The Process of Oximetry in Biomedical Measurements

Oximetry is the determination of the percentage oxygen saturation of the circulating arterial blood.

Where, [HbO₂] is the concentration of oxygenated haemoglobin and [Hb] is the concentration of deoxygenated haemoglobin.

The percentage of oxygen saturation in the blood is very important. The oxygen saturation is a bio-constant and is an indication of the performance of the most important cardio-respiratory functions.

Oximetry is applied in many areas of medical diagnosis; for example it is used in:

• Diagnosis of cardiac and vascular anomalies
• Treatment of post-operative anoxia
• Treatment of anoxia resulting from pulmonary infections.

In Vitro Oximetry

In vitro oximetry is whereby, blood is withdrawn from the subject under anaerobic conditions and the measurement for oxygen saturation is made at a later time in the laboratory.

For the discrete blood samples, spectrophotometric measurement of oxygen saturation can be made by either the transmission or a reflection method.

In Transmission Oximetry, measurement of the degree of oxygen saturation of the blood can be made by spectrophotometric method whereby the concentrations of substances held in a solution are measured by determining the relative light attenuations that the light absorbing substances cause at each of several wavelengths.   

The spectrophotometric technique applies only to hemolyzed blood i.e. to blood that has its individual red cells destroyed and the pigments contained in the cells homogeneously distributed in the whole solution. This is necessary to eliminate artefacts that are associated with multiple scattering of the measuring light from erythrocytes.

Reflection Oximetry is based on the scattering of light by the erythrocytes.

In Vivo Oximetry

Here, the oxygen saturation of blood is measured while the blood is flowing through the vascular system or it may be flowing through a cuvette directly connected with the circulatory system by means of a catheter. The blood used in this case is not hemolyzed. Both the transmission and reflection techniques are utilized in Vivo Oximetry.