Diaphragmatic function during hypoxemia: neonatal and developmental aspects.

The effect of acute hypoxemia on diaphragmatic force output was studied in five young (age 4-8 days, wt 1.3-2.2 kg) and five older (age 16-19 days, wt 2.8-3.3 kg), anesthetized, spontaneously breathing piglets. Diaphragmatic force output was assessed by analysis of the transdiaphragmatic pressure (Pdi) generated during an occluded inspiratory effort, at end-expiratory lung volume, triggered by supramaximal transvenous stimulation of both phrenic nerves at frequencies of 20, 30, 50, and 100 Hz. During pressure measurements, the piglets were fitted with a rigid plaster cast covering the abdomen and lower third of the chest to ensure a consistency in diaphragmatic shortening during phrenic nerve stimulation. Pdi was measured under base-line conditions [inspired O2 fractional concentration (FIO2) = 0.50] and after 10 min of hypoxemia induced by breathing 12-14% FIO2. Pdi was significantly less than base line during acute hypoxemia at all frequencies of stimulation in both young and older piglets. The decline in the older piglets' Pdi during hypoxemia was significantly greater than that seen in younger piglets. We conclude that acute hypoxemia impairs the capacity of the developing piglet diaphragm to generate force. Furthermore, our data suggest that the young piglet is more resistant to the depressant effects of hypoxemia when compared to its older counterpart.