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A Blurb From Bruce

Mixed up About Venous Oxygen Saturations? Go Central or Go Home!

Red Blood Cell

Mixed venous oxygen saturations can help to establish the relationship between oxygen consumption and oxygen delivery.  Understanding the difference between the amount of oxygen being delivered and the amount of oxygen being consumed by the body can lead the clinician to early detection of a soon-to-be-deteriorating patient.  The mixed venous oxygen saturation is a key monitoring parameter for both the in-hospital and pre-hospital critical care provider.  Average mixed venous oxygen saturation in a healthy adult patient is approximately 70%.

When learning about mixed venous oxygen saturations, there are often two variables that are discussed – SvO2 and ScvO2; the difference being the addition of the letter “c” in the name.  The SvO2 is typically the oxygen saturation of mixed venous blood as it is found in the pulmonary artery.  Remember that this blood is a mixture of different sources of venous blood – the inferior and superior vena cava and the coronary sinus; whereas the ScvO2 is primarily just a sample of the vena cava blood.  Knowing this, we can clearly understand why the ScvO2 is slightly higher (3-5%) than the SvO2.  Mixing the vena caval blood with the coronary sinus (used blood from the myocardium) causes a further reduction in the saturation of oxygen on the hemoglobin.

If the body is consuming increased levels of oxygen, the amount of oxygen returning to the heart would be low.  This is typically seen in situations where the body’s cells are hyperactive and demanding more oxygen.  On the contrary, if the SvO2 comes back high, the body is typically being blocked from extracting oxygen from the blood into the tissues.  Situations like drug-induced paralysis or infiltrative microbial disease could be a cause of such.

Overall, the efficacy of circulation can be best measured by a mixed venous oxygen saturation.  The more central you obtain the sample, the more accurate the result.  Remember, the difference between oxygen consumption and oxygen demand, lies in the value of the mixed venous oxygen saturation.

 Peer Review #1:

While many of you may dismiss this blog under the premise that SvO2 (and certainly the more invasive ScvO2) are going the way of the Swan-Ganz catheter, replaced by newer metrics such as lactate and end-tidal CO2. However, even if your daily practice does not require interpretation of central oxygen saturation values, the concept of shock as a mismatch of supply and demand is always worth reviewing, and several new devices are about to hit the airwaves that will play off this theme.

In the world of resuscitation, the sole purpose of the human body is to deliver oxygenated blood to tissues. While oxygenation status is definitely a contributor, the most clinically relevant factor that limits this delivery is a decrease in tissue perfusion via limited blood volume, pump failure, and/or shunting away from the microcirculation. This explains why we generally get away with focusing on the oxygen content of blood returning to the heart, whether through measurement of blood samples taken from the vena cava or coronary sinus or via an oxygen saturation-measuring central venous catheter, rather than always comparing these values to measurements taken from the arterial side of the circulation.

The point is well made that a true systemic sample - either SvO2 or ScvO2 - is necessary to gain insight into the perfusion status of the entire body rather than a single limb (as with oxygenation measurements taken from a peripheral catheter) or specifically from the brain (as with a jugular venous sample). However, new devices are available for noninvasive tissue measurements, requiring careful site selection and understanding of the specific perfusion patterns for that tissue under pathophysiological conditions. Examples include near-infrared measurement of cerebral lactate-to-pyruvate ratios (again, reflecting the delicate balance between supply and demand), gastric pH, rectal mucosal perfusion (surprisingly, not as popular to study!), and skeletal muscle oxygen saturation (SmO2). It is clear that we will need to gain experience with some of these indirect and/or organ-specific devices before we can use them most effectively to care for our patients. In the meantime, keep the SvO2/ScvO2 lessons in mind and understand the basic paradigm of shock as a mismatch of supply and demand.

Dr. Dan Davis, MD

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