![]() If hypoxaemia is present it is normally readily treated with oxygen therapy. It can be caused by any part of the process that controls respiration from the brain, spinal cord, peripheral nerves, neuromuscular junction to the muscles and chest wall.Ĭauses of reduced minute ventilation and alveolar hypoventilation include: This typically results from failure of the respiratory pump. A reduction in minute ventilation results in hypoventilation of the alveoli. Minute ventilation is a product of the respiratory rate x tidal volume. ![]() Hypoventilation of the alveoli therefore leads to hypercapnia. The PaCO 2 (at a steady state of CO 2 production by tissues) is determined by alveolar ventilation. The hypoxaemia normally responds well to oxygen therapy. Hypercapnia is less common than hypoxaemia as carbon dioxide is far more soluble and less likely to to be affected. It is often seen in emphysema and lung fibrosis. Alveolocapillary membrane changes: inflammation and fibrosis of the alveolocapillary membrane impairs diffusion across it.Reduced surface area: reduced surface area of alveoli due to pathological destruction limits the amount of lung tissue available for gaseous exchange.Diffusion limitationĭiffusion limitation refers to the impairment of gaseous exchange across the alveolocapillary membrane. Hypercapnia may occur if there is a high shunt fraction.Ĭauses include pneumonia, ARDS, pulmonary oedema and alveolar collapse. Shunts respond relatively poorly to oxygen therapy when compared to V/Q mismatch as the shunted blood cannot be ventilated. The blood from these capillaries returns to the left side of the heart without having been ventilated. Physiological shunting: refers to unventilated alveoli that are perfused.Pathological conditions causing an anatomical shunt include pulmonary arteriovenous malformations. Anatomical shunting: there is normally a shunt (around 2-3% of cardiac output) as blood from the bronchial and Thebesian veins enter the left side of the heart without having participated in gaseous exchange. ![]() This can be thought of as an extreme version of V/Q mismatch (V/Q = 0) ShuntĪ shunt refers to blood entering the left side of the heart without first having travelled through pulmonary capillaries and participating in gaseous exchange. The hypoxaemia in V/Q mismatch normally responds well to oxygen therapy by improving alveolar oxygenation in low V/Q areas. Blood is diverted to other areas and low V/Q regions are created to compensate. This occurs in areas where perfusion is limited, for example in pulmonary embolism, resulting in alveolar units with a high V/Q.
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