Relative contribution of aortic and carotid baroreflexes to heart rate control in man during steady state and dynamic increases in arterial pressure.

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Abstract
We studied the contribution of carotid vs. extracarotid baroreceptors in control of heart rate in normal humans. We measured heart interval (HI) and arterial pressure during steady-state infusion of phenylephrine (PE). PE increased mean arterial pressure (MAP) by 13 +/- 2 mmHg (mean +/- SEM; n = 10) and thus stimulated both carotid and aortic baroreceptors. Neck pressure (NP) was applied during PE infusion to counter the increase in transmural carotid sinus pressure, thus leaving only aortic baroreceptors stimulated by the increase in arterial pressure. PE infusion alone prolonged HI by 230 +/- 24 ms (P less than 0.05). Application of NP attenuated the HI response to 65 +/- 22 ms above control (P less than 0.05 vs. PE alone). During these steady-state increases in arterial pressure, elimination of the carotid baroreflex contribution reduced the HI prolongation by 41-70% in five subjects and by greater than 93% in five subjects. We also measured the HI response to dynamic ramp elevation of systolic arterial pressure (SAP) using bolus administrations of PE. Baroreflex control was calculated from the slope of the regression correlating SAP to succeeding HI for PE alone (carotid and aortic baroreceptor activation) and for PE plus superimposed dynamic NP at levels equal to the increases in SAP (aortic baroreceptor activation). During PE alone, the baroreflex slope was 20.2 +/- 2.9 ms/mmHg (n = 10). During PE plus NP, the baroreflex slope was reduced by 30% to 14.1 +/- 2.8 ms/mmHg (P less than 0.02 vs. during PE alone). Thus, during dynamic increases in arterial pressure, eliminating the carotid baroreflex contribution reduced the HI response by 30%. These studies indicate that extracarotid (presumably aortic) and carotid baroreflexes both participate in control of heart rate in humans. Extracarotid (aortic) baroreflexes appear to have the greater role in control of heart rate during dynamic increases in arterial pressure.