Where To Draw Mixed Venous Blood Gas

If you lot are in a critical intendance 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 practise you measure out it, and what does it tell united states of america near your patient?

"Mixed gas" is autograph 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…yous can yet get some usable data by using blood from the superior vena cava. Annotation 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 employ PA catheters much , so our mixed gasses are typically taken from the superior vena cava and are used to obtain data related to severe sepsis and septic stupor. Unless you are working in a cardiac-surgery ICU, your mixed gas measurements volition nearly likely be done equally ScVO2 and related to sepsis and/or shock, then that'southward what we'll focus on hither.

To obtain your mixed gas or ScVO2, you're going to draw a VBG or venous blood gas. Information technology's a lot like an ABG, simply it'southward blood from the vein. One of the nifty things near a VBG is that it can as well give you another really valuable information such every bit the claret pH and carbon dioxide levels. Every bit a quick aside, if you need to know your patient's pH and yous're non able to obtain an ABG (maybe they're on max vasopressors and their arteries are all clamped downward), 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 u.s.a. about oxygen consumption and delivery throughout the trunk, and thus provides a fashion to detect global tissue hypoxia. As blood comes out of the centre, all freshly oxygenated, it loses oxygen as it flows throughout the circuit delivering oxygen to the cells. In a normal, good for you, hemodynamically-stable patient, the body uses well-nigh 25-30% of the oxygen in the claret as it travels from the left ventricle through systemic circulation and back to the right side of the middle. So, in our "normal" patient, the ScVO2 will measure 65 to lxxx%. With the ScVO2, this is one instance where we're non trying to shoot for 100%…if that were the case, it would signal that the body is non picking up whatever oxygen as the blood makes its manner effectually the circuit (which would be very bad for your patient indeed) then that's why the range is from 65-80%.

If your mixed gas is less than 65%, be concerned virtually:

Decreased oxygen commitment (anemia, claret loss/hemorrhage, hypoxia, hypovolemia, heart failure)
Increased oxygen consumption (pain, fever, shivering, agitation, respiratory failure, or increased metabolic need)

If your mixed gas is > eighty%, be concerned nigh:

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, prison cell lysiss.

So, what are yous going to do about it, Super Nurse? It depends on what is causing the mixed gas to exist off. Requite blood, requite fluids, requite oxygen, relieve pain, etc.. In the case of a septic patient, if your ScVO2 is too low despite fluids and vasopressors (less than lxx% according to the Surviving Sepsis Guidelines), you're going to add an inotropic agent such as dobutamine which will increment cardiac output. Y'all also want to increase oxygen conveying capacity, and so if your hematocrit is below 30%, you'll be giving i or two units of PRBCs. Of course, in that location are other reasons your patient's mixed gas could be outside normal parameters, then always try to decide what is causing it in the showtime place.

This has been a very bones introduction to mixed venous gas and is certainly plenty to get you through your ICU clinical orientation, only if you are interested in the topic there is a wealth of data out there that is continually existence updated as new knowledge is obtained.

I hope you found this helpful, and be safe out there!

The basics of ScVO2 or "mixed gas."