Key messages

• Infections, cardiac failure, and pulmonary embolism are the most common precipitating factors of acute respiratory failure in chronic obstructive pulmonary disease.

• Hypercapnia develops despite maintenance of minute ventilation because of worsening ventilation-perfusion ( Vl Q) mismatch. Introduction

The evolution of chronic obstructive pulmonary disease (COPD) is characterized by episodes of acute respiratory failure. In the first stages of COPD, the transient impairment in lung function is only perceived as an increase in sputum, the appearance of wheezing, or shortness of breath. As the illness becomes more severe, the number and intensity of the symptoms associated with an episode of acute respiratory failure increase. In the most extreme cases, patients develop severe abnormalities in their blood gases and become unable to sustain spontaneous ventilation. In this scenario, the use of artificial ventilatory support is lifesaving and is the main indication for admission of such patients to the intensive care unit (ICU). Until recently, the most common way of supplying mechanical support was intubation and sedation, with respiratory muscle rest considered as an important aspect of treatment. With the development of non-invasive ventilation, maintenance of a spontaneous breathing pattern has become more important than total respiratory muscle rest. Therefore an understanding of the pathophysiology of acute respiratory failure is useful in the treatment of such patients.

The fact that acute respiratory failure in COPD patients is a severe illness with an associated mortality has been recognized for many years. Nevertheless, the reported mortality of 6 to 40 per cent shows large discrepancies because of the selection criteria used in different studies. Moreover, the main cause of death in these patients is rarely respiratory insufficiency but is coexisting disease, i.e. cardiac failure and infectious processes ( Derenne e.L§L 1988). Some authors have suggested that acute respiratory failure in some varieties of COPD may have lower mortality; thus type B (blue bloaters) would have a better prognosis than type A (emphysematous), but again the different admission rates for each type of patient may have biased this conclusion.

Pathological studies of COPD patients dying during acute respiratory failure have been of little help in clarifying the pathophysiology. Most of the pathological findings have been related to the underlying disease or concurrent illnesses, such as pulmonary embolism, pulmonary edema, and pneumothorax ( Derenneefa/ 1988).

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