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J Thorac Cardiovasc Surg 1994;108:991-992
© 1994 Mosby, Inc.
LETTERS TO THE EDITOR |
Division of Cardio-thoracic Surgery
Rhode Island Hospital
Brown University School of Medicine
Providence, RI 02903
To the Editor:
Capnography has become standard anesthetic technology and end-tidal carbon dioxide tension (PETCO2) is readily available in the operating room PETCO2 is governed by metabolism, ventilation, and circulation. When the first two parameters are controlled, PETCO2 reflects pulmonary flow, therefore cardiac output. A considerable body of literature chronicles the use of PETCO2 to assess the adequacy of cardiopulmonary resuscitation after cardiac arrest.
1,2 In addition, attempts have been made touse PETCO2 measurement as an alternative to the thermodilution technique for determining cardiac output. 3,4 Little has been reported of its use in cardiac surgery. For several years, we have found that PETCO2 could be a highly valuable adjunct to estimate cardiac output, particularly during the weaning from cardiopulmonary bypass (CPB). We present our clinical observations here to share our experience and to stimulate discussion.
When peripheral carbon dioxide production and alveolar ventilation remain relatively constant, PETCO2 is affected by pulmonary blood flow and alveolar dead space (or arterial endotracheal carbon dioxide difference: PaCO2 - PETCO2). The former also influences the latter (formation of West zone 1). Furthermore, in cardiac surgery, additional dead space could result from arterial embolism and barocompression of microvasculature. 5 Nevertheless, the major determinant of PETCO2 remains pulmonary blood flow. During weaning from CPB, PETCO2 becomes constantly available when mechanical ventilation and pulmonic circulation resume. Obviously, cardiac action, chamber size, rhythm and rate, tension of the aorta, and the knowledge of cardiac disease and the degree of its surgical correction are the key factors influencing the weaning process. Electrocardiographic findings, pressure readings, and mixed venous saturation measurement provide additional objective information. From time to time, a high mixed venous saturation could simply reflect peripheral hypoperfusion or systemic hyperoxia. Therefore, we have been increasingly using PETCO2 as an adjunct to assess the adequacy of circulation.
The following are our clinical observations. We find that a PETCO2 less than 20 torr almost invariably is associated with a low cardiac output less than 2 L/min, even when other hemodynamic parameters are seemingly optimal. It would be inadvisable to terminate CPB with this measurement. Further augmentation of preload, reduction of afterload, establishment of an effective atrioventricular synchrony, or increasing inotropy is necessary to raise PETCO2 above 25 torr. In general, we find it is unusual to resume CPB when the patient has a post-CPB PETCO2 consistently above 25 torr. After CPB, the PETCO2 usually stays in the high 20 and low 30 torr range. Eventually, the value of PETCO2 for assessing pulmonary flow becomes self-limited with time, because prolonged low flow state eventually will raise arterial carbon dioxide production and, therefore, PETCO2. Overall, PETCO2 seems to be particularly preload-sensitive and is especially useful to guide preload augmentation when both heart size and chamber pressure appear to be grossly adequate.
Using PETCO2 empirically as an adjunct to guide CPB weaning for the past few years, we have found that it becomes distinctly unusual for us now to resume CPB after its termination, and the need for intraaortic balloon pumping to assist weaning from CPB has been reduced to a rate of less than 1% over the past consecutive 3500 cardiac operations. We believe that gaining experience with PETCO2 application is one of the contributing factors. Although we have not tried to measure pulmonary blood flow directly and simultaneously to correlate PETCO2 findings, it has been our clinical impression that adequate PETCO2 usually indicates adequate blood flow. This impression has reasonable theoretic bases and is partly supported by the usual adequate postbypass cardiac output obtained from the thermodilution technique. Therefore, we would be interested to know if other colleagues have made similar observations and use of PETCO2 during cardiac surgery.
References
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