Effects of airflow resistance variations on resistive load detection in normal subjects

The sensitivity to airflow resistance variations produced by externally added resistive loads, with modification of background load, was studied in 15 normal subjects, using "Sensory Decision Theory" (SDT), a psychophysical method that gives an index of accuracy, P(A), unaffected by response bias. In six subjects, asked to detect three increasingly added loads (delta R) at different levels of basal resistance (1.6 and 3.6 cmH2O X 1(-1) X s), inspiratory resistive load detection (RLD) improved with background loading, and a highly significant relationship was found between P(A) and peak mouth pressure (Pm). The relationship between P(A) and the change in Pm, i.e. delta Pm, was less significant. These results suggest that, although tension developed by the respiratory muscles reflected by mouth pressure may be an important stimulus to respiratory RLD, other factors must also be considered. In seven subjects, in whom airway resistance was increased at least 50% by inhalation of carbachol, inconsistent variations in P(A) were found. In seven subjects, in whom airflow resistance was decreased by inhalation of heliox, and in six of seven subjects after salbutamol inhalation, RLD was significantly impaired. The results of this study lead us to the conclusion that Weber's law is not verified for weak internal or external background loading.