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PREDICTION OF ARTERIAL CARBON DIOXIDE TENSION USING A CIRCLE SYSTEM WITHOUT CARBON DIOXIDE ABSORPTION
Br. J. Anaesth. (1974), 46, 442
PREDICTION OF ARTERIAL CARBON DIOXIDE TENSION USING A CIRCLE SYSTEM WITHOUT CARBON DIOXIDE ABSORPTION E . J. SCHOLFIELD AND N . E . WILLIAMS SUMMARY
The object of this investigation was to determine whether it is possible to predict the arterial carbon dioxide tension (Pa0o2) of a patient having controlled ventilation, using a circle system in which carbon dioxide absorption is not provided. In this investigation the following assumptions were made: (1) In such a system, the composition of alveolar gas and circuit gas will approximate, provided that there is adequate mixing between the two gas compartments. (2) That such mixing can be achieved by moderate mechanical hyperventilation at low frequencies. (3) That under these circumstances carbon dioxide can be eliminated from the combined patient-plus-circuit system only through the expiratory valve and at a rate proportional to the fresh gas flow (FGF), thus Pa C O a ocl/FGF Under normal circumstances, alveolar ventilation is inversely proportional to PaCo23 thus (4) FGF takes over the role of VA in controlling Pa C02 , i.e. the fresh gas flow is equal to the "effective" alveolar ventilation. As the necessary VA for any required Paco2 in a patient of known weight can be predicted with a reasonable degree of accuracy using the Nunn bloodEDWARD J. SCHOLFIELD,* L.R.C.P., M.R.C.S., F.F.A.R.C.S.; NORTON E. WILLIAMS, M.B., CH.B., F.F.A.R.C.S.; Depart-
ment of Anaesthesia, Whiston Hospital, Prescot, Lanes. • Present address: St Gregorius Ziekenhuis, Brunssum, Limburg, Holland. Requests for reprints to N.E.W.
gas predictor (Nunn, 1960, 1962) and as the FGF can be measured to a high degree of accuracy ( + 2%) using ordinary rotameters (Sykes and Vickers, 1970), it is only necessary to set the FGF at the same numerical value as the predicted VA to obtain the required PaC02. To verify this the following investigation was performed. METHOD
Forty-three patients were investigated. They were free from cardiovascular and respiratory disease and were studied in the supine horizontal position during surgery. Each patient's weight was aligned on the Nunn blood-gas slide rule and a PaCo2 between 15 and 45 mm Hg was selected. A FGF corresponding to the alveolar ventilation required for this value was used. All patients were premedicated using oral diazepam 5 mg b.d. and nitrazepam 10 mg the night before operation. Anaesthesia was induced with thiopentone sodium 250-400 mg followed by tubocurarine 25-45 mg to permit intubation and control of ventilation. The patients were ventilated artificially with 30-35% oxygen in nitrous oxide to which 0.5% halothane was added. The circuit was a circle system with the sodalime canister excluded (fig. 1). The ventilator was an East-Radcliffe Mark I, in all cases set at its lowest frequency (13/min). By adjusting the weights on the bellows, the minute volume (MV) was arbitrarily set to be 2-4 times the FGF. The air entrainment valve was occluded and the Heidbrink valve adjacent to the reservoir bag was replaced by a simple flap valve, as it was found that during the upstroke of the bellows room air entrainment could occur at both these points, thus increasing the FGF.
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 14, 2015
A method of predicting arterial carbon dioxide tension in anaesthetized patients having controlled ventilation using a circle system without carbon dioxide absorption is described. Adequate mixing of circuit and alveolar gases is assumed when large minute volumes are provided, and the Nunn blood-gas predictor is utilized to choose the appropriate fresh gas flow. The method has been tested on forty-three patients.
1 443
PaCO2 USING A CIRCLE SYSTEM WITHOUT CO, ABSORPTION DISCUSSION
PATIENT
Fresh Spill va gas inflow
B = Positive pressure bellows R = Reservoir bag •4 = One-way valve
When a steady minute volume had been obtained the time was recorded, and gas samples were taken from the circuit with a 20-ml syringe at 15-min intervals, via a three-way tap connected to the tail of the reservoir bag. In each patient a sample was taken from the radial artery after a minimum of 30 min, at which time a steady ventilatory state had been shown by serial sampling of the circuit gas; this confirmed the equation of Fahri and Rahn (1955), viz.: Pco 2 t=Pco 2 f+(Pco 2 i-Pco 2 f) 0.574 where Pco 2 t=Pco 2 at time t Pco2f=final Pco2 Pco 2 i=initial Pco2 t=time in minutes which predicts a 99% change within 30 min for a step change in ventilation of Pco2 40 mm Hg to 20 mm Hg. All samples were analysed by one technician using a blood-gas analyser (Model 48C, Cambridge Scientific Instruments Ltd). The electrodes were calibrated twice daily using known buffers and gas mixtures, and in addition checked against other laboratories in the region. RESULTS
Nineteen males and twenty-four females were investigated. Figure 2 shows the measured PaCo2 (M) plotted against the predicted Paco2 (P). The regression line, and a line of identity ( M = P ) are included. Statistical analysis of the results showed R = + 0 . 9 3 2 , P=0.01, and a regression equation M=0.76P+8.03.
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 14, 2015
FIG. 1. Diagram of anaesthetic circuit used (after Eger and Ethans, 1968).
Mapleson (1958) introduced the concept of using fresh gas flow instead of alveolar ventilation in the alveolar air equation, when he gave theoretical consideration to the use of his System A (the "Magill attachment") where the minute volume was in excess of FGF (i.e., rebreathing could occur). This was further developed by Nunn and Newman (1964) who stated that, under conditions of rebreathing, hyperventilation cannot alter the alveolar gas composition which is determined by gas exchange and the flow rate and composition of fresh gas. The accuracy of carbon dioxide electrodes when gas calibration rather than tonometered blood is used is considered to be ± 6 mm Hg (Flenley, Millar and Rees, 1967; Miller and Tutt, 1967). If two parallel lines are drawn at + and —6 mm Hg to the "ideal" line it will be seen that forty-one of the forty-three results fall within the range. Accuracy of prediction is greatest at about prediction of 35 mm Hg and this would seem to be a desirable level at which to aim. However, the regression line suggests that there may be a factor or factors causing a lower PaC02 than that predicted at the higher end of the range and a higher PaCo2 at the lower end. This discrepancy is similar to that demonstrated by Baraka (1969) who postulated that a change in PaCO2 influenced the BMR (Gessel, 1930). Fermentation experiments (Rigalatto, 1967) have related the effect of the concentration of the "exhaust" carbon dioxide on fermentation rates and when the environmental concentration of carbon dioxide is 2.5% the rate is about 25% higher than with a carbon dioxide concentration of 5%. An alternative explanation is that at the low values, when PaC02 is higher than predicted, a significant volume of FGF is bypassing the patient. However, the work of Eger and Ethans (1968) suggests that in the system used here, where both the FGF and the "spill" valve are separated from the patient by the inspiratory and expiratory valves, fresh gas will only be lost to the system when it exceeds the patient's minute volume; also the lower values for high predictions could then only be explained by air entrainment into the circuit which the authors felt was prevented by capping the entrainment valve. Further work is in progress to assess the effect of a change of ventilating frequency (f) while keeping minute volume and FGF fixed and predicting PaC02 in the region of 35 mm Hg.
444
BRITISH JOURNAL OF ANAESTHESIA
s
Predicted Pac0! (P)
FIG. 2. Graph showing predicted PaoOS (P) plotted against measured Paeo2 (M). Delta points show multiple results.
The advantages of such a method of prediction are: (1) Good mechanical ventilation can be achieved while the FGF may be regulated to produce normocapnia or moderate hypocapnia. The disadvantages of gross hypocapnia or small ventilatory volumes are thus avoided. (2) Avoidance of difficulties with other methods, i.e. when minute volume must be measured accurately, and carbon dioxide absorbed.
(a) Inaccuracy of respirometers (Nunn and EziAshi, 1962). (b) Changes in compliance during surgery when non-compensating ventilators (e.g. the EastRadcliffe) are being used will affect the minute volume and therefore the PaC02(c) Exhaustion of soda-lime may occur. Apparatus for end-tidal monitoring of carbon dioxide is costly and arterial sampling is not justi-
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 14, 2015
2
PaC02 USING A CIRCLE SYSTEM WITHOUT CO 2 ABSORPTION fied as a procedure for this purpose in routine uncomplicated anaesthesia. REFERENCES
Mapleson, W. W. (1958). Theoretical considerations of the effects of rebreathing in two semi-closed anaesthetic systems. Br. Med. Bull., 14, 64. Miller, J. N., and Tutt, P. (1967). A comparison of four blood gas analysis systems in working conditions. Biotned. Eng., 2, 45. Nunn, J. F. (1960). Prediction of CO2 tension during anaesthesia. Anaesthesia, IS, 123. (1962). Prediction of O2 and CO2 levels during anaesthesia. Anaesthesia, 17, 182. Ezi-Ashi, T. I. (1962). The accuracy of the respirometer and ventigrator. Br. J. Anaesth., 34, 422. Newman, H. C. (1964). Inspired gas, rebreathing and apparatus deadspace. Br. J. Anaesth., 36, 5. Rigalatto, R. C. (1967). Ph.D. Thesis, University of London. (Personal communication.) Sykes, M. K., and Vickers, M. D. (1970). Principles of Measurement for Anaesthetists, 1st edn, p. 122. Oxford and Edinburgh: Blackwell.
THE SOCIETY OF ANAESTHESIOLOGISTS OF EAST AFRICA The Second Annual General Meeting and Conference of the Society of Anaesthesiologists of East Africa will be held in Nairobi from October 18 to 20, 1974. Further information may be obtained from Dr D. Hussein, P.O. Box 45301, Nairobi, Kenya.
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 14, 2015
Baraka, A. (1969). Pco2 control by fresh gas flow during ventilation with a semi-open circuit. Br. J. Anaesth., 41, 527. Eger, E. I., and Ethans, C. T. (1968). The effects of inflow, overflow and valve placement on economy of the circle system. Anesthesiology, 29, 93. Fahri, L. E., and Rahn, H. (1955). Gas stores of the body and the unsteady state. J. Appl. Physiol., 7, 472. Flenley, D. C , Millar, J. S., and Rees, H. A. (1967). Accuracy of oxygen and carbon dioxide electrodes. Br. Med. J., 2, 349. Gessel, R. (1930). The regulation of respiration: a study of the correlation of numerous factors of respiratory control following administration of HC1, of CO2, and the simultaneous administration of CO, and sodium bicarbonate. Am. J. Physiol., 94, 402.
445
PREDICTION OF ARTERIAL CARBON DIOXIDE TENSION USING A CIRCLE SYSTEM WITHOUT CARBON DIOXIDE ABSORPTION E . J. SCHOLFIELD AND N . E . WILLIAMS SUMMARY
The object of this investigation was to determine whether it is possible to predict the arterial carbon dioxide tension (Pa0o2) of a patient having controlled ventilation, using a circle system in which carbon dioxide absorption is not provided. In this investigation the following assumptions were made: (1) In such a system, the composition of alveolar gas and circuit gas will approximate, provided that there is adequate mixing between the two gas compartments. (2) That such mixing can be achieved by moderate mechanical hyperventilation at low frequencies. (3) That under these circumstances carbon dioxide can be eliminated from the combined patient-plus-circuit system only through the expiratory valve and at a rate proportional to the fresh gas flow (FGF), thus Pa C O a ocl/FGF Under normal circumstances, alveolar ventilation is inversely proportional to PaCo23 thus (4) FGF takes over the role of VA in controlling Pa C02 , i.e. the fresh gas flow is equal to the "effective" alveolar ventilation. As the necessary VA for any required Paco2 in a patient of known weight can be predicted with a reasonable degree of accuracy using the Nunn bloodEDWARD J. SCHOLFIELD,* L.R.C.P., M.R.C.S., F.F.A.R.C.S.; NORTON E. WILLIAMS, M.B., CH.B., F.F.A.R.C.S.; Depart-
ment of Anaesthesia, Whiston Hospital, Prescot, Lanes. • Present address: St Gregorius Ziekenhuis, Brunssum, Limburg, Holland. Requests for reprints to N.E.W.
gas predictor (Nunn, 1960, 1962) and as the FGF can be measured to a high degree of accuracy ( + 2%) using ordinary rotameters (Sykes and Vickers, 1970), it is only necessary to set the FGF at the same numerical value as the predicted VA to obtain the required PaC02. To verify this the following investigation was performed. METHOD
Forty-three patients were investigated. They were free from cardiovascular and respiratory disease and were studied in the supine horizontal position during surgery. Each patient's weight was aligned on the Nunn blood-gas slide rule and a PaCo2 between 15 and 45 mm Hg was selected. A FGF corresponding to the alveolar ventilation required for this value was used. All patients were premedicated using oral diazepam 5 mg b.d. and nitrazepam 10 mg the night before operation. Anaesthesia was induced with thiopentone sodium 250-400 mg followed by tubocurarine 25-45 mg to permit intubation and control of ventilation. The patients were ventilated artificially with 30-35% oxygen in nitrous oxide to which 0.5% halothane was added. The circuit was a circle system with the sodalime canister excluded (fig. 1). The ventilator was an East-Radcliffe Mark I, in all cases set at its lowest frequency (13/min). By adjusting the weights on the bellows, the minute volume (MV) was arbitrarily set to be 2-4 times the FGF. The air entrainment valve was occluded and the Heidbrink valve adjacent to the reservoir bag was replaced by a simple flap valve, as it was found that during the upstroke of the bellows room air entrainment could occur at both these points, thus increasing the FGF.
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 14, 2015
A method of predicting arterial carbon dioxide tension in anaesthetized patients having controlled ventilation using a circle system without carbon dioxide absorption is described. Adequate mixing of circuit and alveolar gases is assumed when large minute volumes are provided, and the Nunn blood-gas predictor is utilized to choose the appropriate fresh gas flow. The method has been tested on forty-three patients.
1 443
PaCO2 USING A CIRCLE SYSTEM WITHOUT CO, ABSORPTION DISCUSSION
PATIENT
Fresh Spill va gas inflow
B = Positive pressure bellows R = Reservoir bag •4 = One-way valve
When a steady minute volume had been obtained the time was recorded, and gas samples were taken from the circuit with a 20-ml syringe at 15-min intervals, via a three-way tap connected to the tail of the reservoir bag. In each patient a sample was taken from the radial artery after a minimum of 30 min, at which time a steady ventilatory state had been shown by serial sampling of the circuit gas; this confirmed the equation of Fahri and Rahn (1955), viz.: Pco 2 t=Pco 2 f+(Pco 2 i-Pco 2 f) 0.574 where Pco 2 t=Pco 2 at time t Pco2f=final Pco2 Pco 2 i=initial Pco2 t=time in minutes which predicts a 99% change within 30 min for a step change in ventilation of Pco2 40 mm Hg to 20 mm Hg. All samples were analysed by one technician using a blood-gas analyser (Model 48C, Cambridge Scientific Instruments Ltd). The electrodes were calibrated twice daily using known buffers and gas mixtures, and in addition checked against other laboratories in the region. RESULTS
Nineteen males and twenty-four females were investigated. Figure 2 shows the measured PaCo2 (M) plotted against the predicted Paco2 (P). The regression line, and a line of identity ( M = P ) are included. Statistical analysis of the results showed R = + 0 . 9 3 2 , P=0.01, and a regression equation M=0.76P+8.03.
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 14, 2015
FIG. 1. Diagram of anaesthetic circuit used (after Eger and Ethans, 1968).
Mapleson (1958) introduced the concept of using fresh gas flow instead of alveolar ventilation in the alveolar air equation, when he gave theoretical consideration to the use of his System A (the "Magill attachment") where the minute volume was in excess of FGF (i.e., rebreathing could occur). This was further developed by Nunn and Newman (1964) who stated that, under conditions of rebreathing, hyperventilation cannot alter the alveolar gas composition which is determined by gas exchange and the flow rate and composition of fresh gas. The accuracy of carbon dioxide electrodes when gas calibration rather than tonometered blood is used is considered to be ± 6 mm Hg (Flenley, Millar and Rees, 1967; Miller and Tutt, 1967). If two parallel lines are drawn at + and —6 mm Hg to the "ideal" line it will be seen that forty-one of the forty-three results fall within the range. Accuracy of prediction is greatest at about prediction of 35 mm Hg and this would seem to be a desirable level at which to aim. However, the regression line suggests that there may be a factor or factors causing a lower PaC02 than that predicted at the higher end of the range and a higher PaCo2 at the lower end. This discrepancy is similar to that demonstrated by Baraka (1969) who postulated that a change in PaCO2 influenced the BMR (Gessel, 1930). Fermentation experiments (Rigalatto, 1967) have related the effect of the concentration of the "exhaust" carbon dioxide on fermentation rates and when the environmental concentration of carbon dioxide is 2.5% the rate is about 25% higher than with a carbon dioxide concentration of 5%. An alternative explanation is that at the low values, when PaC02 is higher than predicted, a significant volume of FGF is bypassing the patient. However, the work of Eger and Ethans (1968) suggests that in the system used here, where both the FGF and the "spill" valve are separated from the patient by the inspiratory and expiratory valves, fresh gas will only be lost to the system when it exceeds the patient's minute volume; also the lower values for high predictions could then only be explained by air entrainment into the circuit which the authors felt was prevented by capping the entrainment valve. Further work is in progress to assess the effect of a change of ventilating frequency (f) while keeping minute volume and FGF fixed and predicting PaC02 in the region of 35 mm Hg.
444
BRITISH JOURNAL OF ANAESTHESIA
s
Predicted Pac0! (P)
FIG. 2. Graph showing predicted PaoOS (P) plotted against measured Paeo2 (M). Delta points show multiple results.
The advantages of such a method of prediction are: (1) Good mechanical ventilation can be achieved while the FGF may be regulated to produce normocapnia or moderate hypocapnia. The disadvantages of gross hypocapnia or small ventilatory volumes are thus avoided. (2) Avoidance of difficulties with other methods, i.e. when minute volume must be measured accurately, and carbon dioxide absorbed.
(a) Inaccuracy of respirometers (Nunn and EziAshi, 1962). (b) Changes in compliance during surgery when non-compensating ventilators (e.g. the EastRadcliffe) are being used will affect the minute volume and therefore the PaC02(c) Exhaustion of soda-lime may occur. Apparatus for end-tidal monitoring of carbon dioxide is costly and arterial sampling is not justi-
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 14, 2015
2
PaC02 USING A CIRCLE SYSTEM WITHOUT CO 2 ABSORPTION fied as a procedure for this purpose in routine uncomplicated anaesthesia. REFERENCES
Mapleson, W. W. (1958). Theoretical considerations of the effects of rebreathing in two semi-closed anaesthetic systems. Br. Med. Bull., 14, 64. Miller, J. N., and Tutt, P. (1967). A comparison of four blood gas analysis systems in working conditions. Biotned. Eng., 2, 45. Nunn, J. F. (1960). Prediction of CO2 tension during anaesthesia. Anaesthesia, IS, 123. (1962). Prediction of O2 and CO2 levels during anaesthesia. Anaesthesia, 17, 182. Ezi-Ashi, T. I. (1962). The accuracy of the respirometer and ventigrator. Br. J. Anaesth., 34, 422. Newman, H. C. (1964). Inspired gas, rebreathing and apparatus deadspace. Br. J. Anaesth., 36, 5. Rigalatto, R. C. (1967). Ph.D. Thesis, University of London. (Personal communication.) Sykes, M. K., and Vickers, M. D. (1970). Principles of Measurement for Anaesthetists, 1st edn, p. 122. Oxford and Edinburgh: Blackwell.
THE SOCIETY OF ANAESTHESIOLOGISTS OF EAST AFRICA The Second Annual General Meeting and Conference of the Society of Anaesthesiologists of East Africa will be held in Nairobi from October 18 to 20, 1974. Further information may be obtained from Dr D. Hussein, P.O. Box 45301, Nairobi, Kenya.
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 14, 2015
Baraka, A. (1969). Pco2 control by fresh gas flow during ventilation with a semi-open circuit. Br. J. Anaesth., 41, 527. Eger, E. I., and Ethans, C. T. (1968). The effects of inflow, overflow and valve placement on economy of the circle system. Anesthesiology, 29, 93. Fahri, L. E., and Rahn, H. (1955). Gas stores of the body and the unsteady state. J. Appl. Physiol., 7, 472. Flenley, D. C , Millar, J. S., and Rees, H. A. (1967). Accuracy of oxygen and carbon dioxide electrodes. Br. Med. J., 2, 349. Gessel, R. (1930). The regulation of respiration: a study of the correlation of numerous factors of respiratory control following administration of HC1, of CO2, and the simultaneous administration of CO, and sodium bicarbonate. Am. J. Physiol., 94, 402.
445