Tissue heat content and distribution during and after cardiopulmonary bypass at 31°C and 27°C

Background: Afterdrop following cardiopulmonary bypass results from redistribution of body heat to inadequately warmed peripheral tissues. However, the distribution of heat between the thermal compartments and the extent to which core-to-peripheral redistribution contributes to post-bypass hypothermia remains unknown, Methods: Patients mere cooled during cardiopulmonary bypass to nasopharyngeal temperatures near 31 degrees C (n = 8) or 27 degrees C (n = 8) and subsequently rewarmed by the bypass heat exchanger to approximate to 37.5 degrees C. A nasopharyngeal probe evaluated core (trunk and head) temperature and heat content. Peripheral compartment (arm and leg) temperature and heat content were estimated using fourth-order regressions and integration over volume from 19 intramuscular needle thermocouples, 10 skin temperatures, and "deep" foot temperature. Results: In the 31 degrees C group, the average peripheral tissue temperature decreased to 31.9 +/- 1.4 degrees C (means +/- SD) and subsequently increased to 34 +/- 1.4 degrees C at the end of bypass, The core-to-peripheral tissue temperature gradient was 3.5 +/- 1.8 degrees C at the end of rewarming, and the afterdrop was 1.5 +/- 0.4 degrees C, Total body heat content decreased 231 +/- 93 kcal, During pump rewarming, the peripheral heat content increased to 7 +/- 27 kcal below precooling values, whereas the core heat content increased to 94 +/- 33 kcal above precooling values. Body heat content at the end of rewarming was thus 87 +/- 42 kcal more than at the onset of cooling. In the 27 degrees C group, the average peripheral tissue temperature decreased to a minimum of 29.8 +/- 1.7 degrees C and subsequently increased to 32.8 +/- 2.1 degrees C at the end of bypass. The core-to-peripheral tissue temperature gradient was 4.6 +/- 1.9 degrees C at the end of rewarming, and the afterdrop was 2.3 +/- 0.9 degrees C, Total body heat content decreased 419 +/- 49 kcal, During pump rewarming, core heat content increased to 66 +/- 23 kcal above precooling values, whereas peripheral heat content remained 70 +/- 42 kcal below precooling values. Body heat content at the end of rewarming was thus 4 +/- 52 kcal less than at the onset of cooling. Conclusions: Peripheral tissues failed to fully rewarm by the end of bypass in the patients in the 27 degrees C group, and the afterdrop was 2.3 +/- 0.9 degrees C. Peripheral tissues rewarmed better in the patients in the 31 degrees C group, and the afterdrop was only 1.5 +/- 0.4 degrees C.