300 Section - Heart Rate Variability and Cerebral Blood Flow Velocity in Orthostatic Hypotension Subtypes in Athletes with Spinal Cord Injury

Orthostatic hypotension (OH) after spinal cord injury (SCI) is stratified into three subtypes based on the timing of blood pressure (BP) decline as follows: initial (IOH < 15 sec), classic (COH within 3 min), and delayed (DOH >3 min) subtypes. Although dysregulated autonomic control contributes to the magnitude of BP drop and cerebral blood flow velocity (CBFv) reduction, its severity across OH subtypes remains unclear. This study assessed the relationship between autonomic functions measured with heart rate variability (HRV) and BP/CBFv responses in SCI athletes.

Design:

Forty-one paralympic athletes with chronic SCI above T6 (no OH: n=13; IOH: n=7; COH: n=11; DOH: n=10) underwent continuous BP (plethysmography), heart rate, and ECG monitoring during 10 minutes of supine rest and 10 minutes of sit-up testing. CBFv in the middle and posterior cerebral arteries (MCA, PCA) was assessed in 23 participants (no OH: n=9; IOH: n=2; COH: n=4; DOH: n=8). Changes of HRV metrics (SDNN, RMSSD, HF, LF) and CBFv were analyzed from two-minute pre-tilt average and at nadir systolic BP (SBP).

Results:

All OH subtypes showed significant SBP reduction compared to athletes without OH (p< 0.05). Compared to those without OH, COH and DOH—but not IOH—exhibited significant reductions in ∆SDNN and ∆CBFv of both MCA and PCA (p< 0.05). Only DOH showed a significant reduction in ∆RMSSD (p< 0.05). HRV indices did not differ between COH and DOH. ∆RMSSD and ∆HF were positively associated with ∆SBP (R²=0.23, p=0.04; R²=0.34, p=0.009). Also, ∆SDNN, ∆RMSSD, and ∆LF positively correlated with ∆CBFv in both arteries (R² >0.35, p< 0.05, for all associations).

Conclusion:

COH and DOH are characterized by impaired autonomic regulation and reduced CBFv, unlike IOH. DOH uniquely involves cardiovagal suppression, suggesting a mechanistic link to delayed BP drop in this subtype. Understanding autonomic interplay in each subtype is important to tailor individualized OH management.