Abstract
Intravenous regional anesthesia (IVRA) may be used as a diagnostic method in patients suffering from chronic pain in the forearm or hand to differentiate the origin of pain within the anesthetic area from that above. For this purpose it needs to be proven that all nerve fibers are blocked and that conduction blockade induced by IVRA takes place within the nerve trunks. Therefore the transmission of impulses in a nerve trunk to the central nervous system has been studied. Diagnostic intravenous regional anesthesia (5 mg/kg mepivacaine 1%) of the arm was performed in eight patients for 30 min. Short-latency somatosensory evoked potentials (SSEPs) induced by median nerve stimulation at the wrist were recorded from the scalp at 5-min intervals before, during, and after IVRA. In five patients, sensory nerve action potentials (SNAPs) of the median nerve, at the elbow and axilla, and SSEPs at spinous processus C-7 were recorded simultaneously. During IVRA the function of the nerve fibers which are assumed to mediate pain was tested by the patient's sensation following median nerve stimulation at an intensity which evoked pain before IVRA. During IVRA, peak latencies of the scalp recorded SSEPs (N20) increased progressively and interpeak amplitudes (N20/P25) decreased. After 20 min, SSEPs could no longer be recorded, and median nerve stimulation no longer evoked any sensation at all. After deflation of the cuff, both peak latency (N20) and interpeak amplitude (N20/P25) of SSEPs recovered. The changes in latency and amplitude of SSEPs from the scalp as well as SNAPs and SSEPs from the neck were similar. Because SSEPs, SNAPs, and the pain sensation following median nerve stimulation disappeared during IVRA, it may be concluded that the thick and thin myelinated nerve fibers of the median nerve have been blocked. For diagnostic use, IVRA is superior to peripheral nerve blockade, which has been shown previously to not abolish SSEPs.
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