WO1988006860A1

WO1988006860A1 - A method and a system for the in vivo determination of an electrical potential difference

Info

Publication number: WO1988006860A1
Application number: PCT/DK1988/000043
Authority: WO
Inventors: Liselotte HØJGAARD; Kim Saksager; Einer Krag; Jens Rikardt Andersen; Niels Dürkopp LINNET; Frank Nygaard Olsen
Original assignee: Radiometer A/S
Priority date: 1987-03-13
Filing date: 1988-03-11
Publication date: 1988-09-22
Also published as: DK130187D0

Abstract

The in vivo determination of an electrical potential difference across a human or animal membrane of a fluid-containing cavity, such as the stomach (200), is performed by arranging (e.g. via an endoscope (201)) a measuring half cell (20) (electrode and electrolyte) in its entirety within the cavity and in contact with the cavity fluid, and arranging a reference half cell (50) in contact with a human or animal fluid outside the cavity, typically the blood, suitably via a catheter (87). The concentration of a species such as a H+ concentration in gastric juice contributing significantly to a liquid junction potential between the measuring electrolyte and the cavity fluid is measured, preferably by means of a sensor such as a pH sensor (110), preferably arranged within the cavity, and a relevant correction of the potential difference measured is based on said concentration measurement.

Description

A METHOD AND A SYSTEM FOR THE IN VIVO DETERMINATION OF AN ELECTRICAL POTENTIAL DIFFERENCE

FIELD OF THE INVENTION

The present invention provides a method and a system for measuring electrical potential differences across a human or animal membrane of a fluid-containing cavity in a human or in an animal. Determinations of the potential differences are useful in experimental studies and in clinical routine to characterize the integrity and function of the human or animal membrane.

BRIEF DISCLOSURE OF THE INVENTION

Electrical potential differences across human or animal wall parts are due to the transport of ions across said wall parts and are thus related to the properties and function of the membranes constituting said wall parts. The ion transport function may be impaired by pathological changes in the human or animal membranes. Consequently, measurements of the potential difference (PD) across human or animal membranes are useful in diagnosing a number of diseases in said membranes.

Many human or animal cavities are defined by membranes through which a constant and important ion transport takes place, and this trans¬ port results in potential differences (PD's) across the membranes. Of particular practical interest are PD's across membranes defining a fluid-containing cavity. The present invention provides a reliable, precise, and easily applicable method and system for the in vivo determination of electrical potential differences across a human or animal membrane of a fluid-containing cavity in a human or in an ani¬ mal. According to one main aspect of the invention the PD across a membrane of a fluid-containing cavity is determined by means of a sensor within the cavity, i.e. a PD measuring half cell in its en¬ tirety is positioned within the cavity. When an animal tract leads from the exterior of the human or animal to the interior of the cavi¬ ty it is possible to introduce a PD measuring sensor of the invention to the interior of the cavity via said tract. Thus, PD measurements can be obtained without artificially establishing pathways to the interior of the human or animal, in other words without damaging or hurting the human or animal by invasive surgical procedures.

Particularly interesting for experimental and diagnostic purposes are PD determinations from cavities belonging to the gastrointestinal tract, the hepatic and pancreatic system, the urogenital and respira¬ tory system, from cavities containing cerebrospinal fluid and from peritoneal or pleural cavities. Furthermore, when cavities are formed inside animals due to pathological processes, PD measurements within said cavities may provide valuable clinical information.

In one of its broadest aspects the invention provides a method for for the in vivo determination of an electrical potential difference (PD) across a human or animal membrane of a fluid-containing cavity in a human or in an animal, the method comprising:

arranging a PD measuring half cell comprising a PD measuring elec¬ trode and a PD measuring electrolyte within the fluid-containing cavity so as to establish an electrochemical contact between the PD measuring electrolyte and the fluid contained In the cavity, (the cavity fluid) ,

arranging a PD reference half cell comprising a PD reference elec¬ trode and a PD reference electrolyte so as establish electrochemical contact between the PD reference electrolyte and a fluid of the human or animal but outside the fluid-containing cavity (non-cavity reference fluid) .

detecting the PD between the two PD half cells, and

determining the PD across the animal membrane of the fluid-containing cavity on the basis of the detected PD. The method of the invention makes it possible to perform the PD determination continuously or intermittently over a period of time comprising several hours or days, with the PD measuring sensor remaining in the cavity, without any substantial discomfort to the subject to be examined, and without laborious maintenance and sur¬ veillance of the equipment.

The PD measuring sensor may be introduced safely and easily into the human or animal cavity by means of an instrument such as an endoscope for the visual or photographical examination of the membranes of the cavity, preferably by introducing the PD measuring sensor via a channel situated within the endoscope. This is a valuable feature of the method since endoscopy is important in clinical practice and experimental studies and is being used increasingly.

Theoretical interest has recently been drawn to the fact that the potential difference across an animal membrane may vary depending on the exact position of the site for the electrochemical contact with the fluid contained in the cavity. Thus, the PD value obtained when the PD measuring electrolyte contacts a fluid at some distance from the membrane represents a "summation potential" and it often differs from the PD value obtained when the PD measuring electrolyte contacts a fluid layer immediately adjacent to specific parts of the membrane (a so-called membrane-ad acent fluid layer). The latter PD (the membrane-adjacent PD) is supposed to be a more "informative" PD in that it is supposed to give information about a specific part of the membrane, namely the part in the immediate vicinity of the PD sensor. This makes it possible to diagnose localized pathological changes in said part of the membrane. It is of the utmost clinical importance to identify the site and extent of pathological changes of a membrane, and especially to differentiate between malignant and benignant pathological changes.

Since the method and system according to the present invention permit the use of an endoscope simultaneously with the PD determination, the method and system provide excellent means for the determination of the above-described membrane-adj cent PD's and hence for correlating the determined PD to the visual appearance of a specific part of a membrane.

The PD measuring system of the invention is primarily based on the provision of a PD measuring half, cell which is sufficiently small to be introduced in a convenient and safe manner into a human or animal cavity, preferably small enough to be introduced via a channel within an instrument for visual and/or photographic examination of said cavity.

According to another main aspect of the invention, the problems as- sociated with irrelevant electrical potentials within the measuring electrochemical chain itself, especially liquid junction potentials (PDlj's), are solved by performing an essential adjustment. This ad¬ justment, which will be explained in further detail below, may be performed by a simple and time-saving method based on principles which take into account essential theoretical considerations. Fur¬ thermore, the problems associated with the establishment and use of long salt bridges are eliminated according to the methods of the present invention.

The main advantages of the methods according to the present invention can be summarized as follows:

- the methods are accurate, precise, stable, easily applicable and inexpensive, and they require no special training of personnel,

- the examination procedure is not unduly prolonged due to a time- comsuming calibration of unstable equipment or the laborious obtainment of cavity fluid samples,

- the methods provide quick answers (within seconds or minutes) , thus providing a large examination capacity for routine purposes, which makes it possible to examine several patients or animals within e.g. one hour,

- determination of the above-described PDlj's is made possible. The main advantages obtained by combining the PD measuring procedure with endoscopy are the following:

- the PD value is obtained during the endoscopy examination period, thus, if the evaluation of a specific value obtained makes the ex- amining person want more values, this may be obtained without starting the examination procedure over again,

- determination of the above-described membrane-adjacent PD's is made possible,

- the procedure permits the obtainment of more precise diagnosis of pathological changes and may thus make it possible to differentiate between benignant and malignant changes.

BACKGROUND

The methods hitherto employed for PD determination have hot been satisfactory with regard to ease of use, stability and accuracy. The methodological problems of said methods have restricted their ap¬ plicability to experimental purposes.

A method previously used for measuring electrical potential differen¬ ces across gastrointestinal membranes was a method described by An- dersson & Grossman (S. Andersson & M.I. Grossman, "Profile of pH, pressure, and potential difference at gastroduodenal junction in man", Gastroenterology 49,4, 1965, pp. 364-371). This reference de¬ scribes measurements of potential differences measured from peripher¬ al body fluid to intestinal lumen, the measurements comprising the following features:

A part of a long tube containing a saturated KC1 salt bridge was introduced into the gastrointestinal tract of the subject whose PD was to be determined. Thus, during operation, a first part of the tube was situated inside the subject and was in electrochemical con¬ tact with the fluid contained in the cavity, and a second part of the tube was situated in the environments and converged into the elec- trode chamber of an external calomel electrode, thus connecting the salt bridge comprised in the tube with the external PD measuring electrolyte. A similar external long salt bridge was at one end con¬ nected to cutaneous and subcutaneous tissue fluid or blood of the subject and at the other end In electrochemical contact with an elec¬ trolyte in the electrode chamber of another external calomel elec¬ trode. Thus, both electron-conductive electrodes were situated out¬ side the subject to be examined. Furthermore, the KC1 saturated solu¬ tions were comprised in agar, the salt bridges thus being static bridges. Other methods (see below) employ free-flowing salt bridges.

These methods incur the following problems and disadvantages:

First, the long unshielded salt bridges were influenced by environ¬ mental electrical noise, e.g. static electricity due to movement of synthetic material such as the clothes often employed by hospital personnel. Free-flowing salt bridges required a considerable surveil¬ lance and maintenance work and implicated in practice the risk of disturbances such as air bubble accumulation or using up the flowing liquid, factors which made the measurements uncertain or irrelevant and which might necessitate starting the measuring procedure all over again.

Second, any spillover of the saturated salt bridge solution might sometimes cause damage to the membrane of the subject and thus make a PD measurement across the damaged part of the membrane a clinically irrelevant measurement.

Furthermore, any liquid junction potential difference (PDlj) present in the electrochemical chain influenced the PD measurements. To ob¬ tain the true transmembrane PD from the PD of the whole electrode chain (electrode circuit) the PDlj between the cavity and the PD measuring electrolyte should be compensated for. Thus, any attempt to measure and interpret transmembrane PD's must deal with the compli¬ cated problems of PDl 's and electrode potentials. The employment of salt bridges such as saturated KC1 bridges has commonly been supposed to reduce such PDlj's to small values. However, as described by Barry & Diamond (P.H. Barry & J.M. Diamond, "Junction potentials, electrode standard potentials, and other problems in interpreting electrical properties of membranes", J Membrane Biol 3 , 1970, pp. 93-122), em¬ ployment of salt bridges is unsatisfactory for accurate measurements, as the magnitude of the PDlj cannot be determined accurately employ- ing saturated KCl salt bridges. Especially when the PD's themselves are small, a "false" contribution to the PD value becomes signifi¬ cant.

One approach to estimate the magnitude of the liquid junction poten¬ tial in a special set-up is described by Read and Fordtran (N.W. Read & J.S. Fordtran, "The role of intraluminal junction potentials in the generation of gastric potential difference in man", Gastroenterology 76 , 1979, pp. 932-938). In the described experiments, a tube perfused with either isotonic saline or molar KCl was used as a free-flowing intraluminal electrolyte. The electrolyte was connected to a KCl agar bridge which was connected to a calomel half cell placed on a table next to the subject. The reference electrolyte was isotonic saline ^■ inserted in a cannula in the skin of the subject, the electrolyte then being connected to a KCl agar bridge which in turn was connected to another reference calomel half cell. Both calomel half cells were connected to an electrometer.

In order to perform the calculations of the magnitude of the PDlj 's according to Read & Fordtran, the various concentrations of the various ions present in the cavity fluid had to be carefully deter¬ mined. The determinations required the obtainment of several samples of the cavity fluid, the samples being collected repeatedly by aspiration. After collection, the samples had to be transported to various laboratory equipment for the measurement of the ion concentrations.

The fact that it was necessary to collect several samples and make several concentration measurements made this method unsuited for the simple obtainment of relevant and precise estimates of liquid junc¬ tion potentials. The composition of the ions in the sample of the cavity fluid could undergo changes from the time of collection to the time the measurement was actually performed. Additionally, the means for aspiration of the cavity fluid and the aspiration itself might induce artificial changes in the ion composition of the cavity fluid and in the PD value, changes obviously being irrelevant to the clini¬ cal situation. Furthermore, the moment at which the collection was performed and the moment at which the PD measurement was performed were not necessarily identical, in that the time elapsing from the moment at which the aspiration of sample began to the moment at which the cavity fluid was actually collected into an external container were not very well defined. Thus, in situations when significant changes occurred in the cavity fluid during the sampling period, the sample ion composition might not at all represent the ion composition of the cavity fluid at the moment at which the PD measurement was performed and thus it was obviously absurd to correlate the two com¬ positions. Furthermore, the aspiration of the cavity fluid prolonged the time necessary for the measurement of the PD. This prolongation could be unpleasant for the subject to be examined.

- The present invention provides a method and a system which differ from the ones previously employed and eliminate a major part of the above-described disadvantages of the methods hitherto employed. In addition, the method and system according to the invention provide new advantages which were not obtainable in the methods and systems hitherto employed, such as is mentioned above and explained in greater detail in the following.

Further references are:

- European Journal of Clinical Investigation, Vol. 16, No. 2, April 1986; Abstract from 20th Annual Meeting of European Society for

Clinical Investigation, March 19-22, 1986, Scheveningen, The Netherlands; L. Højgaard, J.R. Andersen, and E. Krag, "A New Method for Gastric Potentential Difference Measurements".

- Scandinavian Journal of Gas troeπtero logy, Abstract from the Nineteenth Scandinavian Conference on Gastroenterology and the

Tenth Scandinavian Conference on Endoscopy, in Torshavn, Føroyar 29th-31st May 1986; L. Højgaard, J.R. Andersen, and E. Krag, "A New method for Gastric Potential Difference Measurements". Scand J Gastroenterol 22 , 1987, pp. 847-858; L. Højgaard, J.R. Andersen, E. Krag, "A new method for measurement of the electrical potential difference across the stomach wall. Clinical evaluation of the gastric mucosal integrity".

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described with reference to the draw¬ ings, in which

FIG. 1 is a diagrammatic view of the main components of the PD and the pH measuring system according to the invention, the system being used for obtaining measurements on a patient who is being examined by means of a gastric endoscope (gastroscope) ,

FIG. 2 is a view of a package containing a kit according to the in¬ vention comprising an insulated cable having at a first end a plug for connection to a potential recording instrument and at its other end an exposed PD measuring electrode, and a PD measuring electrolyte reservoir. Said two components are supplied detached from one another within the sealed package preventing contaminating contact with the surroundings,

FIG. 3 is a view of a sealed package containing a kit according to the invention comprising an insulated cable having at a first end a plug for connection to a potential recording instrument and at its other end an exposed PD reference electrode,

FIG. 4 is an enlarged view of the PD measuring electrode with a part of the cable connected thereto, and an enlarged partly sectional and partly broken away view of the PD measuring electrolyte reservoir, the components being shown in a position ready for being connected to each other,

FIG. 5 is schematic view of the components shown in FIG. 4 sealingly attached to each other, the components thus constituting the PD measuring half cell, FIG. 6 is a (right part sectional view, left part side view) of the PD reference electrode with a connected part of a cable and a connec¬ tion device mounted thereto,

FIG. 7 shows the PD measuring half cell positioned in electrochemical contact with the cavity fluid present on one side of a membrane.

FIG. 8 is a diagrammatic view of the main components of another em¬ bodiment of the PD and the -pH measuring system according to the in¬ vention, wherein the PD and the pH measurements are performed by means of a single unit and wherein the determination of the PD is performed by means of an integrated instrument for determining the corrected PD according to the invention.

FIG. 9 is an enlarged partially sectional and partly broken away view of a single unit used for the measurement of PD and pH.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for the in vivo determination of an electrical potential difference (PD) across a human or animal membrane of a fluid-containing cavity in a human or in an animal. By the term "in vivo determination" is to be understood a determination performed on a living human or animal. The human may be a patient or a healthy volunteer, and the animals in question are primarily mam¬ mals such as dogs, monkeys, cats, pigs, rats, etc.

By the term "membrane" is to be understood any wall or wall part or "layer" of wall part present in a human or animal, e.g. a mucosal membrane, which borders a fluid-containing space or cavity, the fluid being in contact with said membrane. The fluid is a part of any of the body fluids, and the amount of fluid may range from only a very thin fluid layer to large volumes of fluid, such as the fluid present within the lumens of parts of the gastrointestinal cavity. Thus, by the term "fluid-containing cavity" is meant any fluid-containing space or cavity defined within a living human or animal, e.g. the cavity of a hollow viscus. Primarily, the cavity is a cavity natural¬ ly present within the human or animal, or it may be a cavity formed due to pathological processes such as malignant or inflammatory processes.

The PD measuring sensor is introduced into the fluid-containing cavi¬ ty in the human or in the animal via a "tract" by which term is meant any pathway leading from the exterior/the surroundings and into the cavity of the human or animal. Preferably, said tract .is a tract naturally present within the human or animal, e.g. the pharyngo- esophageal pathway leading from the mouth to the stomach. Alterna¬ tively, the tract may be formed due to pathological processes, and e.g. be a fistula, or it may be formed by an invasive procedure such as a tissue puncture.

By the term "an electrical potential difference (PD)" is meant the electrical potential difference between two points at each side of the described human or animal membrane. The in vivo potential dif¬ ference is e.g. caused by "active energy-requiring" transport of sub¬ stances such as ions through said membrane, i.e. transport opposite a gradient, e.g. a concentration gradient.

A main aspect of the present invention is to arrange the PD measuring sensor wi thin the fluid-containing cavity, the sensor being a PD measuring half cell comprising an electron-conductive part (a PD measuring electrode) and an ion-conductive part (PD measuring elec¬ trolyte) . The PD measuring electrode is suitably a metal or metal/metal halide electrode such as an Ag/AgCl electrode or a Hg H 2Cl2 electrode. The PD measuring electrolyte is suitably a diluted KCl solution such as a 0.5 - 2 M, preferably 0.7 - 1.3 M, especially about 1 M KCl solution. The electrochemical contact between the PD measuring electrolyte and the cavity fluid may be established by means of any type of liquid junction element.

When the human or animal cavity in which the PD measuring half cell is to be arranged is a cavity which is usually examined by means of endoscopy, i.e. by means of an instrument for visual and/or photo¬ graphic examination of the membrane of the cavity, the principles for the introduction of the PD measuring half cell will basically be the same as the principles for introducing the endoscope in question. In a preferred embodiment, the PD measuring half cell is introduced into the cavity by means of an endoscope, in other words an instrument for visual and/or photographic examination of the membrane of the cavity. In this context, the term "by means of" indicates both the case where the PD measuring half cell is introduced via a channel situated within the instrument, and the case where the PD measuring half cell is an integrated part of the instrument and thus is introduced together with the instrument itself. Alternatively, routine examina¬ tions of the cavity in question may involve the introduction of plas¬ tic tubes, and in these cases the PD measuring half cell may in prin¬ ciple be introduced in the same way as used for said plastic tubes. Optionally, the PD measuring half cell may then be Introduced via a plastic tube, the tube being used as a sort of "transport vehicle" for the half cell.

Cavities belonging to the gastrointestinal tract or cavities belong¬ ing to the hepatic or pancreatic system are often examined in order to get valuable information about the functional state of the mem- branes and organs connected to said cavities and may therefore also be of particular interest in connection with the method of the pres¬ ent invention. Especially examinations of the upper gastrointestinal tracts such as the esophagus, stomach and duodenum are Important, and the PD measuring half cells may easily be introduced into said cavi- ties, either directly or by means of an endoscope introduced via the esophagus. Also colon and rectum are important cavities, the PD of which may be determined by the method of the invention. Numerous elaborate and multi-potent gastric endoscopes and colonoscopes are commercially available and will provide for an excellent combination of visual examinations of the membranes of human or animal cavities and valid and precise PD measurements across said membranes.

Also endoscopy of the urogenital system is a valuable way of examin¬ ing the membranes and organs connected to said system. Routinely catheters are introduced via the urethra and ureters of the urogeni- tal system. Thus, the Introduction of the PD measuring half cell may be Introduced via said pathways either assisted by plastic tubes or with the PD measuring half cell being introduced by means of the endoscope.

In a further aspect according to the invention, PD measurements may be performed within the respiratory system, the PD measuring half cell being introduced either via tubes or via endoscopes such as the bronchoscope.

Human or animal membranes of peritoneal and pleural cavities may be investigated by means of PD values determined across said membranes, e.g. in combination with examination of a peritoneal or pleural cavi- ty containing an excessive amount of fluid.

In certain cases, examination of cavities containing cerebrospinal fluid are necessary, and tubes are inserted via artificially es¬ tablished thways. It may then be useful to also determine the PD across parts of the membranes defining cerebrospinal fluid containing cavities.

In general, various cavities may be formed inside a human or an ani¬ mal due to pathological processes and it₌ may be valuable to determine the PD across parts of the membranes lining said pathological cavities.

The method according to the invention provides excellent means for monitoring the PD continuously or intermittently over long periods of time such as several hours or days, with the PD measuring half cell remaining in the cavity. For instance, the PD measuring half cell may be situated randomly within a fluid "sea", thus permitting the mea- surement or monitoring of a so-called "summation PD" , measurements which may be very important for instance in monitoring the progres¬ sion or regression of a pathological process as a response to a physical or pharmaceutical treatment. In a further aspect of the in¬ vention the PD measurements may be correlated to the amount of a pharmaceutical administered or to be administered to a human or an animal, since a large number of pharmaceuticals exert their effect by actually changing the "quality" and function of human or animal membranes. As explained above, so-called membrane-adjacent PD's are supposed to be more informative PD values than a summation PD obtained from a random site within the fluid "sea", as a random site may be a con¬ siderable distance from the membrane part of interest. The method and system according to the invention provide for the determination of such membrane-adjacent PD's. By means of an endoscope, a specific part of the animal membrane is visually identified, the PD measuring electrolyte is arranged in electrochemical contact with a fluid layer adjacent to said identified part of the membrane, and the PD across the identified part of the animal membrane is determined. Hence, the determined PD can be. correlated to the visual appearance of the iden¬ tified part of the animal membrane. The method and system according to the invention make it possible to obtain valuable information about localized pathological processes, processes which often pose differential diagnostic problems, cf. above. Specific relevant PD's are obtained by positioning the PD measuring electrolyte at different sites over, alongside, beneath, etc. a pathological localized membrane change.

The PD reference half cell of the invention is a half cell comprising an electron-conductive part (PD reference electrode) and an ion- conductive part (PD reference electrolyte) . The PD reference elec¬ trode is suitably a metal or metal/metal halide electrode such as an Ag/AgCl electrode or a Hg/Hg2Cl2 electrode. The PD reference half cell Is arranged so as to establish electrochemical contact between the PD reference electrolyte and a "non-cavity fluid" . By this term is meant a fluid of the human or animal but outside the fluid- containing cavity, I.e. a body fluid different from the fluid con¬ tained in the cavity in which the PD measuring sensor is arranged during the PD determination, e.g. blood or fluid present in or gener- ated in cutaneous or subcutaneous tissue. The electrochemical contact between the reference electrolyte and the non-cavity reference fluid may be via a liquid junction element, but is preferably a more direct . contact as explained below.

One practical way of establishing electrochemical contact between the PD reference electrolyte and the non-cavity reference fluid is to insert a catheter or a cannula into the non-cavity reference fluid such as into a blood vessel, preferably a vein, especially a vein in an extremity of the human or the animal, filling the catheter or the cannula with a PD reference electrolyte so that the electrolyte in the catheter or cannula is in electrochemical contact with the non- cavity reference fluid of the human or the animal and immersing at least part of the PD reference electrode in the PD reference electro¬ lyte in the catheter or cannula. Preferably, a first end of the catheter or cannula is inserted into the human or the animal and a second end of the catheter or cannula is connected to a container or conduit for containing the PD reference electrolyte and at least part of the PD reference electrode is inserted in an opening in the con¬ tainer or conduit. In a practical embodiment, the container or con¬ duit is a container or conduit of a system of a type which is used for infusion of fluids into a human or an animal. Thus, for example, the opening may be one of the inlets of a two- or several-way tube constituting an integral or removable part of the container or conduit.

The PD reference electrolyte is, thus, preferably a fluid which is physiologically acceptable for use for infusion into the human or animal. Preferably, the PD reference electrolyte is substantially isotonic with the non-cavity fluid with which it is brought into electrochemical contact. The PD reference electrolyte contains, as one of its ions, an ion which participates in the electrochemical electrode process in question. Samples of suitable PD reference elec¬ trolytes are diluted chloride-containing solutions such as about 0.15 M NaCl or an isotonic NaCl/KCl solution. In special cases, the PD reference electrolyte solution is omitted, i.e. that the PD reference electrode is placed in direct electrochemical contact with the non- cavity fluid of the animal, such as in direct contact with the blood of the animal, in which case the non-cavity fluid serves as the PD reference electrolyte. It is, of course, also possible to design the PD reference as a half cell- which in its entirety is inserted in one of the inlets described so that the contact between the PD reference electrode and, e.g., the infusion fluid is via a further liquid and a liquid junction. Alternatively, bio-electrical electrodes, e.g. electrodes used in electrocardiography (ECG electrodes) may be employed as the PD reference electrode. In these cases, the PD reference electrode is arranged so as to establish electrochemical contact between the PD reference electrode and the body fluid (the non-cavity reference fluid), e.g. arranged on a body surface, preferably the skin at an appropriate site of the body.

The ECG reference electrode may preferably be provided with heating and/or thermostating means. The purpose of using a heated ECG reference electrode is to obtain stable reference potential values as well as values Independent of the body site in contact with said ECG reference electrode. A preferred operation temperature may be a temperature which is above normal skin temperature such as above 36°C or 37°C, in particular above 40°C and which is at the same time a temperature at which there Is a minimal risk of heat-provoked skin damage, e.g. a temperature in the interval 42-44°C.

As mentioned above, the Invention provides an essential adjustment solving the problems associated with irrelevant electrical potentials within the measuring electrochemical chains used in the method of the Invention, especially liquid junction potentials (PDlj)'s

Schematically, the PD electrode chain usually employed can be symbol¬ ized as follows:

PD refer¬ PD refer¬ Non-cavity UWL Cavity PD measur¬ PD measur¬

ence ence elec¬ fluid fluid ing elec¬ ing elec¬ electrode trolyte trolyte trode

El PDljl PDtm PDlj2 PDlj3 E2

The term UWL (unstirred water layer) refers to a layer of water and mucus on the lining of a human or animal membrane, a so-called "un¬ s irred water layer" . El and E2 signify the PD reference electrode potential and the PD measuring electrode potential, respectively. The following equation may be applied in the determination of E:

^E - ^Eo ⁺ ^ ^{ln 10} ' ^{lo§ a}ion

a _on = the activity of the relevant ion conducting the current

E₀ = standard potential (the value used being a value relevant to the electrode employed at a relevant temperature)

z = the charge of the ion species

R = the gas constant 8.31 J • degree Kelvin^{" •}*- • mol . T = the absolute temperature (Kelvin degrees) . F = Faraday constant 96,500 coulomb • mol^"1.

"PDljl" is the PDlj between the PD reference electrolyte and the non- cavity reference fluid.

"PDlj2" is the PDlj between the cavity fluid and the UWL.

"PDlj3" is the PDlj between the PD measuring electrolyte and the cavity fluid.

"PDt " (transmembraneous PD) is the PD across the human or animal membrane in question. This quantity is a theoretical quantity, the exact magnitude of which is difficult to establish in practice. What is attempted to establish according to the present invention is a corrected PD (PDcorr) which is the PD detected (PDdet) by means of a voltmeter in the method of the invention corrected for irrelevant PD contributions as described below:

An estimate of a PDlj can be made by means of the Henderson equation (see P.H. Barry and J.M. Diamond, loc . ci t . ) : PDlj = B^L . fϋl - VH - CU - V2⁾ . _ln OJl'+ VI'⁾ F (Ul'+ Vl')- (U2'+ V2') (U2'+ V2')

Where U = Σ m₊ • λ₊ V ^« Σ m_ • λ_

U^f= Σ m₊ • λ₊ • [z+| V- Σ . • λ. • |z-|.

The subscripts 1 and 2 on U, V, U' and V refer to solution 1 (PD measuring electrolyte) and solution 2 (cavity fluid), respectively. m₊ and m_ are the molalities of positive and negative ions of the valencies z-f- and z-, and λ₊ and λ_ are limiting equivalent ionic con¬ ductances, respectively (Handbook of chemistry and physics 4S, ed. Chemical Rubber Publishing Company, Cleveland 1967) .

An estimate of the PDtm may be obtained by correcting the PDdet, e.g. by subtracting the various PDlj's with sign and the temperature- dependent electrode potentials from the PDdet:

PDcorr = PDdet - (E2 - El + PDljl + PDlj2 + PDlj3).

An. aspect of the invention is to make estimates of the PDlj's and adjust the PDdet's on the basis of the estimates. Thus, since the preferred embodiment of the PD measuring system of the invention does not employ attempts to reduce the PDlj 's by means of saturated salt bridges, It is especially important to estimate the magnitude of said PDlj's and adjust the PDdet's on the basis of the estimates. One way to estimate a PDlj is to estimate the concentrations of the cavity fluid of the species which contribute significantly to its magnitude and use said concentrations in the above-described Henderson equa¬ tion. For instance, such concentration estimates may be made from the knowledge of average physiological ranges such as mean values. Alter¬ natively, one or more species concentrations may be measured in the cavity fluid in question.

A main aspect of the present invention is the provision of a simple and relevant method for adjusting the PD based on an estimation of the PDlj between the PD measuring electrolyte and the cavity fluid (PDlj3), an important PDlj contribution in practice such as will be explained below. The method comprises determining the liquid junction potential difference between the PD measuring electrolyte and the cavity fluid (PDlj) existing substantially simultaneously with the PD detection,

adjusting the detected PD, the adjustment being based on the deter- mined PDlj , and

determining the PD across the membrane of the fluid-containing cavity on the basis of the adjusted PD.

A preferred aspect of the method of the invention comprises

measuring the concentration or activity in the cavity fluid of a spe- cies which contributes significantly to the magnitude of the PDlj , and

determining the PDlj using the measured concentration or activity.

In a particularly preferred aspect of the method of the invention, the determination of the PDlj is performed on the basis of a measure- ment performed by means of a sensor (species concentration sensor) arranged within the cavity.

In another preferred aspect of the method of the invention, the determination of the PDlj is performed on the basis of a measurement of the concentration or activity of substantially only one species, the one species being the species which contributes most significant¬ ly to the PDlj . When the one species contributing most significantly to the magnitude of the PDlj is the H⁺ ion, its concentration or ac¬ tivity is preferably measured by means of a H⁺ sensor arranged within the cavity. As will be described below, it is especially relevant to determine the H⁺ concentration or activity when PD's across the stomach of a human are determined.

In one practical aspect of the method of the invention, the PD measuring half cell and the species concentration sensor are combined into a single unit. This permits a substantially simultaneous mea- surement of a PD and of a significant species concentration, thus eliminating an irrelevant time-dependent contribution to the signals obtained. The combined unit may be designed so that the part of the cavity fluid contacting the PD measuring electrolyte corresponds (as far as relevant signals are concerned) to the part of the cavity flu- id contacting the species concentration sensor. Furthermore, the com¬ bined unit may be designed so as to permit its Introduction into the cavity by means of an instrument for visual and/or photographic ex¬ amination of the cavity membrane, i.e. permitting the obtainment of the numerous advantages of the combination of PD measurements and endoscopy, as described above. Thus, the combined unit should have sufficiently small cross-sectional dimensions to be introduced into the fluid-containing cavity via a tract which is present in the human or animal and which leads from the exterior of the human or animal to the cavity. The unit should preferably be sufficiently small to be introduced via a channel situated within the instrument for visual and/or photographic examination. Thus, it is preferred that the unit has a maximum cross-sectional dimension of at the most 4 mm, prefer¬ ably at the most 3 mm, such as about 2 mm. The employment of a com¬ bined unit is especially relevant for the obtainment of membrane- adjacent PD's.

For the purpose of exemplification, the estimation/calculation of the PD across a membrane being part of the stomach of a human is shown below. In the example, the following fluids contributed to the magni¬ tude of the PDlj 's:

PD reference electrode: Ag/AgCl e

PD reference electrolyte: 0.15 M NaCl solution

Non-cavity fluid: blood

UWL: unstirred water layer lining the luminal cellular border of the stomach.

Cavity fluid: gastric juice PD measuring electrolyte: 1 M KCl solution ing

PD reference electrode: Ag/AgCl

El (at 25"C - ambient temperature, corresponding to the temperature of the PD reference half cell placed outside the subject)

E_Q (Cl^") -= 220 mV (25°C = ambient temperature).

(cf. D.J.G. Ives & G.J. Janz, "Reference electrodes", Academic Press,

New York 1961) .

R T In 10 = 59.16 mV (25°C)

For a diluted Cl^* solution:

a_cl- = [Cl^"] • f_Cι-,

(^Cl- being ^tne activity coefficient calculated according to the Debye-Huckel equation, cf. P. Debye; E. Huckel, "Physik 2", 24, 1923, pp. 185-206).

f_cl- = 0.76 M^"1 (25°C)

a_cl- = 0.154 M • 0.76 M^"1 (25°C)

El = 220 mV - 59.16 mV • log (0.154 • 0.76) = 277 mV (25°C)

E2 (at 37°C = body temperature, corresponding to the temperature of the gastric juice)

E₀ (Cl") = 214 mV (37°C) (D.J.G. Ives, op . cit . )

2^ • In 10 = 61.5 mV (37°C) r f_cl- = 0.60 M^"1 (37°C) a_cl- = 1 M • 0.60 M^{" 1} (37°C)

E2 - 214 mV - 61.5 mV • log (1 • 0.60) - 229 mV (37°C)

PDljl (between blood -and the 0.154 M NaCl solution) can by means of the Henderson equation be calculated to be approximately -1 mV (M. Dalmark, "The transmucosal electrical potential difference of rectum in the unanesthetized man", Scan J Gastroenterol 5, 1970, pp. 277- 282).

PDlj2 (between the cavity fluid and the UWL) is uncalculable since the electrolyte concentrations in the UWL are unknown. However, if substantially no food is present in the stomach (i.e. the subject is examined after a fasting period) and if no significant change In the rate of electrolyte secretion into the gastric juice occurs, no steep electrolyte concentration differences will exist between the UWL and the gastric juice, and hence the PDlj2 can be considered negligible.

PDlj3 (between the 1 M KCl solution and the gastric juice). In this case, the species contributing most significantly to the magnitude of the PDlj3 Is the H⁺. The mobility of the H⁺ ions is approximately 7 times greater than the mobilities of the other ions contributing to the PDlj3 and the H⁺ concentration is most prone to change, for which reason It is relevant to measure the magnitude of the H⁺ concentra¬ tion or activity.

Estimations of other relevant ion concentrations are based on the following:

It is assumed that the gastric juice is almost isotonic compared to the blood of the subject and with a constant concentration of potas¬ sium, magnesium and calcium, independent of the acid secretory state (J.N. Hunt & B. Wan, "Electrolytes of mammalian gastric juice", Hand¬ book of Physiology 6, vol . II, 1967, Ed. CF. Code & W. Heidel) .

The sodium concentration is calculated as follows: [Na⁺] = 320 • 0.5 - ([H⁺] + [Mg+÷] + [Ca+÷] + [K⁺])

= 150 - [H⁺] assuming that: [Mg^"*^""1"] + [Ca^"1-1"] + [K⁺] is approximately 10 mM and that an estimate of the isotonic osmolarity of the gastric juice is 320 mM (J.N. Hunt & B. Wan, op . cit . )

The chloride concentration is assumed to be almost unchanged (G. Flemstrόm & E. Kivilaakso, "Demonstration of a pH gradient at the luminal surface of rat duodenum in vivo. Its dependance on mucosal alkaline secretion", Gastroenterology 84 , 1983, pp. 787-794)..

On the basis of these estimates of the ion compositions of the gas¬ tric juice, an estimate of the PDlj3 can be made using the Henderson equation (cf. above). For instance, when the pH is measured to be 1.0, the PDlj3 is found to be 8 mV.

Furthermore, the PD between the serosal surface and peripheral blood has been found to be less than 1 mV (R.N. Grantham, CF. Code & J.F. Schlegel, "Reference electrode sites in determination of potential difference across the gastroesophageal mucosal junction", Mayo Clin Proc 45 , 1970, pp. 265-274.)

Thus, when e.g. the PDdet has been found to be -82 mV, (the PD being recorded with the gastric cavity negative compared to blood) the final calculation of a PDcorr across a gastric membrane is the following:

PDcorr = PDdet - (E2 + PDljl + PDlj3 - El)

= -82 mV - (229 mV - 1 mV + 8 mV - 277 mV) = -41 mV

Thus, in this example, the species, the concentration of which in the cavity fluid is measured, is the H⁺ ion. The measurement of the H⁺ concentration is used for calculating PDlj3, and the detected PD is adjusted by subtracting the PD1J3. Based on this adjusted PD, the corrected PD is calculated by subtraction of the PDljl as an esti¬ mated fixed value and by compensating for El and E2. The determina¬ tion of a PDlj on the basis of a H⁺ concentration measurement, in other words a pH measurement, is, in accordance with an embodiment of the invention, performed on a part of the cavity fluid which is pres¬ ent within the cavity during the PD measurement, the concentration measurement being performed by means of sensor (species concentration sensor) arranged within the cavity, in .this example a pH sensor.

Thereby it is ensured that there is substantial identity between the fluid contacting the PD measuring half cell and the fluid contacting the species concentration sensor. Furthermore, when the PD measure¬ ment is a measurement of a membrane-adjacent PD, the species con- centration sensor is preferably arranged so that its measurement is performed on a fluid layer on which the measurement of the membrane- adjacent PD is performed. As will be understood from this example, the determination of the PDlj used as a basis for the adjustment of the PD measurement may be performed on the basis of a measurement of the concentration of substantially only one species, this one species being the species which contributes most to the PDlj , in the present example the H^"1" ion.

The aspect of the invention according to which the detected PD is adjusted based on a determination of PDlj performed by measuring the concentration of a species which contributes significantly to the magnitude of the PDlj Is of importance irrespective p_f whether or not the PD measurement is performed by means of a PD measuring half cell arranged within the cavity In question, although, evidently, the pre¬ ferred variant is the one where the PD measurement is in fact per- formed by means of a PD measuring half cell arranged within the cavity.

In a most practical embodiment of this method of the invention, the PD measuring half cell and the species concentration sensor are com¬ bined into a single unit. When the species, the concentration of which to be measured, is the H⁺ ion, the single unit may be a com¬ bination of a pH sensor or half cell such as a glass electrode and a PD measuring half cell, in which case the PD measuring half cell may also serve as the reference half cell for the pH sensor.

The present Invention also relates to a system for the in vivo deter- mination of an electrical potential difference (PD) across a human or an animal membrane of a fluid-containing cavity in a human or in an animal, the system comprising:

a PD registration instrument,

a PD measuring cable comprising first and second end parts, the first end part being adapted to be connected to the PD registration instrument,

an electron-conductive body (PD measuring electrode) connected to the second end part of the PD measuring cable and being in electron- conductive contact with the cable,

a PD measuring electrolyte reservoir containing a PD measuring elec¬ trolyte, the reservoir being arranged so as to enable electrochemical contact between at least part of the PD measuring electrode and the PD measuring electrolyte so as to establish a PD measuring half cell, the PD measuring half cell being adapted to establish contact between the PD measuring electrolyte and the fluid contained in the cavity (cavity fluid) ,

the PD measuring cable being sufficiently long to permit the PD measuring half cell to be placed within the fluid-containing cavity in a human or in an animal when the connector means is connected to the PD registration instrument, and the cable and the measuring half cell having sufficiently small cross-sectional dimensions to be introduced into the fluid-containing cavity via a tract which is present in the human or animal and which leads from the exterior of the human or animal to the cavity,

a PD reference cable comprising first and second end parts, the first end part being adapted to be connected to the PD registration instru¬ ment, and

an electron-conductive body (PD reference electrode) connected to the second end part of the PD reference cable and being in electron- conductive contact with the cable, at least part of the PD reference electrode being adapted to be brought into electrochemical contact with a fluid of the human or animal but outside the fluid-containing cavity (non-cavity reference fluid) .

The length of the PD measuring cable is preferably sufficient to per¬ mit the arrangement of the PD measuring half cell in a cavity in a large animal or a human, such as at least 1 , preferably at least 1.5 m, such as about 2 m.

The PD measuring half cell should be sufficiently small to permit its introduction via a tract naturally present in the human or animal, and preferably small enough to permit its introduction via a channel within an endoscope. Thus, the outer maximal cross-sectional diameter of the PD measuring half cell and of the PD measuring cable is preferably at the most 4 mm, more preferably at the most 3 mm, such as about 2 mm. The total volume of the PD measuring electrolyte is at the most 100 μl, preferably at the most 50 μl, such as about 10 μl, or even smaller such as 1 μl or less than 0.1 μl.

The PD measuring cable should have sufficient stiffness and flexibi¬ lity to permit its insertion into the tract by pushing the cable; however, this Is not required when the cable is an integrated part of an endoscope.

It Is often preferred that the PD measuring electrolyte reservoir is detachably arranged, such as detachably mounted on the cable. In one embodiment, it is preferred that the PD measuring second cable end is tapered, and a first end part of the PD measuring electrolyte reser¬ voir Is a hollow cylinder adapted to receive therein at least part of the tapered second PD measuring cable end so as to obtain a liquid- tight connection between the cylindrical wall parts of the PD measur¬ ing electrolyte reservoir and the tapered PD measuring second cable end.

In order to establish electrochemical contact between the PD measur- ing electrolyte and the cavity fluid, the electrolyte reservoir is normally provided with a liquid junction element such as a porous or fibrous or sintered element, or the reservoir may be designed so as to give a "controlled leak". The measuring face of the electrolyte reservoir (a face where the contact between the PD measuring electrolyte and the cavity fluid is established, such as by means of a liquid junction element) may be designed so as to establish an optimum measuring contact surface and at the same time prevent damage of the cells of the membrane. Thus, it is often preferred that an end face of the PD measuring half cell has at least one plane defining an angle differing from 90° in rela¬ tion to a longitudinal axis of the PD measuring electrolyte half cell.

In a particularly preferred embodiment of this kind, the end face of the PD measuring half cell is arranged obliquely in relation to the longitudinal axis of the PD measuring half cell, thus defining an acute angle, preferably an angle in the range of about 10-85°, such as about 30-60°, in particular about 45°. The end face of the PD measuring electrolyte reservoir may have an elliptic or ellipsoid shape.

The PD measuring electrolyte reservoir may be of any suitable design. For example, it may be a chamber, e.g. a cylindrical chamber having a liquid junction element in the form of e.g. a porous or fibrous body such as a porous ceramic plug. Alternatively, it may be constituted by a multiplicity of chambers or spaces containing the PD measuring electrolyte, such as a porous body or a body having "pores" of a microscopic size such as a swellable membrane. The PD measuring elec¬ trolyte reservoir may be supplied devoid of PD measuring electrolyte, the PD measuring electrolyte being applied immediately before use, or the PD measuring electrolyte may be present in the PD electrolyte reservoir as supplied, e.g. in a solid form, for dissolution prior to use or when contacted with the cavity fluid to be investigated.

In accordance with what has been explained above, it is preferred that the system additionally comprises a sensor (species concentra¬ tion sensor) for measuring the concentration or activity in the cavi¬ ty fluid of a species which contributes significantly to the magni¬ tude of the liquid junction potential difference between the PD measuring electrolyte and the cavity fluid (PDlj), and in one practi- cal embodiment, the PD measuring half -cell and the species concentra¬ tion sensor are combined into a single unit, e.g. a combination of a PD measuring half cell and a pH sensor.

Such a combined unit may typically comprise a small centrally ar- ranged pH measuring half cell and a PD measuring reservoir enclosing at least part of the pH measuring half cell, leaving the pH sensing part of the pH measuring half cell exposed for contact with the cavi¬ ty fluid. (The pH measuring half cell may, in this embodiment as well as in other embodiments described herein, be either a half cell using pH-sensitive glass or a half cell using any other pH measuring prin¬ ciple such as metal/metal oxide, e.g. antimony/antimony oxide, iridi- um/iridium oxide, etc.). The PD measuring electrolyte reservoir or a wall part thereof may be constructed as an attachable/detachable part, permitting the re-use of the main central part of the combined unit. For instance, the detachable part may be detached and disposed of after a measurement, and the remaining part may be sufficiently cleaned to permit safe re-use after attaching another PD measurement electrolyte reservoir part thereto with new PD measuring electrolyte.

As indicated above, the electrochemical contact between the PD measuring electrolyte and the cavity fluid may be established by means of a liquid junction element which may be a controlled leak at the edge of a sealing element or it may be a separate element such as a porous or fibrous or sintered element. For example_j a fibre such as a hair may be inserted through the sealing element, or the sealing element may in itself be porous or fibrous and thereby establish the liquid junction. The sealing element may either be an integral part of the electrolyte reservoir, or it may in advance be arranged In position.

In a preferred embodiment of the system of the invention, the PD reg- istration instrument comprises a conversion unit and an input unit, the input unit comprising an input for receiving signals from a PD measuring half cell and an output for transmitting signals derived from the PD measuring half cell to the conversion unit, an input for receiving signals from a species concentration sensor and an output for transmitting signals derived from the species concentration sen¬ sor to the conversion unit, the conversion unit being adapted to per¬ form an adjustment of a detected PD, the adjustment being based on the signals derived from the species concentration sensor, and on algorithms read into a storage means of the conversion unit. In another preferred embodiment, the PD registration unit comprises data collection means with input means for receiving signals from a PD measuring half cell and from a species concentration sensor, and output means for transmitting the collected data to a detached conversion unit such as a standard commercially available microcompu¬ ter programmed to perform an adjustment as explained above.

Furthermore, the input or data collection unit may, if desired, comprise inputs for receiving temperature signals from temperature sensors arranged e.g. at or in the PD reference system such as in the PD reference electrolyte, or the electrolyte may be thermostated by means of thermostating means which may be controlled from the instrument or from a separate thermostating unit. Alternatively, the temperature measured e.g. in the PD reference system (or simply in the room in question) may be introduced into the instrument, the data collection unit or the detached conversion unit such as by means of a keyboard, and the conversion unit may be adapted to perform PD adjustments based on said signals.

As mentioned above, the PD reference electrode may be adapted to be immersed in a PD reference electrolyte placed in a catheter or cannu- la or in a container or conduit connected to the catheter or cannula, the reference electrode preferably being enclosed by a connecting means adapted to connect the electrode sealingly to a standard type inlet of such catheter or cannula or such container or conduit, in particular to a standard type inlet of a two- or several-way tube constituting an integral or removable part of the container or conduit.

Preferably, the PD measuring electrode and the PD measuring electro¬ lyte are kept separate prior to use. Thus, each of said components may be supplied separately in sterilized sealed packages which are not broken until immediately before use. When the PD measuring half cell Is to be used, a preferably sterilized PD measuring electrolyte solution is filled into the PD measuring electrolyte reservoir, e.g. by means of a syringe. Subsequently, the PD measuring electrode is fitted into the PD measuring electrolyte reservoir.

If desired, a pressure transducer may also be arranged in the cavity together with and adjacent to the sensors mentioned above, and the signals from the pressure transducer may be received and processed analogously to the signals from the sensors mentioned above to incorporate the extra information derivable from such pressure transducer, including information which serves to differentiate between different locations of the PD sensor in the body.

An aspect of the invention is the provision of a PD measuring kit comprising a sealed package containing

an electron-conductive body (PD measuring electrode) connected to the second end part of the cable and being in electron-conductive contact with the cable,

a PD measuring electrolyte reservoir adapted to receive a PD measur¬ ing electrolyte and to permit electrochemical contact between at least part of the PD measuring electrode and the PD measuring elec¬ trolyte so as to establish a PD measuring half cell and further adapted to establish contact between the PD measuring electrolyte contained and a cavity fluid,

the PD measuring electrolyte reservoir having cross-sectional dimen¬ sions of at the most 4 mm, preferably at the most 3 mm, such as at the most 2 mm. Preferably the kit PD measuring electrolyte reservoir is detachable.

An advantage of an embodiment of the system according to the inven¬ tion is that it is ensured that no changes in composition have taken place during storage due to contact between the PD measuring elec¬ trode and the PD measuring electrolyte. Also, the risk of contamina¬ tion from the environment is minimized.

A further aspect of the invention is the provision of a PD measuring kit comprising a sealed package containing a PD reference electrode assembly comprising

an electron-conductive electrode and means for connecting the elec¬ trode to a standard type inlet of a container or conduit connected to a catheter or cannula so that at least part of the electrode becomes inserted in the catheter or cannula or container or conduit to permit the electrode to be immersed in an ion-conductive fluid contained in the catheter or cannula or container or conduit.

Preferably the standard type inlet is a standard type inlet of a two- or several-way tube means constituting an integral or removable part of the container or conduit.

To eliminate the risk of accidentally introducing foreign bodies into the circulation of the human or animal when using an infusion type reference system, a filtration system is preferably inserted between the PD reference electrode and the non-cavity fluid so as to permit the catching of the accidentally detached larger part of the PD reference electrode or other detached components. The filtration device may comprise a sterilized microporous filter.

During operation of the infusion type reference system, the PD reference electrolyte may be passed intermittently or continuously from the container to the non-cavity fluid within the animal. Prefer¬ ably the reference electrode is placed within a short distance from the first part of the tube, i.e. the tube length from the point of the tube immediately outside the animal to the point of the tube im¬ mediately prior to the insertion of the PD reference electrode is less than a few centimetres. Such a short distance minimizes the risk of electrical noise conferred to the tubes. Preferably, the PD reference electrode is, as mentioned above, arranged within one of the inlets of a two- or three-way valve means by inserting the elec- trode end of the cable into the inlet and fixing the cable by screw¬ ing a fitting into the inlet. The PD reference electrode is prefer¬ ably placed within the Inlet immediately prior to detecting the PD.

In a special embodiment, the PD reference electrode can be fitted into a conduit or connection device connected to one end of a cannula or a catheter arranged within the subject and in direct contact with the body fluid such as blood.

PREPARATION OF THE EQUIPMENT FOR THE PD MEASUREMENTS

When humans are examined, anaesthesia is usually not necessary. Op- tionally, when performing endoscopy, a slight sedation is performed, whereas the animals are usually anaesthetized.

Further details concerning the preparation of the subject to be exam¬ ined will depend on the particular method chosen.

The equipment is prepared for use easily and quickly, typically within less than 10 minutes. Usually, the components to be contacted with the subject are supplied in a sterilized package.

Preferably all the components are sterilized and precautions are taken during preparation of the equipment to maintain sterilized or aseptic conditions. Both the PD measuring half cell and the PD reference electrode may be used as disposable components with the advantage that no sterilization procedures are needed after use and prior to the use in another subject. Alternatively, the components may be sterilized by a chemical treatment such as treatment with e.g. Corsoline® 3% or by physical treatment such as radiation treatment.

Prior to use, the PD measuring electrode and the PD reference elec¬ trode may be calibrated in sterilized electrolyte solution, e.g. 1 M KCl, to ensure an asymmetry potential of <1 mV.

The connected PD measuring half cell is kept in the PD measuring electrolyte solution sufficiently long for the liquid junction ele- ment, e.g. the porous or fibrous plug, to be soaked through, e.g. a few minutes such as 5 minutes.

In a system of the invention, the asymmetry PD was measured before and after each use of the equipment, and even after long-term mea- surements (> 6 hours) this asymmetry PD did not exceed 2 mV.

DETAILED DESCRIPTION OF THE DRAWINGS

In the drawings, like numerals designate like parts.

In FIG. 1, an embodiment of a PD and pH measuring system according to the invention is shown designated 10 in its entirety. A PD measuring half cell designated 20 in its entirety is, via a cable 21 with a connecting plug (not shown) , connected to an input 22 of a voltmeter 240 comprising a display and connected to a recorder 500 such as a printer or plotter or a mass storage device. The cable 21 is about 2 m long. A PD reference half cell designated 50 in its entirety is, via a cable 51 with a connecting plug (not shown) , connected to an¬ other input 52 of the voltmeter 240. The PD reference half cell 50 is shown connected to an infusion reference system designated 80 in its entirety. The infusion reference system is filled with an infusion fluid 90, such as a 0.15 M NaCl solution, functioning as the PD reference electrolyte. The PD reference half cell 50 comprises a PD reference electrode (shown as 53 in Fig. 3). Connection between the PD reference electrode 53 and the PD reference electrolyte 90 is ob¬ tained by Inserting the PD reference electrode 53 into a tube or valve means 81, preferably a three-way tube or valve device such as a three-way stopcock. The three-way stopcock comprises three standard type inlets: 82, 83, 84. The PD reference electrode 53 is inserted into the three-way stopcock via e.g. the standard type inlet 82 of the three-way stopcock. A tubing 85 connects the inlet 83 to a reser¬ voir 86 for the PD reference electrolyte 90. An end part of a plastic catheter 87 is placed in a hand vein of the patient and, via an en¬ larged diameter part 88 of the catheter connected to the inlet 84, and thus contact between the patient's blood and the PD reference electrolyte 90 contained in the infusion reference system is es- tablished. A filtering device 89 Is inserted between the plastic catheter 87, 88 and the three-way stopcock 81 with the PD reference half cell 50.

Furthermore, FIG. 1 shows for the purpose of exemplification a set-up for measuring the PD across a cavity within the gastrointestinal tract, namely the stomach 200, and additionally, the figure shows a gastric endoscope (gastroscope) 201 introduced into the patient through the mouth so that a part of the gastroscope is placed in the upper gastrointestinal tract, the esophagus 202, and a second ter- minal part of the gastroscope is placed in the stomach 200. In the

Illustrated example the gastroscope, in addition to its optical com¬ ponents (not shown) , comprises two channels, one of which is used for the Introduction of the PD measuring half cell 20, the other one being being used for the Introduction of a pH measuring half cell 110 which via an insulated cable 111 is connected to an input 502 of a pH meter 113 comprising a display and connected to a recorder 112 such as a printer or plotter or a mass storage device. A cable 503 con¬ nects the input 52 of the voltmeter 240 to a reference input 504 of the pH meter 113 so that the reference half cell 50 serves as a reference also for the pH measurement.

FIG. 2 shows an embodiment of a PD measuring electrode 23 protruding from a conical body 24 which is connected to the cable 21, the cable being shown as an insulated solid single conductor cable. The cable terminates in a plug 25. A PD measuring electrolyte reservoir 26 is supplied detached from the PD measuring electrode. All components are supplied in a sealed container 222 preventing contaminating contact with the surroundings, and preferably, all the components are sup¬ plied in a sterilized form. Hence, the components and the sealed con¬ tainer are preferably made from materials which are capable of withstanding sterilizing procedures or processes such as a radiation treatment or a chemical treatment.

FIG. 3 shows an embodiment of a PD reference electrode 53 connected to a cable 51 terminating in a plug 55. A connection device desig¬ nated 56 in its entirety is placed near the PD reference electrode 53 so as to enable insertion of the PD reference electrode 53 into a standard type inlet of the infusion reference system in order to es¬ tablish electrochemical contact between the PD reference electrode 53 and the PD reference electrolyte 90 contained in the infusion reference system. All components are supplied in a sealed container 540 preventing contaminating contact with the surroundings, and preferably supplied in a sterilized form, analogously to what is dis¬ cussed above in connection with FIG. 2.

FIG. 4 shows the PD measuring electrode 23 connected to the cable 21. The PD measuring electrode is a Ag/AgCl electrode. A heat-shrinkable tubing 27 is fitted onto the cable 21 and connects the cable 21 to a conically shaped body 24 preferably made from plastics materials. Said body is adapted to be sealingly fitted into a PD measuring elec¬ trolyte reservoir designated 26 in its entirety, i.e. a liquid-tight connection between wall parts 29 of the electrolyte reservoir 26 and the conical body 24 is achieved. The wall parts 29 of the PD electro¬ lyte reservoir 26 are preferably made from transparent or translucent plastics materials. The electrolyte reservoir 26 is shaped as a hol¬ low cylinder which at one end face 31 defines a PD measuring contact surface 32 and at its opposite end defines an opening 33 through which the PD measuring electrode 23 is to be inserted. The diameter of the PD electrolyte reservoir 26 is about 2 mm to enable the PD measuring half cell to be easily passed through a channel in a gas¬ troscope. A cylinder-shaped plastic filling 34 preferably having a colour different from the surroundings in the cavity such as green, is inserted into the terminal end of the PD measuring electrolyte reservoir, and a ceramic body 35, shaped as a solid cylinder and serving as a liquid junction element, is inserted into the lumen de¬ fined by the cylinder-shaped filling 34. The end face 31 of the ter¬ minal part of the electrolyte reservoir 26 is cut obliquely in rela- tion to the longitudinal axis of the electrolyte reservoir 26 and hence the measuring contact surface 32 is an elliptic or an ellipsoid area. A PD measuring electrolyte 36 is contained in the remaining part of the lumen defined by the wall parts 29 of the PD measuring electrolyte reservoir, and it is in contact with the ceramic body 35 into the pores of which it penetrates. The total volume of the PD measuring electrolyte 36 is about 10 μl. The PD measuring elec¬ trolyte 36 is a diluted KCl solution such as a 1 M KCl solution. FIG. 5 shows the PD measuring electrode 23 and the adjacent part of the conical body 24 inserted into the PD measuring electrolyte reser¬ voir 26, the PD measuring electrode 23 thus being in contact with the PD measuring electrolyte 36.

FIG. 6 shows the PD reference electrode 53, in the embodiment shown being shaped as a closed loop. The electrode 53 is connected to the cable 51 via a soldered connector 58. Contact between the PD reference electrolyte 90 and the soldered connector 58 is prevented by means of -a. glue sealing 60. The cable 51 is centrally arranged in a heat-shrinkable tubing 59. The connection device designated 56 in its entirety comprises two main components: a fitting 70 is screwed into a fitting 80 and thereby compresses fixing meber 71 shown as an 0-ring so that the connection device 56 is fixed to the heat- shrinkable tubing 59. An internal standard type thread 72 in the fit- ting 70 is adapted for mounting the connection device 56 onto the standard type Inlet 82 of the three-way stopcock 81 of FIG. 1 so that the electrode 53 protrudes into the lumen of the three-way stopcock thus permitting the establishment of electrochemical contact with the PD reference electrolyte 90 contained in the infusion reference sys- tern 80 of FIG. 1. All said components are preferably made from sterilizable materials.

FIG. 7 shows a membrane 210 at one side contacting a cavity fluid 211. The PD measuring half cell 20 is arranged so as to establish electrochemical contact between the cavity fluid 211 and the PD measuring electrolyte 36. The embodiment shown of the PD measuring electrolyte reservoir 26 is adapted to measure the so-called membrane-adjacent PD in that it is shaped so as to enable a position in which its ^,rfoot" stands on the surface of a fluid layer lining the membrane, i.e. the end face 32 is positioned substantially parallel to the membrane surface 210. A contour 300 of the lower part of the PD measuring half cell 20 and an arrow 302 indicate a movement by which the end face 32 is placed on the membrane surface 210 by means of the endoscope 201. FIG. 8 is a diagrammatic view similar to Fig. 1 of the main compo¬ nents of another embodiment of the PD and the pH measuring system according to the invention, wherein the PD and the pH measurements are performed by means of a single unit and wherein the determination of the PD is performed by means of an integrated instrument according to the invention.

In this embodiment, a PD measuring half cell is combined with a PH measuring half cell into a single unit 600 so that the PD measuring half cell, in addition to its PD measuring function, also serves as an internal reference half cell for the pH measurement. A cable 602 connected to the unit 600 comprises a solid conductor 700 for the pH signal and a jacket conductor 610 (which is insulated from the solid conductor 700 and covered by an external insulation (which is not shown at the end of the cable)) for the PD signal which, as mentioned above, also serves as the reference signal for the pH measurement.

An instrument for the in vivo determination of PD comprises an input unit 616 and a conversion unit 631. The input unit 616 comprises an input 620 for receiving signals from the PD measuring half cell through a cable 606 which is connected to the jacket conductor 610 through a cable 608 and an input 618 connected to the PD reference half cell through a cable 604, as well as an input 614 for receiving signals from the pH measuring half cell through the conductor 700 and an input 612 connected to the PD measuring half cell through the cable 608 and the jacket conductor 610. From the input unit, outputs. 622 and 624 transfer signals derived from the PD measuring half cell and the pH sensor, respectively, of the single unit 600 to a conver¬ sion unit 631 comprising an interface 630 and a computer 632. The computer 632 calculates the corrected PD on the basis of the pH mea¬ surement and on the basis of correction algorithms read into a storage medium of the computer. Optionally, a temperature sensor or temperature sensors (not shown) may be provided for measuring the temperature of the PD reference electrode 50 and optionally of the combined unit 600, and the signals from the temperature sensor or sensors may be conducted to the input unit and further on to the in- terface and the computer for inclusion in the correction of the determined pH and PD on the basis of further relevant conversion al- gorithms read Into a storage of the computer. As an alternative op¬ tional feature, the temperature of the room in which the measurement is performed may be measured and fed into the computer 632 by means of, e.g. a keyboard (not shown), for inclusion in the correction. From the computer 632, the corrected PD signal and optionally the pH signal may be transferred via a conductor 636 to a display 638, and/or via a conductor 634 to a recorder 640 shown as a printer.

Fig. 9 shows an embodiment of the combined unit 600 partly in section and partly broken away. The unit 600 has a maximum cross-sectional diameter of about 2 mm and comprises a plastic jacket 712 having a central axial bore in which a glass pH half cell 716 terminating in a glass bulb 732 of pH-sensitive glass Is arranged. An Ag/AgCl elec¬ trode 730 extends axially through the pH half cell and is connected to the conductor 700 by means of a soldered connector 710. An elec- trolyte 728 of the pH half cell 716 is 1 M KCl. A PD measuring elec¬ trolyte reservoir jacket 720 is arranged concentrically around a re¬ duced diameter end part 713 of the jacket 712. At one end, the reser¬ voir jacket 720 establishes fluid-tight connection with a tapered part 715 of the plastic jacket 712. At the other end, the reservoir jacket 720 is closed by means of a sealing element 724 such as a rub¬ ber or plastic ring comprising a liquid junction element 726 such as a fibre or a hair ensuring electrochemical contact between a PD mea¬ surement electrolyte 722 and the cavity fluid. A PD measuring elec¬ trode 714 protrudes into the PD measuring electrolyte 722. A part of • the PD measuring electrode 714 is connected to the jacket conductor 610 via a soldered connector 708.

The electrolyte reservoir jacket 720 may be detached from the remainder of the unit 600 by drawing the jacket 720 past the glass bulb 732. Thus, the electrolyte reservoir jacket 720 may be a dis- posable part of the combined unit 600, the remainder of the unit 600 being re-usable several times. Thus, after the use of the combined unit 600, the electrolyte reservoir jacket 720 is detached from the unit, and the unit Is wiped clean and then immersed into a disinfect¬ ing liquid and dried. PD measuring electrolyte may then, e.g., be applied as a drop on the reduced diameter part 713 of the jacket 712, and a fresh, preferably sterile, electrolyte reservoir jacket 720 may be drawn down over the glass bulb 732 and fixed by means of the seal¬ ing element 724 which may either be applied in advance around the reduced diameter part 713 or fixed in advance to the interior of the electrolyte reservoir jacket 720. The diameter of the unit 600 is about 2 mm.

The invention is further illustrated by the following non-limiting examples.

EQUIPMENT EMPLOYED IN THE EXAMPLES

The experiments described in the examples were performed using the equipment illustrated in FIGS 1, 4, 5 and 6. The PD measuring elec¬ trode was an Ag/AgCl electrode, and the PD measuring electrolyte was a 1 M KCl solution. The PD reference electrode was an Ag/AgCl elec¬ trode and the PD reference electrolyte was a 0.15 M NaCl solution.

STATISTICAL METHODS EMPLOYED IN THE EXAMPLES

Regression determination was performed by least square variance anal¬ ysis and by two way variance analysis. Standard deviation was deter¬ mined on double estimates. Data were analyzed using the paired t-test with 95% confidence limits considered significant. ΔPDdet is the difference between two detected PD's, and ΔPDcorr is the difference between two corrected PD's after medical intervention, and they are are listed as maximal values.

Example 1

Validation of the PDlj estimates

Small samples of gastric juice were aspirated from the stomach every 15 minutes during the PD measurements. In the samples, the Na⁺ and K⁺ concentrations were determined by flame photometry, the Ca and Mg^"^ concentrations by atomic absorbtion spectroscopy, and the Cl^" and H⁺ concentrations by titration. A first estimate of PDlj3 towards a 1 M KCl concentration was obtained using a measured H⁺ concentration and "fixed" estimated values of the other ions mentioned, and a second i estimate of PDl 3 was obtained using the ion concentrations measured in the samples. Correlations between the two sets of PDlj3's were satisfactory, (y=x, r=0.92, p<0.05).

IN VIVO TESTING OF THE RELIABILITY OF PD MEASUREMENTS ACROSS A MEMBRANE OF A CAVITY WITHIN THE GASTROINTESTINAL TRACT (GASTRIC PD) .

Tn vivo PD recordings were made in volunteers (who had given informed consent to the procedure) with the PD and pH gastric microelectrodes placed in the "fundic sea" of the stomach by intubation through the nose or mouth. The position was controlled by fluoroscopy. (The total exposure was less than two minutes - i.e. less than 9 mGy (0.9 rads) on a 15.2 cm^ abdominal field) . The investigations were performed in the morning after an overnight fast, and the volunteers were examined in a left side supine position.

Example 2

Precision of the gastric PD measurements

The precision was assessed by comparing the mean values obtained simultaneously from each of two PD measuring systems. Correlations betwen the two sets of PD's were excellent, (y = x, r = 0.99, p < 0.05, n = 8).

Example 3

Stability of the gastric PD measurements

The standard deviation of 30 consecutive PD values obtained with one minute intervals was found to be less than 1 mV. Example 4

Intraindividual day-to-day variation

The intraindividual variation was assessed on eight volunteers who had their PD's recorded on two separate days. The results were: ΔPD - 2 ± 2 mV (mean and standard deviation) . The magnitude of the coeffi¬ cient of variation was independent of the PD value measured.

Example 5

Interindividual variation

The interindividual variation was determined using the PD recordings performed on the first day (cf. example 4). The results were: PDdet - -95 mV ± 10 mV, and PDcorr - -46 mV ± 8 mV, (mean and standard devia¬ tion, respectively) .

Example 6

Gastric PD's after bicarbonate administration

Gastric PD and pH were measured in six fasting volunteers, and an isotonic solution of bicarbonate was instilled to produce a pH of approximately 7 in the gastric juice. The ΔPDdet was compared with the calculated ΔPD1J3 (due to the pH shift) to validate the usability of the PDlj3 calculations. The results are shown in Table I. The values obtained correlated well, (r = 0.85, p < 0.05). This illus¬ trates the validity of the PDlj adjustment method. Example 7

Gastric PD's after cimetidine administration

The cimetidine response was tested by PD and pH measurements in eight volunteers. 200 mg cimetidine was given intravenously after 30 mi- nutes of PD and pH initial measurements. The ΔPDdet and the ΔPDcorr were subsequently determined.

The results are shown in Table II. The PDdet and PDcorr rose signifi¬ cantly, and a significant difference was seen between ΔPDdet and ΔPDcorr.

TABLE I

Detected gastric PD (PDdet) and pH before and immediately after perfusion with an isotonic solution of bicarbonate to produce a neutral pH. PDIJ3, between gastric juice and PD measuring electrolyte, is estimated using the pH measured in the gastric juice.

Before bicarbonate After bicarbonate

Vol. No. PDdet pH PDIJ3 PDdet PH PDIJ3 ΔPDdet ΔPDIJ3 mV mV mV mV mV mV

1 -89 1.3 5 -95 6.8 0 -6 -5 2 -102 2.0 1 -104 4.4 0 -2 -1 3 -87 1.4 4 -89 6.3 0 -2 -4 4 -92 1.3 5 -96 5.1 0 -4 -5 5 -81 1.1 7 -89 7.0 0 -8 -7 6 -76 1.2 6 -82 5.0 0 -6 -6

Correlation between ΔPDdet and ΔPDIJ3: r² = 0.73.

TABLE II

Detected PD (PDdet), gastric juice pH and liquid junction PDcorr before and after bolus injection of cimetidine 200 mg intraveneously.

Before cimetidine After cimetidine

Vol. No. PDdet PH PDcorr PDdet PH PDcorr ΔPDdet ΔPDcorr mV mV mV mV mV mV

1 -95 1.8 -47 -117 6.0 -68 22 21 2 -76 0.8 -38 -106 7.0 -57 30 19 3 -88 1.7 -41 -110 7.3 -61 22 20 4 -83 1.3 -39 -106 5.0 -57 23 18 5 -98 1.3 -55 -118 5.0 -69 20 14 6 -102 1.0 -61 -128 4.0 -79 26 18 7 -82 2.5 -33 -108 7.1 -59 26 26 8 -110 1.0 -69 -140 6.9 -91 30 22

_**

Mean -92^**1) 1.4 -48 -117^**1) 6.0 -68*^*2) 25^*3) 20^*3)

SD 11 0.6 12 12 1.2 12

Gastric lumen negative to blood. *) p < 0.05 *^*) p < 0.001. PDdet (1) and PDcorr (2) rose significantly, and a significant difference was seen between ΔPDdet and ΔPDcorr (3).

Claims

1. A method for the in vivo determination of an electrical potential difference (PD) across a human or animal membrane of a fluid- containing cavity in a human or in an animal, the method comprising:

arranging a PD measuring half cell comprising an electron-conductive part (PD measuring electrode) and an ion-conductive part (PD measur¬ ing electrolyte) within the fluid-containing cavity so as to es¬ tablish an electrochemical contact between the PD measuring electro¬ lyte and the fluid contained in the cavity, (the cavity fluid) ,

arranging a PD reference half cell comprising an electron-conductive part (PD reference electrode) and an ion-conductive part (PD reference electrolyte) so as to establish electrochemical contact between the PD reference' electrolyte and a fluid of the human or ani¬ mal but outside the fluid-containing cavity, (non-cavity reference fluid) ,

detecting the PD between the two PD half cells, and

determining the PD across the animal membrane of the fluid-containing cavity on the basis of the detected PD.

2. A method according to claim 1 wherein the cavity is a cavity within the gastrointestinal tract or a cavity belonging to the hepatic or pancreatic system.

3. A method according to claim 2 wherein the cavity is a cavity within the upper gastrointestinal tract such as the stomach.

4. A method according to any of claims 1-3 wherein the determination of the PD is performed continuously or intermittently over a period of time comprising several hours or days, with the PD measuring half cell remaining in the cavity.

5. A method according to any of claims 1-4, further comprising: correlating the determined PD to the amount of a pharmaceutical ad¬ ministered to the human or animal or to be administered to the human or animal.

6. A method according to any of claims 1-5 wherein the PD measuring half cell is introduced into the cavity by means of an instrument for visual or photographic examination of the membrane of the cavity.

7. A method,according to claim 6 wherein the PD measuring half cell is introduced via a channel situated within the instrument.

8. A method according to claim 6 or 7 wherein

a specific part of the membrane is selected visually by means of the instrument,

the PD measuring electrolyte is arranged in electrochemical contact with a fluid layer adjacent to the selected part of the membrane (a membrane-adjacent fluid layer) ,

the PD between the two PD half cells is detected, and

the PD across the selected part of the membrane is determined.

9. A method according to any of claims 1-8 wherein

a liquid junction potential difference between the PD measuring elec- trolyte and the cavity fluid (PDlj) existing substantially simul¬ taneously with the PD detection is determined,

the detected PD is adjusted, the adjustment being based on the deter¬ mined PDlj , and

the PD across the membrane of the fluid-containing cavity on the ba- sis of the adjusted PD is determined.

10. A method according to claim 9 additionally comprising:

measuring the concentration or activity in the cavity fluid of a spe¬ cies which contributes significantly to the magnitude of the PDlj , and

determining the PDlj using the measured concentration or activity.

11. A method according to claim 9 or 10 wherein the determination of the PDlj is performed on the basis of a measurement performed by means of a sensor (species concentration sensor) arranged within the cavity.

12. A method according to any of claims 9-11 wherein the determina¬ tion of the PDlj is performed on the basis of a measurement of the concentration or activity of substantially only one species, the one species being the species which contributes most significantly to the PDlj.

13. A method according to claim 12 wherein the one species is the H⁺ ion and its concentration or activity is measured by means of a H⁺ sensor arranged within the cavity.

14. A method according to any of claims 11-13 wherein the PD measur¬ ing half cell and the species concentration sensor are combined into a single unit.

15. A method according to any of claims 1-14 wherein the PD reference electrode is immersed in a PD reference electrolyte contained in a container or conduit connected to a cannula or catheter inserted in the human or animal.

16. A method according to any of claims 1-15 wherein the PD reference electrolyte is a fluid which is physiologically acceptable for infu¬ sion into the human or animal.

17. A method for the in vivo determination of an electrical potenti¬ al difference (PD) across a human or animal membrane of a fluid- containing cavity in a human or in an animal, the method comprising:

arranging a PD measuring half cell comprising an electron-conductive part (PD measuring electrode) and an ion-conductive part (PD measur¬ ing electrolyte) so as to establish an electrochemical contact between the PD measuring electrolyte and the fluid contained in the cavity (the cavity fluid) ,

arranging a PD reference half cell comprising an electron-conductive part (PD reference electrode) and an Ion-conductive part (PD reference electrolyte) so as to establish an electrochemical contact between the PD reference electrolyte and a fluid of the human or ani¬ mal but outside the fluid-containing cavity, (non-cavity reference fluid) ,

detecting the PD between the two PD half cells,

measuring the concentration or activity in the cavity fluid of a spe¬ cies which contributes significantly to the magnitude of the liquid junction potential difference between the PD measuring electrolyte and the cavity fluid (PDlj) existing substantially simultaneously with the PD detection, the measurement being performed on a part of the cavity fluid remaining within the cavity during the measurement,

determining the PDlj using the measured concentration or activity,

adjusting the detected PD, the adjustment being based on the deter¬ mined PDlj , and

18. A PD measuring system for the in vivo determination of an elec¬ trical potential difference (PD) across a human or an animal membrane of a fluid-containing cavity in a human or in an animal, the system comprising: a PD registration instrument,

a PD reference cable comprising first and second end parts, the first end part being adapted to be connected to the PD registration instru- ment, and

19. A PD measuring system according to claim 18 wherein the outer maximal cross-sectional diameter of the PD measuring half cell and of the PD measuring cable is at the most 4 mm, preferably at the most 3 mm, such as about 2 mm.

20. A PD measuring system according to claim 18 or 19 wherein the PD measuring electrolyte reservoir is detachably arranged.

21. A PD measuring system according to any of claim 18-20 wherein an end face of the PD measuring half cell has at least one plane defin¬ ing an angle differing from 90° in relation to a longitudinal axis of the PD measuring electrolyte half cell.

22. A PD measuring system according to claim 21 wherein the end face defines a plane which is arranged obliquely in relation to the longi¬ tudinal axis of the PD measuring half cell, thus defining an acute angle, preferably an angle in the range of about 10-85°, such as about 30-60°, in particular about 45°.

23. A PD measuring system according to any of claims 18-22 wherein the PD reference electrode Is adapted to be immersed in a PD reference electrolyte placed in a catheter or cannula or in a con¬ tainer or conduit connected to the catheter or cannula, the reference electrode preferably being enclosed by a connecting means adapted to connect the electrode sealingly to a standard type inlet of such catheter or cannula or such container or conduit, in particular to a standard type inlet of a two- or several-way tube constituting an integral or removable part of the container or conduit.

24. A PD measuring system according to any of claims 18-23 further comprising a sensor (species concentration sensor) for measuring the concentration or activity in the cavity fluid of a species which con¬ tributes significantly to the magnitude of the liquid junction poten¬ tial difference between the PD measuring electrolyte and the cavity fluid (PDlj).

25. A PD measuring system according to claim 24 wherein the species concentration sensor is a H⁺ sensor adapted to be arranged within the cavity.

26. A PD measuring system according to claim 24 or 25 wherein the PD measuring half cell and the species concentration sensor are combined into a single unit.

27. A PD measuring system according to any of claims 24-26 wherein the PD registration instrument comprises a conversion unit and an input unit, the input unit comprising an input for receiving signals from a PD measuring half cell and an output for transmitting signals derived from the PD measuring half cell to the conversion unit, an input for receiving signals from a species concentration sensor and an output for transmitting signals derived from the species con¬ centration sensor to the conversion unit, the conversion unit being adapted to perform an adjustment of a detected PD, the adjustment being based on the signals derived from the species concentration sensor.

28. An instrument for the in vivo determination of an electrical potential difference (PD) across a human or animal membrane of a flu- id-containing cavity in a human or in an animal, comprising a conver¬ sion unit and an input unit, the input unit comprising an input for receiving signals from a PD measuring half cell and an output for transmitting signals derived from the PD measuring half cell to the conversion unit, an input for receiving signals from a species con- centration sensor and an output for transmitting signals derived from the species concentration sensor to the conversion unit, the conver¬ sion unit being adapted to perform an adjustment of a detected PD, the adjustment being based on the signals derived from the species concentration sensor.

29. A PD measuring electrode kit for use in a system according to any of claims 18-27 and comprising a sealed package containing: a PD measuring cable comprising first and second end parts, the first end part being adapted to be connected to the PD registration instrument,

a PD measuring electrolyte reservoir adapted to receive a PD measur¬ ing electrolyte and to permit electrochemical contact between at least part of the PD measuring electrode and the PD measuring elec- trolyte so as to establish a PD measuring half cell and further adapted to establish contact between the PD measuring electrolyte contained and a cavity fluid,

the PD measuring electrolyte reservoir having cross-sectional dimen¬ sions of at the most 4 mm, preferably at the most 3 mm, such as at the most 2 mm.

30. An electrode kit according to claim 29 wherein the PD measuring electrolyte reservoir is detachable.

31. An electrode kit for use in a system according to any of claims 18-27 and comprising a sealed package containing: a PD reference electrode assembly comprising

32. A kit according to claim 31 wherein the standard type inlet is a standard type inlet of a two- or several-way tube means constituting an integral or removable part of the container or conduit.

33. A kit according to any of claims 29-32 which is sterilized.