Measuring tidal volume and functional residual capacity change in sleepinginfants using a volume displacement plethysmograph

The noninvasive measurement of infant lang function during unsedated sleep in infants has been a long-standing objective in paediatric respiratory med icine. This note reports on the design and performance of a head-out volume -displacement plethysmograph (VDP) that overcomes some of the limitations o f traditional lung function apparatus. The VDP comprises a rigid acrylic box with an integral water-sealed spirome ter and a novel neck seal, The bilayer neck seal is of variable compliance and is comfortable and simple to use. The spirometer permits volume resolut ion of 15 mi, and a dynamic range in excess of 100 mi,, The frequency respo nse extends from 0-7 Nz, Spirometer inertance was measured as 0.0015 kPa-L- 1-s(-2), resistance 0.021 kPa-L-1-s(-1) and box capacitance 0.18 L-kPa(-1). Tidal volume, respiratory rate and changes in functional residual capacity can be recorded during unsedated rapid eye movement and nonrapid eye moveme nt whilst monitoring with conventional polysomnographic methods. The head-o ut configuration allows additional instrumentation to be implemented with e ase, avoids facial stimulation and allows unimpeded access to the upper air way, A polysomnograph illustrating the limitations of respiratory inductanc e plethysmography signals and typical changes in functional residual capaci ty are shown.