Pathophysiology of Respiratory Failure and Indications for Respiratory Support

Abstract

Respiratory failure occurs when there is inadequate exchange of O 2 and CO 2 to meet the needs of metabolism, which leads to hypoxaemia, with or without hypercarbia. Diagnosis requires measurement of arterial blood gases. That is, the partial pressure of O 2 in arterial blood (P aO 2) and the partial pressure of CO 2 in arterial blood (P aCO 2). Respiratory failure can be defined as P aO 2 <8 kPa (60 mmHg), or P aCO 2 >6.7 kPa (50 mmHg) in a patient at rest, breathing air at sea level. Respiratory failure can be divided into: • type I respiratory failure, in which processes that impair oxygen transfer in the lung cause hypoxaemia (acute or hypoxaemic respiratory failure) • type II respiratory failure, in which inadequate ventilation leads to retention of CO 2, with hypercarbia and hypoxaemia (chronic, ventilatory or hypercapnic respiratory failure) • ‘mixed’ respiratory failure, in which there is a combination of type I and type II respiratory failure (acute-on-chronic respiratory failure).