The distribution of ventilation-perfusion ratios in the lungs of newborn foals

The distributions of ventilation-perfusion ratios, and the effects of 100% oxygen administration on the distributions, were studied in 3 foals from 4h to 9 days of age, using the multiple inert gas elimination technique. The distributions were calculated from the pulmonary clearance of 6 inert gases following infusion into a peripheral vein of a solution containing the inert gases. The results from a total of 8 studies showed several consistent features. The major findings were (i) the absence of low ventilation-perfusion ratios, i.e. regions where blood flow was greatly in excess of ventilation; (ii) the presence of variable right to left shunt; (iii) a reduction in this shunt with increasing post-natal age; (iv) the presence of a separate high mode of ventilation-perfusion ratios where ventilation was greatly in excess of blood flow and; (v) the observation that breathing enriched oxygen mixtures for 40 min did not increase the right to left shunt in any foal at any age studied. These studies indicate that hypoxaemia in the neonatal foal is attributable to right to left shunt which may be intrapulmonary or intracardiac, or both, rather than overperfusion of poorly ventilated lungs.