Ventilationperfusion

Abstract

Some patients with COPD ‘retain CO2’ when given high concentration or uncontrolled oxygen therapy. The mechanism for this, and why it occurs in only some patients, is unclear. The pathogenesis of chronic hypercapnia in COPD involves multiple, poorly understood mechanisms. During acute exacerbations ‘CO2 retention’ is thought to occur due to redistribution in ventilation-perfusion with oxygen therapy, although some studies suggest reduced ventilation also contributes. Problems arise when inexperienced staff extrapolate this teaching to include all patients with hypercapnia. The most common cause of hypercapnia is ventilatory failure. Teachers need to emphasise how to recognise ventilatory failure and the dangers of hypoxaemia in critically ill patients.