Partial expiratory flow-volume curves in infancy: technical aspects.

We have constructed a simple pressure-jacket with which to produce passive forced expiration in sleeping, supine infants by thoraco-abdominal compression. During expiration, flow and volume were measured at the airway opening with a face-mask and pneumotachograph. From partial expiratory flow-volume (PEFV) curves, peak expiratory flow rate (PEFR) and maximum expiratory flow at a lung volume equal to the functional residual capacity (VmaxFRC) were obtained. Using the jacket in groups of normal and wheezy infants, we have assessed the effects on PEFR and VmaxFRC of variable inflation pressure, rate and duration of jacket inflation, timing of chest compression in relation to the breathing cycle and subject interaction with the compression manoeuvre. The within-subject reproducibility of PEFR and VmaxFRC was measured. Consistent values were produced by inflating the jacket within 100 ms at end inspiration to a pressure of 3-4 kPa and maintaining the inflation (in wheezy infants) for at least 1 s. The median within-subject coefficients of variation of PEFR and VmaxFRC for both normal infants and wheezy infants were 9 and 12% respectively. The technique is clearly reproducible and can provide information about intrathoracic airway function in infancy.