Bilateral lidocaine blocks of glossopharyngeal and vagus nerves in the neck were made in two healthy subjects to achieve deafferentation of arterial and cardiopulmonary baroreceptors. Microelectrode recordings of muscle nerve sympathetic activity (MSA) were made in one peroneal nerve; in one subject skin sympathetic activity (SSA) was recorded simultaneously in the other peroneal nerve. Following the nerve block in the neck there was a strong increase of MSA accompanied by temporary hypertension and tachycardia. The normal cardiac rhythmicity of MSA disappeared and the outflow appeared as bursts of impulses of variable duration occurring in a slow, irregular rhythm. Thus MSA became similar to SSA, but the activities never became synchronous. During the nerve block arousal stimuli evoked single bursts of MSA, a reflex response which normally occurs in SSA but not in MSA. It is concluded that (1) cardiac rhythmicity of MSA is due to baroreceptor influence; (2) a low level of MSA at rest depends on strong baroreceptor inhibition and not on a weak central drive; (3) central sympathetic outflows to skin and muscle are similar though not identical and the different characteristics normally observed are due to a large extent to different modulatory influences from afferent activity (presumably of baroreceptor origin) in glossopharyngeal and vagus nerves; and (4) baroreceptor deafferentation resulting in resting tachycardia and hypertension may explain sympathetic hyperactivity in the Guillain-Barré syndrome.

Propofol has the disadvantage of pain on injection. A higher partition of propofol in the aqueous phase of the preparation causes a higher incidence of pain on injection while addition of 1% lignocaine to propofol reduces pain. The low concentration of this local anaesthetic and the rapid pain relief observed indicates that mechanisms other than local anaesthesia are involved, that is change in pH. We performed a clinical study to investigate the influence of lignocaine and pH on pain during injection of 1% Diprivan. Ten parts of 1% Diprivan were mixed with one part of saline, 1% lignocaine or hydrochloric acid to achieve the same pH as that after addition of lignocaine. Diprivan 1% mixed with 1% lignocaine and with hydrochloric acid gave mean pain ratings (1-10) of 0.32 (SD 0.75) (n = 25) and 0.88 (1.30) (n = 24), respectively. These ratings were significantly lower than ratings after injection of a saline-Diprivan mixture (2.18 (2.06), n = 22). The pH of the 1% Diprivan formulation decreased after mixing with 1% lignocaine. The concentration of propofol in the aqueous phase was lower when 1% Diprivan was mixed with 1% lignocaine (0.376 g litre-1) or HCl (0.392 g litre-1) compared with 1% Diprivan and saline (0.476 g litre-1) mixed in the same proportion. Thus pH changes may modify propofol-induced pain on injection by a mechanism different from the effect of the local anaesthetic on the vascular endothelium. Our findings may explain why lignocaine mixed with propofol causes less pain than injection of lignocaine followed by propofol.

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A 5% eutectic mixture of the two local anaesthetics lidocaine and prilocaine (EMLA, Astra Läkemedel AB) has been tested for application to the skin in the removal of split skin grafts. EMLA is an oil-in-water emulsion cream, containing 50 mg of active substances per ml (25 mg lidocaine, 25 mg prilocaine). The cream has been used on the donor sites of 146 patients and was applied a minimum of 1 h 30 min before surgery. 123 patients (84.3%) experienced adequate analgesia, feeling only the pressure of the dermatome or slight pain without objection to the skin graft being cut. 20 patients (13.7%) described the pain as being moderate, but the operation could be completed without further local or general anaesthesia. Three patients (2.0%) needed additional anaesthesia. Six patients complained of slight transient irritation immediately after application. After removal of the cream, erythema was present in 42 patients, pallor in 62 and oedema in 14. The concentrations of lidocaine and prilocaine in the blood were measured in 106 patients and did not exceed 1100 ng ml-1 for lidocaine and 200 ng ml-1 for prilocaine. There were no systemic side-effects.