Final answer:
High FIO2 can increase lung shunt fraction by reducing the hypoxic pulmonary vasoconstriction response, leading to a mismatch of ventilation and perfusion and allowing blood to bypass the respiratory gas exchange process despite being well-perfused.
Step-by-step explanation:
High fractions of inspired oxygen (FIO2) can increase the shunt fraction in the lungs, which refers to the proportion of blood that bypasses the respiratory gas exchange process. In conditions such as infection or edema in the lungs, an obstruction may occur, diminishing ventilation to the affected areas while perfusion remains unchanged. The result is an altered ventilation/perfusion (V/Q) ratio, negatively impacting gas exchange by creating regions in the lung that contribute to the shunt phenomenon.
Increased FIO2 may exacerbate this shunting effect by causing the partial pressure of oxygen in the blood to rise, subsequently diminishing the stimulus for pulmonary capillary constriction in poorly ventilated areas. This response, known as hypoxic pulmonary vasoconstriction (HPV), normally serves to redirect blood flow from hypoxic or poorly ventilated alveoli to well-ventilated alveoli, optimizing gas exchange. However, with increased oxygen delivery, HPV is dampened or inhibited, leading to a mismatch of ventilation and perfusion and thereby increasing the shunt fraction.
Shunted areas, despite being well-perfused, do not participate in gas exchange
These areas receive oxygenated blood but are unable to contribute to oxygenating venous blood, effectively bypassing the gas exchange process leading to a decrease in the overall oxygen delivery to the body.