In Cardiac Patients Beta-Blockers Attenuate the Decrease in Work Rate during Exercise at a Constant Submaximal Heart Rate.

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Exercise prescription based on fixed heart rate (HR) values is not associated with a specific work rate (WR) during prolonged exercise. This phenomenon has never been evaluated in cardiac patients, and might be associated with a slow component of HR kinetics and -adrenergic activity. The aim was to quantify, in cardiac patients, the work rate decrease at a fixed HR, and to test if it would be attenuated by β-blockers. 17 patients with coronary artery disease in stable conditions (69 ± 9 yr) were divided into two groups according to the presence (BB) or absence (no-BB) of a therapy with β-blockers, and performed on a cycle ergometer: an incremental exercise (INCR); a 15-min "HRCLAMPED" exercise, in which WR was continuously adjusted to maintain a constant HR, corresponding to the gas exchange threshold (GET) +15%. HR was determined by the ECG signal, and pulmonary gas exchange was assessed breath-by-breath. During INCR HRpeak was lower in BB vs. no-BB (p < 0.05), whereas no differences were observed for other variables. During HRCLAMPED the decrease in WR needed to maintain HR constant was less pronounced in BB vs. no-BB (-16 ± 10% vs. -27 ± 10, p = 0.04), and was accompanied by a decreased O2 only in no-BB (-13 ± 6%, p < 0.001). The decrease in WR during 15-min exercise at a fixed HR (slightly higher than that at GET) was attenuated in BB, suggesting a potential role by β-adrenergic stimulation. The phenomenon may represent, also in this population, a sign of impaired exercise tolerance, and interferes with aerobic exercise prescription.