Effects of topical benzocaine and lignocaine on upper airway reflex sensitivity

We studied the degree and duration of effect on upper airway reflex sensitivity of oral benzocaine lozenges, nebulised lignocaine and lignocaine sprayed onto the vocal cords under direct vision, using low concentrations of ammonia as a stimulus to upper airway receptors. Ten minutes after the administration of oral benzocaine 20mg the threshold response of the upper airway to ammonia (NH(3)TR) had risen significantly from baseline mean (SEM) of 680(95) to 975(109)ppm of ammonia with a return to baseline values after 25min (n=8, p<0.05, repeated measures of ANOVA; p<0.001, t-test). A direct spray of lignocaine 100mg onto the vocal cords resulted in a significant elevation in NH3 from a baseline mean (SEM) of 665(81) to a maximum of 1600(88)ppm of ammonia with a significant elevation in the threshold persisting for 100min (n=7, p<0.001, repeated measures of ANOVA; p<0.05, t-test). The application of 4% nebulised lignocaine 4ml significantly increased NH(3)TR from a baseline mean (SEM) of 770(56) to a maximum of 1190(63)ppm of ammonia with a significant elevatin in the threshold persisting for 30min (n=8, p<0.001, repeated measures of ANOVA; p<0.05, t-test) The maximum elevations in NH(3)TR with the two methods of lignocaine delivery were significantly different (p<0.01, 2-way ANOVA).