Cardiac instability during sympathetic or parasympathetic activation following spinal cord injury: evidence for autonomic conflict?

Cardiovascular disease is a leading cause of death following spinal cord injury (SCI), with increased cardiac instability being linked to increased mortality. One theory behind this development after SCI is autonomic conflict, the coactivation of the parasympathetic and sympathetic nervous systems on the heart. To test this, individuals with chronic SCI and uninjured controls underwent the same autonomic testing, aimed at activating pure sympathetic and/or parasympathetic signals to the heart. Throughout testing, heart rate and blood pressure were continuously recorded. Poincaré, blood pressure, and linear mixed model analyses were performed. Cardiac instability was found to significantly increase (ΔSD1) in individuals with SCI compared to uninjured controls for below level of injury, sympathetic stimuli (foot CPT and bladder pressor) but not in systemic parasympathetic (Oxford) or above level of injury, sympathetic stimuli (hand CPT). This increase in cardiac instability with below level of injury stimuli for individuals with SCI is indicative of autonomic conflict. The linear mixed model also suggested injuries at or above T1 and individuals who experienced autonomic dysreflexia during testing are at a greater risk of increased cardiac instability, regardless of the stimulus type. Identification of the causes and consequences of cardiac instability after SCI is imperative for understanding cardiovascular complications and developing treatment methods for this common post-SCI complication.