Desaturation – What’s up with the 90% thing?

Q: Do you have any studies that validate 90% as the minimum safe desat level for everyone?

Dr. Smith: That is an excellent and thought-provoking (for me) question that has me wondering who came up with the 90% rule and how did that become the standard CMCP (conventional morbidity cut point) accepted in the field of sleep. It is oft quoted, and almost all sleep software talks about the time spent under 90%, but after some surfing, I have not yet been able to find out why they chose this many years ago. When they study hypoxemia, they use the time under 90% to define nocturnal hypoxemia.

In a study on dialysis, the abstract states “It is well established that nocturnal hypoxemia in sleep apnea causes an inversion of the circadian arterial pressure rhythm and triggers nocturnal hypertension.”

In a study on advanced cancer, The Prevalence of Nocturnal Hypoxemia in Advanced Cancer they quote:
“Nocturnal hypoxemia was defined as SaO2 < 90% for 2% of the monitored nighttime." They further state "Of 100 patients, 35 had nocturnal hypoxemia. These were more likely to have lung disease (P < 0.05), a lower forced expiratory volume in one second % predicted (P = 0.01), lower daytime SaO2 (P = 0.01) and higher levels of mental fatigue (difficulty concentrating) (P = 0.02), compared to those without nocturnal hypoxemia.

In a study on pulmonary hypertension, they quote “Patients who spent > 10% of the total sleep time with oxygen saturation by pulse oximetry (Spo2) at < 90% or who needed oxygen to maintain their Spo2 level at > 90% were classified as nocturnal desaturators.”

Basically, it is well accepted that 90% is the cut point, but I am still unaware of the original source. I will continue to search, and thanks for the question!