Impaired baroreflex as a cause of chronotropic incompetence during exercise via autonomic mechanism in patients with heart disease

Purpose: It is thought that the mechanisms responsible for impaired chronotropic response to exercise are related to disturbance of cardiovascular autonoric regulation such as the baroreflex. However, it is still unclear whether the baroreflex mechanisms modulate heart rate (HR) responses to exercise via vagal and/or sympathetic alteration. We therefore investigated the effects of baroreflex sensitivity (BRS) on the exercise HR response in the early phase of vagal deactivation and in the later phase of sympathetic excitation via metabolic stimulation. Methods: Twenty-four patients (18 males and 6 females, age 59 9 years) with heart disease underwent symptom-limited treadmill exercise testing according to the Bruce protocol, and BRS was measured utilizing the phenylephrine method. Subjects were grouped by their resting BRS value: 12 with normal BRS (>= 6 ms/mm Hg) and 12 with depressed BRS (< 6 ms/min Hg). The HR response to exercise was assessed using two parameters: the increment in HR during exercise and the ratio of HR response to the metabolic reserve (chronotropic index). Results: (1) In the patients with depressed BRS, the HR responses within 1 min after the start of exercise and from 1 min to peak exercise were attenuated compared with those having a normal BRS (15 +/- 8 vs. 24 +/- 8 bpm and 36 +/- 9 vs. 47 +/- 15 bpm, respectively). (2) The chronotropic index in the patients with depressed BRS was lower than in those with normal BRS (0.50 +/- 0.14 vs. 0.64 +/- 0.08). Conclusion: These findings suggest that impaired BRS modulates both the parasympathetic influence in early exercise and sympathetic effects in the later phase on HR response to exercise. (c) 2004 Elsevier Ireland Ltd. All rights reserved.