Differential effects of alfentanil, fentanyl, pethidine and lignocaine administered intrathecally on nociceptive responses evoked by low and high frequency stimulation of somatic nerves.

We have studied in anaesthetized dogs the effects of alfentanil, fentanyl, pethidine and lignocaine administered intrathecally on nociceptive responses evoked by low and high frequency supramaximal electrical stimulation of the tibial and radial nerves. Doses were selected to abolish both A delta and C fibre somatosympathetic reflexes to single stimuli. Pethidine and lignocaine eliminated reflex pressor and heart rate responses to repeated single stimuli and almost completely abolished responses to train stimulation. The pressor response to single stimuli was abolished by fentanyl and reduced by alfentanil, but these drugs did not reduce significantly this response to train stimulation, suggesting a stimulation rate-dependent effect. Of the opioids, only pethidine had an effect comparable to that of lignocaine. The absence of sympathetic block and antagonism by naloxone imply a lack of significant local anaesthetic effect. We suggest that the greater analgesic efficacy of pethidine is a result of endogenous synergism between a minor local anaesthetic and a major opioid effect.