The Study of Transesophageal Oxygen Saturation Monitoring

Theoretical Principle of SpO₂ Monitoring

Timely monitoring of blood oxygen saturation is an important indicator to determine human respiratory system, circulatory system, or whether there are anoxic obstacles in the surrounding environment. The measurement of SpO₂ is based on the Hb and HbO₂ with different light absorption characteristics (Sola et al., 2006), as shown in Figure 1.

Figure 1.

The light absorption coefficients of HbO₂ and Hb in the red and infrared spectrum

Studies have shown that human blood is sensitive to the light in the range of 600 nm to 1000 nm wavelength (Sola et al., 2006). In Figure 1, HbO₂ and Hb have different light absorption coefficients in different wavelength regions. In the infrared spectrum, their absorption curves changed smoothly and are close to each other, so there is little difference in absorption of Hb and HbO₂. However, in the red spectrum, HbO₂ and Hb are more sensitive to the changes in blood oxygen, because of their large difference in absorption coefficient, especially around 660 nm where the difference between HbO₂ and Hb absorption coefficient is the greatest. With these factors considered, light sources at 660 nm and 940 nm were selected for the oxygen saturation measurement.

Generally, the human’s blood SpO₂ is determined by measuring photoplethysmographic (PPG) signal, which is caused by the fluctuations of vessels volume. The SpO₂ calculation is shown in empirical Equation (1).

A_S, B_S are the coefficients of empirical equation, andλ₁, λ₂ are red and infrared light wavelengths, respectively, and I_AC, I_DC are light intensity. In the experiment, I_AC is calculated by the peak-to-peak values of PPG waveforms, whereas I_DC is a constant. Therefore, as the blood volume alters, the changes of light intensity will be shown in the PPG signal waveforms. For example, when blood backward flow caused by heart beating occurs, the changes of blood volume will be reflected in the PPG waveforms.