Postbypass hypothermia and its relationship to the energy balance of cardiopulmonary bypass

Thermal energy balance as a measure of adequate rewarming from hypothermic cardiopulmonary bypass

OBJECTIVE

To determine whether the amount of heat (thermal energy) used actively to rewarm patients on cardiopulmonary bypass (CPB) was a better indicator of adequate rewarming from hypothermic CPB than core temperature.

DESIGN

Prospective study.

SETTING

Single hospital.

PARTICIPANTS

Fifty-four sequential patients undergoing hypothermic CPB.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Thermal energy balance (TEB) (net heat supplied to or removed from the body, from initiation to termination of CPB) was measured using previously validated apparatus. Adequacy of rewarming was assessed by measuring the coldest postoperative core (tympanic membrane) temperature and the time to rewarm postoperatively to a core temperature of 37.0 degrees C. Core temperature on termination of CPB did not correlate with the degree of postoperative hypothermia as judged by time to rewarm postoperatively to 37.0 degrees C (r = 0.14; p = 0.33), but did correlate with coldest postoperative core temperature (r = 0.47; p = 0.0003). TEB correlated better with time to rewarm to 37.0 degrees C (r = 0.43; p = 0.001) and coldest postoperative core temperature (r = 0.58, p = 0.0001).

CONCLUSION

TEB is a better predictor than corresponding values of core temperature on termination of CPB in predicting the coldest postoperative temperature and time to rewarm to 37 degrees C.

Tissue heat content and distribution during and after cardiopulmonary bypass at 17 degrees C

We measured afterdrop and peripheral tissue temperature distribution in eight patients cooled to approximately 17 degrees C during cardiopulmonary bypass and subsequently rewarmed to 36.5 degrees C. A nasopharyngeal probe evaluated trunk and head temperature and heat content. Peripheral tissue temperature (arm and leg temperature) and heat content were estimated using fourth-order regressions and integration over volume from 30 tissue and skin temperatures. Peripheral tissue temperature decreased to 19.7+/-0.9 degrees C during bypass and subsequently increased to 34.3+/-0.7 degrees C during 104+/-18 min of rewarming. The core-to-peripheral tissue temperature gradient was -5.9+/-0.9 degrees C at the end of cooling and 4.7+/-1.5 degrees C at the end of rewarming. The core-temperature afterdrop was 2.2+/-0.4 degrees C and lasted 89+/-15 min. It was associated with 1.1+/-0.7 degrees C peripheral warming. At the end of cooling, temperatures at the center of the upper and lower thigh were (respectively) 8.0+/-5.2 degrees C and 7.3+/-4.2 degrees C cooler than skin temperature. On completion of rewarming, tissue at the center of the upper and lower thigh were (respectively) 7.0+/-2.2 degrees C and 6.4+/-2.3 degrees C warmer than the skin. When estimated systemic heat loss was included in the calculation, redistribution accounted for 73% of the afterdrop, which is similar to the contribution observed previously in nonsurgical volunteers.

IMPLICATIONS

Temperature afterdrop after bypass at 17 degrees C was 2.2+/-0.4 degrees C, with approximately 73% of the decrease in core temperature resulting from core-to-peripheral redistribution of body heat. Cooling and rewarming were associated with large radial tissue temperature gradients in the thigh.

Pulsatile flow during cardiopulmonary bypass speeds thermal energy transfer: a possible explanation for the reduced afterdrop

The instantaneous thermal energy balance and rates of thermal energy transfer during hypothermic cardiopulmonary bypass were measured for a group of patients receiving continuous flow and compared with a group receiving pulsatile flow. Cooling was more rapid and the rate of thermal energy delivery during rewarming significantly greater in the pulsatile flow group despite similar rewarming times. The final thermal energy balance at the end of cardiopulmonary bypass was larger and the period of postoperative hypothermia shorter in those receiving pulsatile flow. The greater rate of thermal energy transfer may explain the reduced afterdrop.