Reduced heart rate variability and lower cerebral blood flow associated with poor cognition during recovery following concussion

Although physiological deficits such as altered cerebral blood flow (CBF), and autonomic nervous system (ANS) dysregulation have been reported following a concussion, the relationship between CBF and ANS with functional outcome post-injury remains unclear. Our present study was designed to examine heart-rate variability (HRV) using percentage of successive NN intervals (pNN50) and CBF on day-3 (T1), day-21 (T2), and day-90 (T3) following a concussion in collegiate athletes (N = 31) in comparison to non-injured controls (N = 31). Continuous RR-interval (3-lead electrocardiogram), middle cerebral artery blood velocity (MCAV; transcranial Doppler ultrasonography), mean arterial pressure (MAP; finger photoplethysmography) were obtained at rest. Cerebrovascular conductance index (CVCi) was estimated as a ratio of MCAV to MAP. Cognition was evaluated with standard assessment of concussion (SAC), and Trails A & B. Compared to the controls, lower HRV (43 +/- 15 vs. 27 +/- 20%; p < 0.0001) was observed at T1 with normalization at T2 and T3. No difference in MCAV between the control and the concussed groups across the three time points were observed. However, post-hoc analyses indicated a positive relationship between MCAV at T1 phase with HRV and CVCi during T2, and T3 phases. Higher MCAV at T1 was also associated with better cognition scores during the asymptomatic T2 phase in the concussed athletes. Therefore, our results indicate ANS dysregulation during the acute recovery phase after a concussion. Differences in CBF may be one of the underlying causes behind heterogeneity in clinical symptoms and functional outcomes after a concussion and future studies are warranted to validate this finding.