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 of lignocaine 1.5 mg kg-1 i.v. on respiratory responses to airway irritation induced by instillation of distilled water into the trachea in 10 patients anaesthetized with enflurane (1.5% end-tidal). Before administration of lignocaine, airway irritation elicited not only the cough reflex, but also other respiratory reflexes such as expiration, apnoea and spasmodic panting. Immediately after administration of i.v. lignocaine, when plasma concentrations of lignocaine exceeded 4.7 micrograms ml-1, tracheal irritation elicited only brief apnoea. Other reflex responses were suppressed completely; they recovered gradually with progressive decrease in plasma concentration of lignocaine. The apnoeic reflex was not eliminated at plasma lignocaine concentrations greater than 7.0 micrograms ml-1, whereas the expiration reflex, cough reflex and spasmodic panting were eliminated effectively by plasma concentrations of lignocaine greater than 3.5, 2.8 and 2.2 micrograms ml-1, respectively.