MEASUREMENT OF OXYGEN SATURATION

Several species of hemoglobin may exist in blood, such as oxyhemoglobin (HbO₂ ), deoxygenated hemoglobin (HHb), carboxyhemoglobin (HbCO), and methemoglobin (HbMet). To avoid ambiguity, the following definitions have been proposed.^[50]

SO₂ = HbO₂ /(HbO₂ + HHb) (19)

HbO₂ fraction = HbO₂ /Total Hb (20)

SO₂ is most commonly reported as a percentage, obtained by multiplying the value obtained from Equation 19 by 100.

Measurement of SaO₂ is an alternative method to PO₂ for assessing arterial blood oxygenation. The fact that absorption and reflectance spectra of hemoglobin are affected by its oxygenation allows for convenient optical methods of measurement. The traditional method for monitoring SaO₂ is to observe the skin and mucous membranes for cyanosis. In 1947, a systematic comparison of the clinical detection of a cyanosis by medical staff and blood O₂ saturation measurement with an ear oximeter in normal volunteers demonstrated the poor accuracy of cyanosis as an indicator of hypoxemia.^[51] In this study, a total of 7204 observations of skin and mucous membrane color were made in normal volunteers breathing air or hypoxic gas over a measured SaO₂ range of 71% to 100%. False-positive diagnosis of cyanosis was common; cyanosis was diagnosed in 37% of 4587 observations despite a measured SaO₂ of 91% to 100%. However, in 1723 observations of hypoxic volunteers with a measured SaO₂ between 71% and 80%, normal color was observed in 12% of cases.

Cyanosis seems to be correlated best with the quantity of deoxygenated arterial blood. It has been suggested that for cyanosis to be detectable, 5 g/dL of deoxygenated hemoglobin must be present in the arterial blood,^[52] although published evidence is consistent with a lower detection threshold^[53] ( Fig. 36-9 ). Nevertheless, the threshold for visual detection of cyanosis depends on total hemoglobin concentration. If 3 g/dL of desaturated blood is required to detect cyanosis by visual observation, at a total hemoglobin concentration of 15 g/dL, cyanosis occurs when SaO₂ is below 80%, compared with 66% for a hemoglobin level of 9 g/dL. The evident inaccuracy and uncertainty inherent in attempts to assess oxygenation by visual inspection have led to its replacement with highly successful quantitative methods.

Carbon Monoxide Oximetry

Simultaneous measurement of several hemoglobin species with different absorption spectra can be accomplished by using multiple-wavelength absorption with at

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Figure 36-9 Quantity of deoxygenated hemoglobin (Hb) in arterial blood as a function of the degree of cyanosis. Traditionally, it has been assumed that 5 g/dL of deoxygenated hemoglobin is required for the detection of cyanosis, but mild cyanosis can be observed at lower values. (Data from Stadie WC: The oxygen of the arterial and venous blood in pneumonia and its relation to sepsis. J Exp Med 30:215, 1919.)