Effects on gas exchange Intrapulmonary shunt

Regardless of whether atelectasis occurs under mechanical ventilation, it is responsible for changes in gas exchange which often lead to hypoxemia. This type of atelectasis (which is not visible on radiographs but appears on CT sections) causes abnormalities in the ventilation-to-perfusion ratio and creates an intrapulmonary shunt. This is defined as the passage of venous blood into the arterial system through the pulmonary capillaries with a low ventilation-to-perfusion ratio or limited diffusion. This phenomenon can become a true shunt in cases of total bronchial obstruction and hence total absence of ventilation. The degree of atelectasis is correlated with the intrapulmonary shunt and consequently the extent of hypoxemia.

Hypoxic vasoconstriction

Pulmonary vascular resistance is dependent on the oxygen pressure PO2 in the alveolar gas. A reduction in PO2 will cause a contraction of the smooth muscles in the walls of the arterioles in the hypoxic area. The existence of such areas may be due to insufficient ventilation caused by bronchial obstruction. The effect of this vasoconstriction is to divert blood flow from poorly ventilated to well ventilated areas.

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