Effects of elevation on nerve function in an acute upper extremity nerve compression model.

External compression was applied to the palmar surface of the wrist over the carpal tunnel in eight healthy volunteer subjects. With the arm in the maximum elevated position, the carpal tunnel pressure was elevated to 50 mm Hg, being continuously monitored by a slit catheter inserted into the carpal canal. Sensory and motor latencies and amplitudes were evaluated at 1-min intervals. When the sensory amplitude decreased by 50% (correlating with subjective sensory changes), the hand was lowered to heart level, still maintaining the carpal tunnel pressure at 50 mm Hg. Nerve conduction velocity and amplitude monitoring continued at 30-s intervals until complete sensory block. Four subjects demonstrated a transient reversal in the sensory amplitude decline with lowering of the hand to heart level, but soon progressed to a complete sensory block. The remaining four subjects demonstrated no change in the sensory amplitude decline with lowering of the hand to heart level. Our results suggest that with moderately elevated carpal tunnel pressures, once the sensory amplitude drops by 50% and the patient starts experiencing subjective sensory changes, lowering the hand to heart level may not change the local tissue blood flow sufficiently to maintain a sustained reversal in the declining nerve function and impending nerve damage.