Induced Hypothermia during Emergency Department Thoracotomy: An Animal Model

BACKGROUND:Induced hypothermia is used clinically to prevent ischemic injury during elective procedures. We present an animal model of asanguinous hypothermic (10°C) circulatory arrest, induced through a left anterior lateral thoracotomy after exsanguinating uncontrolled hemorrhage. METHODS:Through a left anterior thoracotomy, 26 swine (45–70 kg) sustained a laceration of the descending thoracic aorta, producing exsanguinating uncontrolled hemorrhage. After 5 minutes of severe hypotension (systolic BP 5L/min) through the Foley catheters. Once 10°C was reached, a cannula was placed to the aortic root and the aortic laceration repaired. The animal was maintained at 10°C for a total of 90 minutes. Before the rewarming process, the circulation was rinsed with a solution containing normal levels of electrolytes followed by infusion of whole blood. Rewarming was performed by maintaining a 10-degree gradient on the heat exchanger. The first 16 animals were used in nonsurvival experiments to develop the technique and to record dural temperatures and electroencephalogram tracings. The last 10 animals were used to determine long-term survival and neurologic outcome. Group Iseven animals were kept at < 10°C with flows less than 2L/min. Group IIthree animals underwent 20, 30, and 40 minutes of no flow once they were cooled to 10°C. After 6 weeks of survival and neurologic examinations, the brains were fixed for histologic evaluations. RESULTS:The average time to cool the head to 18°C and 10°C was 6 minutes and 12 minutes, respectively. The hematocrit fell below 2% by the end of the cooling period. A total of 7 of the 10 animals from the long-term study survived. Group Ifive of seven animals survived. Four of the survivors had no appreciable neurologic deficits, were fully functional at 6 weeks, and had no evidence of histologic injury. One of the five survivors in this group had moderate neurologic disability. Of the two animals that died, one died from air embolism from the i.v. line. The second death was in an animal for which maximal cooling to 2.7°C was attempted.