Although lifesaving,
mechanical ventilation can result in
lung injury and contribute to the development of
bronchopulmonary dysplasia. The most critical determinants of
lung injury are tidal volume and end-inspiratory lung volume. Permissive
hypercapnia offers to maintain gas exchange with lower tidal volumes and thus decrease
lung injury. Further physiologic benefits include improved
oxygen delivery and neuroprotection, the latter through both avoidance of accidental
hypocapnia, which is associated with a poor neurologic outcome, and direct cellular effects. Clinical trials in adults with acute
respiratory failure indicated improved survival and reduced incidence of organ failure in subjects managed with low tidal volumes and permissive
hypercapnia. Retrospective studies in low birth weight infants found an association of
bronchopulmonary dysplasia with low PaCO(2). Randomized clinical trials of low birth weight infants did not achieve sufficient statistical power to demonstrate a reduction of BPD by permissive
hypercapnia, but strong trends indicated the possibility of important benefits without increased adverse events. Herein, we review the mechanisms leading to
lung injury, the physiologic effects of
hypercapnia, the dangers of
hypocapnia, and the available clinical data.