JOURNAL
APPLIED
PHYSIOLoGY
Vol. 40, No. 4, April
OF
1 976.
in U.S.A.
Printed
Polymer membrane sensors for continuous intravascular monitoring of blood pH 0. H. LEBLANC, JR., J. F. BROWN, JR., J. F. KLEBE, L. W. NIEDRACH, G. M. J. SLUSARCZUK, AND W. H. STODDARD, JR. General Electric Corporate Research and Development, Schenectady, New York 12301
LEBLANC, 0. H., JR., J. F. BROWN, JR., 3. F. KLEBE, L. W. NIEDRACH, G. M. J, SLUSARCZUK, AND W. H. STODDARD, JR. Polymer membrane sensors for continuous intravascular monitoring of Mood pH. J. Appl. Physiol, 40(4): 644-647. 1976. -A new type of pH sensor suitable for chronic intravascular implantation by virtue of its small size, flexibility, and ruggedness was constructed and evaluated. The pHsensitive element was a thin film of an elastromeric polymer made ion permselective to protons by adding a lipophilic, specific H+-ion carrier. This was coated onto small diameter silver wires to form sensors. In preliminary trials in anesthetized dogs, the sensors permitted continuous, accurate in vivo blood pH measurement with rapid response (CO. 1 s). beagle bonate
dog; hydrogen
ion; intra-arterial
sensor; Pco,;
bicar-
AN INDWELLING pH sensor that could easily be implanted in an artery or vein for continuously monitoring blood pH would be useful both in research and in clinical medicine. Unfortunately, the familiar glass pH electrode does not readily lend itself to the construction of indwelling devices. Although miniature glass electrodes have been mounted in flexible catheters (17), small glass electrodes are inherently fragile and therefore present a serious risk to the subject. Indeed, most investigators of in vivo blood pH have not employed indwelling electrodes, but rather have adopted the somewhat more cumbersome technique in which an arterial-venous shunt is constructed to allow blood flow past a rigidly mounted, mechanically protected glass electrode (l-4, 16). It occurred to us that what was needed to make indwelling sensors was a pH-sensitive material that, unlike glass, would be neither rigid nor fragile. We have found one such material, an elastomeric polymer which can be solvent-cast to form tough, rubbery films (5). These films were made ion permselective to protons by adding mobile, membrane-bound H+ carriers, so that they exhibited the electrical characteristics of a pH-sensitive “liquid membrane” electrode (6). The material was used as the active element in flexible sensors built up on small diameter silver wires by a sequence of wire-dipping steps. From the results of preliminary evaluations in dogs these do appear to be suitable for chronic vascular implantation. Such elastomeric, ion-permselective polymer materials may also be useful in constructing other designs and types of devices for physiological monitoring as well. MATERIALS
AND
METHODS
PH-sensitive membrane. The H+ carrier we have employed was suggested by recent research discoveries made using lipid bilayer membranes (15): certain organic weak acids, all of
which also uncouple oxidative phosphorylation in mitochondria, induce H+ ion transport through bilayers (11). The weak acid uncouplers act as mobile H+ carriers (7, l3), just what we desired. However, the known uncouplers, such as 2,4-dinitrophenol or m-chlorophenylhydrazonemesoxalonitrile (lo), were unsuitable for the present purpose because of their finite water solubilities: they would not be membrane bound. Accordingly, several higher molecular weight homologs of the uncouplers were prepared and examined; one of these, p-octadecyloxy-mchlorophenylhydrazonemesoxalonitrile (OCPH) (5>, exhibited all the properties desired, including water insolubility. The OCPH molecule functioned as a mobile H+ carrier in a variety of elastomers, but pH response characteristics of sufflciently good quality for practical use were only obtained using polymer matrices especially designed for the purpose. These were block copolymer elastomers containing about 60% polysiloxane and 40% poly(bisphenol-A carbonate), which have the useful property that they can be solvent cast to form thin films (19). Such materials are actually two phase systems: the polysiloxane blocks reside principally in a continuous, liquid-like phase through which molecular diffusion can occur rapidly, as in all silicones; the polycarbonate blocks form a discontinuous, crystal-like phase that effectively cross-links the whole structure together (12, 14). The more familiar materials of this type, which contain only dimethylsiloxane in the polysiloxane blocks, did not function as suitable matrices for the OCPH carrier; adding OCPH to them yielded membranes with very high electrical resistances and no pH response. Evidently the dielectric constant of these materials, 2.5-3.0, is too low to permit appreciable electrical charge exchange at the polymer/ water interfaces or significant charge unpairing (8). Therefore, we prepared similar block copolymers that contained a random mixture of dimethylsiloxane and methylcyanoethylsiloxane in the polysiloxane blocks, enough of the latter, polar monomer being introduced to yield final copolymers with dielectric constants in the range 4-7 (5). Materials of this type having siloxane block lengths of 15-20 and intrinsic viscosities in chloroform of 0.5-1.0 dl/g were found to perform well as matrices for the OCPH carrier. The OCPH carrier is soluble in the copolymers to a limit of about 3 wt %; normally a 1 wt % solution was used to prepare sensors. Both OCPH and the copolymers are soluble in CH&, from which rubbery, tough, thin films can be cast. For testing the electrical characteristics of isolated membranes we mounted a fairly large area (= 1 cm2) of a previously cast film on the end of a glass tube with silicone cement, filled the structure with a reference electrolyte, and immersed it in test solutions. pH sensor preparation. Sensors embodying these membranes were prepared by a sequence of wire-dipping steps. We began with a silver wire 0.25-0.5 mm in diameter insulated with, e.g., polyimide, polyester, or polysiloxane. The insula-
644 Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 1, 2019.
SENSORS
FOR
INTRAVASCULAR
MONITORING
OF
BLOOD
PH
tion was stripped from a l-cm length at one end. The bared silver was anodized to form a layer of AgCl, then coated with a =10-2-cm thick layer of a gelled, buffered, chloride-containing internal reference electrolyte solution. Finally, a = 10B2-cm thick film of the pH-sensitive polymer was cast over the electrolyte layer, and also over a short length of the insulation, by dipping into a CH,Cl, solution of OCPH and copolymer, then allowing the solvent to evaporate in dry N,. A Luer-Lok cap and an electrical lead were fitted to the opposite end of the wire to complete the sensor. The composition of the internal reference electrolyte was dictated by the nature of the environment (blood plasma) for which the sensor was intended, and also by the fact that polymer membranes, unlike glass ones, are permeable to water and to carbon dioxide; thus, the internal electrolyte was chosen to be roughly isotonic with plasma (-290 mosM) to minimize osmotic water fluxes, and it was buffered at pH 5 or lower to minimize changes in its pH with physiological variations in carbon dioxide tensions. Typically, we used a solution containing 80 mM NaCl, a pH 5 phosphate buffer in an amount to bring the osmolality to 290 mosM, and 2% by weight Methocel (Dow 90 HG premium grade) as the gelling agent. The completed sensors were stored until use in an isotonic aqueous solution in glass or plastic tubes. Sterilization by gamma-irradiation led to no deterioration in performance. 1n viuo procedures. Preliminary evaluations were performed in 40 beagle dogs, 6-10 kg, anesthetized with sodium pentobarbital, 35 mg/kg, and intubated with a cuffed endotracheal tube. In some experiments the dogs breathed spontaneously; in others they were ventilated with mixtures of O-10% carbon dioxide in air using a Harvard respiration pump. The sensor was implanted in a femoral artery through a 16 gauge catheter-needle with a T connection at its exterior end, either a Hemalock (General Electric Co., Milwaukee, Wis.) or a B-D Longdwel with a Selden adapter attached (Becton, Dickinson, Rutherford, N.J.). The active, pH-sensitive tip of the sensor extended into the blood vessel at least one cm beyond the end of the catheter. The T connection was used to withdraw blood samples for analysis. It also permitted the imposition of a continuous slow flush of lactated Ringer containing 2 IU heparin/ml to prevent clotting within the catheter lumen (9). Usually the reference electrode was simply a chlorided silver wire placed in the Y port of the pressurized venoclysis train (Abbott Venoset-78) used to supply the flush solution; thus, the Ringer solution served both as an electrolyte bridge and as the Cl- ion electrolyte for the AglAgCl reference couple. The electrical potential difference between the sensor and the reference electrode was current amplified by an Instrumentation Laboratories model 245 pH meter, or some similar instrument, and displayed on a strip chart recorder. Calibration was performed in buffers thermostated at 37OC before sensor implantation and was checked by pH’s periodically determined on withdrawn blood samples with a blood gas analyzer (Radiometer model 27GM or Instrumentation Laboratories model 313). RESULTS
Isolated membranes exhibited pH-dependent transmembrane potentials, as illustrated in Fig. 1. Between pH 4 and 9 these potentials had nearly the Nernstian slope (59.4 mV/pH unit at 25°C) and the theoretical absolute magnitude, and they were stable during continuous immersion of the membranes in neutral aqueous solutions for a period of longer than one year* At higher or lower pH’s the potentials were erratic and drifted with time; evidently the OCPH molecule, the copolymer ma-
645
2
4
6
0
IO
PH FIG. 1. Transmembrane potential as a function of external pH observed in vitro for membranes approximately I cm” in area and lo+ cm thick. At time of measurement (0) had been immersed in a neutral electrolyte solution for 37 days, (@) for 250 days. Temperature: 23°C. Inner electrode: Ag/AgCl. Inner electrolyte: 0.15 N Cl-, pH 7.39. Outer electrode: saturated calomel. Outer electrolyte: variable. Theoretical slope: 58.8 mV/pH unit; found: 58.5 & 1.0.
trix, or both are unstable when exposed to aqueous solutions of extreme pH’s. The specificity to H+ ions was extremely high; no interferences by inorganic ions or by most organic ions could be detected in vitro. Certain lipophilic organic ions were, however, found to interfere with the pH response (e.g., phthalate at mM and cetyltrimethylammonium at PM concentrations), a phenomenon to be expected with any organic membrane electrode, if one not always appreciated or acknowledged. Interestingly enough, no intereferences were observed in blood, either in vitro or in vivo, which suggests that lipophilic ions must be present in blood in vanishing concentrations. The sensors exhibited in vitro pH responses identical to those in Fig. 1. They had response times of the order of 0.1 s or less, as illustrated in Fig. 2. Their impedances varied in the range of lo-100 MO, the exact value depending on the exact thickness and geometry of the cast membrane. In the in vivo trials newly implanted sensors indicated blood pH values that agreed with determinations on withdrawn blood samples to within 20.03 pH units. Drift was small: a shift of 0.05 pH units out of calibration during an 8-hr experiment was typical. Changes in arterial pH imposed by CO, breathing were tracked rapidly to within the same accuracy, as illustrated in Fig. 3. Because of the sensitivity and short response time of the sensor, the small oscillations in arterial pH which occur at the respiration frequency (1, 2, 4, 16) were
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 1, 2019.
LEBLANC
ET
AL,
flexible sensors were often bent and distorted badly, none of the 40 tested was broken. No evidence for thrombus formation on the sensors was observed. Tests of the membrane material as prescribed for class VI plastics, US Pharmacopeia (18), for acute toxicity, intracutaneous tissue reaction, and intramuscular tissue reaction were negative (Science Associates, Northwood, Ohio). DISCUSSION
FIG, 2. Time response of a miniature sensor in a fast experiment in which the external pH was suddenly 7.32 to 6.10. Vertical scale: 20 mV per large division; ms per large division. Theoretical potential excursion plete within 100 ms.
flow, in vitro changed from horizontal: 50 was 99% com-
8.0
7.0 IL-
t
i
I
0
1
t 2 TIME
IN
I 4
1
I 6
HOURS
FIG. 3. Comparison of sensor in vivo (n) vs. Radiometer in vitro (0) measurements as a function of time. Changes in pH induced by breathing C0, mixtures. Sensor measurements were actually continuous, but for clarity only the sensor values at the times of sampling for in vitro analysis are plotted here.
observed. In chanical respirators, lation were detected At the termination withdrawn from the readily
monitoring animals supported on meimposed variations in pulmonary ventiwithin a single breath. of an in vivo experiment the sensor was catheter and examined. Although the
The polymer membrane based sensors thus appear to be suitable for intravascular implantation, They measure in vivo blood pH accurately and rapidly. Because they are flexible they can cause little trauma to blood vessel walls, and they are not likely to break, They can readily be made in diflerent diameters and lengths, or in wholly different configurations, for various monitoring purposes. The general concept of using ion-permselective polymer membranes for in vivo devices has several virtues. The structure of these synthetic materials can be chosen or altered to yield the physical and chemical properties desired. For example, although no evidence for bioincompatibility has been found with the polymeric materials we have used here, if such were to be found with it or other similar materials then their chemical structures could be modified to eliminate the problem. Obviously, with suitable modification sensors could be constructed for measuring pH or other ionic activities in tissues besides blood. Even the seemingly annoying fact that polymeric membranes, unlike glass ones, are permeable to gases can be turned to positive advantage. Thus, by using a membrane such as that described here (which is ion permselective to protons and also permeable to carbon dioxide) as the permeation barrier in a Severinghaus Pcoz electrode, and adding an external reference electrode, one has a device that will simultaneously measure pH and carbon dioxide tension (and hence also bicarbonate). An in vivo Pco2 sensor (General Electric) has been converted into a dual-function pH/Pco, in vivo sensor in this fashion (6a). In vivo tests were performed in the laboratories of J. I? Kampine, MD, Medical College of Wisconsin, Milwaukee, Wis., and G. Moss, MD, Rensselaer Polytechnic Institute, Troy, N.Y., to both of whom we are indebted. J. F. Klebe is deceased. Received
for publication
30 September
1975.
REFERENCES D. M., I. R. CAMERON, AND S. J. G. SEMPLE. Oscillations in arterial pH with breathing in the cat. J. Appl. Physiol. 26: 261-267, 1969. 2. BAND, 14. M., I. R. CAMERON, AND S. J. G. SEMPLE. Effect of different methods of CO, administration on oscillations of arterial pH in the cat. J. Appl. Physiol. 26: 268-273, 1969. 3. BAND, D. M., AND S. J. G. SEMPLE. Continuous measurement of blood pH with an indwelling arterial glass electrode. J. AppZ. Physiol. 22: 854-857. 4. BONDI, K. R., AND H. D. VAN LIEW. Fluxes of CO, in the lung gas studied by continuously recorded arterial pH. J. Appl. PhysioZ. 35: 42-46, 1973. 5. BROWN, J. F., G. M. 5. SLUSARCZUK, AND 0. I-3. LEBLANCIO~. Specific Membrane. US Patent 3,743,588, July 3, 1973. 6. BUCK, R. P. Ion selective electrodes, potentiometry, and potentiometric titrations. Anal. Chem. 46: 28R-51R, 1974. 6a.CooN, R. L., N. C. J. LAI, AND J. P. KAMPINE. Evaluation of a dual-function pH and Pco2 in vivo sensor. J. Appl. Physiol. 40: 625-629. 1976. 1. BAND,
7. FINKELSTEIN, A. Weak acid uncouplers of oxidative phosphorylation. Mechanism of action on thin lipid membranes. Biochim. Biophys. Acta 205: 1-6, 1970. 8. Fuoss, R. M., AND C. A. KRAUS. Properties of electrolytic solutions. XV. Thermodynamics of very weak electrolytes. J. Am. Chem. Sot. 57: 1-4, 1935. 9. GARDNER, R. M., R. SCHWARTZ, H. C. WONG, AND J. P. BURKE. Percutaneous indwelling radial-artery catheters for monitoring cardiovascular function. N. Engl. J. Med. 290: 1227-1231, 1974. 10. HEYTLER, P. CL, AND W. W. PRITCHARD. A new class of uncouphenylhydrazones. Biochem. pling agents - carbonylcyanide Biophys. Res. Commun. 7: 272-275, 1962. 11. HOPFER, U., A. L. LEHNINGER, AND T. E. THOMPSON. Protonic conductance across phospholipid bilayer membranes induced by uncoupling agents for oxidative phosphorylation. Proc. Natl. Acad. Sci., US 59: 484-490, 1968. 12. KAMIBOUR, R. P. Microdomains in alternating block copolymers of dimethylsiloxane and bisphenol-A carbonate. J. Polymer Sci. B7: 573-577, 1969.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 1, 2019.
SENSORS
FOR
INTRAVASCULAR
MONITORING
OF
BLOOD
13. LEBLANC, 0. H. The effect of uncouplers of oxidative phosphorylation on lipid bilayer membranes: carbonylcyanide m-chlorophenylhydrazone. J. Membrane BioZ. 4: 227-251, 1971. 14. LEGRAND, D. G, Mechanical and optical studies on poly (dimethylsiloxane) bisphenol-A polycarbonate copolymers, J. Polymer Sci. ~7: 579-585, 1969. 15. MUELLER, P., D. 0. RUDIN, H. T. TIEN, AND W. C. WESCOTT. Reconstitution of cell membrane structure in vitro and its transformation into an excitable system. Nature 194: 979-980, 1962.
pH
647 16. NIMS, L. F., AND C. S. MARSHALL. Blood pH in vivo I. Changes due to respiration. Yule J. BioZ. Med. 10: 445-448, 1938, 17. STAEHELIN, H. B., E. N. CARLSEN, D. B., HINSHAW, AND L. L. SMITH. Continuous blood pH monitoring using an indwelling catheter. Am. J. Surg. 116: 280-285, 1968, 18. United States Pharmacopeia. xix: 645, 1975. 19. VAUGHN, H. A. The synthesis and properties of alternating block polymers of dimethylsiloxane and bisphenol-A carbonate. J. Polymer Sci. ~7: 569-572, 1969.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 1, 2019.
APPLIED
PHYSIOLoGY
Vol. 40, No. 4, April
OF
1 976.
in U.S.A.
Printed
Polymer membrane sensors for continuous intravascular monitoring of blood pH 0. H. LEBLANC, JR., J. F. BROWN, JR., J. F. KLEBE, L. W. NIEDRACH, G. M. J. SLUSARCZUK, AND W. H. STODDARD, JR. General Electric Corporate Research and Development, Schenectady, New York 12301
LEBLANC, 0. H., JR., J. F. BROWN, JR., 3. F. KLEBE, L. W. NIEDRACH, G. M. J, SLUSARCZUK, AND W. H. STODDARD, JR. Polymer membrane sensors for continuous intravascular monitoring of Mood pH. J. Appl. Physiol, 40(4): 644-647. 1976. -A new type of pH sensor suitable for chronic intravascular implantation by virtue of its small size, flexibility, and ruggedness was constructed and evaluated. The pHsensitive element was a thin film of an elastromeric polymer made ion permselective to protons by adding a lipophilic, specific H+-ion carrier. This was coated onto small diameter silver wires to form sensors. In preliminary trials in anesthetized dogs, the sensors permitted continuous, accurate in vivo blood pH measurement with rapid response (CO. 1 s). beagle bonate
dog; hydrogen
ion; intra-arterial
sensor; Pco,;
bicar-
AN INDWELLING pH sensor that could easily be implanted in an artery or vein for continuously monitoring blood pH would be useful both in research and in clinical medicine. Unfortunately, the familiar glass pH electrode does not readily lend itself to the construction of indwelling devices. Although miniature glass electrodes have been mounted in flexible catheters (17), small glass electrodes are inherently fragile and therefore present a serious risk to the subject. Indeed, most investigators of in vivo blood pH have not employed indwelling electrodes, but rather have adopted the somewhat more cumbersome technique in which an arterial-venous shunt is constructed to allow blood flow past a rigidly mounted, mechanically protected glass electrode (l-4, 16). It occurred to us that what was needed to make indwelling sensors was a pH-sensitive material that, unlike glass, would be neither rigid nor fragile. We have found one such material, an elastomeric polymer which can be solvent-cast to form tough, rubbery films (5). These films were made ion permselective to protons by adding mobile, membrane-bound H+ carriers, so that they exhibited the electrical characteristics of a pH-sensitive “liquid membrane” electrode (6). The material was used as the active element in flexible sensors built up on small diameter silver wires by a sequence of wire-dipping steps. From the results of preliminary evaluations in dogs these do appear to be suitable for chronic vascular implantation. Such elastomeric, ion-permselective polymer materials may also be useful in constructing other designs and types of devices for physiological monitoring as well. MATERIALS
AND
METHODS
PH-sensitive membrane. The H+ carrier we have employed was suggested by recent research discoveries made using lipid bilayer membranes (15): certain organic weak acids, all of
which also uncouple oxidative phosphorylation in mitochondria, induce H+ ion transport through bilayers (11). The weak acid uncouplers act as mobile H+ carriers (7, l3), just what we desired. However, the known uncouplers, such as 2,4-dinitrophenol or m-chlorophenylhydrazonemesoxalonitrile (lo), were unsuitable for the present purpose because of their finite water solubilities: they would not be membrane bound. Accordingly, several higher molecular weight homologs of the uncouplers were prepared and examined; one of these, p-octadecyloxy-mchlorophenylhydrazonemesoxalonitrile (OCPH) (5>, exhibited all the properties desired, including water insolubility. The OCPH molecule functioned as a mobile H+ carrier in a variety of elastomers, but pH response characteristics of sufflciently good quality for practical use were only obtained using polymer matrices especially designed for the purpose. These were block copolymer elastomers containing about 60% polysiloxane and 40% poly(bisphenol-A carbonate), which have the useful property that they can be solvent cast to form thin films (19). Such materials are actually two phase systems: the polysiloxane blocks reside principally in a continuous, liquid-like phase through which molecular diffusion can occur rapidly, as in all silicones; the polycarbonate blocks form a discontinuous, crystal-like phase that effectively cross-links the whole structure together (12, 14). The more familiar materials of this type, which contain only dimethylsiloxane in the polysiloxane blocks, did not function as suitable matrices for the OCPH carrier; adding OCPH to them yielded membranes with very high electrical resistances and no pH response. Evidently the dielectric constant of these materials, 2.5-3.0, is too low to permit appreciable electrical charge exchange at the polymer/ water interfaces or significant charge unpairing (8). Therefore, we prepared similar block copolymers that contained a random mixture of dimethylsiloxane and methylcyanoethylsiloxane in the polysiloxane blocks, enough of the latter, polar monomer being introduced to yield final copolymers with dielectric constants in the range 4-7 (5). Materials of this type having siloxane block lengths of 15-20 and intrinsic viscosities in chloroform of 0.5-1.0 dl/g were found to perform well as matrices for the OCPH carrier. The OCPH carrier is soluble in the copolymers to a limit of about 3 wt %; normally a 1 wt % solution was used to prepare sensors. Both OCPH and the copolymers are soluble in CH&, from which rubbery, tough, thin films can be cast. For testing the electrical characteristics of isolated membranes we mounted a fairly large area (= 1 cm2) of a previously cast film on the end of a glass tube with silicone cement, filled the structure with a reference electrolyte, and immersed it in test solutions. pH sensor preparation. Sensors embodying these membranes were prepared by a sequence of wire-dipping steps. We began with a silver wire 0.25-0.5 mm in diameter insulated with, e.g., polyimide, polyester, or polysiloxane. The insula-
644 Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 1, 2019.
SENSORS
FOR
INTRAVASCULAR
MONITORING
OF
BLOOD
PH
tion was stripped from a l-cm length at one end. The bared silver was anodized to form a layer of AgCl, then coated with a =10-2-cm thick layer of a gelled, buffered, chloride-containing internal reference electrolyte solution. Finally, a = 10B2-cm thick film of the pH-sensitive polymer was cast over the electrolyte layer, and also over a short length of the insulation, by dipping into a CH,Cl, solution of OCPH and copolymer, then allowing the solvent to evaporate in dry N,. A Luer-Lok cap and an electrical lead were fitted to the opposite end of the wire to complete the sensor. The composition of the internal reference electrolyte was dictated by the nature of the environment (blood plasma) for which the sensor was intended, and also by the fact that polymer membranes, unlike glass ones, are permeable to water and to carbon dioxide; thus, the internal electrolyte was chosen to be roughly isotonic with plasma (-290 mosM) to minimize osmotic water fluxes, and it was buffered at pH 5 or lower to minimize changes in its pH with physiological variations in carbon dioxide tensions. Typically, we used a solution containing 80 mM NaCl, a pH 5 phosphate buffer in an amount to bring the osmolality to 290 mosM, and 2% by weight Methocel (Dow 90 HG premium grade) as the gelling agent. The completed sensors were stored until use in an isotonic aqueous solution in glass or plastic tubes. Sterilization by gamma-irradiation led to no deterioration in performance. 1n viuo procedures. Preliminary evaluations were performed in 40 beagle dogs, 6-10 kg, anesthetized with sodium pentobarbital, 35 mg/kg, and intubated with a cuffed endotracheal tube. In some experiments the dogs breathed spontaneously; in others they were ventilated with mixtures of O-10% carbon dioxide in air using a Harvard respiration pump. The sensor was implanted in a femoral artery through a 16 gauge catheter-needle with a T connection at its exterior end, either a Hemalock (General Electric Co., Milwaukee, Wis.) or a B-D Longdwel with a Selden adapter attached (Becton, Dickinson, Rutherford, N.J.). The active, pH-sensitive tip of the sensor extended into the blood vessel at least one cm beyond the end of the catheter. The T connection was used to withdraw blood samples for analysis. It also permitted the imposition of a continuous slow flush of lactated Ringer containing 2 IU heparin/ml to prevent clotting within the catheter lumen (9). Usually the reference electrode was simply a chlorided silver wire placed in the Y port of the pressurized venoclysis train (Abbott Venoset-78) used to supply the flush solution; thus, the Ringer solution served both as an electrolyte bridge and as the Cl- ion electrolyte for the AglAgCl reference couple. The electrical potential difference between the sensor and the reference electrode was current amplified by an Instrumentation Laboratories model 245 pH meter, or some similar instrument, and displayed on a strip chart recorder. Calibration was performed in buffers thermostated at 37OC before sensor implantation and was checked by pH’s periodically determined on withdrawn blood samples with a blood gas analyzer (Radiometer model 27GM or Instrumentation Laboratories model 313). RESULTS
Isolated membranes exhibited pH-dependent transmembrane potentials, as illustrated in Fig. 1. Between pH 4 and 9 these potentials had nearly the Nernstian slope (59.4 mV/pH unit at 25°C) and the theoretical absolute magnitude, and they were stable during continuous immersion of the membranes in neutral aqueous solutions for a period of longer than one year* At higher or lower pH’s the potentials were erratic and drifted with time; evidently the OCPH molecule, the copolymer ma-
645
2
4
6
0
IO
PH FIG. 1. Transmembrane potential as a function of external pH observed in vitro for membranes approximately I cm” in area and lo+ cm thick. At time of measurement (0) had been immersed in a neutral electrolyte solution for 37 days, (@) for 250 days. Temperature: 23°C. Inner electrode: Ag/AgCl. Inner electrolyte: 0.15 N Cl-, pH 7.39. Outer electrode: saturated calomel. Outer electrolyte: variable. Theoretical slope: 58.8 mV/pH unit; found: 58.5 & 1.0.
trix, or both are unstable when exposed to aqueous solutions of extreme pH’s. The specificity to H+ ions was extremely high; no interferences by inorganic ions or by most organic ions could be detected in vitro. Certain lipophilic organic ions were, however, found to interfere with the pH response (e.g., phthalate at mM and cetyltrimethylammonium at PM concentrations), a phenomenon to be expected with any organic membrane electrode, if one not always appreciated or acknowledged. Interestingly enough, no intereferences were observed in blood, either in vitro or in vivo, which suggests that lipophilic ions must be present in blood in vanishing concentrations. The sensors exhibited in vitro pH responses identical to those in Fig. 1. They had response times of the order of 0.1 s or less, as illustrated in Fig. 2. Their impedances varied in the range of lo-100 MO, the exact value depending on the exact thickness and geometry of the cast membrane. In the in vivo trials newly implanted sensors indicated blood pH values that agreed with determinations on withdrawn blood samples to within 20.03 pH units. Drift was small: a shift of 0.05 pH units out of calibration during an 8-hr experiment was typical. Changes in arterial pH imposed by CO, breathing were tracked rapidly to within the same accuracy, as illustrated in Fig. 3. Because of the sensitivity and short response time of the sensor, the small oscillations in arterial pH which occur at the respiration frequency (1, 2, 4, 16) were
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 1, 2019.
LEBLANC
ET
AL,
flexible sensors were often bent and distorted badly, none of the 40 tested was broken. No evidence for thrombus formation on the sensors was observed. Tests of the membrane material as prescribed for class VI plastics, US Pharmacopeia (18), for acute toxicity, intracutaneous tissue reaction, and intramuscular tissue reaction were negative (Science Associates, Northwood, Ohio). DISCUSSION
FIG, 2. Time response of a miniature sensor in a fast experiment in which the external pH was suddenly 7.32 to 6.10. Vertical scale: 20 mV per large division; ms per large division. Theoretical potential excursion plete within 100 ms.
flow, in vitro changed from horizontal: 50 was 99% com-
8.0
7.0 IL-
t
i
I
0
1
t 2 TIME
IN
I 4
1
I 6
HOURS
FIG. 3. Comparison of sensor in vivo (n) vs. Radiometer in vitro (0) measurements as a function of time. Changes in pH induced by breathing C0, mixtures. Sensor measurements were actually continuous, but for clarity only the sensor values at the times of sampling for in vitro analysis are plotted here.
observed. In chanical respirators, lation were detected At the termination withdrawn from the readily
monitoring animals supported on meimposed variations in pulmonary ventiwithin a single breath. of an in vivo experiment the sensor was catheter and examined. Although the
The polymer membrane based sensors thus appear to be suitable for intravascular implantation, They measure in vivo blood pH accurately and rapidly. Because they are flexible they can cause little trauma to blood vessel walls, and they are not likely to break, They can readily be made in diflerent diameters and lengths, or in wholly different configurations, for various monitoring purposes. The general concept of using ion-permselective polymer membranes for in vivo devices has several virtues. The structure of these synthetic materials can be chosen or altered to yield the physical and chemical properties desired. For example, although no evidence for bioincompatibility has been found with the polymeric materials we have used here, if such were to be found with it or other similar materials then their chemical structures could be modified to eliminate the problem. Obviously, with suitable modification sensors could be constructed for measuring pH or other ionic activities in tissues besides blood. Even the seemingly annoying fact that polymeric membranes, unlike glass ones, are permeable to gases can be turned to positive advantage. Thus, by using a membrane such as that described here (which is ion permselective to protons and also permeable to carbon dioxide) as the permeation barrier in a Severinghaus Pcoz electrode, and adding an external reference electrode, one has a device that will simultaneously measure pH and carbon dioxide tension (and hence also bicarbonate). An in vivo Pco2 sensor (General Electric) has been converted into a dual-function pH/Pco, in vivo sensor in this fashion (6a). In vivo tests were performed in the laboratories of J. I? Kampine, MD, Medical College of Wisconsin, Milwaukee, Wis., and G. Moss, MD, Rensselaer Polytechnic Institute, Troy, N.Y., to both of whom we are indebted. J. F. Klebe is deceased. Received
for publication
30 September
1975.
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SENSORS
FOR
INTRAVASCULAR
MONITORING
OF
BLOOD
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pH
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