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 develop wheezing lower respiratory tract illness (LRI) in the first year of 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 pulmonary function (PFT) was made before the age of 6 months and before the occurrence of any LRI. Occurrence of LRI was evaluated by standardized questionnaires at well-baby visits, through biweekly telephone calls to mothers, and review of all visits to physicians. T-me/T-e was derived from 10 tidal breathing loops during stable respiration. Partial expiratory flow-volume curves were obtained with the rapid compression technique, and passive respiratory mechanics were evaluated by the single 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 take into account the age-related decline in T-me/T-e. Among 80 infants first tested at less than or equal to 10 weeks, T-me/T-e was 12.4% shorter 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 those studied at less than or equal to 10 weeks (16%, P = 0.16). For females, decreases were observed for those tested at less than or equal to 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 measurements of T-me/T-e over the first year of life and adjusted for age-at-test and maternal smoking during pregnancy also demonstrated a greater 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 residual 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 associated with the development of LRI in the first year of life; (2) this ratio is a less precise and an epidemiologically less useful measure than is V-FRC to investigate groups of infants with and without LRI and without clinically significant underlying lung disease. (C) 1995 Wiley-Liss, Inc.