Selective convective brain cooling during hypothermic cardiopulmonary bypass in dogs

Background. Neurologic complications, primarily resulting from ischemic insults, represent the leading cause of morbidity and disability, and the second most common source of death, after cardiac operations. Previous studies have reported that increases (as occur during the rewarming phase of cardiopulmonary bypass [CPB]) or decreases in brain temperature of a mere 0.5° to 2°C can significantly worsen or improve, respectively, postischemic neurologic outcome. The purpose of the present study was to evaluate a novel approach of selectively cooling the brain during hypothermic CPB and subsequent rewarming. Methods. Sixteen dogs were anesthetized with either intravenous pentobarbital or inhaled halothane (n = 8 per group). Normocapnia (alpha stat technique) and a blood pressure near 75 mm Hg were maintained. Temperatures were monitored by placing thermistors in the esophagus (ie, core), parietal epidural space, and brain parenchyma at depths of 1 and 2 cm beneath the dura. During CPB, core temperature was actively cycled from 38°C to 28°C, and then returned to 38°C. Forced air pericranial cooling (air temperature of approximately 13°C) was initiated simultaneous with the onset of CPB, and maintained throughout the bypass period. Brainto-core temperature gradients were calculated by subtracting the core temperature from regional brain temperatures. Results. In halothane-anesthetized dogs, brain temperatures at all monitoring sites were significantly less than core during all phases of CPB, with one exception (2 cm during systemic cooling). Brain cooling was most prominent during and after systemic rewarming. For example, during systemic rewarming, average temperatures in the parietal epidural space, and 1 and 2 cm beneath the dura, were 3.3° ± 1.3°C (mean ± standard deviation), 3.2° ± 1.4°C, and 1.6° ± 1.0°C, cooler than the core, respectively. Similar trends, but of a greater magnitude, were noted in pentobarbital-anesthetized dogs. For example, during systemic rewarming, corresponding brain temperatures were 6.5° ± 1.7°C, 6.3° ± 1.6°C, and 4.2° ± 1.3°C cooler than the core, respectively. Conclusions. The magnitude of selective brain cooling observed in both study groups typically exceeded the 0.5° to 2.0°C change previously reported to modulate ischemic injury, and was most prominent during the latter phases of CPB. When compared with previous research from our laboratory, application of cold forced air to the cranial surface resulted in brain temperatures that were cooler than those observed during hypothermic CPB without pericranial cooling. On the basis of the assumption that similar beneficial brain temperature changes can be induced in humans, we speculate that selective convective brain cooling may enable clinicians to improve neurologic outcome after hypothermic CPB.