Factors influencing the measurement of conduction velocity in the human peripheral nerve

Abstract

Conduction velocity in the ulnar nerve was determined in normal people and in patients with peripheral neuropathy. The nerve was stimulated percutaneously by square pulses at the wrist and recorded by surface electrodes above the elbow. Conduction velocities were consistently reduced in a group of patients with neuropathy. The mean velocity was 63.9 m/sec in normal subjects and 45.9 m/sec in patients with nerve lesions. The difference of these means was highly significant ( P <0.001 by t test). From the velocity histogram it may be concluded that a slowing of conduction appeared in some of the population of formerly rapidly-conducting fibres as a result of the peripheral nerve lesion; this explanation seemed more likely than a complete block of conduction in the fastest fibres with simultaneous sparing of more slowly conducting fibres. Different latency measurements were compared. The following latencies were measured on the records: L 1: latencies from the start of the stimulus artefact to the beginning of the action potential; L 2: to the peak of the initial positive phase; L 3: to the point at which the negative deflection crosses the baseline; L 4: to the peak of the negative phase of the triphasic nerve response. They were expressed in relation to the conduction distance measured from the stimulating cathode to the distal recording electrode. According to the velocity histogram only conduction velocities which were calculated from latencies to the peak of the initial positive deflection, i.e. to the onset of the negative potential deflection (as defined by Gilliatt et al. 1965) represented true maximal conduction velocity in relation to the above-mentioned distance. Similarly, the latency graphs, obtained by plot of decreasing conduction distance against corresponding latency, showed that latencies to the onset of the negative deflection were the most accurately proportional to estimated conduction distance and corresponded to the onset of active depolarization of some of the fastest-conducting fibres below the lower recording electrode. The junction of the negative deflection and the baseline used as point of reference occurred about 0.4 msec later than the initial positive peak and denoted that the depolarization process under the lower recording electrode became predominant.