Paradoxical improvement in exercise tolerance and peak VO2 consumption after treatment with ivabradine and beta-blockers in a patient with mild dilated cardiomyopathy and inappropriate sinus tachycardia-a case report.

Background: Left bundle branch block (LBBB) is a rare conduction disorder in athletes associated with ventricular dyssynchrony, which can lead to left ventricular systolic dysfunction and exercise intolerance. Inappropriate sinus tachycardia (IST) is characterized by an excessive heart rate (HR) that is not related to physiological needs, often resulting in reduced exercise capacity. Managing these conditions in athletes can be challenging, as standard treatments like beta-blockers and ivabradine, while effective in controlling HR, are described to be associated with a reduction in maximal exercise performance. Case summary: A 50-year-old amateur athlete presented with exercise intolerance, LBBB, and mild dilated cardiomyopathy due to ventricular dyssynchrony. Resting electrocardiogram and 24-h monitoring confirmed IST. Initial cardiopulmonary exercise testing (CPET) off-therapy showed rapid HR increase during exertion, an early plateau in oxygen pulse, and reduced peak oxygen consumption (VO2, 22.1 mL/kg/min, 76% of the predicted value). After 1 month of ivabradine 5 mg b.i.d., there was some improvement in these parameters. At the third follow-up, with combined therapy of ivabradine (5 mg b.i.d.) and metoprolol (50 mg b.i.d.), the HR response during exercise normalized, and CPET parameters significantly improved, with peak VO2 reaching 29.2 mL/kg/min (101% of the predicted value). Discussion: This case highlights a paradoxical improvement in exercise tolerance and peak VO2 with combined ivabradine and beta-blocker therapy in a patient with IST. The treatment optimized the HR response during exercise, suggesting that individualized strategies can enhance exercise performance in patients with IST and mild cardiomyopathy, despite the expected limitations of these medications.