Background and objectives: The effects of β-blocking agents on exercise tolerance in cardiopulmonary exercise testing (CPX) have not been fully identified. Because the negative chronotropic effects of these agents produce a sluggish increase in heart rate (HR) during CPX, exercise capacity is actually underestimated by methods that depend on HR-related variables such as peak oxygen uptake (V̇o2) and anaerobic threshold (AT). The aim of this study was to clarify the efficacy of β-blocking agents by means of V̇o2 kinetics, a parameter independent of HR, in patients with dilated cardiomyopathy (DCM).
Design and patients: The exercise capacity of 12 patients (9 men and 3 women; mean ± SD age, 54 ± 12 years; New York Heart Association class I [n = 1], NYHA class 2 [n = 4], and NYHA class III [n = 6]) with DCM, who were treated with β-blocking agents, was evaluated by CPX. V̇o2 was calculated from respiratory gas analysis on a breath-by-breath basis. Nine patients were treated with metoprolol (30 mg or 60 mg), two with carteolol (10 mg or 20 mg), and one patient with atenolol (25 mg).
Results: All patients showed a significantly favorable results (ie, improvement in symptoms of congestive heart failure). Peak V̇o2 (20.4 ± 5.1 to 18.8 ± 5.8 mL/min/kg), AT (12.7 ± 3.5 to 12.1 ± 2.1 mL/min/kg), and exercise time (4.8 ± 2.2 to 4.5 ± 2.1 s) were unchanged. The time constant of V̇o2 kinetics (τ) on response to constant low-dose work loading (warm up) decreased significantly (64 ± 30 to 44 ± 24 s; p < 0.01) and ejection fraction increased (30 ± 14 to 44 ± 15%, p < 0.01) significantly following treatment with β-blocking agents. In spite of excluding two NYHA I patients, these changes were also statistically correlated.
Conclusion: In the low level of exercise, τ was prolonged in patients with DCM. Although indexes of total exercise time and AT were not useful markers for clinical improvement in cardiac performance as assessed by echocardiography, measuring can validly assess the beneficial effects in heart failure treated with β-blocking agents.