Causal Analysis Is Needed to Evaluate Cardiorespiratory Interaction Alterations in Postural Orthostatic Tachycardia Syndrome Patients

Respiratory and cardiac activities are known to be linked by several mechanisms, resulting in a variety of patterns of heart rate variability. Cardiorespiratory interactions can be evaluated from spontaneous variability of heart period (HP) and respiration (R) through noncausal and causal approaches. The aim of this study is to describe cardiorespiratory interactions in a population known to feature an exaggerated sympathetic response to orthostatic stressors, such as patients affected by postural orthostatic tachycardia syndrome (POTS). Twelve female POTS patients $(age: 36\pm 10\ yrs)$ and 14 female healthy controls $(age: 37\pm 8\ yrs)$ underwent electrocardiogram and respiratory movement recordings while supine and during head-up tilt. Cardiorespiratory interactions were assessed via a model-based noncausal approach, squared coherence $(K^{2})$, and a model-based causal method, transfer entropy (TE). TE was found to be significantly decreased in POTS patients compared to healthy controls during head-up tilt, while $K^{2}$ was similar. We conclude that causal approaches are better suited than noncausal methods to evaluate modifications in the magnitude of HP-R variability interactions in POTS patients during orthostatic challenge, with possible future applications in post-acute COVID-19 patients exhibiting symptoms of dysautonomia.