Transcutaneous Oximetry

The fact that different hemoglobin species, particularly reduced hemoglobin and oxyhemoglobin (HbO₂ ) have different absorption spectra suggests the possibility of using absorption of light in vivo to calculate arterial hemoglobin O₂ saturation (SaO₂ ). A dual-wavelength system can be used to estimate SaO₂ if the following conditions are met:

1. The light is transilluminating arterial blood.
2. There are no significant quantities of other hemoglobin species, such as HbMet and HbCO.
3. The absorption of light by tissue is negligible.

The first condition may be met, for example, by transilluminating the earlobe, provided the tissue is kept warm. Under these circumstances, the ratio of blood flow to tissue O₂ consumption is relatively high, and the capillary blood is therefore predominantly arterial. The second condition is met under most clinical circumstances, in which the total of other hemoglobin species is usually less than 5% of the total. The third condition can be ensured by appropriate choice of wavelength.

Dual-wavelength oximeters became available in the 1940s. An eight-wavelength ear oximeter was produced by Hewlett-Packard,^[58] in which active heating of the ear lobe by the sensor maintained a high proportion of arterial blood in the capillary bed. However, its cumbersome size prevented widespread use.