EFFECT OF APPARATUS ON FUNCTIONAL RESIDUAL CAPACITY

As the route of breathing and use of airway apparatus such as mask, mo uthpiece and noseclip can alter breathing pattern, this study has used the helium dilution method to estimate the effects of mouthpiece and mask breathing on functional residual capacity (FRC) in the supine pos ition, and the change in FRC that occurs between the sitting and supin e positions while breathing by mouthpiece. In 13 normal subjects, brea thing by mouthpiece, FRC was smaller, by a median of 1.07 litre (inter quartile values 0.73-1.43 litre) in the supine compared with the sitti ng position (P < 0.01), but residual volume (RV) did not change signif icantly. FRC measured in the supine position was significantly greater when breathing by mask than by mouthpiece (0.25, 0.04-0.38 litre) and RV was greater by similar amounts (0.20, -0.02 to 0.49 litre). This d ifference may result from increased inspiratory activity while breathi ng via the mask.