The major function of the respiratory system is to supply the blood with adequate oxygen and to remove carbon dioxide. This process is achieved by three distinct mechanisms:
• ventilation - the delivery and removal of air to and from the alveoli
• diffusion - oxygen and carbon dioxide cross the alveolar-capillary wall
• circulation - transported from the site of gas exchange to the cells and back again.
To accomplish this, the lungs function as an area of gas exchange and as a pump for effective ventilation. Failure of these two functions results in hypoxaemia or hypercapnia respectively. Hence respiratory failure is traditionally defined as hypoxaemic respiratory failure (PaO2 < 8 0 kPA or 60 mmHg) or hypercapnic respiratory failure (hypoxaemia with PaCO2 above 6 0 kPA or 50 mmHg).
Respiratory failure may be classified as three main types:
• hypoxaemia - mainly an imbalance between ventilation and perfusion in the lung
• hypercapnia - an imbalance between ventilatory capacity and demand
• a mixture of hypoxaemia and hypercapnia, commonly seen in clinical practice.
Figure 4.1 Oxygen partial pressure (vertical lines) in parts of the body (horizontal lines). A, inspired dry gas; B, humidified at 37°C; C, mixed with expired gas; D, alveolar ventilation and O2 consumption; E, venous mixing and V/Q mismatch; F, capillary blood concentration depends on blood flow and haemoglobin concentration; G, mitochondria
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