Mechanism of improvement in pulmonary gas exchange during growth in awake piglets.

Previous studies have shown a lower arterial PO2 (PaO2) in infants and young animals than in adults. To investigate the mechanism of this impairment of gas exchange we studied 13 piglets from 12 to 65 days of age. Two days after instrumentation we measured the distribution of ventilation-perfusion ratios (VA/Q) by use of the multiple inert gas technique on awake animals. We showed that PaO2 is lower in young animals, increasing from 72 +/- 11.5 Torr before 2 wk to 102 Torr at 2 mo. This hypoxemia is due to an enlarged alveolar-arterial O2 pressure difference that significantly decreases with age. This impairment in gas exchange is not due to shunting (0.6 +/- 1.3%). Mean dead space (36 +/- 11%) was not related to age. Mean modes of perfusion and ventilation did not differ significantly between age groups. However, the dispersion of perfusion as expressed by its logSD decreased significantly with age, whereas dispersion of ventilation remained constant. Furthermore, in the young animals only, a significant difference was evidenced between measured alveolar-arterial PO2 gradient and the value predicted by the inert gas model. We therefore conclude that the impairment of gas exchange in piglets is due to two mechanisms: VA/Q mismatch and diffusion limitation for O2.