Minute ventilation, Vt and respiratory frequency were measured during an average of at least 20 breaths during quiet breathing using an oronasal mask. The door of the body plethysmograph was closed and the infant breathed air from the box. When thermal equilibrium had been achieved, the pneumatic sleeves were sealed. An infant oronasal mask connected to a rebreathing bag (inspired air at body temperature and pressure saturated) and a heated pneumotachograph with a shutter was sealed over the infants nose and mouth. The mask was checked for leakage both visually and by appearance of the oscilloscope tracings.
All measurements were corrected for apparatus dead space (46 ml) and resistance (1.8 cm H20/L/s). Thoracic gas volume at FRC and Raw were measured by the method of DuBois et al,* suitably modified for infants. The maximal intrasubject coefficient of variation for Raw in our laboratory is 6.7 ±1.7 percent. Specific airway conductance was calculated as a reciprocal of Raw divided by TGV The maximal intrasubject coefficient of variation for SGaw in our laboratory is 6.2 ±9.2 percent. After conclusion of plethysmographic measurements, the infant was repositioned in a right lateral position.