Influence of driving pressure on raised-volume forced expiration in infants

The raised-volume forced-expiration technique measures infant lung function over an extended volume range. To improve comparisons between individuals and populations, we investigated the influence of jacket pressure on outcome variables in 21 infants. To quantify pressure transmitted from the jacket to the pleural space at a given lung volume, the jacket was inflated against an occluded airway, and the increase in pressure at the mouth was measured. Flow-volume curves were recorded at transmitted pressure (Ptrans) values ranging from 0 to 41.9 cm H20. The effect of Ptrans on the FEV measures of FEV0.5, FEV0.75, and FVC, and on the forced expiratory flow measures of FEF25%, FEF50% and FEF75% was assessed. At Ptrans values between 0 to 20 cm H20, a significant positive relationship existed between transmitted pressure (Ptrans) and all outcome variables except FVC. At higher Ptrans values, all outcome variables demonstrated pressure independence, with the exception of FEF25% (which remained positive) and FVC (which was negative in a subgroup of wheezy infants). FEF75% values tended to decrease at Ptrans values > 25 cm H20. At Ptrans values between 20 and 25 cm H20, most outcome variables are pressure independent. This range is therefore the most suitable for use with the raised-volume forced expiration technique.