Supporting Data for: Physiological and cardiac rhythm responses during a maximal breath-hold in a world champion freediver

This dataset contains a two-lead electrocardiogram (ECG) printout collected during a dry, maximal voluntary breath-hold performed by a world champion freediver. The ECG recording captures the full duration of the breath-hold (6 minutes and 7 seconds), including both the easy-going phase (2 minutes and 20 seconds) and the struggle phase (3 minutes and 47 seconds), as well as pre- and post-breath-hold periods. The dataset was collected as part of a physiological case study investigating ventilatory, gas exchange, neuromuscular, oxygenation, and cardiac responses to prolonged breath-holding. During the struggle phase of the breath-hold, the ECG revealed transient, asymptomatic cardiac arrhythmias, including sinus-junctional rhythm competition, multiple supra- and ventricular extrasystoles, and marked bradycardia. Normal sinus rhythm resumed after breath-hold cessation. This dataset provides a manually verifiable reference of cardiac rhythm changes during the maximal voluntary breath-hold.