Metabolic Rate, Temperature, and Acid-Base Control: The Best Strategy and Our Needs to Achieve It

__________ _ Important recent knowledge concerning acid-base status during hypothermia is briefly summarized. A basic diagram of pH-C02 relationships during cooling with diluted blood is presented to illustrate the three possible acid-base management strategies during hypothermic perfusion. On the basis of an extensive review of pertinent literature on the effect of excess C02 and acidosis on the hypothermic brain and heart during hypothermic cardiopulmonary bypass (H-CPB), it is recommended that no C02 be added to any ventilatory gas and that respiratory alkalosis is the optimal acidbase strategy during perfusion cooling with circulatory arrest. The great need for on-line blood gas monitoring instrumentation, so that perfusionists may actually identify and control the acid-base status of the patient during hypothermia, is emphasized. Discussion _____________ _ The effects of cold tissue temperatures on acid-base balance have long been poorly understood by physiologists and clinicians alike. In the last decade, giant strides in the basic understanding of the effect of temperature on biologic acid-base equilibria have been made. Recently, I have reviewed and evaluated this newer knowledge and its implications in Surgery, Gynecology, and Obstetrics. 1.2 Evidence is now available that common current management of patients during perfusion either ignores or improperly manages the acid-base relationships of hypothermic patients, because of several erroneous assumptions regarding the measurement of temperature, and the effect of blood gases on the blood pH Presented at AmSECT's Third National Symposium, August 24-26, 1984, Boston, MA. Direct communications to Henry Swan, M.D., D.Sc (Hon.) FACS, Research Institute for Biological Science, 6700 W. Lakeridge Rd., Lakewood, CO 80227. Volume 17, Number2, 1985 at various temperatures. Thus, for years, deliberate respiratory acidosis induced by adding C02 to the respiratory gas mixture has been the accepted custom for the conduct of hypothermic cardiopulmonary bypass (HCPB) in spite of recent evidence that such a patient management strategy is harmful to the heart of the patient. There is strong evidence that an acid-base management strategy that uses no added C02 and emphasizes the achievement of hyperventilation alkalosis is more appropriate during perfusion hypothermia. Moreover, perfusionists do not currently have proper tools for acid-base control. Prompt adequate blood gas information is lacking. For example, blood samples must be sent from the operating room to a laboratory, often at a remote location, which can introduce considerably delay in receiving the information in the operating room. Occasionally, errors are made in the reports delivered to the operating room. Current instrumentation for gas analysis makes the measurement on blood samples which have been warmed to 37 o C. This measurement is commonly "corrected" mathematically to the so-called "body temperature" of the patient, and is reported as a "corrected" measurement. Although the analyzers can be quite precise, a blood sample submitted with a temperature reading taken from some remote site which is called "body temperature" will not produce realistic "corrected" blood gas values. We often speak of "the body temperature." What does this term mean? Probably in poikilotherms the terms has meaning, since physiologic mechanisms to increase, preserve, or lose body heat are poorly developed. Superficial fat layers that provide a thermal buffer zone are absent. The thermodynamic lifestyle ofturtles, frogs, snakes, and fish is based on passive acceptance of equilibration with environment temperature. Usually changes in the thermal environment occur slowly. Body temperature control is almost entirely locomotor; the animal seeks more acceptable surroundings by moving to a new location. Internal body thermal gradients are The Journal of Extra-Corporeal Technology 65 minimal or absent. Thus we may excuse physiologists who study turtles when they use the term ''body temper-