Effects of airway anaesthesia on the ability to detect added inspiratory resistive loads.

1. The effects of airway anaesthesia on the ability to detect added inspiratory resistive loads were studied in normal subjects. A 4% solution of lignocaine hydrochloride was used for anaesthesia of the airways. 2. After anaesthesia of the mouth and upper airways to the level of the vocal cords there was a significant deterioration in the detection ability expressed in terms of the absolute added resistance (deltaR), with a concomitant increase in pulmonary resistance (Rint.). However, there was no significant change in the detection ability expressed in terms of the ratio of deltaR to the sum of Rint. and the minimal resistance of the apparatus (deltaR/Ro). 3. After combined anaesthesia of the upper and lower airways there was no significant change in pulmonary resistance or in the detection ability expressed either as deltaR or as deltaR/Ro. 4. We conclude that, in normal subjects, the main site of detection of added inspiratory resistive loads does not lie in the upper or lower airways. Our results and those of previous studies suggest that the diaphragm is the most likely site of detection of added resistive loads.