Exploring the relationship between forced maximal flow at functional residual capacity and parameters of forced expiration from raised lung volume in healthy infants

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Abstract
The raised volume rapid thoraco‐abdominal compression technique (RVRTC) is being increasingly used to assess airway function in infants, but as yet no consensus exists regarding the equipment, methods, or analysis of recorded data. The aim of this study was to explore the relationship between maximal flow at functional residual capacity (V′maxFRC) and parameters derived from raised lung volumes, and to address analytical aspects of the latter technique in an attempt to assist with future standardization initiatives. Forced vital capacity (FVC) from lung volume raised to 3 kPa, timed forced expiratory volumes (FEVt), and forced expiratory flow parameters at different percentages of expired FVC (FEF%) were measured in 98 healthy infants (1–69 weeks of age). V′maxFRC using the tidal rapid thoraco‐abdominal compression (RTC) technique was also measured. The within‐subject relationships and within‐subject variability of the various parameters were assessed.Duration of forced expiration was < 0.5 sec in 5 infants, meaning that FEV0.3 and FEV0.4 were the only timed volume parameters that could be calculated in all infants during the first months of life, and even when it could be calculated, FEV0.5 approached FVC in many of these infants. It is recommended that FEV0.4 be routinely reported in infants less than 3 months of age. Contrary to previous reports, within subject variability of V′maxFRC was less than that of FEF75 (mean CV = 6.3% and 8.9%, respectively).A more standardized protocol when analyzing data from the RVRTC would facilitate comparisons of results between centers in the future. Pediatr Pulmonol. 2002; 33:419–428.