SELECTIVE DEPRESSION BY ALFENTANIL OF GROUP III AND IV SOMATOSYMPATHETIC REFLEXES IN THE DOG

The effect of alfentanil on sympathetic reflexes evoked by supramaximalelectrical stimulation of the radial nerve has been observedin five dogs anaesthetized with -chloralose, paralysed withsuxamethonium and artificially ventilated. In five dogs duringthe infusion of alfentanil at a rate of 20 µg kg^–1min^–1 the late long latency sympathetic response evokedby unmyelinated fibres (group IV, C) was abolished at a meandose of 68.8 (SE 2.85) µg kg^–1. The infusion ratewas then increased to 200 µg kg^–1 min^–1 andthe early, short latency response evoked by small myelinatedfibres (group III, A) was eliminated at mean total dose of 809(SE 72) µg kg^–1. When the infusion was stopped thegroup III reflex returned within 1–5 min and recoveryto approximately 50% of control for both reflexes occurred within15–60 min in different preparations. Mean arterial pressureand mean heart rate decreased from 140 (6) mm Hg and 132 (11)beat min^–1 to 100 (7) mm Hg and 70 (6) beat min^–1,respectively, by the time the group IV response was eliminated;that is, after a mean infusion time of 3.4 min. Thereafter,there was no further cardiovascular depression. Within 3 minof the administration of naloxone 2 mg i.v., the sympatheticreflexes, arterial pressure and heart rate returned to withincontrol values.