The Study of Transesophageal Oxygen Saturation Monitoring

The Study of Transesophageal Oxygen Saturation Monitoring

Zhiqiang Zhang (Sichuan University, China), Bo Gao (Sichuan University, China), Guojie Liao (Sichuan University, China), Ling Mu (West China Hospital, China) and Wei Wei (West China Hospital, China)
DOI: 10.4018/978-1-60960-064-8.ch015
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Abstract

In this chapter, the transesophageal oxygen saturation (SpO2) monitoring system was proposed based on the early experiments, to provide a new program of SpO2 acquisition and analysis and avoid the limitation of traditional methods. The PPG (photoplethysmographic) signal of descending aorta and left ventricular was monitored in the experiment. The analysis of the peak-to-peak values, the standard deviation and the position of peaks in signal waveforms showed that in vivo signal was more stable and sensitive; and the physiological information was reflected in the left ventricular PPG waveform. Therefore, it can be concluded that the transesophageal SpO2 monitoring technology has better guidance in clinical applications.
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Spo2 Monitoring System

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).

(1)

AS, BS are the coefficients of empirical equation, andλ1, λ2 are red and infrared light wavelengths, respectively, and IAC, IDC are light intensity. In the experiment, IAC is calculated by the peak-to-peak values of PPG waveforms, whereas IDC 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.

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