Longitudinal Assessment of Autonomic Function during the Acute Phase of Spinal Cord Injury: Use of Low-Frequency Blood Pressure Variability as a Quantitative Measure of Autonomic Function.

High-level spinal cord injury (SCI) can disrupt cardiovascular autonomic function. However, the evolution of cardiovascular autonomic function in the acute phase following injury is unknown. We evaluated the timing, severity, progression, and implications of cardiovascular autonomic injury following acute SCI. We tested 63 individuals with acute traumatic SCI (aged 48 ± 2 years) at five time-points: <2 weeks, and 1, 3, 6-12, and >12 months post-injury. Supine beat-to-beat systolic arterial pressure (SAP) and R-R interval (RRI) were recorded and low-frequency variability (LF SAP and LF RRI) determined. Cross-spectral analyses were used to determine baroreflex function (low frequency) and cardiorespiratory interactions (high frequency). Known electrocardiographic (ECG) markers for arrhythmia and self-reported symptoms of cardiovascular dysfunction were determined. Comparisons were made with historical data from individuals with chronic SCI and able-bodied controls. Most individuals had high-level (74%) motor/sensory incomplete (63%) lesions. All participants had decreased LF SAP at <2 weeks (2.22 ± 0.65 mm Hg2). Autonomic injury was defined as high-level SCI with LF SAP <2 mm Hg2. Two distinct groups emerged by 1 month: autonomically complete SCI with sustained low LF SAP (0.76 ± 0.17 mm Hg2) and autonomically incomplete SCI with increased LF SAP (5.46 ± 1.0 mm Hg2, p < 0.05). Autonomically complete injuries did not recover over time. Cardiovascular symptoms were prevalent and worsened with time, especially in those with autonomically complete lesions, and chronic SCI. Baroreflex function and cardiorespiratory interactions were impaired after SCI. Risk of arrhythmia increased immediately after SCI, and remained elevated throughout the acute phase. Acute SCI is associated with severe cardiovascular dysfunction. LF SAP provides a simple, non-invasive, translatable, quantitative assessment of autonomic function, and is most informative 1 month after injury.