Diaphragmatic function during hypercapnia: neonatal and developmental aspects

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Abstract
The effect of acute hypercapnia on diaphragmatic force output was studied in 6 young (4-8 days) and 6 older (16-20 days) anesthetized, spontaneously breathing piglets. Diaphragmatic force output was assessed by analysis of the transdiaphragmatic pressure (Pdi) generated during phrenic nerve stimulation. Pdi was measured under base-line conditions (50% O2-50% N2) and after 10 min of hypercapnia induced by breathing 5, 10, or 15% CO2 balanced with N2 and 50% O2. Pdi was significantly less than base line during the 10 and 15% hypercapnic conditions in the young (P < 0.05) but not the older piglets. End-expiratory lung volume was noted to decrease during 15% CO2 hypercapnia. Force output augmentation occurred at this lower end-expiratory lung volume and was significantly greater in the older piglet compared with its younger counterpart (P < 0.05). When the effects of lung volume on Pdi were corrected for, there was no age-related difference in the response to 15% CO2 hypercapnia. We conclude that 1) severe hypercapnia has a depressant effect on diaphragmatic force output in both young and older piglets, and 2) a differential augmentation in diaphragmatic force-output gain occurs at lower end-expiratory lung volume between young and older piglets, with the greater output occurring in the more mature animal.