Unilateral carotid sinus stimulation and muscle sympathetic nerve activity in man

The carotid baroreflex influences sympathetic outflow to the peripheral circulation, yet this net response is the result of information received from both right and left carotid sinuses. To assess their individual contributions on sympathetic activity, direct measurements of muscle sympathetic nerve activity (SNA) from the peroneal nerve of the right leg were made during unilateral carotid sinus stimulation. A sustained neck pressure (approximately 25 +/- 2 mm Hg) was applied to right and left carotid sinuses, as well as bilaterally, for 5 s in 10 healthy subjects using a customized neck collar device partitioned to allow for stimulation of only one side of the neck. In addition to muscle SNA, blood pressures (radial artery) and R-R interval (RRI) changes were recorded. Muscle SNA was greater for left side neck pressure with respect to burst amplitude (left = 4.07 +/- 0.67 vs right = 2.48 +/- 0.46 microV; P < 0.05), normalized burst amplitude (388 +/- 63 vs 269 +/- 37 units; P < 0.05), burst ratio (0.64 +/- 0.05 vs 0.46 +/- 0.04; P < 0.05), and a SNA index of normalized amplitude and ratio (24832 +/- 3455 vs 6566 +/- 3259 units; P < 0.05). The combined values of the muscle SNA index for right and left sides (25590 +/- 4531 units) did not differ from the bilateral value (21906 +/- 3855 units; P > 0.05). These findings suggest that afferent input from the left carotid sinus may have a greater influence on efferent muscle sympathetic outflow (as measured in the right leg) and that the bilateral response may represent the summed right and left carotid inputs.