Vol. 184 . No. 5
LETTERS TO THE EDITOR
standard error of the mean is clearly inapplicable if the sample population is very small; say, less than 20 individuals, and must still be interpreted with some caution in samples of less than 100 individuals. This paper was initially well designed and could have made an important contribution to the physiology of hemodilution. It has been rendered almost valueless by the simple failure to critically examine whether or not the results are internally consistent. The use of a totally inappropriate statistical method does not add to reader confidence. Aaron F. Kopman, M.D. Reference 1. Laks, H., Pilon, R. N., Klovekorn, W. P., et al.: Acute Hemodilution: Its Effects on Hemodynamics and Oxygen Transport in Man. Ann. Surg., 180:103-109, 1974.
May 4, 1976 Dear Editor: Dr. Kopman states that the major thrust of the paper was that "acute hemodilution . . . was well tolerated and not accompanied by major changes in oxygen utilization." If he will review the original paper, he will find no data pertaining to oxygen consumption and no conclusions drawn on oxygen utilization. We did state that oxygen transport was well maintained and feel that these data are reliable as they are based on multiple measurements of cardiac output by thermodilution, and on the arterial oxygen content. The calculation of oxygen consumption from cardiac output and the arteriovenous oxygen content difference is generally unreliable. Fluctuations would be more likely in the venous than in the arterial oxygen content in a patient on 50% oxygen. The patient with the extremely low arteriovenous oxygen content difference had the pulmonary artery catheter positioned to obtain balloon wedge pressures. The obviously high venous oxygen content may have been due to partial wedging of the catheter resulting in withdrawal of highly oxygenated capillary blood. An attempt was made to avoid this in subsequent studies. For these reasons, data on oxygen consumption were not included in the paper. Oxygen consumption can be measured directly under anesthesia, but the technical problems in the clinical situation are considerable. Our data do support the conclusion that in terms of oxygen transport the reduction in hemoglobin is compensated for by an increase in cardiac output. As we suggested in the paper, the lactate, pyruvate and muscle surface pH measurements suggest that tissue oxygenation is adequate. Dr. Kopman has also drawn some erroneous conclusions regarding the use of statistics. The Student's "t" test is particularly applicable to studies with a small number of samples, and the standard error on the mean
657
is based on an estimate of the population standard deviation as described in some recent texts.' The literature is replete with reports using the standard error ol the mean in small sample populations.2 Regrettably Dr. Kopman has drawn some unfounded conclusions on the first careful and detailed study oi intra-operative hemodilution in man. It is hoped that the challenge of conducting further physiologic studies in man in the clinical situation will be undertaken by Dr. Kopman and others. Hillel Laks, M.D. Saint Louis University Department of Surgery Saint Louis, Missouri 63104
References 1. Colton, T: Statistics in Medicine. Boston, Little Brown and Co., 1974. 2. Saini, U. K., Hood, W. B., Hechtman, H. B., and Berger, R. L.: Nutrient Myocardial Blood Flow in Experimental Myocardial Ischemia. Circulation, 52:1086-1090, 1975.
June 4, 1976 Dear Editor: In their recent article "Evaluation of Catheter Placement in the Treatment of Venous Air Embolism" (Ann. Surg., 183:58-61, 1976), Dr. Sink and colleagues conclude that placing a Swan-Ganz catheter in the right ventricle or the pulmonary artery for the purpose of treating (aspiration) air embolism is not merited. It appears that the authors fail to appreciate the primary value of a right heart catheter, namely early detection of air within the pulmonary circulation. In our experience, venous air embolism during neurosurgical procedures in the sitting position results from repeated small showers of intravascular air rather than from a single large bolus of air. Increase in pulmonary artery pressure indicates the onset of venous air embolism. When the embolized air is curtailed and the intrapulmonic air is dissipated, pulmonary artery pressure returns toward normal. Using this monitoring technic we have seldom needed to aspirate air from the right heart. However, we suggested (Anesthesiology, 42:223-226, 1975) that intracardiac gas could be aspirated using the distal or central venous port of the Swan-Ganz catheter by withdrawing the catheter while applying syringe aspiration until gas is obtained. The data of Sink et al. support this concept since they found that this technic was an effective in retrieving embolized gas as waE aspiration with a catheter placed either in the right ventricle or atrium. Edwin S. Munson, M.D. University of Florida College of Medicine Gainesville, Florida 32610
LETTERS TO THE EDITOR
standard error of the mean is clearly inapplicable if the sample population is very small; say, less than 20 individuals, and must still be interpreted with some caution in samples of less than 100 individuals. This paper was initially well designed and could have made an important contribution to the physiology of hemodilution. It has been rendered almost valueless by the simple failure to critically examine whether or not the results are internally consistent. The use of a totally inappropriate statistical method does not add to reader confidence. Aaron F. Kopman, M.D. Reference 1. Laks, H., Pilon, R. N., Klovekorn, W. P., et al.: Acute Hemodilution: Its Effects on Hemodynamics and Oxygen Transport in Man. Ann. Surg., 180:103-109, 1974.
May 4, 1976 Dear Editor: Dr. Kopman states that the major thrust of the paper was that "acute hemodilution . . . was well tolerated and not accompanied by major changes in oxygen utilization." If he will review the original paper, he will find no data pertaining to oxygen consumption and no conclusions drawn on oxygen utilization. We did state that oxygen transport was well maintained and feel that these data are reliable as they are based on multiple measurements of cardiac output by thermodilution, and on the arterial oxygen content. The calculation of oxygen consumption from cardiac output and the arteriovenous oxygen content difference is generally unreliable. Fluctuations would be more likely in the venous than in the arterial oxygen content in a patient on 50% oxygen. The patient with the extremely low arteriovenous oxygen content difference had the pulmonary artery catheter positioned to obtain balloon wedge pressures. The obviously high venous oxygen content may have been due to partial wedging of the catheter resulting in withdrawal of highly oxygenated capillary blood. An attempt was made to avoid this in subsequent studies. For these reasons, data on oxygen consumption were not included in the paper. Oxygen consumption can be measured directly under anesthesia, but the technical problems in the clinical situation are considerable. Our data do support the conclusion that in terms of oxygen transport the reduction in hemoglobin is compensated for by an increase in cardiac output. As we suggested in the paper, the lactate, pyruvate and muscle surface pH measurements suggest that tissue oxygenation is adequate. Dr. Kopman has also drawn some erroneous conclusions regarding the use of statistics. The Student's "t" test is particularly applicable to studies with a small number of samples, and the standard error on the mean
657
is based on an estimate of the population standard deviation as described in some recent texts.' The literature is replete with reports using the standard error ol the mean in small sample populations.2 Regrettably Dr. Kopman has drawn some unfounded conclusions on the first careful and detailed study oi intra-operative hemodilution in man. It is hoped that the challenge of conducting further physiologic studies in man in the clinical situation will be undertaken by Dr. Kopman and others. Hillel Laks, M.D. Saint Louis University Department of Surgery Saint Louis, Missouri 63104
References 1. Colton, T: Statistics in Medicine. Boston, Little Brown and Co., 1974. 2. Saini, U. K., Hood, W. B., Hechtman, H. B., and Berger, R. L.: Nutrient Myocardial Blood Flow in Experimental Myocardial Ischemia. Circulation, 52:1086-1090, 1975.
June 4, 1976 Dear Editor: In their recent article "Evaluation of Catheter Placement in the Treatment of Venous Air Embolism" (Ann. Surg., 183:58-61, 1976), Dr. Sink and colleagues conclude that placing a Swan-Ganz catheter in the right ventricle or the pulmonary artery for the purpose of treating (aspiration) air embolism is not merited. It appears that the authors fail to appreciate the primary value of a right heart catheter, namely early detection of air within the pulmonary circulation. In our experience, venous air embolism during neurosurgical procedures in the sitting position results from repeated small showers of intravascular air rather than from a single large bolus of air. Increase in pulmonary artery pressure indicates the onset of venous air embolism. When the embolized air is curtailed and the intrapulmonic air is dissipated, pulmonary artery pressure returns toward normal. Using this monitoring technic we have seldom needed to aspirate air from the right heart. However, we suggested (Anesthesiology, 42:223-226, 1975) that intracardiac gas could be aspirated using the distal or central venous port of the Swan-Ganz catheter by withdrawing the catheter while applying syringe aspiration until gas is obtained. The data of Sink et al. support this concept since they found that this technic was an effective in retrieving embolized gas as waE aspiration with a catheter placed either in the right ventricle or atrium. Edwin S. Munson, M.D. University of Florida College of Medicine Gainesville, Florida 32610
