Mixed Venous Oxygen Saturation

Mixed venous oxygen saturation can be monitored continuously by reflection spectrophotometry with the use of a fiberoptic pulmonary artery catheter. Light of two specific wavelengths, one of which is sensitive to changes in oxygen saturation of hemoglobin and the other which is used as a reference, is transmitted to the pulmonary artery blood, reflected, then transmitted back to a photodetector. The oxygen saturation of this blood, which represents mixing of blood from the coronary sinus, pulmonary artery, and the superior and inferior vena cavae, is then measured as the absorption ratio of the wavelengths.

Continuous mixed venous oxygen saturation monitoring is useful for the evaluation of oxygen consumption relative to supply. It therefore provides useful information regarding tissue oxygenation

At the resting metabolic state the Sv02 is normally 75%. Under anesthesia the Sv02 may rise to 85% as oxygen consumption substantially diminishes. Decreases in Sv02 during surgery can be due to low cardiac output, as tissue supply is reduced in relation to demand, or to insufficient anesthesia, where the oxygen consumption increases relative to supply.

v=fl, where v=1540m/sec in soft tissues. The frequency of ultrasound waves is greater than 20,000 Hz, making them inaudible to the human ear. Transducer frequencies of 3.5 MHz or 5MHz are generally used in echocardiography.

As the ultrasound beam wave traverses a medium its intensity will attenuate through absorption, reflection, or scatter. Most of the energy which is absorbed is converted to heat.

Reflection is the redirection of sound in a single direction back to the medium where it originated after striking an interface, such as a blood-tissue interface of a cardiac structure. The physical dimensions of blood -tissue interfaces in the heart are large with smooth, flat surfaces. Imaging is based primarily on sound reflection.

Scatter results when the sound beam is redirected in many directions after striking an interface which has small physical dimensions with a rough surface. The liver and blood stasis in a cardiac chamber may be imaged through scattering.

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