In Reply:

—I would like to thank Drs. Introna and Blair for making several interesting comments regarding the reported case of vagotonia and cardiac arrest during a spinal anesthetic. I agree that in most individuals the sympathetic and parasympathetic systems remains remarkably balanced, despite significant blockade of the sympathetic afferent and efferent pathways during spinal anesthesia. Otherwise, as pointed out, completely unopposed vagal tone would lead to asystole in many patients during spinal and epidural anesthesia. However, it appears that in a select group of patients the balance between the sympathetic and parasympathetic systems is not as well-controlled. Even in the absence of spinal anesthesia, these patients can experience significant hypothension and bradycardia when exposed to noxious stimuli. Perhaps these individuals lack the pathways that are responsible for down-regulating the parasympathetic system when sympathetic blockade occurs? Studying heart rate variability in these patients using the methods described by Introna et al . would be interesting. 1,2If preoperative abnormalities could be detected or predictive of adverse events during spinal anesthesia, then this technology could be used to identify patients in whom spinal or epidural anesthesia should be avoided.

Introna R, Yodlowski E, Pruett J, MOntano N, Porta A, Crumrine R: Sympathovagal effects of spinal naesthesia assessed by heart rate variability analysis. Anesth Analg 1995; 80:315–21
Malliani A, Pagani M, Lombardi R, Cerutti S: Cardiovascular neural regulation in the frequency domain. circulation 1991; 84:482–92