Maturation of respiratory control in unanesthetized newborn rabbits

The maturation of control of breathing was studied in unanesthetized rabbit pups breathing room air in a body plethysmograph during the first 8 postnatal days. Measurements include pulmonary ventilation (VE), tidal volume (VT), inspiratory (TI) and total breathing cycle (Ttot) durations, TI/Ttot, mean inspiratory flow (VT/TI), and tracheal pressure developed by the inspiratory muscles 0.1 and 0.2 s after the onset of inspirations with airways occluded at functional residual capacity (P0.1 and P0.2). All of the above variables increased progressively from the 1st to the 8th day, except for P0.1, P0.2, and TI/Ttot which remained constant. The constancy of TI/Ttot implies that the increase in VE with age was due entirely to increased VT/TI. The constancy of P0.1 and P0.2 implies that the increase in VT/TI with age was due to decreased "effective" impedance of the respiratory system. The latter probably mainly reflects increased compliance and decreased flow resistance with growth. The results also show that during the first 8 days of life there is a progressive shift to the right in the VT vs. TI relationship.