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 degrees 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 <20 mm Hg), a 22 French Foley catheter was directed cephalad through the enlarged aortic wound. A solution (containing 42.5 mmol/L K+ and precooled to 1 degrees C) was infused to arrest/preserve the heart and brain. A second 24 French Foley catheter was then directed caudally through the same wound. The right atrium was opened to drain the venous system. The animal was cooled with a cardiopulmonary bypass pump (>5L/min) through the Foley catheters. Once 10 degree C was reached, a cannula was placed to the aortic root and the aortic laceration repaired. The animal was maintained at 10 degree 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 I: seven animals were kept at < 10 degrees C with flows less than 2L/min. Group II: three animals underwent 20, 30, and 40 minutes of no flow once they were cooled to 10 degrees 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 degrees C and 10 degrees 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 I: five 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 degrees C was attempted. Group II: The first two animals that had "no flow" for 20 and 30 minutes were fully functional and had normal neurologic examinations. However, the second animal was found to have brain injury on histologic examination. The last animal in this group died of accidental extubation during recovery.

Conclusion: Induction of hypothermic arrest through the chest after exsanguination is possible. The further development of this technique may provide an extended state of "suspended animation" to allow for repairs of hemorrhaging injuries in trauma patients who require emergency department thoracotomy.