Question

Thinking in terms of evolutionary advantage, why does the ventilatory response to hypoxemia not show an increase until the PaO₂ decreases below 60 mmHg?

Answer 1

Hypoxemia indicates low levels of oxygen(PaO2) in the blood. Pulse oximetry is the test which is done to measure oxygen saturation (SpO2) of the blood. SpO2 is related to PaO2 with the help of oxyhemoglobin dissociation curve. The SpO2 of 90% is approximately equivalent to PaO2 of 60 mm Hg under standard conditions. Hence when PaO2 decreases below 60 mm Hg then the oxygen dissociation curve becomes steeper which results in a large fall in SpO2 for a small decrease in PaO2. The decrease in PaO2 below 60 mmHg leads to increase in PaCO2 due to which ventilatory responses increases.