Interaction of temperature with hematocrit level and pH determines safe duration of hypothermic circulatory arrest.

OBJECTIVE: Previous studies have demonstrated that both hematocrit level and pH influence the protection afforded by deep hypothermic circulatory arrest. The current study examines how temperature modulates the effect of hematocrit level and pH in determining a safe duration of circulatory arrest. The study also builds on previous work investigating the utility of near-infrared spectroscopy as a real-time monitor of cerebral protection during circulatory arrest.
METHODS: Seventy-six piglets (9.3 +/- 1.2 kg) underwent circulatory arrest under varying conditions with continuous monitoring by means of near-infrared spectroscopy (hematocrit level of 20% or 30%; pH-stat or alpha-stat strategy; temperature of 15 degrees C or 25 degrees C; arrest time of 60, 80, or 100 minutes). Neurologic recovery was evaluated daily by a veterinarian, and the brain was fixed in situ on postoperative day 4 to be examined on the basis of histologic score in a blinded fashion.
RESULTS: Multivariable analysis of total histologic score revealed that higher temperature, lower hematocrit level, more alkaline pH, and longer hypothermic circulatory arrest duration were predictive of more severe damage to the brain (P <.01). Regression modeling revealed that higher temperature exacerbated the disadvantage of a lower hematocrit level and longer arrest times but not pH strategy. Normalized oxyhemoglobin nadir time, derived from near-infrared spectroscopy, was positively correlated with neurologic recovery on the fourth postoperative day and with total histologic injury score (P <.0001).
CONCLUSION: Hematocrit level and pH, as well as temperature, determine the safe duration of hypothermic circulatory arrest. Near-infrared spectroscopy is a useful real-time monitor of safe duration of circulatory arrest.