Acute hypercapnic respiratory failure in adults (AHRF) results from an inability of the respiratory pump, in concert with the lungs, to provide sufficient alveolar ventilation to maintain a normal arterial PCO2. Co-existent hypoxaemia is usually mild and easily corrected. Conventionally, a pH <7.35 and a PCO2 >6.5 kPa define acute respiratory acidosis and, when persisting after initial medical therapy, have been used as threshold values for considering the use of non-invasive ventilation. More severe degrees of acidosis, such as pH<7.25, have been used as a threshold for considering provision of IMV.
AHRF complicates around 20% of acute exacerbations of COPD. It signals advanced disease, a high risk of future hospitalisations and limited long-term prognosis. The median survival following recovery from AHRF was 1 year in a large case series. Around 12% of patients with hypercapnic COPD died during the index admission and this increased to 33% if the respiratory acidosis developed after hospitalisation. In asthma, acute hypercapnia also signals an increased risk of death and an increased likelihood of future life-threatening attacks. The same risks apply to AHRF complicating CF and bronchiectasis, although this has not been formally reported. In the neuromuscular and CWDs, including morbid obesity, respiratory pump failure is often insidious in its onset, but AHRF may be acute and unexpected. Acute on chronic ‘decompensated’ episodes of AHRF are more common and normally indicate the future need for domiciliary NIV.