EVALUATION OF A TIDAL EXPIRATORY FLOW INDEX IN HEALTHY AND DISEASED INFANTS

Patterns of tidal respiratory flow have been shown to relate well to airway function in adults, and one epidemiological study in infants has demonstrated the value of the ratio of time to reach peak tidal expiratory flow to the total expiratory time (t(pef)/t(e)) in predicting subsequent wheezing. The aim of this study was to evaluate t(pef)/t(e) as a measure of lung function, by sequential observations over the first year, on a group of 22 healthy infants and on 32 infants with a history of mild recurrent lower respiratory illness (LRI), and by single observations on 20 infants with asthma and 20 with severe chronic lung disease of prematurity. We compared t(pef)/t(e) measured in quiet, supine sleep (under sedation) through a face mask and pneumotachograph, with a measure of airway function, maximal flow at functional residual capacity (V-maxFRC), obtained from partial forced expiratory flow volume loops using the ''squeeze'' technique. In healthy infants t(pef)/t(e) was significantly longer at 1 month than at 6 months (median values, 0.38 (95% CI, 0.36-0.43) and 0.28 (95% CI, 0.26-0.33), respectively). Between 6 and 12 months t(pef)/t(e) did not alter significantly and it was independent of V-maxFRC. Both t(pef) and t(e) as well as their ratio varied with frequency of breathing over the first year of life, but not within each individual age band, due to the narrow spread of frequencies at each age. In assessing airway obstruction, t(pef)/t(e) was less sensitive than V-maxFRC. There was no difference between healthy infants, those with LRI, and infants with asthma. Values outside the 95% CI for our control group of healthy infants were only seen in the group of infants with severe chronic lung disease of prematurity (median value, 0.16; 95% CI, 0.12-0.22), most of whom demonstrated expiratory flow limitation during tidal breathing. We found the tidal breathing index t(pef)/t(e) to be an insensitive measure of airway function in infants, compared with V-maxFRC. (C) 1994 Wiley-Liss, Inc.