MODULE SUPPLEMENT: PULMONARY SYSTEM
The Oxy-Hemoglobin Dissociation Curve

The Oxy-Hemoglobin Dissociation Curve underpins the relationship between PaO₂ and O2 saturation.

Normally about 97% of the oxygen is carried by hemoglobin whereas only 3% is carried dissolved in the blood (Guyton & Hall, 2000). As the PaO₂ increases, the amount of oxygen that is bound to Hemoglobin, or the percent saturation of hemoglobin, also increases; normally at a PaO₂ of about 95 mmHg the saturation of hemoglobin is about 97%. Each gram of hemoglobin can carry about 1.34ml of O₂ so that if the hemoglobin were completely saturated, that is, if it carried every bit of oxygen that it could, it would carry about 20ml of oxygen. Normally it is only about 97-98% saturated so it actually carries about 19.4ml of oxygen. Then when pO₂ drops to about 40mmHg in the capillaries the hemoglobin is about 75% saturated, or about 14.4ml of oxygen. This indicates that the blood normally delivers about 5ml of oxygen per 100ml of blood to the tissues. Shifting of the oxyhemoglobin curve in response to physiological changes is called the Bohr effect.

If we consider the PaO₂ of an 85 year old as predicted by Enright's view that PaO2 plateaus around 80, the O₂ saturation should be about 95%.

This isn't a big decrease. However, it's good to understand the potential implications of even a small decrease IF this is combined with a decrease in hemoglobin.

For example, if the hemoglobin is 98% saturated it carries about 19.7 ml of oxygen.

If we consider an older person with a saturation of about 95 and a hemoglobin level at the low end of normal--about 13, what would happen?

The amount of oxygen carried goes down to 16.5ml/100ml of blood.

Then if we consider a hemoglobin of 10, we end up with only 12.7 ml of oxygen being transported to the tissues. This can place the older adult at a significant risk if they encounter any pulmonary compromise.

It also suggests that we have to be somewhat cautious when using O₂ saturation (pulse oximetry) as a guide in our assessments--this doesn't take into account hemoglobin level. Thus, someone may have well saturated hemoglobin but if there is not very much hemoglobin to saturate, not very much oxygen will be available to the tissues.