### Description

Oxygen saturation is a ratio of the volume of oxygen actually carried in the blood versus the maximum volume which could be carried.

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.

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