Reflex Vagal Withdrawal After Sympathetic Blockade

In Response: I appreciate the interest Introna et al. have shown in our work (1) and highly respect their insight into autonomic nervous system modulation by neuroaxial blockade. First, I would like to apologize that we inappropriately cited their previous observations (2). As Introna et al. correctly stated, their findings, that subarachnoid block involving cardiac sympathectomy resulted in diminutions of both high-frequency and low-frequency powers of heart rate variability with unchanged low/high ratio, were consistent with our major finding seen after cervical epidural anesthesia. In addition, they proposed a reasonable mechanism that central sympathetic outflow reflexively augmented, while vagal outflow reflexively inhibited, a reciprocal relationship between sympathetic and parasympathetic activity proposed by Malliani et al. (3). Alternatively, reduced high-frequency power after sympathetic blockade may also be explained by diminished “accentuated antagonism,” a phenomenon described more than three decades ago (4). Heart rate response to vagal nerve stimulation is accentuated when sympathetic tone is elevated, and vice versa. Thus, cardiac sympathetic withdrawal by high spinal or epidural blockade may have resulted in diminished beat-to-beat fluctuations of R-R intervals without alteration of actual vagal nerve activity. To draw a definitive conclusion regarding the mechanism, determinations of central vagal/sympathetic outflow would be mandatory by an animal experiment. Makoto Tanaka, MD Department of Anesthesia Akita University School of Medicine Akita, Japan [email protected]