Mixed Venous Saturation Monitoring

Mixed venous oxygen saturation monitoring (SvO2) typically utilizes reflective spectrophotometric technology to measure the amount of oxygen in mixed venous blood. Yet, a true mixed venous blood sample is measured in the pulmonary artery. Systemic venous blood with different oxygen extraction ratios returns to the right atrium via the superior and inferior venae cavae, mixes and equilibrates in the right ventricle, and flows out past the pulmonic valve to the pulmonary artery. As blood travels past the SvO2, catheter light emitted from the catheter tip is reflected off the red blood cells and is detected by a photode-tector. The difference in wavelengths of emitted and reflected light is processed to estimate SvO2 (Fig. 22).

Continuous venous saturation (SvO2) monitoring has been made possible with the adaptation of a pulmonary artery catheter with fiberoptic technology (see JPEG 9 on the Companion CD). Such monitoring utilizes the principle of reflectance spectrophotometry, which uses multiple wavelengths of transmitted light at specific intensities that is then reflected from red blood cells. For example, oxygenated hemoglobin absorbs most infrared light (940 nm) and reflects or transmits most red light (660 nm); this is the reason that oxyhemoglobin looks red, and deoxyhemoglobin appears blue. The tip of the SvO2 catheter emits light with specific wavelengths, which measures both oxyhemoglobin and deoxyhemoglobin as red blood cells flow past the tip of the catheter. The difference between absorption of light between saturated and desaturated hemoglobin results in the calculated SvO2 value.

Spectrophotometric Catheter

Fig 22. Mixed venous saturation monitoring (SvO2). Spectrophotometric technology such as pulse oximeter and mixed venous oxygen saturation monitors are utilized to measure the amount of oxygenated hemoglobin in circulating blood. A specific wavelength (infrared) is emitted, and the reflected wavelength off the red blood cells is detected and processed.

Fig 22. Mixed venous saturation monitoring (SvO2). Spectrophotometric technology such as pulse oximeter and mixed venous oxygen saturation monitors are utilized to measure the amount of oxygenated hemoglobin in circulating blood. A specific wavelength (infrared) is emitted, and the reflected wavelength off the red blood cells is detected and processed.

The SvO2 equation is a modification of the Fick equation; SvO2 is derived by rearranging the Fick equation as follows:

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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