Arterial baroreflex control of the sinus node during dobutamine exercise stress testing

The contributions of increases in circulating catecholamines, changes in ce ntral command, and muscle afferents on baroreflex control of the sinus node during exercise are unclear. We used a dobutamine infusion to induce hemod ynamic changes comparable to those of moderate physical exercise in the abs ence of changes in central command and muscle afferents in 13 healthy subje cts. Dobutamine (up to 9 mu g/kg body weight per minute) increased systolic blood pressure, shortened the RR interval, increased systolic blood pressu re variability, but blunted RR interval variability (P < 0.05 versus placeb o). Consequently, dobutamine decreased the coherence between variations in systolic blood pressure and RR interval and decreased arterial baroreflex s ensitivity from 12 +/- 2 to 3 +/- 1 ms/mm Hg (P < 0.01). The largest increa ses in systolic blood pressure with dobutamine were paralleled by the great est impairments in arterial baroreflex sensitivity (0.50 < r < 0.56, P < 0. 01). The chronotropic effects of dobutamine prevented a reflex bradycardia in response to the blood pressure increase. However, less predominant low-f requency oscillations in systolic blood pressure (P < 0.0001) suggested pre served sympathetic withdrawal in response to the blood pressure increase in duced by dobutamine. In conclusion, this study revealed that a shift in the operating point of the arterial baroreceptors and the chronotropic effects of adrenergic stimulation impair baroreflex control of the sinus node duri ng dobutamine exercise stress testing. Baroreflex control of the sinus node is not reset when hemodynamic characteristics of exercise are reproduced i n the absence of modifications in central command and muscles afferents.