Sensory and Motor Responses to Acute Ischemia in Healthy Individuals

Abstract

1132 Decreased endoneurial blood flow has been implicated in nerve dysfunction in diseased individuals. The theory that diminished blood flow leads to nerve dysfunction is provocative, however, no causal relationship has been established. PURPOSE: To examine the effect of acute ischemia on peripheral nerve function in healthy subjects. It was hypothesized that acute ischemia would produce changes in the ability of sensory and motor nerves to propagate an impulse. METHODS: Twelve adults (23.6 ± 2.15 yrs) with no history of diabetes, peripheral, or central nerve disease or injury participated in the study. Surface electrodes were placed over the soleus and tibialis anterior muscles with electrical stimulation occurring through the skin on the tibial nerve behind the knee. The H-wave and M-wave recruitment curves were determined for each subject while in a prone position at rest and after 5 minutes of ischemia. Ischemia was induced by occluding the femoral artery just above the knee with a pneumatic cuff infiated to 50 mm Hg above systolic blood pressure. Ischemia was confirmed when tissue oxygen saturation levels dropped to zero percent as measured by a near infrared spectrometer. RESULTS: Hmax/Mmax ratios were significantly reduced with acute ischemia (mean ± SE) 66.29 ± 5.4 percent and 58.81 ± 6.7 percent respectively for rest and ischemia. This was due to a decrease in Hmax during acute ischemia with no change in Mmax. In addition, the stimulus intensity required to evoke a motor response significantly decreased with ischemia from 6.4 mA ± 0.04 to 5.2 ± 0.03 mA. The stimulus intensity required to evoke an H-reflex also significantly decreased from rest 5.3 mA ± 0.04 to 4.2 ± 0.03 mA. CONCLUSION: Acute ischemia increases both motor and sensory nerve excitability by decreasing motor and sensory thresholds in healthy individuals. Surprisingly, this increased excitability is accompanied by a decrease in H-reflex amplitude, suggesting that acute ischemia has differential effects on sensory nerve propagation and synapse transmission.