14- oxygen saturations

KYJ14- Sats
Now before you just scroll past thinking you know oximetry and that random number generator we call a Sats probe, just consider that there might be a little something in today's episode of "Knowing your Jargon".

Oxygen is carried primarily in your red blood cell. Inside your RBCs (all 75 trillion) you have millions of iron rich molecules of protein called haemoglobin.

Form a picture of delivery van. Got that image?

This van is the red blood cell.
Now imagine opening the double doors of the van- and in the back of the van are millions of small lunch boxes. Each of these is haemoglobin.

93% of all our Oxygen is carried inside the haemoglobin. Some is dissolved into the plasma (water) of our blood and exerts a pressure (partial pressure).

If I measure that little bit of plasma dissolved oxygen, it is recorded as a partial pressure of oxygen. In arteries this is expressed as paO2 and normally 80-100 mmHg. In room air at sea level in a patient with a normal temperature (36-38 deg).

Back to the Haemoglobin (Hb).
Hb bound oxygen is responsible for 93% of the oxygen carried on blood. Understanding how these boxes of Hb helps us to grasp the jargon of measuring it,

Each Hb is only capable of holding 4 oxygen molecules.  When each of the 4 receptor binding sites is attached to an oxygen then we say that this Hb is 100% "saturated", meaning it can't hold any more oxygen.

If 3 sites are attached =75% saturated.
2=50% and 1=25%.

In arterial blood, or blood fresh from the lungs- Hb is usually 100% saturated. In Venous (deoxygenated) blood, the saturations are 75% saturated.  So we really only use a quarter of the oxygen we carry.

Now when we measure our saturations "sats" we can look at blood pulsing through our finger/toes/earlobes, non-invasively using a sats probe or we can take a sample of arterial blood, and measure saturation directly on a blood gas machine or portable iStat device.

Normal sats are 95-99% and reflect an average of millions of molecules of haemoglobin, not just one (which mathematically can only be either 75% or 100%).

If measuring sats using a probe on a finger, then the value is recorded as s saturation of pulsatile oxygen (SpO2).

If measuring sats of arterial blood you are recording it as a saturation of arterial oxygen, and the lowercase "a" denotes this, or SaO2.

Summary.
Saturations (SaO2 and SpO2) measure haemoglobin (RBC) bound oxygen.

Plasma bound oxygen is measured as a pressure not percentage. The abbreviation is PaO2 and normal is 80-100mmHg.