Cardiac arrhythmia during cold-water immersion: a self-controlled field study with extended rhythm monitoring.

Cold-water immersion (CWI) is increasingly practiced for its physical and psychological benefits, yet its cardiac safety profile remains poorly defined. This study aimed to assess the arrhythmic burden associated with acute CWI in middle-aged recreational athletes using continuous extended rhythm monitoring. We conducted a self-controlled observational field study during the 2023 Geneva Christmas Cup, the world's largest cold-water swimming event. Twenty middle-aged recreational athletes (mean age 56 ± 6 years; 11 women) underwent baseline cardiovascular assessment and continuous 10-day ambulatory ECG monitoring using a waterproof single-lead device (BodyGuardian Mini®). Thirteen participants (65%) had low-to-moderate and seven (35%) high 10-year cardiovascular risk based on SCORE2. A total of 64 CWI sessions (mean water temperature 7 °C) were detected and analysed. Heart rate increased significantly during CWI, rising by 40 ± 13 bpm (43% ± 16%; p < 0.001). Only one arrhythmic event, a brief non-sustained atrial tachycardia (8 beats at 156 bpm), occurred during immersion. Over the 10-day monitoring period, 17 participants (85%) experienced brief, predominantly benign arrhythmias, with no significant difference in event rates between CWI and non-CWI periods (0.12 vs 0.037 per hour, p = 0.25). No sustained arrhythmias or clinically significant events were observed. In the context of a large, non-competitive cold-water event, CWI was associated with a low arrhythmic burden.