Journal of Medical Engineering & Technology

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A catheter-tip potassium-selective electrode T. Treasure & D. M. Band To cite this article: T. Treasure & D. M. Band (1977) A catheter-tip potassiumselective electrode, Journal of Medical Engineering & Technology, 1:5, 271-273, DOI: 10.3109/03091907709162193 To link to this article:

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A catheter-tip potassium-selective electrode t’. Treasure and D. M. Band iherrington School of Physiology, St. Thomas’s Hospital Medical School, London S E l , UK. ~~

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In the management of patients in the intensive care unit a measurement of potassium is required more frequently than any other plasma electrolyte. W e have developed an ion selective electrode to provide rapid results on whole blood samples and have used the method to monitor continuously plasma levels. Potassium is one of the principal cations of the body and its distribution across the cell membranes is predominantly *esponsible for maintenance of the membrane potential. The plasma concentration is between 3.5 and 5.5 mmol 1-1 and levels outside this range begin to affect adversely the function of nerve and muscle tissue. In health the daily intake is balanced by an appropriate urinary loss which averages about 80 m moles in 24 hrs. Kidney or endoxine diseases, diarrhoea and vomiting or treatment with diuretic drugs can disturb this day-to-day balance and result in an overall excess or deficit. In some patients abnormalities of distribution can result in alterations of the critical gradient of potassium across cell membranes. This can occur, for example, in the treatment of diabetic coma or following cardio-pulmonary bypass surgery and may result in impairment of the contractility or rhythmicity of the heart. These changes of distribution can occur quite rapidly and the availability of an electrode which can be used to make several whole blood estimations within an hour has proved very valuable in the intensive :are unit of this hospital.’-* We have therefore extended the uses of this electro-analytical method to in vivo monitoring. Continuous measurement of blood potassium in animal experiments has previously been attempted using glass3 and liquid membrane4 electrodes, but poor selectivity and problems with pressure and flow artifacts5 have limited their usefulness. By using a highly selective ion exchanger ivstemu in a supported PVC membrane’ we have overcome these problems. Catheter-tip electrodes made as described in this paper have proved satisfactory for continuous recording in animal experiments and initial attempts at clinical use have been promising.

Principle of the method The measurement of potassium with an ion-selective electrode depends on measuring the membrane potential established when a selectively permeable membrane is used to separate two dissimilar solutions. The potential is proportional to the natural logarithm of the potassium ion ratio across the membrane and for a perfect electrode is given by the Nernst equation :

If one surface of the membrane is exposed to blood and the other to a solution of unchanging potassium concentration then the potential across the membrane changes with the logarithm of the concentration of the potassium in the blood. The membrane potential is recorded between two silver/silver chloride electrodes by a high impedance electrometer.

Methods of construction Polyvinyl chloride (PVC) bilumen tubing of 1.6 mm 0.d. was used as the basis of the catheter (Fig. 1). Lengths of about 50 cm were taken and the lumen which was to contain the external reference electrode was opened by side holes at 2 cm and 40 cm from the tip. Diamel coated silver wires in 60 cm lengths were bared at one end and chlorided by electrolysis with a current density of 10 mA cm-2 for 15 minutes in a solution of KC1 0.1M. These chlorided silver wires were used, one in each lumen as the internal and external reference electrodes. One silver wire was introduced into the external reference lumen. This lumen was then sealed with PVC at the tip and the reference electrode secured proximally with silicone rubber. A second chlorided silver wire (the internal reference electrode) was introduced into the other lumen. A porous ceramic plug was inserted into the tip of the catheter so that it was in contact with the “internal” lumen and the ion-selective membrane was dip cast onto its outer surface. The membrane recipe is given in Table 1. Its consistency was adjusted by evaporation or further addition of tetrahydrofuran (THF) so that a smooth‘even film of PVC was obtained after dipping the porous ceramic into the mixture two or three times. When the membrane was quite dry (at least 24 hours) the internal reference solution (KC1 4 mmol 1-1) was injected through fine needle tubing passed as far as the ceramic plug which is hydrophilic and readily takes up solutions. The lumen was then filled from the tip backwards and sealed with silicone rubber. A subminiature two pin plug was attached to the two silver wires. A length of PVC tubing was glued onto the proximal side hole for injection of the reference salt bridge (NaCl 0.14 M) so that a liquid junction could be formed with the blood at the catheter tip. The catheters were calibrated in potassium standards made up in a background of NaCl 0.14 M. This has the

Table 1. Materials used in construction of the membrane where E

equilibrium potential R = gas constant T = absolute temperature F = Faraday’s number Converting from the natural logarithm of the base 10 logarithm and replacing the constants with numerical values we can restate the equation:

~ _ _ _ ~~



September, 1977

1.5 mg 0.025 mg

150 mg 50 mg 1 5 mg

3 ml

27 1

:ffect of bringing the activity coefficient for KCI to about he same value in all solutions. Their response was better han 97% of the theoretical Nernst relationship between and 1 0 mmol 1-1 KCI standards confirming high selectirity for potassium over sodium. They had a d.c. resistance )f 5-10 Mohm measured at 20°C. In animal experiments measurements were made with iish input impedance differential amplifiers and the signal iisjjlajiei: sii aii OSC~!!OSCO~C b c f x e recordizg. Fer neasurements in patients a full floating battery operated neter was used. The catheter was connected to the dual ield effect transistor a t the input stage of the circuit and he low impedence signal was then taken to the amplifier md the potassium read directly from a scale gradulated in nmol 1-l.



130 a


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hnimal experiments 2atheters of this construction have been used in experinents on anaesthetised cats and dogs. Figure 2 shows an :xperimental record obtained from two of these devices in he venous circulation of a greyhound. The response to in injection of potassium is illustrated by the rapid resIonse of the catheter in the inferior vena cava and then, rfter two capi1la;y circulations, the rise in potassium is ieen in blood returning from the left hind limb in the 'emoral vein. A dircct comparison was obtained using a )otassiurn electrode in a cat and estimations of potassium n blood samples with the flame photometer. Injections if KCI 0.1 M were made and as soon as the electrode ndicated a plateau in the potassium level a blood sample vas aspirated from the inferior vena cava. The voltage eading was plotted against the log of concentration as neasured by flame photometry (Fig. 3).


relerence electrode liquid junction

saline injected lor salt b r a e

:ig 1. A catheter in the course o f construcfion. The silver/ ilver chloride electrodes are seen, one in each lumen of a iilumen PVC tube. The membrane is dip cast onto porous ,eramic so that its inner surface is exposed to KCI reference 'oluriori and its ouier surface is exposed to blood.








4 5



2.0 30 mmotes I I

Fig. 3. A calibration of a potassium eleclrode in a cat. The voltage from the catheter is plotted against the logarithm oj the coiicentration o f potassium as estimated with ait emission flame photometer.







0 1 2 Potas siurn m moles / I



Tig. 2. The response to an injection o f 2 mls o f 0.1 M KCI ecorded by poiassium sensing catheiers in the inferior vena ava (above) and the femoral vein (below). 272


I 4 time in horn

I 5

Fig. 4. An injection of 30 g glucose and 30 u insulin in ar anesthefised greyhound A produced a fall in blood potassium Potassium measurements with the catheter 0 are comparec with estimations made on blood samples with a bend electrode


Journal of Medical Engineering and I echnology

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Various manoeuvres which are known to produce changes in plasma potassium were performed and the response of the catheters was compared with estimations on intermittent samples by the bench electrode. Figure 4 shows an example where an injection of glucose and insulin was made intravenously in a greyhound. Blood samples were withdrawn intermittently during the five hour period and fall in potassium measured. The results were compared with the measurements made by a catheter in the inferior vena cava. During experiments of this kind the electrodes continued to function without anti-coagulants for more than ten hours. Their great advantage of course is that the catheter continues to give information about transient changes or steady trends in between these spot measurements. Previous work on rapid potassium changes in the blood has usually depended on intermittent sampling and flame photometry. This is difficult and tedious and the information is necessarily limited if only by the quantity of blood that can be withdrawn from a small animal. Using this device we have been able to make repeated measurements of the potassium changes associated with haemorrhage and catecholamines. We have also observed the potassium changes associated with asphyxia and muscle depolarisation following suxamethonium injection. As was seen in Figure 2, potassium can be measured in more than one site. Monitoring in a peripheral vein will give information about the potassium efflux from a particular vascular bed. A catheter in the right atrium or the vena cava gives an index of mixed venous potassium. At times there are marked discrepancies in the level of potassium in different fractions of the returning venous blood.

amplifiers to permit recording on mains powered equipment and this is an acceptable method of making such devices patient safe. We are also exploring the use of F M telemetering as a means of isolation. The selectivity of the PVC membrane is impaired by exposure to x-rays and ethylene oxide so these methods of sterilising are not applicable to this device. Glutaraldehyde was the only method of those investigated which did not reduce the membrane response. We have no doubt that this and similar devices will continue to be of great interest to research workers. In experimental work on animals where sterilisation and electrical isolation do not present a problem ion-selective electrodes can be used to follow potassium changes associated with a number of circulatory and metabolic changes. They may also find a place in clinical research. For example, the effect on venous potassium of halting limb perfusion during vascular surgery could be investigated with devices of this type. The value of continuous monitoring of ECG and blood pressure has long been recognised in the operating theatre and intensive care unit. Oxygen monitoring with cathetertip polarographic electrodes is now an established technique.* The results of further clinical trials of the potassium catheter will tell if the technique is sufficiently reliable and the information of enough clinical use to merit application to routine patient care. However, there is little doubt that these devices would be of enormous help to those involved in the care of patients following cardiac surgery.

Temperature response Changing temperature will affect the e.m.f. of the catheter in two ways. The voltage of the two silver chloride reference electrodes will change with temperature but, since they lie side by side in the catheter and are both exposed to the same fluctuations in body temperature, no net change in e.m.f. occurs. The other temperature-dependent part of the system is the selective membrane. Its response to a decade change in potassium concentration will increase from 61.4 mV at 37°C to 62 mV at 40°C.Since potassium changes are well within a decade and the calibration is performed near the operating range temperature effect on the membrane voltage never exceeds 0.5%.

Time constant When tested in v i f r o the voltage change is complete within a second. In vivo the limiting factor in the catheter response is flow in the vessel. As can be seen from Figure 2 the response in a large vessel is rapid. This is more than would be required clinically and adequate for experimental purposes.

Future use and clinical applications Before the device was tried clinically two particular difficulties had to be overcome; those of electrical isolation and sterilisation of the device. By using a battery operated fully floating meter as described we were able to obtain steady interference free readings in the operating theatre and intensive care unit. We have used optically isolated September, 1977

REFERENCES 1. Band, D. M., Kratochvil, J. and Treasure, T. (1977) An ion selective electrode for the determination of potassium. Journal of Physiology, 265, 5-6P. 2. Treasure, T. and Band, D. M. (1977) Measurement of plasma potassium using ion selective electrodes. Proceedings of rhe Analytical Division of rhe Chemical Society (in press). 3. Friedman, S. M., larnieson, J. D., Nakashima, M. and Friedman, C. L. (1959) Sodium and potassium glass electrodes for biological use. Science, 130, 1252-4. 4. Hnik, P., Kriz, N., Vyskocil, F., Smiesko, V., Mejsnar, J., Ujec, E. and Holas, M. (1973) Work induced potassium changes in muscle venous effluent blood measured by ionspecific electrodes. Pflugers Archives, 338, 177-181. 5. Portnoy, H. D., Thomas, L. M. and Gurdjian, E. S. (1962) Reducing flow artifact when recording from glass electrodes. Journal of Applied Physiology, 17, 175-6. 6. Band, D. M. and Kratochvil, J. (1974) A potassium ionselective liquid membrane. Journnl of Physiology, 239, 1OP. 7. Band, D. M. and Treasure, T. (1977) Continuous monitoring of blood potassium demonstrated in an animal. Journal of Physiology, 266, 12-13P. 8. Soutter, L. P., Conway, M. J. and Parker, D. (1975) A system for monitoring oxygen tension in sick new born babies. Biomedical Engineering, 10, 257-260. 273

A catheter-tip potassium-selective electrode.

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