Abstract 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 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 heart rate, 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 ECG and 24-hour monitoring confirmed IST. Initial cardiopulmonary exercise testing (CPET) off-therapy showed a rapid heart rate increase during exertion, an early plateau in oxygen pulse, and reduced peak VO2 (22.1 ml/kg/min, 76% of the predicted value). After one month of ivabradine 5 mg bid, there was some improvement in these parameters. At the third follow-up, during combined therapy of ivabradine (5 mg bid) and metoprolol (50 mg bid), the heart rate 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 heart rate 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.
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