Does the Heart Fall Asleep? – Diurnal Variations of Heart Rate Variability in Patients with Disorders of Consciousness

The current study investigated heart rate (HR) and heart rate variability (HRV) across day and night in patients with disorders of consciousness (DOC). We recorded 24-h ECG in 26 patients with DOC (i.e., unresponsive wakefulness syndrome [UWS; n =16] and (exit) minimally conscious state [(E)MCS; n =10]). To examine diurnal variations, HR and HRV indices in the time, frequency, and entropy domains were computed for periods of clear day- (forenoon: 8am-2pm; afternoon: 2pm-8pm) and nighttime (11pm-5am). Results indicate that patients’ interbeat intervals (IBIs) were larger during the night than during the day indicating HR slowing. Additionally, higher HRV entropy was associated with higher EEG entropy during the night. Patients in UWS showed larger IBIs compared to patients in (E)MCS, and patients with non-traumatic brain injury showed lower ECG entropy than patients with traumatic brain injury. Thus, cardiac activity varies with a diurnal pattern in patients with DOC and can differentiate between patients’ diagnoses and etiologies. Moreover, also the interaction of heart and brain appears to follow a diurnal rhythm. Thus, HR and HRV seem to mirror the integrity of brain functioning and consequently might serve as supplementary measures for improving the validity of assessments in patients with DOC.