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Carbon dioxide tensions in physiological salt solutions: Direct measurements
Clinica Chimica Acta, 185 (1989) 1-6 Elsevier
CCA 04556
Carbon dioxide tensions in physiological salt solutions: direct measurements Jan Abrahamsen Departments
‘, Birgitte Norrie ‘, Poul K. Andersen and Ove A. Nedergaard ’
*, Dag B. Stokke *
of ’ Pharmacologv and 2 Anesthesiology and Intensive Care, Odense University, Odense University Hospital, Odense C (Denmark) (Received
Key words: Carbon
29 January
dioxide
1989; accepted
tension;
Bicarbonate;
10 June 1989) Physiological
salt solutions
Carbon dioxide tensions were measured directly in organ baths and a tonometer aerated in parallel with 6 different gas mixtures of 0, and CO,, 3 gas flows, 3 equilibration periods, and 3 bicarbonate concentrations. The measured partial pressure of carbon dioxide differed systematically from expected values, probably due to errors in the carbon dioxide measurement system. In conclusion, carbon dioxide equilibrates with the bubbling gas in the baths as well as in the tonometer to an almost perfect equilibration.
Introduction
For pharmacological and physiological experiments, organ baths are commonly bubbled with a mixture of 5% carbon dioxide and 95% oxygen. According to the Henderson-Hasselbalch equation, this is assumed to yield a normal pH (7.40) and tension of carbon dioxide (36 mm Hg) in the bath solution when employing the usual modifications of Krebs solutions [l]. However, most baths are open to diffusion of the dissolved gases to the surrounding atmosphere and there may be substantial loss of carbon dioxide through permeable tubings. In addition, it is doubtful whether the gas content of the bubbles within the limited time space of ascent reaches an equilibrium with the surrounding electrolyte solution. Furthermore, we are unaware of literature on carbon dioxide titration curves based on direct measurements of aerated Krebs solutions.
Correspondence and requests for reprints to: Jan Abrahamsen Odense University, Odense University Hospital, J.B. Winliiwsvej
0009-8981/89/$03.50
0 1989 Elsevier Science
Publishers
MD, Department of Pharmacology, 19, DK-5000 Odense C, Denmark.
B.V. (Biomedical
Division)
2
We therefore investigated the influence of 6 different tions, 3 bicarbonate concentrations, and 3 aerating gas dioxide tensions in the organ baths filled with modified Equilibration time-periods were chosen to be 1, 2 and used at least 1 h of equilibration.
carbon dioxide concentraflows on the actual carbon Krebs-Henseleit solutions. 3 h, as most investigators
Material and methods Physiological salt solutions (PSS) and gases Three PSS were prepared. The concentrations
of ions of the normally used standard PSS was (in mmol/l): Na+, 144.1; K+, 4.9; Ca’+, 1.3; Mg2+, 1.2; Cl-, 125.5; HCO;, 25.0; SO:-, 1.2; H,PO;, 1.2. CaNa, EDTA (3 x 10e5 mol/l) and L-( +)ascorbic acid (1.1 x lop4 mol/l) were added. By mutual replacement of sodium bicarbonate and sodium chloride, two additional PSS with sodium bicarbonate contents of 10 and 40 mmol/l, respectively, were made. By means of a warm water circulation pump (Heto, Birkerod, Denmark) the temperatures of the salt solutions were kept at 37.0 o C k 0.1 throughout the experiments. Gas mixtures of carbon dioxide and oxygen were commercially available with a fractional carbon dioxide content (FCO,) of 0.015, 0.030, 0.050, 0.080, 0.130 and 0.180. However, by means of mass spectrometry the following FCO, were measured (mean of three analyses): 0.0164, 0.0283, 0.0480, 0.0815, 0.1305 and 0.1827, respectively. These values were taken as reference values. Syringes,
baths, tubings
2-ml plastic syringes with gum plunger were used. To compare gas tightness, a special gas-tight glass syringe with Teflon plunger (Hamilton, Switzerland) was used. When the gas mixture with FCO, : 0.1827 and a flow rate of 150 ml/mm was used, the carbon dioxide tension was identical in all three bicarbonate solutions at any equilibration time regardless of the syringes used. The baths used were jacketed tissue baths with an inner diameter of 22 mm and a total depth of 110 mm. When the baths contained 20 ml PSS there was 61 mm to the open end. The organ baths were gassed through single lumen gastight tubings (Radiometer, Copenhagen) with an internal diameter of 1.14 mm. CO, investigations
For the tonometer (Radiometer Copenhagen@ 7034 bubble tonometer) a fixed flow rate of 70 ml/mm was used throughout the experiments according to the manufacturer’s instruction, whereas the organ baths (Cappelen, Odense, Denmark) were bubbled with 3 flow rates of 50, 100 and 150 ml/mm. The precision flowmeters (Fischer and Porter, UK) were checked for accuracy and precision [2]. With each flow rate equilibration periods of 1, 2 and 3 h were used. The whole system consisted of the gas source, from which the mass spectrometric analyzed gases were delivered through flowmeters, a bubble tonometer, three parallel organ baths and a Radiometer BMS III, Mk II acid base laboratory. The tonometer and the uncovered organ baths were aerated in parallel.
3
Each of the three salts solutions was aerated one at a time with the six different gas mixtures, using flow rates in increasing order. Before sampling equilibration was allowed for 1, 2 and 3 h, respectively, for each combination of gas mixture, salt solution and flow rate. Samples were drawn anaerobically and simultaneously from tonometer and organ bath. The mean of six successive samples-after which solution volume was restored -was used for regression analyses. Samples were taken from the outlet port of the tonometer and from the open end of the organ baths, and immediately analyzed with the BMS III.
Statistics
Stepwise multiple linear regression was used to test the correlation between the measured partial pressure of carbon dioxide (PCO,) and fractional content of carbon dioxide (FCO,) in the bubbling gas mixture of 0, and CO,, gas flow (ml/mm), bicarbonate concentration (mmol) and equilibration time (h) in the baths and the tonometer, respectively. The Statistical Package for the Social Sciences (SPSS) [3] was used for the statistical analysis of the data on a digital computer (UNIVAC 1162, Sperry Univac). The fit of the model is given by the determination coefficient, r2 and a simultaneous F test. The F test is a test of the null-hypothesis where the coefficients bi are equal to zero. Student’s t test is a test of the null-hypothesis where each particular b,-coefficient, significantly different from zero, is entered into the model. P < 5% was regarded as significant.
TABLE
I
A. Correlation
between
PCO,
and FCO,
in the Tonometer
a
Variable
Coefficient
t
P
”
FCO,
428
139
< O.oool
322
-=zo.ooo1 < O.oool c O.oool
969 969 969
B. Correlation FCO, HCO, Flow
Between PCO, 425 - 0.05 0.02
and FCO,
in the Organ 232 -5.71 5.69
Baths b
a Constant u =13.4. b Constant (I =14.1. Results using a linear model: PCOa = u + b, * FCOa + b, * (HCO; ) + b, *gas flow + b4 * equil. period. Fit of the two models: r* = 0.98, r* = 0.98, F(1322) = 19190, F(3968) = 17994 for the tonometer and the baths, respectively. PCO, = partial pressure of carbon dioxide in tonometer and baths (mm Hg). n = total number of samples. FC02 = fractional content of carbon dioxide in oxygen. HCO; = concentration of bicarbonate (mmol). Flow = gas flow (mI/min) in uncovered organ bath. Equil. period = time allowed for equilibration. The table shows variables of which the coefficients differed from zero. I is the r value of Student’s test. P is the level of significance.
4
As the regression analyses for the carbon dioxide tensions of the baths and in the tonometer appeared to be the same (Table I) the regression lines were assumed to be parallel. A dummy variable (= 0 for the tonometer values; = 1 for the organ baths values) was introduced and the data were pooled. As the coefficient of the dummy variable (D) came out as zero, this was interpreted to be strong evidence for identical regression lines. Results For the variables that entered the statistic model, Table I shows the coefficient, the t value, the P value, the value of the constant ‘a’, and the Y*. Measured PCO, was in the range 13.1-99.7 mm Hg. Table I shows a significant linear correlation between PCO, and FCO, in both organ bath and tonometer. Only those coefficients which influence the carbon dioxide tension by at least 0.1 mm Hg are included in the formulas. For the tonometer the only factor gaining significance is the coefficient for the FCO,. For the organ baths the coefficient for the equilibrium time disappears. Regression analysis showed a significant systematic deviation of measured values of carbon dioxide tension from reference line (Fig. 1) with a correlation coefficient of 0.98 (see Table I). The line of ‘Reference’ expresses the expected PCO, calculated from the mass-spectrometric analyzed gases. Discussion Measurements of the carbon dioxide tension of the physiological salt solution in an organ bath have no absolute accuracy since they are limited by the accuracy of the calibrating standards and errors inherent in the analytical method [4]. Moreover, during practical laboratory work numerous factors may lead to partial equilibration and dilution of the gas mixture used for aeration. In addition, the Henderson-Hasselbalch equation is only valid under ‘ideal’ circumstances. Thus, the solubility coefficient and the pKa may vary with electrolytes in solution, ionic strength, acidity, temperature and concentration of proteins, lipids, hemoglobin, urea etc. [5-81. Therefore, assumption of full equilibrium between gas mixturethe carbon dioxide content of which may even substantially deviate from the declaration given by the manufacturer-and the physiological salt solution of an organ bath and use of the Henderson-Hasselbalch equation to calculate pH or carbon dioxide tension may lead to erroneous results. Thus, as acidosis and alkalosis may have profound impact on organ function in tissue baths [9] our main conclusion is that measurements of actual pH and PCO, with proper calibration standards must be considered mandatory. If the carbon dioxide content of the aerating gases is mass-spectrometrically measured, the PCO, may be accurately calculated using the formula for PCO, exp. in Fig. 1. This is justified only when the behaviour of the laboratory equipment is sufficiently well known. Imitating others must be avoided because any reference and test system has its own limitations, which in addition may change with time.
PC02 mm Hg kPa 150-20
1 0
I 0.05
I I 0.10 0.15 CO*-Fraction
8 0.20
Fig. 1. The line of ‘Reference’ expresses the expected PCO, @‘CO2 exp.) calculated spectrometric analyzed gases by means of the formula P,-47 PCO, exp. (mm Hg) = -
loo
from the mass
* FCO, ,
where P, is barometric pressure, 47 = saturated water vapour pressure at 37 o C in mm Hg, FCO, = fractional content of CO, in oxygen. The organ bath behaves like the tonometer with regard to equilibrium between aerating gas mixture and PSS. The PCO,-electrode has a systematic error of measurement, yielding too high values below and too low values above normal physiological area. Directly measured PCO,-values can be converted into ‘true’ values by the formula
PCO, corr. =
*(Pa-47) PC%bath
* FCO, ,
~%tono * 100
PC%*,tvdirectly measured
measured PCO, (mm Hg) in the aerated organ bath solution (PSS). PCO,,,,,, directly PCO, (mm Hg) in the aerated PSS of the Radiometer” tonometer. The ‘true’ PCO, value of the tonometer is supposed to be equal to PCO, exp.
Measuring the actual values of PCO, in our set-up, the multiple regression model yielded the equations in Table I. The influence on PCO, by changes of the bicarbonate concentrations, gas flows and equilibration periods was negligible, the main determinant being the carbon dioxide content of the aerating gas mixture. The measured PCO, values differed systematically from the expected values calculated from the mass-spectrometric analyzed gases, indicating that the PCO, readings are in error probably due to the PC02, measurement system of the BMS3. Some tentative explanations for these discrepancies could be: (a) time aloud for equilibration in the electrode chamber was too short [lo]; (b) the PCO, of the calibrating gases were inaccurate; (c) the temperature of the electrode was not exact 37.0 “C; (d) the electrode membrane was defective [ll]. Explanations (c) and (d) seem less likely since the temperature was checked several times through the day
6
and the electrode membrane was changed at least every day. Explanations (a) and (b) can, however, not be ruled out. In summary, this study has proven, that carbon dioxide equilibrates with the bubbling gas in the bath as well as in the tonometer to an almost perfect equilibrium. Acknowledgements
This study was supported by the Fund of E. Danielsen Regional Medical Research Fund of the County of Funen. offered by Professor Dr. Oecon. Barge Obel, Department of University, is gratefully acknowledged. The authors also wish Larsen for her secretarial skills.
and his wife and the The statistical advice Management, Odense to thank Mrs. Kirsten
References 1 Umbreit WW, Burris RH, Stauffer JF. The solubility of carbon dioxide. Manometric and biochemical techniques. Minneapolis: Burgess Publ. Co., 1972;20-29. 2 Waaben J, Stokke DB, BrinkIoev MM. Accuracy of anaesthetic gas flowmeters determined by the bubble meter method. Br J Anaesth 1978;50:1251-1256. 3 Nie NH, Hull CH, Jenkins JG, Steinbrenner K, Bent DH. Statistical package for the social sciences, 2nd ed. New York: McGraw Hill, 1975. 4 Hood I, Cambell EJM. Is pK OK? N Engl J M 1982;306:864-866. 5 Robinson JR. Fundamentals of acid base regulation. 3rd ed. Oxford: Blackwell,1969. 6 Mass AHJ, Van Heist ANP, Visser BF. The determination of the true equilibrium constant (pK,,) and the practical equilibrium coefficient ( pK&) for the first ionization of carbonic acid in solutions of bicarbonate, cerebrospinal fluid, plasma and serum at 25” and 38O. Clin Chim Acta 1971;33:325-343. 7 Howorth P. RI pH revisited. Lancet 1974;1:253-254. 8 Natelson S, Nobel D. More on blood bicarbonate measurement. Clin Chem 1978;24:1082-1083. 9 Stokke DB, Andersen PK, Brinkloev MM, Nedergaard OA, Hole P, Rasmussen NJ. Acid-base interactions with noradrenaline induced contractile response of the rabbit isolated aorta. Anesthesiology 1984;60:400-404. 10 Bainton CR, Severinghaus JW. Modification of the Radiometer thermal stability. Anesthesiology 1970;33:548-550. 11 Siggaard-Andersen 0. The acid-base status of the blood, 1974;170-175.
BMS-3 Electrode 4th
ed.
system
Copenhagen:
to improve
Munksgaard,
CCA 04556
Carbon dioxide tensions in physiological salt solutions: direct measurements Jan Abrahamsen Departments
‘, Birgitte Norrie ‘, Poul K. Andersen and Ove A. Nedergaard ’
*, Dag B. Stokke *
of ’ Pharmacologv and 2 Anesthesiology and Intensive Care, Odense University, Odense University Hospital, Odense C (Denmark) (Received
Key words: Carbon
29 January
dioxide
1989; accepted
tension;
Bicarbonate;
10 June 1989) Physiological
salt solutions
Carbon dioxide tensions were measured directly in organ baths and a tonometer aerated in parallel with 6 different gas mixtures of 0, and CO,, 3 gas flows, 3 equilibration periods, and 3 bicarbonate concentrations. The measured partial pressure of carbon dioxide differed systematically from expected values, probably due to errors in the carbon dioxide measurement system. In conclusion, carbon dioxide equilibrates with the bubbling gas in the baths as well as in the tonometer to an almost perfect equilibration.
Introduction
For pharmacological and physiological experiments, organ baths are commonly bubbled with a mixture of 5% carbon dioxide and 95% oxygen. According to the Henderson-Hasselbalch equation, this is assumed to yield a normal pH (7.40) and tension of carbon dioxide (36 mm Hg) in the bath solution when employing the usual modifications of Krebs solutions [l]. However, most baths are open to diffusion of the dissolved gases to the surrounding atmosphere and there may be substantial loss of carbon dioxide through permeable tubings. In addition, it is doubtful whether the gas content of the bubbles within the limited time space of ascent reaches an equilibrium with the surrounding electrolyte solution. Furthermore, we are unaware of literature on carbon dioxide titration curves based on direct measurements of aerated Krebs solutions.
Correspondence and requests for reprints to: Jan Abrahamsen Odense University, Odense University Hospital, J.B. Winliiwsvej
0009-8981/89/$03.50
0 1989 Elsevier Science
Publishers
MD, Department of Pharmacology, 19, DK-5000 Odense C, Denmark.
B.V. (Biomedical
Division)
2
We therefore investigated the influence of 6 different tions, 3 bicarbonate concentrations, and 3 aerating gas dioxide tensions in the organ baths filled with modified Equilibration time-periods were chosen to be 1, 2 and used at least 1 h of equilibration.
carbon dioxide concentraflows on the actual carbon Krebs-Henseleit solutions. 3 h, as most investigators
Material and methods Physiological salt solutions (PSS) and gases Three PSS were prepared. The concentrations
of ions of the normally used standard PSS was (in mmol/l): Na+, 144.1; K+, 4.9; Ca’+, 1.3; Mg2+, 1.2; Cl-, 125.5; HCO;, 25.0; SO:-, 1.2; H,PO;, 1.2. CaNa, EDTA (3 x 10e5 mol/l) and L-( +)ascorbic acid (1.1 x lop4 mol/l) were added. By mutual replacement of sodium bicarbonate and sodium chloride, two additional PSS with sodium bicarbonate contents of 10 and 40 mmol/l, respectively, were made. By means of a warm water circulation pump (Heto, Birkerod, Denmark) the temperatures of the salt solutions were kept at 37.0 o C k 0.1 throughout the experiments. Gas mixtures of carbon dioxide and oxygen were commercially available with a fractional carbon dioxide content (FCO,) of 0.015, 0.030, 0.050, 0.080, 0.130 and 0.180. However, by means of mass spectrometry the following FCO, were measured (mean of three analyses): 0.0164, 0.0283, 0.0480, 0.0815, 0.1305 and 0.1827, respectively. These values were taken as reference values. Syringes,
baths, tubings
2-ml plastic syringes with gum plunger were used. To compare gas tightness, a special gas-tight glass syringe with Teflon plunger (Hamilton, Switzerland) was used. When the gas mixture with FCO, : 0.1827 and a flow rate of 150 ml/mm was used, the carbon dioxide tension was identical in all three bicarbonate solutions at any equilibration time regardless of the syringes used. The baths used were jacketed tissue baths with an inner diameter of 22 mm and a total depth of 110 mm. When the baths contained 20 ml PSS there was 61 mm to the open end. The organ baths were gassed through single lumen gastight tubings (Radiometer, Copenhagen) with an internal diameter of 1.14 mm. CO, investigations
For the tonometer (Radiometer Copenhagen@ 7034 bubble tonometer) a fixed flow rate of 70 ml/mm was used throughout the experiments according to the manufacturer’s instruction, whereas the organ baths (Cappelen, Odense, Denmark) were bubbled with 3 flow rates of 50, 100 and 150 ml/mm. The precision flowmeters (Fischer and Porter, UK) were checked for accuracy and precision [2]. With each flow rate equilibration periods of 1, 2 and 3 h were used. The whole system consisted of the gas source, from which the mass spectrometric analyzed gases were delivered through flowmeters, a bubble tonometer, three parallel organ baths and a Radiometer BMS III, Mk II acid base laboratory. The tonometer and the uncovered organ baths were aerated in parallel.
3
Each of the three salts solutions was aerated one at a time with the six different gas mixtures, using flow rates in increasing order. Before sampling equilibration was allowed for 1, 2 and 3 h, respectively, for each combination of gas mixture, salt solution and flow rate. Samples were drawn anaerobically and simultaneously from tonometer and organ bath. The mean of six successive samples-after which solution volume was restored -was used for regression analyses. Samples were taken from the outlet port of the tonometer and from the open end of the organ baths, and immediately analyzed with the BMS III.
Statistics
Stepwise multiple linear regression was used to test the correlation between the measured partial pressure of carbon dioxide (PCO,) and fractional content of carbon dioxide (FCO,) in the bubbling gas mixture of 0, and CO,, gas flow (ml/mm), bicarbonate concentration (mmol) and equilibration time (h) in the baths and the tonometer, respectively. The Statistical Package for the Social Sciences (SPSS) [3] was used for the statistical analysis of the data on a digital computer (UNIVAC 1162, Sperry Univac). The fit of the model is given by the determination coefficient, r2 and a simultaneous F test. The F test is a test of the null-hypothesis where the coefficients bi are equal to zero. Student’s t test is a test of the null-hypothesis where each particular b,-coefficient, significantly different from zero, is entered into the model. P < 5% was regarded as significant.
TABLE
I
A. Correlation
between
PCO,
and FCO,
in the Tonometer
a
Variable
Coefficient
t
P
”
FCO,
428
139
< O.oool
322
-=zo.ooo1 < O.oool c O.oool
969 969 969
B. Correlation FCO, HCO, Flow
Between PCO, 425 - 0.05 0.02
and FCO,
in the Organ 232 -5.71 5.69
Baths b
a Constant u =13.4. b Constant (I =14.1. Results using a linear model: PCOa = u + b, * FCOa + b, * (HCO; ) + b, *gas flow + b4 * equil. period. Fit of the two models: r* = 0.98, r* = 0.98, F(1322) = 19190, F(3968) = 17994 for the tonometer and the baths, respectively. PCO, = partial pressure of carbon dioxide in tonometer and baths (mm Hg). n = total number of samples. FC02 = fractional content of carbon dioxide in oxygen. HCO; = concentration of bicarbonate (mmol). Flow = gas flow (mI/min) in uncovered organ bath. Equil. period = time allowed for equilibration. The table shows variables of which the coefficients differed from zero. I is the r value of Student’s test. P is the level of significance.
4
As the regression analyses for the carbon dioxide tensions of the baths and in the tonometer appeared to be the same (Table I) the regression lines were assumed to be parallel. A dummy variable (= 0 for the tonometer values; = 1 for the organ baths values) was introduced and the data were pooled. As the coefficient of the dummy variable (D) came out as zero, this was interpreted to be strong evidence for identical regression lines. Results For the variables that entered the statistic model, Table I shows the coefficient, the t value, the P value, the value of the constant ‘a’, and the Y*. Measured PCO, was in the range 13.1-99.7 mm Hg. Table I shows a significant linear correlation between PCO, and FCO, in both organ bath and tonometer. Only those coefficients which influence the carbon dioxide tension by at least 0.1 mm Hg are included in the formulas. For the tonometer the only factor gaining significance is the coefficient for the FCO,. For the organ baths the coefficient for the equilibrium time disappears. Regression analysis showed a significant systematic deviation of measured values of carbon dioxide tension from reference line (Fig. 1) with a correlation coefficient of 0.98 (see Table I). The line of ‘Reference’ expresses the expected PCO, calculated from the mass-spectrometric analyzed gases. Discussion Measurements of the carbon dioxide tension of the physiological salt solution in an organ bath have no absolute accuracy since they are limited by the accuracy of the calibrating standards and errors inherent in the analytical method [4]. Moreover, during practical laboratory work numerous factors may lead to partial equilibration and dilution of the gas mixture used for aeration. In addition, the Henderson-Hasselbalch equation is only valid under ‘ideal’ circumstances. Thus, the solubility coefficient and the pKa may vary with electrolytes in solution, ionic strength, acidity, temperature and concentration of proteins, lipids, hemoglobin, urea etc. [5-81. Therefore, assumption of full equilibrium between gas mixturethe carbon dioxide content of which may even substantially deviate from the declaration given by the manufacturer-and the physiological salt solution of an organ bath and use of the Henderson-Hasselbalch equation to calculate pH or carbon dioxide tension may lead to erroneous results. Thus, as acidosis and alkalosis may have profound impact on organ function in tissue baths [9] our main conclusion is that measurements of actual pH and PCO, with proper calibration standards must be considered mandatory. If the carbon dioxide content of the aerating gases is mass-spectrometrically measured, the PCO, may be accurately calculated using the formula for PCO, exp. in Fig. 1. This is justified only when the behaviour of the laboratory equipment is sufficiently well known. Imitating others must be avoided because any reference and test system has its own limitations, which in addition may change with time.
PC02 mm Hg kPa 150-20
1 0
I 0.05
I I 0.10 0.15 CO*-Fraction
8 0.20
Fig. 1. The line of ‘Reference’ expresses the expected PCO, @‘CO2 exp.) calculated spectrometric analyzed gases by means of the formula P,-47 PCO, exp. (mm Hg) = -
loo
from the mass
* FCO, ,
where P, is barometric pressure, 47 = saturated water vapour pressure at 37 o C in mm Hg, FCO, = fractional content of CO, in oxygen. The organ bath behaves like the tonometer with regard to equilibrium between aerating gas mixture and PSS. The PCO,-electrode has a systematic error of measurement, yielding too high values below and too low values above normal physiological area. Directly measured PCO,-values can be converted into ‘true’ values by the formula
PCO, corr. =
*(Pa-47) PC%bath
* FCO, ,
~%tono * 100
PC%*,tvdirectly measured
measured PCO, (mm Hg) in the aerated organ bath solution (PSS). PCO,,,,,, directly PCO, (mm Hg) in the aerated PSS of the Radiometer” tonometer. The ‘true’ PCO, value of the tonometer is supposed to be equal to PCO, exp.
Measuring the actual values of PCO, in our set-up, the multiple regression model yielded the equations in Table I. The influence on PCO, by changes of the bicarbonate concentrations, gas flows and equilibration periods was negligible, the main determinant being the carbon dioxide content of the aerating gas mixture. The measured PCO, values differed systematically from the expected values calculated from the mass-spectrometric analyzed gases, indicating that the PCO, readings are in error probably due to the PC02, measurement system of the BMS3. Some tentative explanations for these discrepancies could be: (a) time aloud for equilibration in the electrode chamber was too short [lo]; (b) the PCO, of the calibrating gases were inaccurate; (c) the temperature of the electrode was not exact 37.0 “C; (d) the electrode membrane was defective [ll]. Explanations (c) and (d) seem less likely since the temperature was checked several times through the day
6
and the electrode membrane was changed at least every day. Explanations (a) and (b) can, however, not be ruled out. In summary, this study has proven, that carbon dioxide equilibrates with the bubbling gas in the bath as well as in the tonometer to an almost perfect equilibrium. Acknowledgements
This study was supported by the Fund of E. Danielsen Regional Medical Research Fund of the County of Funen. offered by Professor Dr. Oecon. Barge Obel, Department of University, is gratefully acknowledged. The authors also wish Larsen for her secretarial skills.
and his wife and the The statistical advice Management, Odense to thank Mrs. Kirsten
References 1 Umbreit WW, Burris RH, Stauffer JF. The solubility of carbon dioxide. Manometric and biochemical techniques. Minneapolis: Burgess Publ. Co., 1972;20-29. 2 Waaben J, Stokke DB, BrinkIoev MM. Accuracy of anaesthetic gas flowmeters determined by the bubble meter method. Br J Anaesth 1978;50:1251-1256. 3 Nie NH, Hull CH, Jenkins JG, Steinbrenner K, Bent DH. Statistical package for the social sciences, 2nd ed. New York: McGraw Hill, 1975. 4 Hood I, Cambell EJM. Is pK OK? N Engl J M 1982;306:864-866. 5 Robinson JR. Fundamentals of acid base regulation. 3rd ed. Oxford: Blackwell,1969. 6 Mass AHJ, Van Heist ANP, Visser BF. The determination of the true equilibrium constant (pK,,) and the practical equilibrium coefficient ( pK&) for the first ionization of carbonic acid in solutions of bicarbonate, cerebrospinal fluid, plasma and serum at 25” and 38O. Clin Chim Acta 1971;33:325-343. 7 Howorth P. RI pH revisited. Lancet 1974;1:253-254. 8 Natelson S, Nobel D. More on blood bicarbonate measurement. Clin Chem 1978;24:1082-1083. 9 Stokke DB, Andersen PK, Brinkloev MM, Nedergaard OA, Hole P, Rasmussen NJ. Acid-base interactions with noradrenaline induced contractile response of the rabbit isolated aorta. Anesthesiology 1984;60:400-404. 10 Bainton CR, Severinghaus JW. Modification of the Radiometer thermal stability. Anesthesiology 1970;33:548-550. 11 Siggaard-Andersen 0. The acid-base status of the blood, 1974;170-175.
BMS-3 Electrode 4th
ed.
system
Copenhagen:
to improve
Munksgaard,