Br. J. Anaesth. (1975), 47, 530
CORRESPONDENCE SOLUBILITY OF HALOTHANE IN DOG BLOOD
ALAN L. COWLES
Mountain View, California. REFERENCE
Cowles, A. L., Borgstedt, H. A., and Gillies, A. J. (1971). Solubilities of ethylene, cyclopropane, halothane and diethyl ether in human and dog blood at low concentrations. Anesthesiology, 35, 203. ARTERIAL OXYGEN TENSION
IN PARTURIENT
WOMEN
Sir,—Archer and Marx (1974) compared the rate of decrease of Pao t during apnoea in parturient and nonparturient women. They reported that Pa Ot decreased more rapidly in the parturient women and claimed that this was a result of the increased oxygen consumption associated with pregnancy. However, they provided no evidence that the parturient patients had a higher oxygen consumption than the control group, nor did they discuss other possible explanations for their findings. The rate of increase of Pacoi was similar in both groups of patients. This suggests that there was little difference in oxygen consumption between the two groups or, indeed, between parturient patients in labour and those having elective Caesarean section. Whether this deduction is justified or not, the observation does indicate that the difference in the rate of decrease of Paoi could not have been the result of differences in the rate of increase of alveolar Pco,. However, the patients were ventilated with 97.5% oxygen for only 4 min and were then disconnected from the source of oxygen. If there had been a difference in oxygen consumption in the two groups, there would have been a greater "aventilatory mass flow" of room air down the trachea in the group with the higher oxygen consumption. This would have resulted in a more rapid dilution of the alveolar gas in the parturient group of patients and a greater decrease in Paoi. Since oxygen consumption was not measured, it would have been helpful if the authors had considered other possible explanations for their finding* For example, it is known that the functional residual capacity is reduced in preenancy (Cugell et al., 1953) and that the functional residual capacity may decrease on induction of anaesthesia (Hewlett et al., 1974). Bevan and colleagues (1974)
have demonstrated that airway closure is often present in the tidal volume range during late pregnancy. This might provide an explanation for the reduction in Paoi often found at term, and for the smaller initial Paoi values in the parturient group in the present studies. Changes in functional residual capacity after induction of anaesthesia or greater reductions in cardiac output after the induction of anaesthesia in the parturient women could have accounted also for the observed differences between the two groups of patients. It is apparent that there are many possible explanations for the authors' results. Surely they might have mentioned just a few of them? M.
K.
SYKES
London REFERENCES
Archer, G. W., and Marx, G. P. (1974). Arterial oxygen tension during apnoea in parturient women. Br. J. Anaesth., 46, 358. Bevan, D. R., Holdcroft A., Loh, L., MacGregor, W. G., O'Sullivan, J. C , and Sykes, M. K. (1974). Closing volume and pregnancy. Br. Med. J., 1, 13. Cugell, D. W., Frank, N. R., Gaensler, E. A., and Badger, T. L. (1953). Pulmonary function in pregnancy: serial observations in normal women. Am. Rev. Tuberc, 67, 568. Hewlett, A. M., Hulands, G. H., Nunn, J. F., and Milledge, J. S. (1974). Functional residual capacity during anaesthesia. I l l : Artificial ventilation. Br. J. Anaesth., 46, 495. Sir,—We thank Professor Sykes for pointing out other possible explanations for our findings. The concept that our results may be explained on the basis of change in oxygen consumption assumes that the data by Gemzell and colleagues (1957) are valid and do not require confirmation. The arterial carbon dioxide tension (Pacoi) is of no value as a measurement of oxygen consumption, since vast volumes of carbon dioxide can be stored in the body without significant changes in Paco, (Farhi and Rahn, 1960). Therefore, major changes in oxygen consumption can occur without being reflected by changes in Pacoi. New explanations for any physiological event are part of the continuous development of knowledge. An exposition of all of the possible explanations of our findings would have produced a long review paper. Most important, our findings provide more evidence for the concept that adequate oxygenation is particularly important in the obstetric patient. GIRVICE W. ARCHER JR. GERTIE F. MARX
Bronx, New York REFERENCES
Farhi, L. E., and Rahn, H. (1960). Dynamics of changes in carbon dioxide stores. Anesthesiology, 21, 604. Gemzell, C. A., Robbe, H., Stern, B., and Strom, G. (1957). Observations on circulatory changes and muscular work in normal labour. Acta Obstet. Gynecol. Scand., 36, 75.
Made and printed in Great Britain by John Sherratt & Son Ltd
© Macmillan Journals Ltd 1975
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Diego on August 31, 2015
Sir,—In response to an inquiry by Dr W. W. Mapleson regarding the solubility of halothane in dog blood (sec article in this issue), I would like to make note of the following additional statistical information which we obtained (Cowles, Borgstedt and Gillies, 1971). The relation between haemoglobin and solubility showed a mean solubility at 37.5°C of 3.11 at a haemoglobin concentration of 15.0 g/100 ml and 95% confidence Limits for the true mean of 2.94 to 3.27 (±0.17). The regression line had a slope of —0.003 with 95% confidence limits of —0.097 to +0.090. The relation between haematocrit and solubility gave the following: mean solubility at a haematocrit of 45% : 3.10, 95% confidence Limits for the true mean: 2.92 to 3.29 (±0.18), slope: —0.002, and 95% confidence limits: —0.033 to +0.029. The standard errors given in our publication were for a single observation of solubility.
CORRESPONDENCE SOLUBILITY OF HALOTHANE IN DOG BLOOD
ALAN L. COWLES
Mountain View, California. REFERENCE
Cowles, A. L., Borgstedt, H. A., and Gillies, A. J. (1971). Solubilities of ethylene, cyclopropane, halothane and diethyl ether in human and dog blood at low concentrations. Anesthesiology, 35, 203. ARTERIAL OXYGEN TENSION
IN PARTURIENT
WOMEN
Sir,—Archer and Marx (1974) compared the rate of decrease of Pao t during apnoea in parturient and nonparturient women. They reported that Pa Ot decreased more rapidly in the parturient women and claimed that this was a result of the increased oxygen consumption associated with pregnancy. However, they provided no evidence that the parturient patients had a higher oxygen consumption than the control group, nor did they discuss other possible explanations for their findings. The rate of increase of Pacoi was similar in both groups of patients. This suggests that there was little difference in oxygen consumption between the two groups or, indeed, between parturient patients in labour and those having elective Caesarean section. Whether this deduction is justified or not, the observation does indicate that the difference in the rate of decrease of Paoi could not have been the result of differences in the rate of increase of alveolar Pco,. However, the patients were ventilated with 97.5% oxygen for only 4 min and were then disconnected from the source of oxygen. If there had been a difference in oxygen consumption in the two groups, there would have been a greater "aventilatory mass flow" of room air down the trachea in the group with the higher oxygen consumption. This would have resulted in a more rapid dilution of the alveolar gas in the parturient group of patients and a greater decrease in Paoi. Since oxygen consumption was not measured, it would have been helpful if the authors had considered other possible explanations for their finding* For example, it is known that the functional residual capacity is reduced in preenancy (Cugell et al., 1953) and that the functional residual capacity may decrease on induction of anaesthesia (Hewlett et al., 1974). Bevan and colleagues (1974)
have demonstrated that airway closure is often present in the tidal volume range during late pregnancy. This might provide an explanation for the reduction in Paoi often found at term, and for the smaller initial Paoi values in the parturient group in the present studies. Changes in functional residual capacity after induction of anaesthesia or greater reductions in cardiac output after the induction of anaesthesia in the parturient women could have accounted also for the observed differences between the two groups of patients. It is apparent that there are many possible explanations for the authors' results. Surely they might have mentioned just a few of them? M.
K.
SYKES
London REFERENCES
Archer, G. W., and Marx, G. P. (1974). Arterial oxygen tension during apnoea in parturient women. Br. J. Anaesth., 46, 358. Bevan, D. R., Holdcroft A., Loh, L., MacGregor, W. G., O'Sullivan, J. C , and Sykes, M. K. (1974). Closing volume and pregnancy. Br. Med. J., 1, 13. Cugell, D. W., Frank, N. R., Gaensler, E. A., and Badger, T. L. (1953). Pulmonary function in pregnancy: serial observations in normal women. Am. Rev. Tuberc, 67, 568. Hewlett, A. M., Hulands, G. H., Nunn, J. F., and Milledge, J. S. (1974). Functional residual capacity during anaesthesia. I l l : Artificial ventilation. Br. J. Anaesth., 46, 495. Sir,—We thank Professor Sykes for pointing out other possible explanations for our findings. The concept that our results may be explained on the basis of change in oxygen consumption assumes that the data by Gemzell and colleagues (1957) are valid and do not require confirmation. The arterial carbon dioxide tension (Pacoi) is of no value as a measurement of oxygen consumption, since vast volumes of carbon dioxide can be stored in the body without significant changes in Paco, (Farhi and Rahn, 1960). Therefore, major changes in oxygen consumption can occur without being reflected by changes in Pacoi. New explanations for any physiological event are part of the continuous development of knowledge. An exposition of all of the possible explanations of our findings would have produced a long review paper. Most important, our findings provide more evidence for the concept that adequate oxygenation is particularly important in the obstetric patient. GIRVICE W. ARCHER JR. GERTIE F. MARX
Bronx, New York REFERENCES
Farhi, L. E., and Rahn, H. (1960). Dynamics of changes in carbon dioxide stores. Anesthesiology, 21, 604. Gemzell, C. A., Robbe, H., Stern, B., and Strom, G. (1957). Observations on circulatory changes and muscular work in normal labour. Acta Obstet. Gynecol. Scand., 36, 75.
Made and printed in Great Britain by John Sherratt & Son Ltd
© Macmillan Journals Ltd 1975
Downloaded from http://bja.oxfordjournals.org/ at University of California, San Diego on August 31, 2015
Sir,—In response to an inquiry by Dr W. W. Mapleson regarding the solubility of halothane in dog blood (sec article in this issue), I would like to make note of the following additional statistical information which we obtained (Cowles, Borgstedt and Gillies, 1971). The relation between haemoglobin and solubility showed a mean solubility at 37.5°C of 3.11 at a haemoglobin concentration of 15.0 g/100 ml and 95% confidence Limits for the true mean of 2.94 to 3.27 (±0.17). The regression line had a slope of —0.003 with 95% confidence limits of —0.097 to +0.090. The relation between haematocrit and solubility gave the following: mean solubility at a haematocrit of 45% : 3.10, 95% confidence Limits for the true mean: 2.92 to 3.29 (±0.18), slope: —0.002, and 95% confidence limits: —0.033 to +0.029. The standard errors given in our publication were for a single observation of solubility.
