EFFECTS OF I.V. LIGNOCAINE ON AIRWAY REFLEXES ELICITED BY IRRITATION OF THE TRACHEAL MUCOSA IN HUMANS ANAESTHETIZED WITH ENFLURANE

We have investigated the effects of bolus administration oflignocaine 1.5 mg kg^–1 i.v. on respiratory responses toairway irritation induced by instillation of distilled waterinto the trachea in 10 patients anaesthetized with enflurane(1.5% end-tidal). Before administration of lignocaine, airwayirritation elicited not only the cough reflex, but also otherrespiratory reflexes such as expiration, apnoea and spasmodicpanting. Immediately after administration of i.v. lignocaine,when plasma concentrations of lignocaine exceeded 4.7 µgml^–1, tracheal irritation elicited only brief apnoea.Other reflex responses were suppressed completely; they recoveredgradually with progressive decrease in plasma concentrationof lignocaine. The apnoeic reflex was not eliminated at plasmalignocaine concentrations greater than 7.0 ug ml^–1, whereasthe expiration reflex, cough reflex and spasmodic panting wereeliminated effectively by plasma concentrations of lignocainegreater than 3.5, 2.8 and 2.2 ug ml^–1, respectively.