16-Oxyhaemoglobin dissociation curve

KYJ16- Oxyhemoglobin Dissociation curve.

It doesn't seem to matter how many courses on respiratory or ventilation I attend, there is always a teacher who makes a hash job of explaining the difficult concept of the oxyhaemoglobin dissociation curve.  After one of you requested this poison chalice, thought I'd have a go of explaining it my way.

First, know that haemoglobin binds to oxygen and we record this as saturation percentage (normal now believed to be SaO2 = 90-98%).

The plasma also dissolves oxygen into solution, and this exerts a pressure in plasma. In arteries we measure this in the normal range of 80-100mmHg.

The OHD curve achieves three things
First
The curve predicts that if Sats are normal, then pressures are normal. There is a direct relationship between the present age of Hb that is saturated, and the amount of plasma dissolved O2.  As one goes up, so does the other and if one goes down, the other follows.

Second
Being a sigmoid shape, the curve is flat at the top, and steep in the middle. For a drop in oxygen in the top 20mmhg of PaO2, there is little effect on saturation, but once a PaO2 drops below 80, the patients saturations drop below 95% and rapidly fall.

It works backwards too. If I look at your sats and they are in the high 90% (normal) I should be able to predict that your plasma O2 is also normal.  And nurses do that every shift.  We grab a sat probe, and use this as a guide to the patients oxygenation.

What we need to understand is that a 10% drop in sats from 99-89% is no where near as dramatic as a 10% drop between 89-79%!!
In fact sats below 88% is very dangerous, because blood is not carting enough oxygen to perfuse cells. With sats lower than 88% we predict a PaO2 at below 60mmHg.  This is technically the diagnostic criteria for Respiratory Failure.

Third
The last concept of the curve is called shift.
The graph curve can be skewed to the left or right of normal prediction, by things like pH, blood temperature, CO2, and a chemical on red blood cells called 2,3 DPG which allows red blood cells to release oxygen to cells.

When the curve is shifted to the left, then haemoglobin binds tightly to oxygen and is reluctant to give it up to the cell. It means sats will be high, but cells may be hypoxic, as oxygenated blood won't give up the goods.  I like to remember that a Left Shift means that oxygen is "left hanging on" to haemoglobin.

Left shift is caused when blood is
cold (<36C),
alkaline (pH >7.45),
And when a chemical called 2,3 DPG is lacking (banked blood products).

The opposite to these
High DPG
Acidosis (pH<7.35)
Hyperthermia
Causes a shift of the curve to the right. That means that for a given PaO2 the Hb saturation is less than predicted.
We may see low saturations in these patients meaning less oxygen is being carried in the blood. Cells naturally become hypoxic if blood is reluctant to carry oxygen.