AFTERDROP AFTER HYPOTHERMIC CARDIOPULMONARY BYPASS - THE VALUE OF TYMPANIC MEMBRANE TEMPERATURE MONITORING

Objectives: After weaning from cardiopulmonary bypass (CPB), a decreas e in nasopharyngeal temperature (NPT) occurs (afterdrop). The pathophy siology of the afterdrop remains unclear: It might be caused by either inadequate total body rewarming on CPB or to heterogenous distributio n of heat during CPB, with subsequent redistribution of heat from the warmer core to the cooler shell tissues. The study objectives were (1) to determine whether post-CPB afterdrop is the result of a negative C PB thermal balance, and (2) to investigate which sites (if any) could best predict the afterdrop. Design: Prospective evaluation using withi n-patient comparisons during CPB cooling, CPB rewarming, and 45 minute s post-CPB. Setting: Adult patients gave informed consent before a car diac surgical procedure in a university hospital. Participants: Eight patients undergoing CABG or valvular replacement with hypothermic CPB (NPT near 29 C) and standardized general anesthesia. Interventions: Ea ch patient was studied with temperature monitors (Mon-a-therm 7000; Ma llinckrodt-Medexel, Gemenos, France) attached to disposable thermocoup le probes placed as follows: urinary bladder, rectum, deltoid, esophag us, nasopharynx, tympanic membrane, and four skin sites. In addition, the temperatures from the thermistors of the pulmonary artery catheter , and the arterial and venous lines of the CPB circuit were considered . Thirteen sites for monitoring temperature were studied. Measurements and Main Results: Temperatures were recorded every 5 minutes, from th e beginning of CPB to the 45th minute after CPB, and thermal exchanges were calculated: change in body heat (OBH), thermal exchanges between the patient and the pump (QCPB), metabolic heat production (Qm) (equa l to calculated VO2 at the pump level), and heat loss to the environme nt (QS) (equal to QBH-QCPB-Qm). Thermal exchanges were obtained in six patients during the plateaus of cooling acid rewarming, during the wh ole CPB phase, and after CPB. It was found that despite a change in QB H during rewarming (1,017 +/- 88 kJ) that was slightly greater than du ring cooling (-1,008 +/- 104 kJ) (mean +/- SEM), a significant decreas e in post-CPB ''core'' temperature occurred (afterdrop: -1.4 degrees C ). Magnitude of the afterdrop was directly related to the magnitude of tympanic membrane cooling and was negatively correlated to the temper ature difference between the warmest site (tympanic membrane) and the coolest site (cutaneous thigh temperature) observed at the end of rewa rming (r = -0.667; p < 0.05). Conclusions: It is suggested that beside s post-CPB heat loss, redistribution of heat may be involved in the me chanism of the afterdrop and that measurements of tympanic membrane an d cutaneous thigh temperatures are the best monitors of adequacy of re warming during CPB. Copyright (C) 1996 by W.B. Saunders Company.