What oxygen level is too low for a 91 year old women who is 5 foot 4 inches and weighs 151 pounds?

Hypoxia. As measured by a pulse oximeter, saturation below 88 % indicates needs for supplemental oxygen. On an arterial blood gas study, po2 < 55 would be too low. If she has cor pulmonale or class IV chf, then slightly higher readings would also be considered too low. Weight and height are irrelevant for oxygen level.