Flight Surgeon Module B Practice Test

Severe hypoxemia marks the final, most dangerous stage. When oxygen saturation falls to about 60–69%, tissues are not receiving enough oxygen and the brain and heart are at real risk. At this point cognitive function deteriorates, judgment and coordination collapse, and unconsciousness can occur if the situation isn’t corrected immediately by descent and high-flow oxygen. This is why the 60–69% range is associated with the critical stage—it signals imminent failure of the body's compensatory mechanisms and the urgent need for corrective action.

Earlier stages involve progressively lower oxygen delivery but with enough reserve and compensatory responses (like increased breathing and heart rate) to avoid loss of consciousness, so SpO2 typically remains higher and symptoms are milder.