Measurement of Lung Function
All iPFTs were performed using the Jaeger BabyBody device (CareFusion, San Diego, California) equipment. Infants were sedated with 50-100 mg/kg oral chloral hydrate and cardiopulmonary monitoring was performed throughout testing. FRC was measured using plethysmography24. Forced expiratory flows were obtained using the RVRTC technique. All lung function parameters were obtained according to published guidelines 24,25. Infant PFT values were converted to z-scores based on previously published reference equations26,27. Respiratory rate data were obtained from tidal breathing analysis performed during iPFT testing for a minimum period of 20-30 breaths 27 or from the vital signs flowchart. Only iPFT data that met published quality control criteria were used for analysis 24,25.