Gas exchange

Worsening of gas exchange is the initial factor of acute respiratory failure in COPD patients. Stable COPD patients show alterations in ventilation-perfusion ( Vl Q)

a mismatch, mainly as low and high Vl Q areas. When acute respiratory failure develops, this effect is increased because of increased resistance and hyperinflation.

The former reduces ventilation in some zones of the lungs, which increases the number of low Vl Q units. Conversely, hyperinflation reduces blood flow in distended

alveoli, increasing the number of high Vl Q units. These alterations in Vl Q mismatch are the main reason for the increase in PaC02 despite maintenance of minute ventilation.

Worsening of Vl Q mismatch is limited by hypoxic pulmonary vasoconstriction. This vascular reflex reduces blood flow to hypoventilated areas, redistributing flow to

better ventilated areas. Administration of oxygen at high doses abolishes this reflex, leading to an impairment in Vl Q mismatch and a further increase in PaC02.

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