliseconds (75 bpm).
Corresponding to the pacing rate). The T _PI timer 102 also issues a T _B pulse during the pacing interval following the T _P pulse with a predetermined isolation time t _S. The value of t _S may be, for example, 60-100 ms. The T _P pulse is used as a trigger pulse applied to the pulse amplifier 106. Similarly, the T _B pulse is used as a trigger pulse provided to the pulse amplifier 108. The pulse amplifiers 106 and 108 generate V _i and V _B pulses, respectively, when triggered by the T _P and T _B pulses. The amplitude (or energy content) of the pulse thus generated is controlled by the amplitude modulator 116.
Amplitude modulator 116 defines the amplitude of the V _i pulse generated by the two signals, (1) V _i amplifier 106 A
The V _i signal and (2) the AV _B signal that determines the amplitude of the backup pulse V _B are generated. The amplitude modulator is controlled in part by a pulse-to-index counter or “i counter” 118. Counter 118 is, for example, T _PI counter 10 each time a pair of V pulses is generated.
It is incremented by counting the number of T _B trigger pulses emitted by 2. Therefore, the parameter or flag "i" keeps track of how many pulse pairs have been emitted.

A "stop timer" for pacemakers 1
04 is also included. The stop timer 104 has the same function as a "stop watch". That is, it measures the time interval from the time the start pulse is received to the time the stop pulse is received. As can be seen in the timing diagram of FIG. 5, T _PI counter 102 has T _P -T _B
The stop timer 104 is reset at a point during the pacing cycle immediately before the generation of the pair of T _P pulses. The stop timer 104 is stopped by the T _T pulse. The T _T pulse is generated by the T wave detection amplifier 110.

The T wave detection amplifier 110 is connected to a lead 19 which is connected to the heart 12 of the pacemaker patient so as to detect the occurrence of the T wave. T wave detection amplifier 110
To protect against large ventricular pulse pairs V _i -V _B and between during and after the example 10-20 ms moieties present pulse pair V _i -V _B in pacing cycle blanking signal T _BLANK Generated by _TPI timer 102. The blanking signal is thus the T-wave detection amplifier 11
Useful for the ability to enable or disable (and protect) zeros. When enabled after the T _BLANK signal is terminated, the T-wave sense amplifier thus monitors the ventricle for the occurrence of a T-wave. When a T-wave is detected, the T-wave detection amplifier produces a T _T pulse. The presence of the T _T pulse thus indicates that the T wave is being detected.

The stop timer 104 thus provides a T _P pulse and a T _P pulse during each pacing cycle in which a pulse pair is generated.
The time interval T _i between the _T pulse is measured. That is,
Stop timer 104 measures the time between the preceding pulse and the T wave of each pulse pair. As can be seen in the timing diagram of FIG. 5, the first time interval T ₁ is measured corresponding to the _first pulse pair V ₁ -V _B and the second time interval T ₂ is the second pulse pair. V ₂ -V _B is measured and a third time interval T ₃ is measured corresponding to the _third pulse pair V ₃ -V _B.

The time interval T _i measured by the stop timer 104 is compared with the reference time interval T _R by the difference circuit 112. The difference circuit 112 outputs an output signal ΔT _i representing the time difference between the measured time interval T _i and the reference time interval T _R.
Occurs. The determined time difference ΔT _i then determines whether it exceeds a predetermined tolerance value,
It is compared with the reference acceptance signal T _TOL . If not,
No lost capture (LOC) signal is generated. If yes, the LOC signal is generated. In FIG. 5, the first measured time interval T ₁ is approximately equal to the reference time interval T _R , so that the difference signal ΔT ₁ is equal to the predetermined tolerance signal T 1.
_It is a very narrow signal that is smaller than the _TOL . Therefore, LO
No C signal is generated. Similarly, since the second measured time interval T ₂ is approximately equal to the reference time interval T _R , the difference signal ΔT ₂ is a very narrow signal that is smaller than the predetermined tolerance signal T _TOL . Therefore, similarly, no LOC signal is generated. However, the third measured time interval T ₃
Is substantially larger than the reference time interval T _R , so that the difference signal ΔT ₃ is a large (wide) signal that exceeds a predetermined tolerance signal T _TOL . Therefore, a LOC signal is generated, indicating that the loss of capture signal occurred for the third pulse pair when the previous stimulation pulse was V ₃ .

The LOC signal is used by the pacemaker to end the automatic threshold mode. The LOC signal can also be provided to the "i counter" to hold the count therein. This count is then used by the amplitude modulator 116 or other processor to determine the energy lost capture. For example, the amplitude modulator 11
6 can be programmed to reduce the amplitude of the preceding pulse of each pulse pair by a predetermined step size ΔAV _P.
Thus, the amplitude of the preceding pulse of each pulse pair is AV _i = AV
_It has an amplitude that can be represented by _P- i (ΔAV _P ). Where AV _P is the initial amplitude of the first pulse. Therefore,
Once the capture is lost, the capture can be done by simply knowing the number or index i of the pulse pair with the preceding pulse that loses such capture, and by using the above relationship to determine the amplitude. It is straightforward to determine the amplitude of the missing preceding pulse, which is just below the capture threshold. The index is simply the count held in counter 118.

Other techniques also determine the energy lost capture by calculating the lost capture energy based on the above techniques, or the number of times the preceding pulse has been depleted by a fixed increment. May be used in addition to, or instead of. For example, the amplitude of the preceding pulse of each pulse pair could be held in a register (if digital) or stored in a capacitor (if analog) and updated each time a new pulse pair was generated. Such amplitudes could then be retrieved and used as an indication of a loss of capture threshold when the LOC signal is generated.

Once the capture loss threshold is known, requesting an automatic capture mode can be a simple task. In automatic capture mode, the energy of the V-pulse is a predetermined safety factor (SF) rather than a predetermined loss-of-capture threshold.
Only set to the value above.

In the above example, the energy content of the preceding pulse of the pulse pair is adjusted by reducing its amplitude. Amplitude regulation is not the only way to control the energy of an electrical pulse. Energy can also be adjusted by changing the pulse width of the stimulation or both the amplitude and the pulse width.

As further seen in FIG. 4, the operating parameters required to operate the pacemaker in the autothreshold and / or autocapture modes are preferably stored in pacemaker memory 120. Such operating parameters can then be programmed using conventional pacemaker programming techniques to set them to the appropriate values for a given patient. The operating parameter is typically the amplitude AV of the backup stimulation pulse.
_B , the amplitude of the preceding stimulation pulse AV _{P of} the first pulse pair (which may be equal to AV _B ), the step size ΔAV _{P at which} the amplitude of the preceding pulse is reduced per pulse pair, the basic pacing interval T _PI (this Can be generated by other pacing circuits), T
Wave detector amplifier blanking period T _BLANK , isolation time t _S between the leading and backup pulses of each pulse pair, reference time interval T _R , between measured time interval T _i and reference time interval T _R. It contains the magnitude T _TOL of the change or tolerance that may exist prior to the declaration of the LOC condition. T
The value of _R can be determined based on the average of the stimulation-T interwave time intervals measured during the previous 3-10 pacing cycles, as described above.

Again, it is emphasized that what has been described with reference to FIG. 4 is a description of the function. One of ordinary skill in the art could readily configure various types of circuits and programmed processors that perform the functions described above. For example, the V _i and V _B pulse generators 106, 108 and the T-wave detection amplifier 110 can be easily incorporated into a digital controller circuit or programmed processor.

The pulse generators 106, 108 may be of conventional design and are typically the same ventricular pulse generators used by the pacemaker 10 (FIG. 1).
In this regard, it should be noted that separate pulse generators need not be used to generate the pulse pairs, but separate pulse generators could be used if desired.
May be used. Rather, a single pulse generator triggered twice (once with a T _P pulse and once with a T _B pulse after t _S seconds) may be used.

The T-wave detector amplifier 110 may likewise be of conventional design, that is, a detector amplifier of the type used to detect ventricular depolarizations (R amplifier in FIG. 1). However, preferably an amplifier other than the one used for R-wave or P-wave detection should be used for the T-wave amplifier 110. This is because the bandwidth of the T wave amplifier must be selected to be slightly wider than that of the R wave or P wave amplifier. In addition, the use of a separate T-wave amplifier allows the T-wave to be of any lead configuration, ie monopole, normal dipole (tip-ring) or modified dipole (ring-case or tip-case) lead configurations. Can be detected.

Advantageously, the present invention may be used with most types of pacemakers. If the pacemaker in which the present invention is used is a rate response (RR) pacemaker, that is, one or more physiological sensors are used in an attempt to detect the metabolic demand of the patient and the pacemaker's basic pacing interval is used. If the sensor indication rate (SIR) is generated from the information thus detected to be changed, the automatic threshold feature of the present invention is preferably
It should only be done when the patient is at rest.
Thus, a preliminary step in practicing the present invention with a rate responsive pacemaker is to focus on the SIR signal and
Retaining the automatic threshold mode of the present invention until the R signal indicates that the patient is resting.

As mentioned above, for an automatic threshold routine to be successfully called, it is in ventricular pacing mode, that is, it monitors the naturally occurring R-waves and only occasionally generates V-pulses. In contrast to the mode, it is necessary to be in the mode in which the V pulse is generated to pace the ventricle. To ensure ventricular pacing, it is preferred that the pacer include the ability to measure the natural heart rate, a feature commonly available on many RR pacers. If such a measurement results in a heart rate of, for example, 75b
pm, but if it is clear that the pacing rate is set to 60 bpm (which means that pacing has not occurred), the present invention preferably determines that ventricular pacing does occur. To ensure that the basic pacing rate is greater than the measured heart rate, eg 75 bp
Includes the ability to increase to m. If RR pacing is used, this adjustment of the basic pacing interval can be linked with a check of the SIR signal to determine if the patient is active. If active, a faster heart rate is taken into account. If inactive, the pacer automatically adjusts the pacing rate to a value greater than the measured rate.

Alternatively, the overall basic pacing interval (VA
Interval or VA interval plus AV delay or AV delay) is adjusted, but the overall basic pacing interval is kept at the same value (ie, AV delay is of a magnitude and approximate The AV delay of the pacer can be shortened (while increasing the VA interval by the same amount).

In addition, if the pacer is operating in the pacer mode where the P wave is being tracked, the P wave or atrial rate can be monitored. If a high atrial rate is present, and if the SIR signal indicates a fast rate, the automatic threshold feature of the present invention is reserved until the patient is at rest (ie, the atrial rate slows down). Should be. However, if the SIR signal indicates that the patient is at rest at the same time when the atrial rate is high (such as if the SIR signal is based on the detected activity of the patient and the patient is emotionally To ensure that pacing occurs in the ventricle prior to the initiation of the automatic threshold routine, which may occur when in rest while experiencing a charged event in the AV.
The delay may be shortened and / or the basic pacing interval may be shortened.

In fact, there are a variety of techniques that can be used to ensure ventricular pacing, any of which could be used in the automatic threshold assessment according to the present invention. Regardless of which technique is used and what type of pacing mode is used, the automatic threshold routine of the present invention will operate when needed (eg, 1
(1-4 times a day), with a preceding pulse that simply (1) starts with an energy level sufficient to achieve capture, and then reduces the energy in a predetermined manner, and an energy level sufficient to achieve capture Emitting a series of stimulation pulse pairs, each pair having a subsequent pulse starting at, and staying at that energy level, and (2) a constant reference point, eg, the first pulse of each pulse pair to each pacing cycle. It is performed by measuring the time interval until the T wave of. As long as the time interval to the T-wave remains relatively constant as the energy of the leading pulse of each pulse pair is reduced, the capture threshold has not yet been reached by the leading pulse. As soon as the time interval to the T-wave is significantly longer, the capture threshold has been exceeded by the preceding pulse, and the leading pulse of Energy can be used as an indicator of capture threshold.

Referring now to FIG. 6, there is shown a block diagram of the main hardware components of a preferred pacemaker in which the present invention may be used. The system includes an external programmer 48, implantable pacemaker 10 and lead system 19. Lead system 19 includes conventional atrial and ventricular leads and electrodes, as described above. Lead system 19 may also include an oxygen sensor lead, which includes an LED detector used to measure the oxygen content of blood. Such leads are described, for example, in US Pat. No. 4,815,469, the contents of which are incorporated herein by reference.

The external programmer 48 includes a telemetry head 49 that is located near the implantable pacemaker 10 whenever a communication link is to be established between the pacemaker 10 and the external programmer 48. The external programmer may be of conventional design such as that described in U.S. Pat. No. 4,809,697. The contents of this patent specification are hereby incorporated by reference.

The components of pacemaker 10 are housed in a suitable sealed case or housing 400 (which case or housing is shown in FIG. 6 by dashed line 400). Case 400 is preferably a titanium metal case. The components inside the case 400 are the RF coil 402, the memory chip 404, and the battery 40.
6, including one or more sensors in sensor circuit 408, crystal 412, output / protection circuitry 412, analog chip 420 and digital chip 440.

Battery 406, which is the largest component in a pacemaker in terms of volume, can be of conventional design and is a lithium battery that provides operating power to all of the electronic circuitry within the pacemaker. The RF coil 402 is used to establish the communication link 50 with the telemetry head 49. The crystal 412 is a digital chip 440.
Used in combination with the crystal oscillator circuit above (below) to provide a stable clock frequency for the pacemaker circuit. In the preferred embodiment, the frequency of the crystal oscillator is 32 kHz, although any suitable frequency may be used. The sensor circuit 408 includes suitable sensors used by the pacemaker in performing the rate responsive pacing function. For example, in one embodiment sensor circuit 408 includes an accelerometer configured to detect patient activity.

The memory chip 404 is a low power static random access memory (RAM) chip in which pacemaker operating parameters such as control variables and sensed data may be stored as desired. Analog chip 420 and digital chip 44
0 contains the main pacemaker processing and control circuitry. These chips are advantageously designed to minimize the number of components externally required for pacemaker operation. The analog chip 420 is connected to the lead system 19 through the output and protection network 412.
Interface with. The circuitry 412 includes output capacitors, such as are commonly used in implantable medical devices, and suitable feedthroughs to allow electrical connection through a sealed case.

Next, referring to FIG. 7, the analog chip 4
Twenty block diagrams are shown. The analog chip contains all of the subsystems and modules necessary to interface with the lead system 19 and digital chip 440. For example, the start-up / bias current / reference module 422 includes a power-up signal used to initialize the pacer circuit when the battery is first used. The low battery module 424 detects four voltage levels of the battery voltage to determine the state of the battery. Case amplifier 426 produces a case bias voltage that is used as a reference for detection and IEGM (intracardiac electrogram) amplifier module 428. Module 428 includes P wave amplifier 22, R wave amplifier 24 and T wave amplifier 110 described in FIG. The measured data module 430 measures battery voltage and current and other analog parameters of the pacing system. AD
The C and logic module 432 contains the analog-to-digital converter and timing logic used to convert the pacemaker analog signal to an 8-bit digital word. These digital words are then passed to digital module 434. This digital module is used to generate all of the basic timing and bus control functions as data travels back and forth between analog chip 420 and digital chip 440.

Continuing to refer to FIG. 7, it can be seen that the run-away protection (RAP) circuit oscillator 436 is also connected to the digital module 434. Such oscillator 436 provides an independent time base for limiting the maximum pacing rate allowed by the pacemaker.
Further, the digital circuit 434 is connected to the sensor circuit network 40.
8 are connected. The sensor circuitry 408 includes suitable sensors for detecting activity and other parameters. For example, O ₂ sensor circuit 409 may be used in conjunction with an oxygen sensor lead to measure blood oxygen in a patient. Activity sensor 408 may also be used to detect patient activity, as measured by, for example, an accelerometer. The charge pump circuit 438 produces an output voltage for the stimulation pulse delivered to the patient's heart. The output switch network 439 connects the charge generated by the pump circuit 438 to the output lead at the appropriate time to form the appropriate stimulation pulse.

The analog chip 420 thus detects an atrial or ventricular event, digitizes the IEGM waveform, the measured data and various other analog signals, and such detected and digitized signal is provided by the digital chip 440. It contains the circuitry necessary to provide the digital module 434 for use. Charge pump circuit 438 acts as a voltage doubler / triple for high output pulse capability. The output pulse width is controlled by the output switch 439. Battery conditions are monitored and independent runaway protection is provided.

Referring now to FIG. 8, it can be seen that the main control element of the pacemaker is the microprocessor 442. This microprocessor is included in digital chip 440. Digital chip 440 contains all of the logic necessary to interface between analog chip 420 and internal microprocessor 442. Microprocessor 442 includes a basic CPU (Central Processing Unit) and 8K static RAM. In addition, 8K × 8 static RAM44
6 is connected to the microprocessor 442 for storing data and programs. The microprocessor support logic 444, also connected to the microprocessor 442, includes interrupt logic, timer logic,
Includes noise / detected event logic and magnet state logic. In addition, a bus controller 448 is included on digital chip 440 to provide DMA timing and control of data transfer with analog chip 420, including timing and control of analog-to-digital converter 432 (FIG. 7) and telemetry data. There is. Telemetry channel logic 450 includes clock logic, IEGM and marker logic, telemetry command protocol logic, telemetry interrupt logic, error checking logic and CPU reset logic. The RF transceiver 452 connected to the RF coil 402 transmits / receives data to / from the external programmer 48 through the telemetry head 49 (FIG. 6).
A crystal oscillator circuit 456 is associated with the crystal (external to the digital chip 440) to provide the time base for the pacemaker system. Current generator 454 provides a bias current for the digital chip. Reed switch circuit 458 detects the presence of a magnetic field. This magnetic field is present whenever the telemetry head 49 is placed on the patient's skin over the implanted position of the pacemaker.

The pacemaker circuit described above in connection with FIGS. 6-8 provides the basic functions of the pacemaker described in connection with FIG. 1 and other pacing / sensing functions known in the art. For the purposes of the present invention, the pacemaker circuits of Figures 6-8 set the basic timing of the pacing interval, including the setting of the AV and VA intervals. This pacemaker circuit is used for natural ventricular events (R waves), natural atrial events (P waves).
Wave), also for the detection of repolarization events (T waves), the measurement of the time interval between the pacing ventricular event 8 (V pulse) and the resulting T wave (ie repolarization time measurement). To take necessary measures for this.

As noted above, capture is assessed without the need to detect the evoked response immediately following a given stimulus, and at the potentially absence of potentially interfering polarization voltages at the electrode / tissue interface. An implantable pacemaker is provided. Further, the present invention provides a simple capture determination technique that does not require bipolar pacing leads or special low polarization electrode materials.

While the present invention has been described in terms of particular embodiments and applications, it will be understood by those skilled in the art that various modifications can be made within the scope of the invention as claimed. See.

[Brief description of drawings]

FIG. 1 is a block diagram of an exemplary pacemaker in which the present invention may be used.

FIG. 2 is a timing waveform chart conceptually showing a method of determining a capture threshold according to the present invention.

3 is a flow chart illustrating the method of the present invention as shown conceptually in FIG.

FIG. 4 is a functional block diagram illustrating components of a pacing system used to implement the present invention, according to one embodiment.

5 is a timing diagram illustrating the operation of the pacing system of FIG.

FIG. 6 is a block diagram of a preferred pacemaker in which the present invention may be used.

7 is a block diagram of an analog chip of the pacer of FIG.

FIG. 8 is a block diagram of a digital chip of the pacer of FIG.

[Explanation of symbols]

 10 Pacemaker 12 Heart 14, 16 Lead 15, 17 Electrode 49 Telemetry Head 400 Case 420 Analog Chip 440 Digital Chip

Claims (20)

[Claims] 1. A method for automatically assessing the capture threshold of an implantable pacemaker electrically connected to cardiac tissue, the method comprising: (a) paired stimulation sequences at a predetermined pacing rate. The pair of stimuli, including the process that occurs, has a tunable energy content, with the preceding stimulus of each pair of paired stimuli having an adjustable energy content beginning with the energy content on one side of the capture threshold. Subsequent stimuli of each pair of s always had an energy content above the capture threshold, and (b) were paired while adjusting the energy content of the preceding stimulus towards the capture threshold. (C) measuring a repolarization time as the length of time it takes for the heart tissue to repolarize following each pair of paired stimuli; ) Substantial changes during repolarization time Of the implantable pacemaker capture threshold, characterized by including a process of defining a capture threshold approximately equal to the energy content of the preceding stimulus of each pair of immediately preceding paired stimuli. Automatic evaluation method. 2. The step of generating a sequence of paired stimuli comprises the step of generating a subsequent stimulus of each pair of stimuli 60-100 ms after the preceding stimulus. The method according to item 1. 3. The process of adjusting the sequence of paired stimuli first sets the energy content of the preceding stimulus equal to the energy content of the subsequent stimulus and thereafter only by a predetermined magnitude. 3. The method of claim 2 including the step of systematically reducing the energy content of the preceding stimulus. 4. The process of adjusting the sequence of paired stimuli first sets the energy content of the preceding stimulus to a value below the capture threshold and then precedes it by a predetermined amount. 3. The method of claim 2 including the step of systematically increasing the energy content of said stimulus. 5. The step of adjusting the energy content of the preceding stimulus comprises the step of altering the energy content of the preceding stimulus each time a new paired stimulus is generated. The method of claim 2. 6. The step of defining a capture threshold is a step of defining a capture threshold approximately equal to an energy content of a preceding stimulus of a paired stimulus immediately preceding a change in repolarization time of at least 20%. The method of claim 1, comprising: 7. The method of claim 6 wherein the step of measuring repolarization time comprises the step of measuring the time interval between the preceding stimulus of each pair of stimuli and the occurrence of the T wave. Method. 8. The method of claim 6, wherein the step of measuring repolarization time includes the step of measuring the time interval between the subsequent stimulation of each pair of stimulation and the occurrence of the T wave. Method. 9. The method of claim 1, wherein the step of providing the paired stimulation sequence to the cardiac tissue comprises the step of providing the paired stimulation to the ventricular tissue. 10. The method of claim 1, wherein the step of providing the paired stimulation sequence to the cardiac tissue comprises the step of providing the paired stimulation to the atrial tissue. 11. (e) Once the capture threshold has been determined, the process of terminating the generation of paired stimuli, and (f) capturing the energy of the electrical stimulus subsequently generated by the pacemaker. 2. The method according to claim 1, further comprising the step of setting the sum equal to the safety factor. 12. A means for selectively generating ventricular stimulation at a rate defining a cardiac cycle to request depolarization of ventricular muscle tissue, and detecting a T wave representative of the repolarization of ventricular muscle tissue following the depolarization. A method of determining a capture threshold in an implantable pacemaker, the method comprising: (a) generating a pair of ventricular stimulation pulses during each cardiac cycle, the pair of ventricular stimulation pulses being 60 from the ventricular stimulation pulse (V _i ) and the original ventricular stimulation pulse
Backup stimulation pulse (V
_B ) and the original ventricular stimulation pulse has a first energy that decreases according to a set schedule, the backup stimulation pulse has a second energy, and the second energy has a ventricular Measure whether there is sufficient energy to capture and (b) there is a time interval between the delivery of the original ventricular stimulation pulse (V _i ) of each pair of ventricular stimulation pulses and the detection of the T wave And (c) defining a loss of capture conditions if the time interval measured in step (b) undergoes an approximate 60-100 millisecond change from the previous measurement. Of determining the capture threshold of an implantable pacemaker, characterized by: 13. An implantable cardiac pacemaker for automatically determining a capture threshold, comprising: timing means for defining a cycle of the heart; and means for generating a pair of ventricular stimulation pulses during each cardiac cycle. And the pair of ventricular stimulation pulses is the original ventricular stimulation pulse (V _i ) and the time T from the original ventricular stimulation pulse.
_A backup stimulation pulse (V _B ) that lasts within _S , where T _S is less than the natural refractory period of the heart tissue, and the native ventricular stimulation pulse has a first energy, and The backup stimulation pulse has a second energy, the first energy is adjustable energy, the second energy is always sufficient energy to capture the ventricles, and a predetermined schedule Means for systematically altering the energy of the original ventricular stimulation pulse of each pair of ventricular stimulation pulses, means for detecting the occurrence of a T wave, and each cardiac cycle in which the pair of ventricular stimulation pulses is generated. what includes a means for measuring the time period T _i of between ventricular stimulation pulse -T wave (V-T), the time period T _i is specified pair of ventricular stimulation pulses for Includes the time interval until the occurrence of the T-wave between the Luo same cardiac cycle, and the cardiac for monitoring the measured time period T _i and the substantial change in the measured time period T _i occurs And a means for defining a capture threshold approximately equal to the energy of the native ventricular stimulation pulse during the cycle. 14. The method of claim 13 wherein T _S comprises a time period of 60 milliseconds to 100 milliseconds.
Pacemaker listed. 15. The first energy of the original ventricular stimulation pulse is initially equal to the second energy of the backup stimulation pulse, and the means for systematically altering the energy of the original ventricular stimulation pulse is the original ventricle. 14. A pacemaker according to claim 13, including means for systematically reducing the energy of the stimulation pulse. 16. The first energy of the original ventricular stimulation pulse is initially low energy insufficient to capture the ventricle, and means for systematically altering the energy of the original ventricular stimulation pulse are provided. 14. A pacemaker according to claim 13, including means for systematically increasing the energy of the native ventricular stimulation pulse. 17. The means for monitoring includes means for comparing the measured time interval T _i with a reference time interval T _W , and the reference time interval T _W is for the generation of a pair of ventricular stimulation pulses. 14. The pacemaker of claim 13, including an average VT inter-T time period for at least three consecutive pacing cardiac cycles. 18. The means for comparing defines a capture threshold only when the difference between the measured time interval T _i and the reference time interval T _W is at least 0.9 × T _S. The pacemaker according to claim 17. 19. In an implantable pacemaker
In a capture threshold evaluation system for use, a timer defining a pacing interval and, in response to the timer, a stimulation pattern synchronized with the pacing interval.
And a pulse generator that generates a pair of lus.
The first pulse of the pair of scans has a first programmed amplitude
And the second pulse of the pair of pulses has a second program
The first and second pulses have a
Period t_SAre separated by, where t _SOf heart tissue
Less than the refractory period and the second programmed swing
The width is always set to a high value that causes capture,
To detect T waves that represent repolarization of depolarized cardiac muscle tissue
Starts when the pulse generator generates the first stimulation pulse, with the sense amplifier configured to
Also, the clock that stops when the detection amplifier detects the T wave
Occurs at each tuned interval, thus
The first stimulus of the pulse pair is the T wave as the size of the interval
The time it takes to occur at each pacing interval following the pulse
A time interval measuring means for providing a measure of the length of the interval, and a first programmed oscillation of the first pulse of the pulse pair.
Energy adjusting means for systematically adjusting the width, and the energy adjusting means for the first program of the first pulse
Measured systematic while adjusting the measured amplitude systematically.
The time interval is defined by the stimulation pulse that causes capture and such capture.
A reference meter that defines the rated delay to and from the T-wave as a result of the catch.
A comparing means for comparing with the time interval, and the measured time interval is t_SA size approximately equal to
Generates a loss-of-capture signal whenever it differs from the reference time interval
Capture means characterized by including means for
Threshold evaluation system. 20. Means for the energy adjusting means to initially set the first programmed amplitude of the first pulse of the pair of pulses to a value that causes capture, followed by the first.
20. A capture threshold assessment system according to claim 19 including means for systematically reducing the programmed amplitude of.