Assessing the capacity of the sympathetic nervous system to respond to a cardiovascular challenge in human spinal cord injury

Study design: Measurement of haemodynamic responses and cutaneous blood flow during an inspiratory-capacity apnoea following spinal cord injury (SCI). Objective: To assess the capacity of the sympathetic nervous system to respond to a cardiovascular challenge following SCI. Setting: Prince of Wales Medical Research Institute, Australia. Subjects: Thirteen spinal cord injured subjects with injuries ranging from C5-T8 and eight able-bodied control subjects. Methods: Continuous blood pressure, an electrocardiogram, respiration and cutaneous blood flow were recorded during a static maximum inspiratory breath-hold for 40 s. Results: On average, systolic blood pressure decreased 26% from baseline in the spinal group during the breath-hold and remained below baseline throughout the entire apnoeic period. Heart rate in this group had an initial decrease from baseline but quickly increased throughout the breath-hold, being 17% above baseline in the recovery period. Systolic pressure in the control group decreased 12% from baseline at the beginning of the breath-hold but quickly stabilized for the remainder of the apnoea, with heart rate initially decreasing 22% and remaining below baseline throughout the breath-hold. Conclusion: A maximal inspiratory breath-hold, which is known to cause a sustained increase in muscle sympathetic nerve activity, is a simple test to perform in supine spinal cord-injured subjects, and provides information on the capacity of muscle and splanchnic vasoconstrictor activity to increase blood pressure in SCI. A sustained decrease in blood pressure, coupled with an increase in heart rate, infers interruption of sympathetic vasoconstrictor pathways to muscle and splanchnic vascular beds.