Dangers of Hypoxemia: How low is too low?
No one knows. Natives of the Andes mountains tolerate SpO2 of 60-70% without having heart attacks or strokes. SaO2 measured in "normal" people at the top of Pikes Peak (14,000 feet) averages 82%. Medicare pays for home-use of oxygen if patients are below 88%. Some organs, like the brain are more vulnerable to oxygen starvation and some people are more at risk than others in ways impossible to predict.
Those with preexisting diseases may have less tolerance for prolonged hypoxemia. If there is abnormal narrowing of blood vessels blood flow will be decreased, and if that blood contains less oxygen, damage to vital organs may occur.
Most physicians agree that that an SpO2 below 90% is concerning and below 85% needs to be acted upon urgently. It's important to know if SaO2 is decreasing and to stop the decline. Without reversing such a trend there is no question that death, stroke, or heart attack will follow.
Conrad Anker, a climber of Himalayan peaks over 28,000 feet without oxygen, recently had a heart attack on Denali below 20,000 feet. His misfortune was an example of unpredictable and unexpected oxygen starvation. Luckily he survived to get angioplasty that opened blocked arteries in his heart. Mr. Anker was one of many who had no idea he was sitting on a time bomb. The AHA (American Heart Association) says that for 47% of those who have Coronary Artery Disease the first sign of the problem is Sudden Death. The diagnosis is only made at autopsy.
What this means is that both hypoxemia and heart disease are very common. Our hearts may not be normal but as long as we don't make excessive demands they may not be the "weak link" that kills us. But even if your heart is good enough for superhuman activity that does not mean that it won't fail you the next time there is a serious lack of oxygen supplying it. Even young and perfectly healthy hearts fail if oxygen levels drop far enough and for long enough. Plenty of young people are among today's overdose deaths.
If patients are at 85-90% in hospitals they are routinely given extra oxygen by nasal prongs or a mask. Sometimes Continuous Positive Airway Pressure (CPAP) is used to keep the airway open so air can flow into and out of the lungs.
Patients may need more than extra oxygen or CPAP if their breathing is too shallow or slow (hypoventilation). They may need resuscitation and the immediate placement of an endotracheal (breathing) tube.
Note: Slow, shallow breathing (hypoventilation) or apnea (no breathing) may not be detected immediately by pulse oximetry if patients are using extra oxygen. "Normal" SaO2 readings might sometimes lead to a false sense of security if people are using extra oxygen--dangerous hypoventilation or apnea could be occurring.