Effects of Autonomic Blockade on Heart Rate Variability after Spinal Cord Injury: A Case Series

The purpose of this study was to examine the effects of pharmacological cardiac autonomic blockade on heart rate variability (HRV) in individuals with SCI and compare these effects to age, sex, and weight-matched able-bodied controls. Three males (45.8 years) with incomplete tetraplegia and three male (33.0 years) able-bodied controls underwent 10 min of electrocardiogram recordings before and after beta-blockade (metoprolol tartrate) and cholinergic blockade (atropine sulphate), in both the 0 deg−tilt position and during cardiovascular stress (40 deg head-up tilt, cold pressor, and isometric jaw contraction). The cardiac response to such blockade was determined using power spectral analysis. In the three participants with SCI, atropine sulphate was associated with a 99.9, 98.2, and 99.2% reduction in resting high-frequency (HF) power, while metoprolol tartrate was associated with a 72.5, 73.1, and 39.6% decrease in the low-frequency to high-frequency ratio (LF:HF) during cardiovascular stress. Similar changes were experienced by the able-bodied participants. HF power is a valid estimate of cardiac parasympathetic outflow, and cardiac sympathovagal balance contributes to the LF:HF ratio in individuals with SCI. Large-scale studies on the validity of HRV after SCI are warranted, since such measures may gauge autonomic impairment and subsequent improvement.