The effects of surface anaesthesia on the autonomic dysreflexia response during functional electrical stimulation

Recently, increases in blood pressure (BP) and concomitant bradycardia, suggestive of autonomic dysreflexia (AD), have been documented during functional electrical stimulation (FES) in individuals with a high spinal cord injury (SCI). If uncontrolled, this response could preclude the safe use of FES among such individuals. FES induced pain is partly related to stimulation of skin nociceptors. Therefore, measures to reduce skin sensitivity may reduce the risk of AD during FES. The purpose of this study was to determine if topical anaesthetic applied over the site of electrical stimulation could minimize the AD cardiovascular and hormonal responses to FES in individuals with SCI above the T6 level. Seven subjects with a SCI above T6 received FES to the quadriceps muscle of each leg under two conditions on two different testing days. The two treatment conditions, topical anaesthetic and placebo creams, were double blinded and randomized. The cream was administered to an area the size of the electrode (10 x 10 cm) 1 h prior to stimulation. Stimulation began at 0 mAmps and increased by 16 mAmps every 2 min until an intensity of 160 mAmps was achieved. HR and BP were measured at each stimulation intensity level. Catecholamines were analyzed three times during the stimulation protocol (pre, mid and post stimulation intensities). At the end of the stimulation protocol, FES induced isometric quadriceps contraction force at 160 mAmps intensity was measured using a hand held dynamometer. As FES stimulation intensity increased, significant rises in systolic and diastolic BP were seen, with a concomitant progressive drop in HR. The AD response to stimulation was not significantly different between the topical anaesthetic and placebo conditions. Serum catecholamine (epinephrine and norepinephrine) levels tended to rise with increasing FES intensity levels but did not reach statistical significance. The two treatment conditions did not significantly affect serum catecholamine levels or FES-induced quadriceps contraction force. In summary, FES application to the quadriceps muscle in high level SCI subjects resulted in significant increases in BP, decreases in HR (AD-like response), a trend towards elevations in catecholamine levels, and no difference in quadriceps muscular strength. However, these responses were unaffected by the use of topical anaesthetic cream on the skin at the stimulation site. This suggests that other mechanisms than skin nociception are operative in FES-induced AD.