The effects of convective cooling and rewarming on systemic and central nervous system physiology in isoflurane-anesthetized dogs

Recent studies have demonstrated that small (i.e., 2-5°C) reductions in temperature may protect the brain and spinal cord from ischemic injury. The present study evaluated the physiologic response of anesthetized animals to convective-based cooling and warming. Six shaved, isoflurane-anesthetized (1.50% end-expired; 1 MAC), pancuronium-paralyzed dogs were subjected to temperature manipulation. The flow of cool (13-14°C) or warm (39-41°C) air was uniformly applied to the the dorsal and lateral surfaces of the dog using an inflatable blanket with perforations in the interior surface. Convective cooling reduced pulmonary artery temperature (Tpa) from 37.0 ± 0.2°C (Mean ± S.D.) to 33.0 ± 0.0°C over a 93 ± 18 min period. Thereafter, the active cooling was discontinued and passive cooling resulted in a further reduction in Tpa to 32.4 ± 0.3°C over the next 60 min. Institution of convective warming resulted in an increase in Tpa from 32.4 ± .03 to 33.0 ± 0.0°C in 23 ± 14 min and from 33.0 to 37.0 ± 0.0 in an additional 137 ± 26 min. During the periods of active cooling, passive cooling and active warming, there were strong correlations between Tpa and temperature within the brain, cisterna magna, parietal epidural space, lumbar subarachnoid space and other commonly used temperature measurement sites non-invasively monitored (e.g. tympanic membrane, esophagus, rectum) (r ≥ 0.97; P < 0.0001). The combination of isoflurane anesthesia (a potent EEG-suppressor) plus mild hypothermia ( < 34°C) resulted in an EEG attenuation in five dogs, two of which progressed to burst suppression. The magnitude of EEG changes correlated with the degree of temperature reduction. Upon rewarming to 37°C, all dogs had normal EEG activity and normal brain concentrations of high energy phosphates, glucose and lactate. Blood pressure and cardiac output did not change during the study and no dog exhibited acid-base anomalies or blood lactate accumulation. Whole body oxygen consumption and heart rate decreased in a temperature-dependent fashion. Cardiac rhythm disturbances were rare. The authors conclude that convection-based corporeal cooling and rewarming are efficacious methods for non-invasively and uniformly altering CNS temperatures without adversely affecting cerebral or systemic physiology.