Monitoring of cardiorespiratory vagal desynchrony using novel biomarkers derived from smartwatch electrocardiograms in a patient recovering from long COVID: Case report

Abstract

Background Long COVID and cardiovascular autonomic dysfunction, including postural orthostatic tachycardia syndrome (POTS), present significant healthcare challenges. Long-term monitoring is challenging due to the evolving nature of symptoms and the limited availability of objective diagnostic tools. With over 200 million electrocardiogram (ECG)-enabled smartwatches sold worldwide, these devices offer a promising solution for at-home diagnostics and disease tracking. Methods and results This study examines a 35-year-old male with long COVID, POTS, and chronic fatigue syndrome (CFS), who recorded 328 ECGs over using a Samsung smartwatch. The protocol required ECG recordings to be taken first in a sitting posture, followed by a standing position, with slow, controlled breathing. For testing, the patient used a Samsung smartwatch to perform a 30-s hand-to-hand single-lead ECG while engaging in 0.1 Hz diaphragmatic controlled breathing, consisting of 5 s of inhalation followed by 5 s of exhalation (Appendix 1). S-/R-peak amplitude ratios, heart rhythm changes, and other biomarkers were analysed to assess autonomic function. Fatigue levels were self-reported via the BREATHE FLOW app using a three-grade scale, and health status was tracked monthly with the EQ-5D-5L model. Initially, the patient experienced severe fatigue and heart rhythm changes consistent with POTS. Electrocardiogram analysis revealed an increased S-wave amplitude and higher S/R ratio in standing posture, along with worsening respiratory sinus arrhythmia (RSA), indicating cardiorespiratory desynchrony. Over time, as symptoms improved, heart rate responses between sitting and standing normalized, and S/R ratio and RSA index followed self-reported fatigue levels, including fluctuations due to post-exercise fatigue. Conclusion Smartwatch-derived S-/R-wave amplitude ratio may serve as an accessible biomarker for tracking disease progression in long COVID. Given the widespread availability of smartwatches, standardized at-home protocols could improve diagnostics and monitoring for autonomic dysfunction.