Impaired baroreflex control of vascular resistance and heart rate in acute myocardial infarction.

The baroreflex control of vascular resistance and heart rate was studied in 11 patients to determine whether it is impaired in patients with acute myocardial infarction. Reflex forearm vasoconstriction in response to lower body negative pressure at 40 mm Hg was less in the early convalescent phase (mean seven days) than in the late convalescent phase (mean 41 days). Pressor as well as vasoconstricting responses to the cold pressor test did not differ between the early and late convalescent phases. The slope of the regression line relating systolic blood pressure and the RR interval during a transient rise in blood pressure produced by intravenous phenylephrine was appreciably reduced in the early convalescent phase of myocardial infarction. These results suggest that baroreflex control of vascular resistance and heart rate is impaired in patients with acute myocardial infarction.