In Reply:-We have no data regarding the age dependence of the ventilatory response to hypoxia during epidural anesthesia. The rabbits (approximately 2 kg) we studied are not mature, heightening the relevance of Dr. Gunter's comparison to human infants. Although we did not typically see the hyperventilatory phase before arrest as described by Dr. Gunter, that mirrors the pattern of neural activity identified in the central nervous system. [1,2]Regarding the mechanism of apnea, we noted a similar response to hypoxia in the absence of sympathetic blockade in animals with hypotension caused by problems with surgical preparation, such as excessive blood loss. This suggests that circulatory compromise, not sympathetic block per se, is the critical component that sets the stage for hypoxic arrest. Findings from our study [3]that support (but do not prove) the key role of circulatory compromise include the lack of a change in ventilation after sympathetic block alone and the effectiveness of hemodynamic resuscitation in preventing arrest.

Quinn H. Hogan, M.D.

Department of Anesthesiology; Medical College of Wisconsin; Milwaukee, Wisconsin;

(Accepted for publication July 7, 1998.)

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