Permissive hypercapnia

Mechanical ventilation is normally adjusted so that pCO2 is maintained within normal limits (35-45 mmHg). However, the intensity of ventilation required to sustain a normal pCO2 can cause mechanical damage to the lungs of patients with acute respiratory distress syndrome (ARDS). 

One of the strategies used to minimize ventilator-induced lung damage in these patients is adjustments to mechanical ventilation which result in reduced alveolar ventilation and an inevitable increase in pCO2 (hypercapnia). This therapeutic approach, termed "permissive hypercapnia" has been shown to reduce morbidity and mortality among ARDS patients and, more recently, among patients with other respiratory disease requiring mechanical ventilation. 

Since "permissive hypercapnia" was first introduced to critical care around a decade ago, it has become clear that a raised pCO2 is not simply tolerated but has beneficial effect independent of the ventilation strategy that induces it. 

In a review of 105 referenced articles, the authors describe how the therapeutic approach of "permissible hypercapnia" is applied in routine critical care and summarize current understanding of the ways in which hypercapnia might modulate the pathogenesis of acute lung injury. 

Of wider significance is the research indicating that hypercapnia has specific anti-inflammatory effects which are beneficial in the context of ARDS but potentially detrimental in the context of sepsis. This is a wide-ranging review, not only of the clinical application of "permissible hypercapnia", but also of the research which continues to reveal ways in which a raised pCO2 may modulate the course of respiratory, as well as non-respiratory disease.