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Ann Thorac Surg 2001;71:28
© 2001 The Society of Thoracic Surgeons
a Division of Cardiothoracic Surgery, College of Physicians & Surgeons of Columbia University, 177 Ft Washington Ave, Suite 7-435, New York, NY 10032, USA
Stecker and coauthors have presented an elegant description of the natural history of brain cooling, assessed by a variety of neurophysiologic parameters infrequently applied in a clinical setting. In Part I, the variability of cooling to achieve electrocerebral silence (ECS) is striking. In Part II, the "hysteresis" of warming and cooling seen in neural tissue is beautifully illustrated by the disappearance of sensory evoked potentials (SEP) prior to ECS during cooling, with reappearance of SEP while ECS persists during warming. An interesting subanalysis suggests that retrograde cerebral perfusion produces a more rapid rate of rewarming, when comparing results from this series to previous published experience. Not surprisingly, the duration of circulatory arrest appears to correlate with the rate of SEP recovery, and a lower temperature during circulatory arrest is associated with a trend toward later return of neurologic function.
No doubt the authors hoped to connect observation to practice by linking conduct of circulatory arrest to outcome. This is something of a Holy Grail in the treatment of conditions requiring circulatory arrest. Specifically, is there a "best temperature" for circulatory arrest? The conventional answer to this question would be 16° to 18°C, clouded by uncertainty about the relationship between brain temperature and the temperature recorded at various peripheral monitoring sites. One implication of this pair of articles is that a significant number of patients arrested at 18°C will not have achieved ECS. Does this mean that EEG monitoring, to allow confirmation of ECS, is essential for safe conduct of circulatory arrest? Alternatively, should all patients be cooled to 12°C, or cooled for 50 minutes? The authors are careful to warn against leaping to these conclusions, which their data do not support. They can only conclude that "appearance of ECS is consistent from patient to patient and is neither ‘too low’ or ‘too high’."
The difficulty linking observation to practice lies in isolating the variables. "Best temperature" variables such as ECS are dwarfed by other clinical variables. Where neurologic outcome is concerned, the most striking finding in this series is that mean circulatory arrest time was 36.6 ± 12 minutes in neurologically normal patients, versus 51.6 ± 21 minutes in patients with postoperative neurologic impairment (p = 0.006). Therefore, duration of circulatory arrest is the most important predictor of outcome. Assuming reasonable technical facility, duration of circulatory arrest is determined by the complexity of the surgical pathology, not by the management of warming and cooling. The authors make no claims to have assessed the relationship between surgical complexity and outcome, which is difficult to accomplish, and was not the goal of this study. As a result, the findings fall short of a mandate for a change in practice, while still contributing to our understanding of warming and cooling.
Related Article
Ann. Thorac. Surg. 2001 71: 22-28.
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