[Conduction block in man is stimulation frequency dependent]

The action of local anaesthetics on isolated nerves is enhanced by high stimulation frequencies. The aim of our study was to investigate whether high-frequency stimulation enhances regional anaesthesia in man. METHODS. Seven healthy volunteers underwent three ulnar nerve blocks while non-noxious electrical stimulation with high (10, 50 Hz) or low (0.2 Hz) frequencies was applied via a widespread cutaneous electrode in the area supplied by the ulnar nerve. Perception was monitored continuously by means of a visual analogue scale. Skin temperature (infrared telethermometry) was evaluated every minute as an indicator of vasomotor block. After complete loss of perception or after 45 min, the spread of anaesthesia was determined by sharp-dull discrimination. The nerve block was preceded by a control stimulation of 45 min under otherwise identical conditions. RESULTS. During stimulation with 10 and 50 Hz, perception was lost significantly earlier than with 0.2 Hz. The spread of sensory block at the end of the experiments was also enhanced by stimulation with high frequencies, whereas the onset of vasomotor block (rise in skin temperature) remained unaltered. CONCLUSION. Non-oxious electrical stimulation with high frequencies significantly accelerates the onset of anaesthesia and extends the spread of sensory block.