Reflex control of sympathetic activity during simulated ventricular tachycardia in humans

Background-Ventricular tachyarrhythmias present a unique set of stimuli to arterial and cardiopulmonary baroreceptors by increasing cardiac filling pr essures and decreasing arterial pressure. The net effect on the control of sympathetic nerve activity (SNA) in humans is unknown. The purpose of this study was to determine the relative roles of cardiopulmonary and arterial b aroreceptors in controlling SNA and arterial pressure during ventricular pa cing in humans. Methods and Results-Two experiments were performed in which SNA and hemodyn amic responses to ventricular pacing were compared with nitroprusside infus ion (NTP) in 12 patients and studied with and without head-up tilt or pheny lephrine to normalize the stimuli to either the arterial or cardiopulmonary baroreceptors in 9 patients. In experiment 1, the slope of the relation be tween SNA and mean arterial pressure was greater during NTP (-4.7+/-1.4 U/m m HE) than during ventricular pacing (-3.4+/-1.1 U/mm Hg). Comparison of NT P doses and ventricular pacing rates that produced comparable hypotension s howed that SNA increased more during NTP (P=0.03), In experiment 2, normali zation of arterial pressure during pacing resulted in SNA decreasing below baseline (P<0.05), whereas normalization of cardiac filling pressure result ed in a greater increase in SNA than pacing alone (212+/-35% versus 189+/-3 7%, P=0.04). Conclusions-These data demonstrate that in humans arterial baroreflex contr ol predominates in mediating sympathoexcitation during ventricular tachyarr hythmias and that cardiopulmonary baroreceptors contribute significant inhi bitory modulation.