Analysis of baroreflex control of heart rate in conscious dogs with pacing-induced heart failure. The autonomic components of the baroreflex control of heart rate were evaluated in conscious mongrel dogs before and after 4-6 weeks of ventricular pacing (250 beats/min). Arterial baroreflex sensitivity (BRS) was determined by the slopes of linear regression of pulse interval versus the preceding systolic arterial pressure in response to bolus injections of either phenylephrine or nitroglycerin. BRS was significantly depressed in the heart failure state [nitroglycerin slope, 5.0 +/- 2.7 (mean +/- SD) versus 16.6 +/- 5.1 msec/mm Hg, p less than 0.005; phenylephrine slope, 15.0 +/- 14.8 versus 32.0 +/- 26.7 msec/mm Hg, p less than 0.005]. There was no depression in BRS in dogs that were used as time controls or were acutely paced for 30 minutes. After beta 1-adrenergic blockade with metoprolol, the resting heart rate in the heart failure state was depressed more than in the normal state (-17.0 +/- 5.0% versus -3.2 +/- 3.4%, p less than 0.001). Atropine significantly increased resting heart rate more in the normal state than in the heart failure state (115.8 +/- 36.7% versus 25.4 +/- 14.5%, p less than 0.005). Thus, dogs in the heart failure state appear to have high resting cardiac sympathetic tone and low resting vagal tone. For nitroglycerin administration, metoprolol depressed BRS by 47.6 +/- 26.3% in the normal state and by 63.6 +/- 58.5% in the heart failure state. Atropine decreased the BRS by 86.7 +/- 7.8% in the normal state and by 39.5 +/- 30.2% in the heart failure state. 