WO1993012710A1

WO1993012710A1 - Gastric probe

Info

Publication number: WO1993012710A1
Application number: PCT/GB1992/002392
Authority: WO
Inventors: Andrew Roy Webb
Original assignee: Medicina Ltd
Priority date: 1991-12-24
Filing date: 1992-12-23
Publication date: 1993-07-08
Also published as: GB2276939A; GB9127390D0; GB9410761D0

Abstract

A gastric probe comprises a light source (23) connected by a first fibreoptic connector (22) to a light dispersing member (21), said member being disposed in a body of liquid contained in a gas permeable container (20), said liquid containing a compound adapted to fluoresce upon excitation by said light source, the intensity of the fluorescence being dependent on the concentration of gas within said liquid, a second fibreoptic connector (29) extending from a location within the container to a detector adapted to produce a signal indicative of the concentration of gas in said liquid.

Description

GASTRIC PROBE

This invention relates to a gastric probe. The invention is particularly, though not exclusively suited for providing a continuous indication and record of gastric intramucosal pH.

Many seriously ill patients suffer major complications or die following trauma or major surgery. A major cause of death is multi-organ failure. It is believed that ischaemia is one of the major, complications which can lead to multi-organ failure. Following major surgery, trauma or in otherwise critically ill patients, the blood flow to and around the intestine may be severely reduced causing the gut wall to break down. If the gut is ischaemic for any length of time, for example 20 minutes or more, it does not form an effective barrier to endotoxins or bacteria. Release of endotoxins or bacteria from the gastrointestinal tract into the blood stream can induce multi-organ failure.

A fall in intramucosal pH may precede development of ischaemia. Intramucosal pH may give a better indication of a patient's haemodynamic status than other commonly used indicators such as cardiac output, blood pressure, heart rate or pulmonary capillary wedge pressure.

In order to prevent multi-organ failure it is desirable to measure the intramucosal pH. Unfortunately there is no direct method of measurement of this parameter although it is known that intramucosal pH can be calculated on an intermittent basis from pC0₂ (partial pressure of C0₂) or other indicia of pH in luminal fluid and the bicarbonate concentration in arterial blood.

In known methods of measuring the pC0₂ in the lumen of the gut it has been necessary to obtain a sample of fluid that has been in contact with the wall of the gut for a certain period, usually at least half an hour.

In a previous arrangement this was achieved by introducing a catheter into the stomach of the patient. The catheter had a gas permeable liquid impermeable balloon at its tip. The balloon is placed against the stomach wall and filled with a saline solution. The apparatus is left in contact with the stomach wall sufficient time for the gases to equilibrate (usually 30 minutes). Removal of a sample of the saline solution allows the concentration of carbon dioxide to be measured. In this way, the pC0₂ within the lumen of the gut can be measured and used in conjunction with the bicarbonate ion concentration (HC0₃ ^~) obtained from an arterial blood sample to determine the pH.

This known sampling technique- ~-rttay involve a risk of operator and blood gas analyzer error and also has a major clinical disadvantage in that it is slow due to the equilibration time intervals of 30 minutes.

In the care of patients undergoing major surgery or in intensive care it is possible for major blood flow changes to occur rapidly and therefore be missed by the catheter because of the necessarily long sampling time. Even when using correction factors to compensate for the equilibration time, measurements can only be made at 30 minute intervals.

Alternative procedures involving withdrawal of liquid from the gut have the disadvantage that liquid withdrawal alters the equilibrium of the remaining liquid so that liquid replacement becomes necessary and time must be taken for the physiological system to re—equilibrate.

It would be advantageous to have a method of continuous monitoring of the intramucosal pH which would remove the need for sampling and which would allow the physician to have a continuous recording of the status of the patient.

According to one aspect of the present invention there is provided a gastric probe comprising a light source connected by a first fibre optic connector to a light dispersing member, said member being disposed in a body of liquid contained in a gas permeable container, said liquid containing a compound adapted to fluoresce upon excitation by said light source, the intensity of the fluorescence being dependent on the concentration of a gas within said liquid, a second fibre optic connector extending from a location within the container to a detector, the detector being adapted to produce a signal indicative of fluorescence of said liquid.

The invention confers the advantage that concentrations of a gas which may be carbon dioxide or oxygen can be measured constantly allowing rapid detection of the onset of ischaemia. This has the highly desirable and beneficial advantage of providing a physician with early and vital warning of possible multi-organ failure. The gastric probe, according to the invention, has the further advantage that it is not necessary to extract any fluids from the digestive trac _-_i/bich could cause a resulting imbalance in the equilibrium state of the fluids present in the stomach and render any subsequent pH measurement inaccurate. A further important benefit is that the device can be easily calibrated and operate over a range of pH conditions which may be encountered in a stomach of a critically ill patient. This removes any need to carry out time consuming calibrations of the device allowing more time to be devoted to other important and necessary tasks. None of the known prior art techniques confer any of these highly desirable advantages.

According to a preferred embodiment of the invention the gastric probe includes a catheter tube.

This embodiment has the advantage that fluids may be introduced via the catheter tube into a patients stomach. Aspiration of a patients stomach can be performed whilst monitoring the pH and preventing the undesirable necessity of having to remove the probe while aspiration is performed.

The signal may be analyzed by a microcomputer to provide a continuous display. The continuous display may be a VDU, a plotter or both. Furthermore, the use of a microcomputer may allow the simultaneous analysis of arterial blood pH or bicarbonate content together with other important physiological parameters such as body temperature and pulse.

In preferred embodiments of the invention the permeable container may include a gas permeable membrane adapted to be located in the patient's digestive tract, the pH of the liquid within the membrane being dependent on the concentration of carbon .dioxide or oxygen in the tract. The intensity of fluorescence may be dependent upon pH within the container which in turn may be dependent on the pC0₂ within a patient's stomach .

The fluorescent compound may be fluorescein. This has the advantages of strong fluorescent properties, the intensity of the fluorescence being pH dependent. Fluorescein has the important property that it is non-toxic and this ensures that in the event that a leak or rupture of the permeable container wall, occurs the patient will not suffer from poisoning.

Preferably, the fluorescent compound contains a light dispersant compound. This helps to ensure that the body of liquid in the permeable container is excited by the light emitted from the light source.

Preferably, the light dispersant compound is an intra¬ lipid. This type of compound has been found to be an effective light dispersant.

A suitable fluorescent compound may comprise fluoroscein diluted to a concentration of about 60mg/l with water and to which about 5% intralipid is added as a suitable light scattering material.

The fluorescent compound may be buffered to a predetermined pH.

The permeable container may be elastic. This has the advantage that the permeable container could be deflated to enable easy insertion of the device. This may be particularly important if there is a partial restriction or obstruction in the oesophagus or digestive tract of a patient. Furthermore, once the device has been located in the desired position the permeable container may be inflated thereby presenting a larger surface area to surrounding gastric intra-mucosal fluids.

Alternatively the permeable container may comprise a tubular or otherwise shaped container having a plurality of perforations, each perforation being sealed by a gas permeable material. This construction of permeable container is less prone to damage and has the advantage that it may be easier to repair in the event that the permeable membrane is perforated. The permeable membrane may be provided on the internal surface of the perforated tube. This further reduces the risk of damage to the permeable membrane especially damage which may occur from abrasion during insertion or removal of the probe from a patient.

The gas permeable membrane may comprise polyvinyl chloride. Alternatively the gas permeable membrane may comprise a silicone elastomer. Polyvinyl chloride and silicone elastomer have the appropriate gas permeable properties.

Preferably-the permeable polyvinyl chloride membrane has a thickness of about 3 microns.

A guide wire may be located in the device to facilitate insertion and positioning of the device in a patient. It is preferable to position the permeable container adjacent to a portion of the stomach wall.

A chemically responsive field effect transistor (chemfet) may be provided on the external surface of the tube. This may provide additional information on the condition of a patient, in particular, it may provide information on gastric luminal pH.

The light source may comprise a laser, selected to emit at an excitation wavelength of the fluorescent compound.

In preferred embodiments the light source further comprises, light chopping means to produce regular pulsed light and a lens for collimating the pulsed light for transmission via a fibre optic connector.

Preferably the wavelength of light emitted by the light source is greater than about 400 n , more preferably greater than about 450 nm.

Preferably, the wavelength of light emitted by the light source is less than about 570 nm, more preferably less than about 520 nm.

A preferred wavelength of light emitted by the light source is 488 nm.

The detector may contain a photo-electric cell. This provides a suitable electrical signal which may be processed to provide an indication of the gastric intra-mucosal pH.

The first and second fibre optic connectors may both terminate at the light diffusing member. Alternatively the connectors may terminate separately. The light diffusing member provides an initial scattering of light from the light source with further scattering taking place due to the presence of a light dispersing compound within the fluorescent compound. Graduations may be provided along the length of the tube. This may give an indication of the location or position of the device inside a patient during and after insertion.

The gastric probe may be manufactured from materials which are opaque to X—rays to allow the device to be clearly identified on an X—ray photograph.

The invention also provides a method of measurement of a gas concentration in intestinal mucosal fluid including the step of use of the apparatus in accordance with the first aspect of this invention.

The method confers the advantages of ease of calibration and absence of interference by electronic equipment. Instrumental drift, such as is encountered with PC0₂ or pH electrodes is not encountered. In addition the apparatus is robust and inexpensive.

The invention is further described, by means of example only, but not in any limitative sense with reference to the accompanying drawings, in which:

Figure 1 is a plan view of the first embodiment of the gastric probe;

Figure 2 is a schematic of the workings of the fibreoptics and light source incorporating a micro-computer and display system;

Figure 3 is a plan view of another embodiment including a combined nasogastric feeding and gastric aspiration tube;

Figure 3a is a cross sectional view of Figure 3 taken along the line 3A—3A of Figure 2;

Figure 4 is a plan view of another embodiment incorporating an electrical sensor for gastric luminal pH;

Figure 4a is a cross sectional view of Figure 4 taken along the line 4A-4A;

Figure 5 is a plan view of another embodiment incorporating a nasogastric feeding or aspiration tube together with optical and electrical sensing elements; Figure 5a is a cross sectional view of Figure 5 taken along the line 5A-5A;

Figure 6 is a plan view of an embodiment which uses a plurality of apertures in the catheter wall covered with a semi-permeable membrane; and

Figure 7 illustrates the catheter in situ for measurement of pH of the stomach;

Figure 1 illustrates a first embodiment of the gastric probe according to the invention.

The catheter comprises a length of suitable tubing 10 one end of which is closed and the opposite end has a connector 11, an example of which is known as a luer lock, to which can be attached a three-way tap 12. This three-way tap may be used to selectively connect various methods for introducing or aspirating liquid 15 into the gastric probe and into the sampling chamber 16, for example a syringe 14.

The sampling chamber 16 is made up of a balloon like membrane 20 which is permeable to gas but not to liquid. In the preferred embodiment it is permeable to C0₂ but not hydrogen or oxygen. It is also impermeable to other materials such as gases and proteins which may interfere with the measurement of pH and pC0₂. In the preferred embodiment a suitable permeable membrane material is silicone elastomer although polyvinyl chloride may be employed. This membrane is bonded to the tube 10 at 19 such that no leakage of liquid 15 is possible.

Tube 10 has perforations 28 enclosed within the sampling chamber such that the sampling chamber is in contact with the inside of the tube. These perforations are close to the edges of the balloon so that aspiration of the balloon is made easier.

It should be noted that because the measurement of pH and pC0₂ are not be liable to drift then the sampling chamber 16 need not be in contact with the tubing. However, whilst calibration, which involves removal of the fluid is necessary, it will be important to be able to aspirate and introduce liquid 15 from the chamber. The tube 10 should be made up in part of C0₂ impermeable material. It should also be radio-opaque from viewing under X— ray conditions.

The sampling chamber is filled with a liquid 15 which undergoes changes in fluorescent properties in response to corresponding changes in pH. This liquid is preferably able to be--excited into fluorescence by a light source and change its fluorescence over the pH range present in the sampling chamber. The fluorescent liquid must be non-toxic and the fluorescence should preferably not display hysteresis or drift. The fluorescent liquid may contain a light dispersing compound to scatter light such that the changes in fluorescence can be monitored by a detecting fibreoptic system. A suitable light dispersing compound is an intralipid which may comprise 5% content by volume of the fluorescent liquid.

In the preferred embodiment the liquid is fluorescein diluted to a concentration of 60mg/l with sterile water then buffered to the appropriate pH and to which has been added a light scattering liquid which, in this case, is 5% intralipid. The liquid 15 in the chamber 16 is excited by a light source 23. This equipment, shown in Figure 2, operates in the wavelength range of 450-520 nm; suitable light may be provided by a blue line Argon laser 23a operating at 488 nm. The light is chopped into pulses by a chopper 24 connected to a phase sensitive detector 31. The light is focused into a fibre optic cable 22 by a lens 25 and passed down the cable to the isotropic detector 21. The isotropic detector 21 scatters the light in a 360° arc. The changes in pH in the sample chamber 16 cause changes in the fluorescence of the liquid 15. These changes are detected by the second fibreoptic cable 29 joined to the isotropic detector 21. The light is processed using band pass filters 28 to cut out the exciting light and any external fluorescence which may be caused by extraneous factors, for example from the stomach wall or its contents. In a preferred embodiment these filters operate in the 550-650 mm range.

In other embodiments it may be necessary to use two, three or four light sources operating at different wavelengths or one light source with different filters. It may also be necessary to pass these wavelengths down either one or more fibreoptic cables into more than one isotropic detector to give a reading which is free from drift.

Once past the filters the light passes through a phot- multiplier 3-0a--and a monochromator 30b before passing into the phase sensitive detector 31. The phase sensitive detector produces an output electrical current which is passed into the microcomputer 32 via a suitable interface. The microcomputer analyses the signal and displays it on either a CRT screen 33 or a chart recorder 34 or both. The microcomputer may also have the facility to allow manual input of the patient's arterial blood pH or HC0₃ ^" concentration or this may be done electrically from a continuous detector 35. The computer may have the facility to monitor the pH values and display them on a CRT screen for a comparison with the present value and also with selected upper and lower alarm limits. An alarm may sound if the measured levels exceed those set.

Referring to Figure 1 the fibreoptic cables exit the catheter at a sealed aperture 9 via a special connector 7 which splits the fibres into separate paths. This connector 7 fits into a connector 6 which takes the individual fibres into the processing equipment and light source.

A second embodiment of the catheter is shown in Figure 3 and 3A. The catheter is appropriately configured to also serve as a nasogastric feeding/gastric aspiration tube. With reference to Figure 3 the catheter comprises a double lumen tube 36 which has two individual non-communicating lumens 37 and 39. The catheter lumen 37 contains the fibreoptic cables 22 and 29 and is in communication with the sampling chamber 16. A feeding/aspiration lumen 39 is provided through which a guide wire 40 passes to aid insertion and placement of the catheter. A semi-permeable membrane 20 is attached as in the first embodiment but a portion of tubing 42 to extends beyond the end of the membrane to form the nasogastric feeding/aspiration tube. The tubing 42 has a number of holes 43 through which the feed can be passed or the stomach aspirated. At the opposite end the tube splits to form three connectors. The lumen 37 communicates with the tubing 45. The fibreoptic cables 22 and 29 exit through a sealed aperture 46. The feeding/aspiration lumen exits via the feeding port 47. The tube 36 ends with a three-way tap identical in function and purpose to the three- way tap described previously and- shown in Figure 1. The fibreoptic cables 22 and 29 end in a connector 7 as shown in Figure 1. The feeding lumen 39 ends in an adaptor 48 which allows connection to standard feeding equipment. The guide wire 40 ends in a connector through which aspiration can take place to determine whether the feeding tube 42 is placed correctly in the stomach. All connections are preferably made of moulded plastic material and comply with regulations related to connection of fittings (for example for luer and nasogastric feeding).

A third embodiment of the catheter is shown in Figure 4 and 4a. In this embodiment the catheter, as well as being configured to measure gastric intramucosal pH, can also be used to measure gastric luminal pH using a chemically responsive field effect transistor (a chemfet) . The chemfet may provide additional information on the condition of a patient. The second lumen of a double lumen catheter 55 proceeds to the end of the tube 56. The chemfet sensor 51 is mounted such that it is in communication with the outside of the tube 56 and therefore in direct contact with luminal fluid but isolated from the inside of the tube 56. A suitable electrical conductor 50 is then used and routed through the lumen 55 to exit through a sealed aperture at 52. These electrical signals can be processed in the microcomputer 32 to give either on screen 33 or chart recorder 34 information, or both.

In a further embodiment, shown in Figure 5 and 5a, the catheter utilises intramucosal pH measurement (as shown in Figure 1 and described above) with a nasogastric feeding or gastric aspiration tube and a chemfet sensor 63 for luminal pH measurement. In this embodiment a triple lumen catheter 62 is used. The sampling lumen 37 contains the fibreoptic cables 22 r

- 1 1 -

and 29. The feeding lumen 39 contains a guide wire 40. The chemfet lumen 60 contains an electrical wire 61. In this embodiment the chemfet 63 is positioned on the outside of the catheter wall away from the feeding ports 43 so that the feed does not interfere with the luminal pH reading. At the other end of the catheter the tube 62 splits into four with the fibreoptic cables 22 and 29 and elecJric≥,l «cable 64 coming through sealed apertures 46 and 52 respectively. A feeding lumen adaptor 48 and sampling lumen three-way tap 12 is used as described above.

In yet a further embodiment, which is shown in Figure 6, the sapling chamber is formed by making a plurality of apertures 70 in the catheter tube 71. These apertures are covered in a semi-permeable membrane material of similar material used and described above for the balloon. Such suitable material is polyvinyl chloride or a silicone elastomer. The permeable membrane may, alternatively, be provided on the internal surface of the perforated tube. This further reduces the risk of damage to the permeable membrane especially damage which may occur from abrasion during insertion or removal of the probe from a patient.

An isotropic detector 73 and fibreoptic cables 22 and 29 are located inside the tube 71 where they are less susceptible to damage during use, insertion and removal.

This form of sampling chamber may be used in previous embodiments already discussed above.

Graduations may be provided along the length of the catheter tube to give an indication of the location or position of the device in a patient during and after insertion.

Claims

1. A gastric probe comprising a light source connected by a first fibreoptic connector to a light dispersing member, said member being disposed in a body of liquid contained in a gas permeable container, said liquid containing a compound adapted to fluoresce upon excitation by said light source, the intensity of the fluorescence being dependent on a concentration of gas within said liquid, a second fibreoptic connector extending from a location within the container to a detector, the detector being adapted to produce a signal indicative of fluorescence of said liquid.

2. A gastric probe as claimed in claim 1 wherein said gas is oxygen or carbon dioxide.

3. A gastric probe as claimed in any preceding claim further comprising a catheter tube.

4. A gastric probe as claimed in any preceding claim wherein said fluorescent compound is aqueous fluoroscein.

5. A gastric probe as claimed in any preceding claim wherein the fluorescent compound includes a light dispersant compound.

6. A gastric probe as claimed in claim 5 wherein the light dispersant compound is an intralipid.

7. A gastric probe as claimed in any preceding claim wherein the permeable container includes a plurality of perforations, each perforation being sealed with a gas permeable membrane.

8. A gastric probe as claimed in any preceding claim wherein the membrane comprises polyvinyl chloride or a silicone elastomer.

9. A gastric probe as claimed in any preceding claim wherein the first and second fibreoptic connectors both terminate at the light diffusing member.

10. A method of measurement of gas concentration in intestinal mucosal fluid including the step of use of apparatus in accordance with any preceding claim.