Effect of apparatus on functional residual capacity

As the route of breathing and use of airway apparatus such as mask,mouthpiece and noseclip can alter breathing pattern, this study has usedthe helium dilution method to estimate the effects of mouthpiece and maskbreathing on functional residual capacity (FRC) in the supine position, andthe change in FRC that occurs between the sitting and supine positionswhile breathing by mouthpiece. In 13 normal subjects, breathing bymouthpiece, FRC was smaller, by a median of 1.07 litre (interquartilevalues 0.73-1.43 litre) in the supine compared with the sitting position (P< 0.01), but residual volume (RV) did not change significantly. FRCmeasured in the supine position was significantly greater when breathing bymask than by mouthpiece (0.25, 0.04-0.38 litre) and RV was greater bysimilar amounts (0.20, -0.02 to 0.49 litre). This difference may resultfrom increased inspiratory activity while breathing via the mask.