Associations between multidimensional fatigue and wearable-derived cardio-respiratory variables in post-COVID-19 patients: an observational study

Introduction Psychophysiological research increasingly recognizes the multidimensional nature of subjective experiences such as fatigue and their relevance for autonomic regulation. However, little is known about how different dimensions of fatigue are associated with changes in core cardio-respiratory variables in clinical populations. This longitudinal study aimed to investigate the associations between three dimensions of fatigue – physical, cognitive, and emotional – and core physiological variables in post-COVID-19 patients. Physiological variables were collected during both daytime and nighttime periods to examine the individual and combined effects of fatigue dimensions. Methods To this end, thirty post-COVID-19 patients (male: 11, female: 19, age: 44.1 ± 12.7 years) participated in the study during their 29.6 ± 10.2 days stay in a rehabilitation clinic. Four times a day, they reported their fatigue level using a mobile app. Physiological variables, i.e., electrocardiogram (ECG), respiratory rate, and blood oxygen saturation were recorded continuously via wearable sensors during at least 2 monitoring weeks that were 2 weeks apart. Linear mixed models were used to assess associations between daily averages of each fatigue dimension and physiological variables, adjusted for potential confounders. Results The interaction of multiple fatigue dimensions demonstrated more consistent associations with physiological changes than single dimensions independently. Daily changes in fatigue were associated with fluctuations in physiological variables, particularly during sleep. Specifically, daily changes in resting heart rate ( P < .01), sample entropy of RR intervals ( P = .04), and LF peak frequency ( P = .047), as well as overnight changes in LF/HF ratio ( P = .046) and sample entropy of RR intervals ( P < .001), showed significant linear associations with changes in the interaction among fatigue dimensions. Patterns suggest a shift toward sympathetic dominance with higher physical fatigue, and a shift toward parasympathetic dominance with increases in both cognitive and emotional fatigue. Additional trends (adjusted P < .10) supported the interpretation that interaction effects between fatigue components play a critical role in autonomic dynamics. Discussion This study highlights the importance of investigating fatigue as a multidimensional construct in this patient group, enhancing understanding of its relationship with autonomic regulation and supporting more personalized approaches to monitoring and managing fatigue in post-COVID-19 patients.