The Various Oximetric Techniques Used for the Evaluation of Blood Oxygenation

Abstract

Adequate oxygen delivery to a tissue depends on sufficient oxygen content in arterial blood and blood flow to the tissue. Oximetry is a technique for the assessment of blood oxygenation by measurements of light transmission through the blood, which is based on the different absorption spectra of oxygenated and deoxygenated hemoglobin. Oxygen saturation in arterial blood provides information on the adequacy of respiration and is routinely measured in clinical settings, utilizing pulse oximetry. Oxygen saturation, in venous blood (SvO₂) and in the entire blood in a tissue (StO₂), is related to the blood supply to the tissue, and several oximetric techniques have been developed for their assessment. SvO₂ can be measured non-invasively in the fingers, making use of modified pulse oximetry, and in the retina, using the modified Beer-Lambert Law. StO₂ is measured in peripheral muscle and cerebral tissue by means of various modes of near infrared spectroscopy (NIRS), utilizing the relative transparency of infrared light in muscle and cerebral tissue. The primary problem of oximetry is the discrimination between absorption by hemoglobin and scattering by tissue elements in the attenuation measurement, and the various techniques developed for isolating the absorption effect are presented in the current review, with their limitations.

Keywords: accuracy; blood oxygenation; light absorption spectra; modified Beer–Lambert Law; near infrared spectroscopy; oximetry; oxygen saturation; oxygenated and deoxygenated hemoglobin; photoplethysmography; pulse oximetry.

Figure 1

Molar extinction coefficients of oxygenated (blue curve) and deoxygenated hemoglobin (red curve) as a function of the wavelength in the visible and near infrared regions. Prepared by Scott Prahl from a variety of sources [3] and is presented with permission from Dr. Prahl.

Figure 2

Reflection (left) and transmission (right) fiber-optic intravenous blood oximeters. The emission and measuring optic fibers are drawn in grey, the catheter wall is drawn in black.

Figure 3

The measured parameters of the PPG pulses. The transmitted light through the tissue decreases during systole and increases during diastole. AC is the difference between the maximal (I_D) and minimal (I_S) light transmission through the tissue; DC is the mean light transmission during the pulse. This Figure was published in Yossef Hay et al. Sensors 2018 [41].

Figure 4

Schematic cross-section of the retina (grey), which includes cross-sections of a retinal vessel (arteriole or venule) and retinal capillaries (red), and possible trajectories of light that illuminates the retina. The incident light (broad black rays) illuminates the vessel and its neighborhood. The light that strikes the vessel and the neighboring tissue is scattered and backscattered by the blood and the tissue (thin rays). Most of the scattered/reflected light from the neighboring tissue is not affected by absorption in the blood, but some photons are affected by blood in the retinal vessel or the retinal capillaries.

Figure 5

Schematic presentation of the relationship between cerebral blood flow CBF and mean arterial pressure MAP, affected by autoregulation. The autoregulation is only effective between two MAP limits, presented by dashed lines. For MAP lower than the left limit, the brain tissue is ischemic, and for MAP greater than the right limit, the brain tissue is hyperemic, potentially leading to edema with its adverse consequences.