Selective convective brain cooling during hypothermic cardiopulmonary bypass in dogs

Background. Neurologic complications, primarily resulting from ischemic ins ults, represent the leading cause of morbidity and disability, and the seco nd most common source of death, after cardiac operations. Previous studies have reported that increases las occur during the rewarming phase of cardio pulmonary bypass [CPB]) or decreases in brain temperature of a mere 0.5 deg rees to 2 degrees C can significantly worsen or improve, respectively, post ischemic neurologic outcome. The purpose of the present study was to evalua te a novel approach of selectively cooling the brain during hypothermic CPB and subsequent rewarming. Methods. Sixteen dogs were anesthetized with either intravenous pentobarbit al or inhaled halothane (n = 8 per group). Normocapnia (alpha stat techniqu e) and a blood pressure near 75 mm Hg were maintained. Temperatures were mo nitored by placing thermistors in the esophagus (ie, core), parietal epidur al space, and brain parenchyma at depths of 1 and 2 cm beneath the dura. Du ring CPB, core temperature was actively cycled from 38 degrees C to 28 degr ees C, and then returned to 38 degrees C. Forced air pericranial cooling la ir temperature of approximately 13 degrees C) was initiated simultaneous wi th the onset of CPB, and maintained throughout the bypass period. Brain-to- core temperature gradients were calculated by subtracting the core temperat ure from regional brain temperatures. Results. In halothane-anesthetized dogs, brain temperatures at all monitori ng sites were significantly less than core during all phases of CPB, with o ne exception (2 cm during systemic cooling). Brain cooling was most promine nt during and after systemic rewarming. For example, during systemic rewarm ing, average temperatures in the parietal epidural space, and 1 and 2 cm be neath the dura, were 3.3 degrees +/- 1.3 degrees C (mean +/- standard devia tion), 3.2 degrees +/- 1.4 degrees C, and 1.6 degrees +/- 1.0 degrees C, co oler than the core, respectively. Similar trends, but of a greater magnitud e, were noted in pentobarbital-anesthetized dogs. For example, during syste mic rewarming, corresponding brain temperatures were 6.5 degrees +/- 1.7 de grees C, 6.3 degrees +/- 1.6 degrees C, and 4.2 degrees +/- 1.3 degrees C c ooler than the core, respectively. Conclusions. The magnitude of selective brain cooling observed in both stud y groups typically exceeded the 0.5 degrees to 2.0 degrees C change previou sly reported to modulate ischemic injury, and was most prominent during the latter phases of CPB. When compared with previous research from our labora tory, application of cold forced air to the cranial surface resulted in bra in temperatures that were cooler than those observed during hypothermic CPB without pericranial cooling. On the basis of the assumption that similar b eneficial brain temperature changes can be induced in humans, we speculate that selective convective brain cooling may enable clinicians to improve ne urologic outcome after hypothermic CPB. (C) 1998 by The Society of Thoracic Surgeons.