Abstract : ABSTRACT: INTRODUCTION: Protective ventilation using limited airway pressures and ventilation may result in moderate and prolonged hypercapnic acidosis, as often observed in critically ill patients. Because allow of moderate and prolonged hypercapnia may be considered as a protective measure for the lungs, we hypothesized that moderate and prolonged hypercapnic acidosis may protect diaphragm against ventilator induced diaphragmatic dysfunction (VIDD). The aim of our study was to evaluate the effects of moderate and prolonged (72-h of mechanical ventilation) hypercapnic acidosis on in-vivo diaphragmatic function. METHODS: Two groups of anesthetized piglets were ventilated during a 72-h period. Piglets were assigned to the Normocapnia group (n=6), ventilated in normocapnia, or to the Hypercapnia group (n=6), ventilated with moderate hypercapnic acidosis (PaCO2 from 55 to 70 mmHg) during the 72-h period of the study. Every 12h, we measured transdiaphragmatic pressure (Pdi) after bilateral, supramaximal trans-jugular stimulation of the two phrenic nerves to assess in vivo diaphragmatic contractile force. Pressure-frequency curves were drawn after stimulation from 20 to 120Hz of the phrenic nerves. The protocol was approved our institutional animal care committee's. RESULTS: Moderate and prolonged hypercapnic acidosis was well tolerated during the study period. The baseline pressure-frequency curves of the two groups were not significantly different (Pdi at 20 Hz = 32.7+/-8.7 vs. 34.4+/-8.4 cm H2O; and at 120 Hz = 56.8+/-8.7 vs. 60.8+/-5.7 cm H2O, for Normocapnia and Hypercapnia groups respectively). After 72-h of ventilation, Pdi decreased by 25% of its baseline value in the normocapnia group, whereas Pdi did not decrease in the hypercapnia group. CONCLUSION: Moderate and prolonged hypercapnic acidosis limited the occurrence of VIDD during controlled mechanical ventilation in a healthy piglet model. Consequences of moderate and prolonged hypercapnic acidosis should be better explored with further studies before being tested on patients.
