Forced oscillation technique (FOT) is a non-invasive means of measuring resistance and reactance. It can be made using a mouthpiece or with a mask placed over mouth or nose. The development of commercially-available equipment has revived interest in FOT and, since measurements vary according to equipment used, it is important to re-evaluate the impact of the connection to the airway.
To measure resistance at 5Hz (R5), frequency dependence of resistance (R5-20) and the Reactance area (Ax) in adults breathing through a mouthpiece or mask.
We studied 10 subjects using the TremoFlo (Thorasys Inc) under 3 settings: 1) a mouthpiece plus nose clip (mouthpiece); 2) a face mask plus nose clip, imposing mouth breathing (mouth mask) and 3) a face mask allowing only nasal breathing (nasal mask). At least 3 measurements were made in each setting.
Comparing the mouthpiece to the mouth mask, there were no significant differences in any parameter (Table). Use of nasal mask led to a significant increase in R5 Hz compared with mouthpiece or mouth mask (p < 0.001 for both) and Ax (p < 0.01 for both). There was a non-significant trend for an increase in R5-20 when measured with a nasal mask
Using a mask instead of a mouthpiece did not result in significant changes when positioned over the mouth. Placing the mask over the nose resulted in increased resistance and reactance. This is important when considering measurements in subjects who cannot co-operate.