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.