Reflex Vagal Withdrawal After Sympathetic Blockade

To the Editor: We would like to thank Tanaka et al. for their interesting report (1). The authors referred to a previous study by our group (2) as having results that were conflicting with theirs. Instead, we believe that our work not only was consistent with that of the authors but that we also provided a likely explanation of their findings (2). The initial hypothesis of our groups, that anesthetic induced blockade of the sympathetic nervous system would result in evidence of parasympathetic nervous system dominance, was wrong. Instead, sympathetic blockade either by high spinal anesthesia (2) or cervical epidural anesthesia (1), while leaving the parasympathetic pathways intact, resulted in decreased activity from both neural systems. In our report (2), we discussed why these results actually made physiologic sense. Although both sympathetic and parasympathetic systems generally oppose each other, these two systems are also dependent on the continuous interactions between their two neural systems (3). Thus the parasympathetic nervous system activity seems to be inhibited centrally in the absence of an opposing sympathetic outflow. This may explain why cardiovascular collapse, after neuroaxial blockade, is so rare. We believe there is an as yet unnamed “reflex” that protects against an unopposed vagal response. As the saying goes, “it is always better to be lucky than good.” Robert P. S. Introna, MD John R. Blair, MD Northside Anesthesiology Consultants Northside Hospital Atlanta, GA [email protected] Dan C.