Efficacy of two methods for reducing postbypass afterdrop

Background: Afterdrop, defined as the precipitous reduction in core tempera ture after cardiopulmonary bypass, results from redistribution of body heat to inadequately warmed peripheral tissues. The authors tested two methods of ameliorating afterdrop: (1) forced-air warming of peripheral tissues and (2) nitroprusside-induced vasodilation. Methods: Patients were cooled during cardiopulmonary bypass to approximatel y 32 degrees C and subsequently rewarmed to a nasopharyngeal temperature ne ar 37 degrees C and a rectal temperature near 36 degrees C. Patients in the forced-air protocol (n = 20) mere assigned randomly to forced-air warming or passive insulation on the legs. Active heating started with rewarming wh ile under-going bypass and was continued for the remainder of surgery. Pati ents in the nitroprusside protocol (n = 30) were assigned randomly to eithe r a control group or sodium nitroprusside administration. Pump flow during rewarming was maintained at 2.5 l . m(-2) . min(-1) in the control patients and at 3.0 l . m(-2) . min(-1) in those assigned to sodium nitroprusside. Sodium nitroprusside was titrated to maintain a mean arterial pressure near 60 mmHg. In all cases, a nasopharyngeal probe evaluated core (trunk and he ad) temperature and heat content. Peripheral compartment (arm and leg) temp erature and heat content mere estimated using fourth-order regressions and integration over volume from 18 intramuscular needle thermocouples, nine sk in temperatures, and ''deep'' hand and foot temperature. Results: In patients warmed with forced air, peripheral tissue temperature was higher at the end of warming and remained higher until the end of surge ry. The core temperature afterdrop was reduced from 1.2 +/- 0.2 degrees C t o 0.5 +/- 0.2 degrees C by forced-air warming. The duration of afterdrop al so was reduced, from 50 +/- 11 to 27 +/- 14 min. Ln the nitroprusside group , a rectal temperature of 36 degrees C was reached after 30 +/- 7 min of re warming. This was only slightly faster than the 40 +/- 13 min necessary in the control group. The afterdrop was 0.8 +/- 0.3 degrees C with nitroprussi de and lasted 34 +/- 10 min which was similar to the 1.1 +/- 0.3 degrees C afterdrop that lasted 44 +/- 13 min in the control group. Conclusions: Cutaneous warming reduced the core temperature afterdrop by 60 %. However, heat-balance data indicate that this reduction resulted primari ly because forced-air heating prevented the typical decrease in body heat c ontent after discontinuation of bypass, rather than by reducing redistribut ion. Nitroprusside administration slightly increased peripheral tissue temp erature and heat content at the end of rewarming, However, the core-to-peri pheral temperature gradient was low in both groups, Consequently, there was little redistribution in either case.