Postbypass hypothermia and its relationship to the energy balance of cardiopulmonary bypass.
Using a newly developed computerized intraoperative data acquisition system, the apparent adequacy of rewarming and its relation to the energy exchange between the patient and the bypass system was investigated. Retrospective analysis of comparable patients identified two groups that had, at the end of surgery, either a nasopharyngeal temperature (NPT) of 36 degrees C or more ("warm" group, n = 19), or a NPT of 35 degrees C or less ("cold" group, n = 19). Temperatures from the nasopharynx, thenar eminence skin, and bypass pump arterial and venous lines were continually recorded and sent to the computer data base together with the pump flow rate. There were no significant differences between the groups regarding time on perfusion, time taken to cool, time of hypothermia, or the time interval from end of perfusion to the end of surgery. However, rewarming time was greater in the warm group (P less than 0.01). The cold group were subjected to more profound hypothermia (P less than 0.001), and had lower NPTs and skin temperatures at the end of bypass (P less than 0.0001 and P less than 0.01, respectively). However, the difference between NPT and thenar skin temperature in each group at either the end of bypass or the end of surgery was the same. The net energy exchange between patient and pump was significantly different (mean in warm, 130 kJ [SD = 530]; in cold, -389 kJ [SD = 427]; P less than 0.003). In conclusion, the adequacy of rewarming can be expressed in terms of the energy exchanged in the bypass system, and cannot be assessed by the nasopharynx:skin temperature gradient.