Oxygen Saturation

 

The oxyhemoglobin dissociation curve shows the relation between pO2 and percent oxygen saturation of hemoglobin. At a body temperature of 37 °C and with a blood pH of 7.4, the oxygen saturation of hemoglobin varies with pO2 as follows:

pO2	% oxygen saturation of hemoglobin
10	13.5%
20	35%
30	57%
40	75%
50	83.5%
60	89%
70	92.7%
80	94.5%
90	96.5%
100	97.4%

 

Variation in body temperature and pH can shift the dissociation curve and affect oxygen release (see: 08.05.04 Oxygen Delivery and Consumption).

 

oxygen saturation as percent =

= ((oxygen content) / (oxygen capacity)) * 100% =

= (A / B ) * 100

 

where:

• A = (X^4) - (15 * (X^3)) + (2045 * (X^2)) + (2000 * X)

• B = (X^4) - (15 * (X^3)) + (2400 * (X^2)) - (31100 * X) + (2.4 * (10^6))

• X = (pO2) * (10 ^ ((0.48 * ((pH) - 7.4)) - (0.0013 * (in vitro base excess))))

 

With co-oximetry the concentrations of deoxyhemoglobin, oxyhemoglobin, hemoglobin-CO, and methemoglobin can be measured by light absorption.

 

oxygen saturation as percent =

= ((oxyhemoglobin in g/dL) / (total hemoglobin in g/dL)) * 100%

 

Interpretation:

• normal arterial blood shows an oxygen saturation of 95-100%

• normal venous blood shows an oxygen saturation of 70-75%

 

Limitations:

• Patients with abnormalities in 2,3-diphosphoglycerate or carbon monoxide have alterations in the affinity of hemoglobin for oxygen, and these patients may have measured oxygen saturation different from that estimated by the above equation.