We studied the degree and duration of effect on upper airway reflex se
nsitivity of oral benzocaine lozenges, nebulised lignocaine and lignoc
aine sprayed onto the vocal cords under direct vision, using low conce
ntrations of ammonia as a stimulus to upper airway receptors. Ten minu
tes after the administration of oral benzocaine 20mg the threshold res
ponse 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 th
e vocal cords resulted in a significant elevation in NH3 from a baseli
ne 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 ba
seline mean (SEM) of 770(56) to a maximum of 1190(63)ppm of ammonia wi
th 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 eleva
tions in NH(3)TR with the two methods of lignocaine delivery were sign
ificantly different (p<0.01, 2-way ANOVA).