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Mixed venous oximetry
419
LETTERS TO THE EDITOR
REFERENCES
1. Macintosh RR: A new laryngoscope. Lancet 1:205,1943 2. Wilson RS: Endobronchial intubation, in Kaplan JA (ed): Thoracic Anesthesia. New York, NY, Churchill Livingstone, 1983, pp 389-402
3. Racz GB: Improved vision modification of the Macintosh laryngoscope. Anaesthesia 39:1249-1250,1984 4. Brodsky JB, Adkins MO, Gaba DM: Bronchial cuff pressure of double-lumen tubes. Anesth Analg 69:608-610, 1989
Mixed Venous Oximetry To the Editor: The position taken by Boylan and Teasdale on the issue of SvO, is shared equally by some who believe its unproven usefulness.’ This apparent disbelief is based on the failure to consider two factors. The first one is the integration of
hemodynamics, oxygen consumption, oxygen carrying capacity of the blood, and ventilation. Compensation that follows deviation in any of the above parameters may make SvO, irrelevant for a given clinical situation. Obviously, this requires careful evaluation and analysis of circumstances for any given SvO, value within the clinical context. To these can be added the second group of factors that are technical and related to misdiagnosis and misinterpretation. Under these circumstances it is not difficult to come to the same conclusion as the authors have. The majority of reported studies attempt to correlate cardiac output (CO) and SvO, in a linear fashion. SvO, is not intended for that purpose. Any change in SvO, must be interpreted with all the factors affecting its value. A properly obtained SvO, is nothing more than an indicator of the balance between oxygen consumption and demand for a given degree of oxygen transport to the tissues. Before arriving at any clinical conclusions, one must search and consider the factors influencing SvO,, namely, oxygen consumption, ventilation, and oxygen carrying capacity of the blood in addition to CO.* The inability to accurately determine the interplay of these factors leads to misunderstanding and misinterpretation of SvO,. This issue has been discussed very clearly in the article of Gutierrez and Pohil.3 The conclusion of the study of Pearson et al4 should be focused on the fact that blood gas and hematocrit costs were identical in all three groups despite the statistically significant left ventricular dysfunction in groups II and III. If patients of group II and III had the same cost as group I, one can speculate on the overutilization of laboratory services for group I and underutilization for groups II and III. It is conceivable that such underutilization, particularly in group III, may represent savings due to the use of SvO, oximetry. The cost factor indeed is real, albeit reasonable for the final product. Today, the cost of an uncomplicated coronary bypass procedure is in the range of $30,000 to $40,000. The cost difference between a regular and an oximetric pulmonary artery catheter is about $150.00. Even with the hospital mark-up, its contribution to the total cost is small. As clinicians we don’t have a “crystal ball” to determine which patient will do well with or without the use of expensive technology. The data given to us preoperatively may have some prognostic value. However, they are static and reflect findings obtained during the test. This is evident in the study of Pearson et al in which a fair number of patients originally assigned to group I were reassigned either to group II or III. It is for this reason that one needs an on-line real time system that is dynamic and capable of detecting changes early, for an equally early therapeutic intervention. Monitoring tissue oxygenation is the ultimate monitoring modality. Such an endeavor is like Percival’s quest for the Holy Grail in the absence of proper respect for the Almighty.’ Lacking such a precise and practical modality, SvO,, the pooled average of multiple venous saturations, is, for the time being, a viable alternative. Istrati Kupeli, MD
Department of Anaesthesia New England Deaconess Hospital Harvard Medical School Boston, MA
REFERENCES 1. Boylan JF, Teasdale SJ: Con: Perioperative continuous monitoring of mixed venous oxygen saturation should not be routine in high-risk cardiac surgery. J Cardiothorac Anesth 5651-654, 1990 2. Kupeli IA, Sahvicz PR: Mixed venous oximetry. Int Anesth Clin 27:176-183,1989 3. Gutierrez G, Pohil RJ: Oxygen consumption is linearly
related to oxygen supply in critically ill patients. J Crit Care 16:655-658, 1988 4. Pearson KS, Gomez M, Moyers JR, et al: A cost/benefit analysis of randomized invasive monitoring for patients undergoing cardiac surgery. Anesth Analg 69:336-341,1989 5. Laver MB: The Arthurian legend. Anesthesiology 40:523, 1974
LETTERS TO THE EDITOR
REFERENCES
1. Macintosh RR: A new laryngoscope. Lancet 1:205,1943 2. Wilson RS: Endobronchial intubation, in Kaplan JA (ed): Thoracic Anesthesia. New York, NY, Churchill Livingstone, 1983, pp 389-402
3. Racz GB: Improved vision modification of the Macintosh laryngoscope. Anaesthesia 39:1249-1250,1984 4. Brodsky JB, Adkins MO, Gaba DM: Bronchial cuff pressure of double-lumen tubes. Anesth Analg 69:608-610, 1989
Mixed Venous Oximetry To the Editor: The position taken by Boylan and Teasdale on the issue of SvO, is shared equally by some who believe its unproven usefulness.’ This apparent disbelief is based on the failure to consider two factors. The first one is the integration of
hemodynamics, oxygen consumption, oxygen carrying capacity of the blood, and ventilation. Compensation that follows deviation in any of the above parameters may make SvO, irrelevant for a given clinical situation. Obviously, this requires careful evaluation and analysis of circumstances for any given SvO, value within the clinical context. To these can be added the second group of factors that are technical and related to misdiagnosis and misinterpretation. Under these circumstances it is not difficult to come to the same conclusion as the authors have. The majority of reported studies attempt to correlate cardiac output (CO) and SvO, in a linear fashion. SvO, is not intended for that purpose. Any change in SvO, must be interpreted with all the factors affecting its value. A properly obtained SvO, is nothing more than an indicator of the balance between oxygen consumption and demand for a given degree of oxygen transport to the tissues. Before arriving at any clinical conclusions, one must search and consider the factors influencing SvO,, namely, oxygen consumption, ventilation, and oxygen carrying capacity of the blood in addition to CO.* The inability to accurately determine the interplay of these factors leads to misunderstanding and misinterpretation of SvO,. This issue has been discussed very clearly in the article of Gutierrez and Pohil.3 The conclusion of the study of Pearson et al4 should be focused on the fact that blood gas and hematocrit costs were identical in all three groups despite the statistically significant left ventricular dysfunction in groups II and III. If patients of group II and III had the same cost as group I, one can speculate on the overutilization of laboratory services for group I and underutilization for groups II and III. It is conceivable that such underutilization, particularly in group III, may represent savings due to the use of SvO, oximetry. The cost factor indeed is real, albeit reasonable for the final product. Today, the cost of an uncomplicated coronary bypass procedure is in the range of $30,000 to $40,000. The cost difference between a regular and an oximetric pulmonary artery catheter is about $150.00. Even with the hospital mark-up, its contribution to the total cost is small. As clinicians we don’t have a “crystal ball” to determine which patient will do well with or without the use of expensive technology. The data given to us preoperatively may have some prognostic value. However, they are static and reflect findings obtained during the test. This is evident in the study of Pearson et al in which a fair number of patients originally assigned to group I were reassigned either to group II or III. It is for this reason that one needs an on-line real time system that is dynamic and capable of detecting changes early, for an equally early therapeutic intervention. Monitoring tissue oxygenation is the ultimate monitoring modality. Such an endeavor is like Percival’s quest for the Holy Grail in the absence of proper respect for the Almighty.’ Lacking such a precise and practical modality, SvO,, the pooled average of multiple venous saturations, is, for the time being, a viable alternative. Istrati Kupeli, MD
Department of Anaesthesia New England Deaconess Hospital Harvard Medical School Boston, MA
REFERENCES 1. Boylan JF, Teasdale SJ: Con: Perioperative continuous monitoring of mixed venous oxygen saturation should not be routine in high-risk cardiac surgery. J Cardiothorac Anesth 5651-654, 1990 2. Kupeli IA, Sahvicz PR: Mixed venous oximetry. Int Anesth Clin 27:176-183,1989 3. Gutierrez G, Pohil RJ: Oxygen consumption is linearly
related to oxygen supply in critically ill patients. J Crit Care 16:655-658, 1988 4. Pearson KS, Gomez M, Moyers JR, et al: A cost/benefit analysis of randomized invasive monitoring for patients undergoing cardiac surgery. Anesth Analg 69:336-341,1989 5. Laver MB: The Arthurian legend. Anesthesiology 40:523, 1974