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An analgesic action of intranvenously administered lidocaine on dorsal-horn neurons responding to noxious thermal stimulation.

Using extracellular single-unit recording techniques, effects of intravenously administered lidocaine on dorsal-horn nociceptive neurons were studied in cats made decerebrate whose spinal cords had been transected. Thirty-seven neurons in Rexed lamina V responding to high-threshold mechanical and noxious thermal stimuli (radiant heat, using Hardy-Wolff-Goodell dolorimeter) were studied. Lidocaine hydrochloride, 2.5, 5, and 10 mg/kg, iv, produced dose-related suppression of both spontaneous activity and responses of these neurons to noxious thermal stimulation. Spontaneous discharge frequencies at maximum suppression, observed 3--7 min after administration of each of the three doses of lidocaine were 64 +/- 14 (mean +/- 1 SE), 32 +/- 8, and 25 +/- 9 per cent of control values, respectively; responses to noxious thermal stimuli were 83 +/- 5, 52 +/- 8, and 39 +/- 7 per cent of the control values, respectively. Threshold skin temperature to noxious thermal stimulation increased from 44.7 +/- 0.4 C (control) to 46.3 +/- 0.7 C with lidocaine, 5 mg/kg (P less than 0.05), to 47.8 +/- 0.8 C with lidocaine, 10 mg/kg (P less than 0.01). The times necessary for recovery varied in a dose-related fashion. Plasma lidocaine concentrations 5 min after lidocaine, 5 mg/kg, averaged 3.6 +/- 0.7 microgram/ml. These data support the clinical impression that intravenously administered lidocaine produces analgesia at plasma concentrations of 3--10 microgram/ml. It is suggested that lidocaine may block conduction of nociceptive impulses, at least in part, by suppression of spinal-cord nociceptive neurons.