Changes in venous blood lactate, venous blood gases, and somatosensory evoked potentials after tourniquet application

Abstract

The effects of inflation of a 7-cm tourniquet applied to the upper arm of eight volunteers on venous lactate, venous blood gases, and ulnar nerve somatosensory evoked potentials (SSEPs) were investigated. The inflation pressure was 100 mmHg over the systolic pressure. Venous blood samples for lactate and blood gas determinations were withdrawn before tourniquet inflation; immediately and at 2, 5, 10, 15 min after tourniquet deflation; and additionally at 30, 45, and 60 min after deflation in the last four volunteers. SSEP stimulating surface electrodes were placed over the ulnar nerve at the wrist. Recording electrodes were placed over the ipsilateral ulnar groove of the elbow, Erb's point, and on the contralateral cortex. Averaged responses were acquired before inflation of the tourniquet, every 5-10 min during tourniquet inflation, and every 5-10 min for 45-60 min after tourniquet deflation. The tourniquet was inflated for 36 ± 11 min. After deflation of the tourniquet, postdeflation pain and paresthesias were felt by five volunteers; these occurred at 30-120 s after deflation and lasted for 75-120 s. The postdeflation pains were characterized as burning, cramping, paresthesias, buzzing, or severe expansion of the hand. The venous blood lactate levels were significantly elevated for 10 min, and the time course of its change did not correlate with reperfusion pain. The Po2 and O2Hb saturation in venous blood were significantly elevated for 10-15 min after deflation. The elevated lactate and Po2 levels in the presence of a restored blood flow probably result from continued anaerobic muscle metabolism secondary to capillary closure from the tourniquet-induced ischemia. The Erb's and cortical SSEPs were abolished after inflation of the tourniquet; time to 100% depression of the amplitude was 22 ± 6 min for the Erb's and 25 ± 6 min for the cortex. These changes probably resulted from nerve compression and ischemia. The percentage depression of the ulnar nerve SSEP at the elbow was 84 ± 16%, presumed to result solely from nerve ischemia. The SSEPs recovered after deflation of the tourniquet.