Originally posted by @arvxnd on Threads

One of the more anxiety-inducing pages during call:

“Doctor, the patient desaturated to 70% on room air.”

You apply a nasal cannula, and the saturation increases to 93%. Crisis averted?

Not necessarily. A blood gas shows a PaO₂ of 55 mmHg. This is Type 1 respiratory failure.

Let’s unpack what that means.

SpO₂ vs. PaO₂ vs. PO₂ — What’s the Difference?

• SpO₂: Percent of hemoglobin saturated with oxygen, measured via pulse oximeter

• PaO₂: Partial pressure of oxygen dissolved in arterial blood, measured via arterial blood gas

• PO₂: General term for oxygen partial pressure (can refer to air, gas, or blood)

SpO₂ is a percentage, whereas PaO₂ is a pressure.

They are related — but not linearly.

Oxygen Transport Mechanisms

Oxygen in blood is carried in two forms:

• 98% is bound to hemoglobin

• <2% is dissolved in plasma — this is what PaO₂ measures

PaO₂ tells us how much free oxygen is in plasma.

SpO₂ tells us how saturated hemoglobin is with oxygen.

Hemoglobin and Cooperative Binding

Hemoglobin is a tetramer (2 alpha and 2 beta subunits).