ANALGESIC ACTION OF INTRAVENOUSLY 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, i.v., produced dose-related suppressions activity and responses of these neurons to noxious thermal stimulation. Spontaneous discharge frequencies at maximum suppression, observed 3-7 after administration of each of the three doses of lidocaine were 64 ± 14 (mean ± 1 SE), 32 ± 8, and 25 ± 9% of control values, respectively; responses to noxious thermal stimuli were 83 ± 5, 52 ± 8, and 39 ± 7% 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<0.05), to 47.8 ± 0.8 C with lidocaine, 10 mg/kg (P< 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 μg/ml. These data support the clinical impression that intravenously administered lidocaine produces analgesia at plasma concentrations of 3-10μg/ml. It is suggested that lidocaine may block conduction of nociceptive impulses, at least in part, by suppression of spinal-cord nociceptive neurons.