Blood Gases: 4-5-6, 7,8,9 Rule

Your Question:  What is the 4-5-6, 7-8-9 rule?

My answer:  The idea for this rule is that by using the oxyhemoglobin dissociation curve you can estimate the patients PO2.  Basically you take the patient's SpO2 and subtract 30, such as follows:

SpO2               PO2

70%                 40

80%                 50

90%                 60

This is a good way of using a noninvasive pulse oximetry reading to "guestimate" that same patient's PO2 which is invasive to get by an arterial blood gas.