If you are in a critical care clinical rotation, or new to the ICU, you may hear the term “mixed gas” thrown around here and there. What is a mixed gas, why do you measure it, and what does it tell us about your patient?
“Mixed gas” is shorthand for an SVO2 measurement or “mixed venous gas,” which looks at the oxygen saturation levels of blood obtained from the pulmonary artery. However, not every ICU uses PA catheters regularly, but never fear…you can still get some usable data by using blood from the superior vena cava. Note that when the blood is drawn from a central line placed in the superior vena cava vs the PA catheter, the measurement is referred to as an ScVO2. In the ICU where I work, we don’t use PA catheters much , so our mixed gasses are typically taken from the superior vena cava and are used to obtain information related to severe sepsis and septic shock. Unless you are working in a cardiac-surgery ICU, your mixed gas measurements will most likely be done as ScVO2 and related to sepsis and/or shock, so that’s what we’ll focus on here.
To obtain your mixed gas or ScVO2, you’re going to draw a VBG or venous blood gas. It’s a lot like an ABG, only it’s blood from the vein. One of the nifty things about a VBG is that it can also give you some other really valuable information such as the blood pH and carbon dioxide levels. As a quick aside, if you need to know your patient’s pH and you’re not able to obtain an ABG (maybe they’re on max vasopressors and their arteries are all clamped down), you can grab a VBG off your central line and voila…pH for everyone!
So, what does a mixed gas tell us? Basically, it tells us about oxygen consumption and delivery throughout the body, and thus provides a way to detect global tissue hypoxia. As blood comes out of the heart, all freshly oxygenated, it loses oxygen as it flows throughout the circuit delivering oxygen to the cells. In a normal, healthy, hemodynamically-stable patient, the body uses about 25-30% of the oxygen in the blood as it travels from the left ventricle through systemic circulation and back to the right side of the heart. So, in our “normal” patient, the ScVO2 will measure 65 to 80%. With the ScVO2, this is one instance where we’re not trying to shoot for 100%…if that were the case, it would indicate that the body is not picking up any oxygen as the blood makes its way around the circuit (which would be very bad for your patient indeed) so that’s why the range is from 65-80%.
If your mixed gas is less than 65%, be concerned about:
- Decreased oxygen delivery (anemia, blood loss/hemorrhage, hypoxia, hypovolemia, heart failure)
- Increased oxygen consumption (pain, fever, shivering, agitation, respiratory failure, or increased metabolic demand)
If your mixed gas is > 80%, be concerned about:
- Increased oxygen delivery (high FiO2, increased cardiac output, hypervolemia, blood transfusions)
- Decreased oxygen consumption (sedation, pain meds, hypothermia, mechanical ventilation)
- Decreased oxygen extraction (shunting in sepsis, cell lysiss.
So, what are you going to do about it, Super Nurse? It depends on what is causing the mixed gas to be off. Give blood, give fluids, give oxygen, relieve pain, etc.. In the case of a septic patient, if your ScVO2 is too low despite fluids and vasopressors (less than 70% according to the Surviving Sepsis Guidelines), you’re going to add an inotropic agent such as dobutamine which will increase cardiac output. You also want to increase oxygen carrying capacity, so if your hematocrit is below 30%, you’ll be giving one or two units of PRBCs. Of course, there are other reasons your patient’s mixed gas could be outside normal parameters, so always try to determine what is causing it in the first place.
This has been a very basic introduction to mixed venous gas and is certainly enough to get you through your ICU clinical orientation, but if you are interested in the topic there is a wealth of information out there that is continually being updated as new knowledge is obtained.
I hope you found this helpful, and be safe out there!