Hypoxic Drive

The hypoxic drive is a form of respiratory drive in which the body uses oxygen chemoreceptors instead of carbon dioxide receptors to regulate the respiratory cycle.

Normal respiration is driven mostly by the levels of carbon dioxide in the arteries, which are detected by central chemoreceptors, and very little by the oxygen levels. An increase in carbon dioxide will cause chemoreceptor reflexes to trigger an increase in respirations. Hypoxic drive accounts normally for 10% of the total drive to breathe. This increases as the PaO₂ goes to 70 torr and below, while hypoxic drive is no longer active when PaO2 exceeds 170 torr. The hypoxic drive is so weak that unconsciousness will develop before respiratory distress is noted and is therefore a risk for pilots flying at high altitudes. For this reason, supplemental oxygen is required by Federal Aviation Regulations for pilots flying above about 12,500 feet altitude in unpressurized airplanes.

In the past, it was believed that in cases where there are chronically high carbon dioxide levels in the blood such as in COPD patients, the body will begin to rely more on the oxygen receptors and less on the carbon dioxide receptors. And that in this case, when there is an increase in oxygen levels the body will decrease the rate of respiration.

Recent studies have proven that COPD patients who have chronically compensated elevated CO₂ levels (known as "CO₂ Retainers") are not in fact dependent on hypoxic drive to breathe. However, when in respiratory failure and put on high inspired oxygen, the CO₂ in their blood may increase via three mechanisms, namely the Haldane Effect, the Ventilation/Perfusion mismatch (where the regional pulmonary hypoxic vasoconstriction is released) and by the removal or reduction of the hypoxic drive itself.