Effect of Local Ischemia upon Human Nerve Fibers in vivo. ∗

Abstract

In each experiment ischemia was induced in the forearm and hand by inflating to a pressure of about 160 mm. Hg. a Riva-Rocci sphygmomanometer applied above the elbow. The median, ulnar or radial nerve was stimulated at the wrist by an alternating current of 160 cycles per second, using the generator and electrode described by Thompson et al. 1 ; the minimal current (expressed as ma X v) evoking sensation was recorded as an indicator of the threshold for stimulation of sensory fibers. A threshold reading was taken every 2 minutes after inflation of the sphygmomanometer, for about 16 minutes.

Thirteen such experiments were performed, involving 4 subjects. The results are exemplified in the following experiment, wherein the right radial nerve was stimulated; the first figure of each pair denotes the number of minutes after the induction of ischemia, the second figure is the threshold at that time: 0, 5.5; 2, 7.2; 4, 5.2; 6, 2.8; 8, 2.4; 10, 1.1; 12, 0.8; 14, 1.6; 16, 11.5. In every experiment there was, on the whole, a rapid and progressive lowering of the threshold for about the first 12 minutes. The average fall in threshold per unit of time significantly exceeded the corresponding value in each of 4 groups of control experiments; the most important group consisted of 14 experiments wherein nerves were similarly stimulated proximal to a sphygmomanometer arresting the circulation. In 5 of the 13 instances the threshold drop progressed throughout the duration of the experiment; but on 8 occasions (though in no control experiment) there was a terminal rise (exemplified above) to values equalling or exceeding the initial figure. We infer that the ischemia of the nerve first increased and later diminished the irritability of those fibers first stimulated by a current increasing from zero.

These results confirm the opinion of Lewis, Pickering and Roths-child 2 that such ischemia affects the nerve fibers, not merely the nerve endings.