RELATIONSHIP BETWEEN AN INDEX OF TIDAL FLOW AND LOWER RESPIRATORY ILLNESS IN THE FIRST YEAR OF LIFE

The ratio of time to tidal peak flow (T-me) to total tidal expiratory time (T-e) has been reported to be decreased in infants who later deve lop wheezing lower respiratory tract illness (LRI) in the first year o f life. The relationship between T-me/T-e to the subsequent occurrence of LRI was studied in 98 infants in whom the first measurement of pul monary function (PFT) was made before the age of 6 months and before t he occurrence of any LRI. Occurrence of LRI was evaluated by standardi zed questionnaires at well-baby visits, through biweekly telephone cal ls to mothers, and review of all visits to physicians. T-me/T-e was de rived from 10 tidal breathing loops during stable respiration. Partial expiratory flow-volume curves were obtained with the rapid compressio n technique, and passive respiratory mechanics were evaluated by the s ingle breath occlusion technique. Analysis of T-me/T-e was stratified by age (less than or equal to 10 weeks, > 10 weeks to 6 months) to tak e into account the age-related decline in T-me/T-e. Among 80 infants f irst tested at less than or equal to 10 weeks, T-me/T-e was 12.4% shor ter in those who developed a LRI vs. those who did not (P = 0.46); for 18 infants tested after 10 weeks, the difference was 1.9% (P = 0.39). Among male infants, the decrease in T-me/T-e was observed only for th ose studied at less than or equal to 10 weeks (16%, P = 0.16). For fem ales, decreases were observed for those tested at less than or equal t o 10 weeks (11%, P = 0.83) and those tested after 10 weeks (17.5%, P = 0.09). Poisson regression analysis which included data for multiple m easurements of T-me/T-e over the first year of life and adjusted for a ge-at-test and maternal smoking during pregnancy also demonstrated a g reater decrease in T-me/T-e in female infants who subsequently develop an LRI (P = 0.08). Level of T-me/T-e was not consistently related to level of respiratory system resistance (R(RS)) or flow at functional r esidual capacity (V-FRC). Level of V-FRC has been shown previously to be related to the occurrence of LRI and in this study to R(RS) (P = 0. 007). The results indicate (1) a shortened T-ma/T-a is only weakly ass ociated with the development of LRI in the first year of life; (2) thi s ratio is a less precise and an epidemiologically less useful measure than is V-FRC to investigate groups of infants with and without LRI a nd without clinically significant underlying lung disease. (C) 1995 Wi ley-Liss, Inc.