A NEW TECHNIQUE TO GENERATE AND ASSESS FORCED EXPIRATION FROM RAISED LUNG-VOLUME IN INFANTS

We have developed a new technique that allows assessment of infant lun g function over an extended volume range. The lungs are rapidly inflat ed to a predetermined inflation pressure (PP) using a modified diaphra gm pump. Forced expiratory flow-volume (FEFV) curves are then generate d from raised lung volumes using an inflatable plastic jacket. We stud ied 26 normal infants with a median age of 14 mo (range, 3 to 23 mo). FEFV curves were obtained in each infant from end-tidal inspiration an d from lung volumes set by a range of PP (15 to 20 cm H2O). Mean (SE) volume above FRC was 107 ml (9 ml), and mean forced expiratory time wa s 0.73 s (0.05 s) at end-tidal inspiration. Both measurements increase d progressively with increases in PP to 251 ml (13 ml) and 1.04 s (0.0 6 s), respectively, at 20 cm H2O PP (p < 0.0001). Mean intrasubject co efficient of variation was 15.5% (95% confidence interval, 12 to 19%) for maximal flow at FRC, but it was less than 6% (95% CI, 4 to 8%) for forced expiratory volume-time (FEV(t)) measurements at all levels of PP. Twenty-seven recurrently wheezy infants with a median age of 13 mo (range, 6 to 18 mo) were subsequently studied using a PP of 17.5 cm H 2O. Wheezy infants had a lower VmaxFRC [mean (1.39 ml/s/cm) and 95% CI (1.15 to 1.63 ml/s/cm)] than did normal infants (1.78, ml/s/cm; CI, 1 .51 to 2.05) (p < 0.05). FEV(1) measurements were all lower in wheezy infants than in normals infants: mean FEV(0.5), 1.86 ml/cm (CI, 1.73 t o 1.98) and 2.31 ml/cm (CI, 2.15 to 2.48), respectively (p < 0.0001); FEV(0.75), 2.20 ml/cm (CI, 2.07 to 2.32) and 2.72 ml/cm (CI, 2.52 to 2 .91), respectively (p < 0.0001); FEV(1.0), 2.42 ml/cm (CI, 2.26 to 2.5 8) and 2.84 ml/cm (CI, 2.63 to 3.06), respectively (p < 0.005). The CI values of each FEV(t) measurement did not overlap between the wheezy and normal groups; however, the CI values of VmaxFRC overlapped marked ly. in addition, FEV(1) parameters showed greater sensitivity in detec ting reduced lung function (71 to 89%) than did VmaxFRC parameters (56 %). We conclude that (1) FEV(t) measurements derived from a lung volum e set by a standardized pressure are more reproducible than flow measu rements in the tidal volume range; (2) FEV(t) measurements are signifi cantly lower in wheezy infants than in normal infants, show less overl ap than flow measurements in the tidal volume range, and therefore are better able to separate the two populations.