Failure to maintain adequate gas exchange across the alveolar capillary membrane gives rise to problems with oxygenation and excretion of carbon dioxide. Acute respiratory failure is one of the most common reasons for deterioration, leading to admission to intensive care. Early identification, with a systematic assessment and the development of an appropriate treatment plan whilst the patient is ward-based, is essential to prevent deterioration. Respiratory failure can be a result of failure of the gaseous exchange mechanism at alveolar level (lung failure), or as failure of ventilation, limiting the flow of gases through the respiratory system (ventilatory failure) (Figure 16.1). The diagnosis of respiratory failure is made on assessment of hypoxaemia (low levels of oxygen in the blood). Hypoxaemia is measured clinically either by SpO₂ monitoring or by evaluation of ABGs as PaO₂. The relationship between oxygen saturations and PaO₂ is demonstrated in Figure 16.2 and Box 16.1. A SpO₂ of <90% is a point at which Hb starts to lose affinity for oxygen and a consequent drop in oxygen content and delivery occurs, a marker for respiratory failure. Obtaining an arterial sample can be uncomfortable for the patient, but extra information obtained regarding PaO₂ pH, PaCO₂ and bicarbonate (HCO₃⁻) levels, assist in determining the type of failure present and appropriate management strategies (Table 16.1). Failure of the lung can be caused by impedance of airflow through the airways or obstructive lung disorders. Changes in the lung that reduce the lung volumes are known as restrictive lung disorders; this could be due to damage to the lung tissue itself, or a disease of the chest wall (Table 16.2).

Inadequate alveolar ventilation