Mixed venous sample from a Swan

[CABG patch kid, BSN, RN]

Okay, I've looked in my books, searched online, and I still can't find the answer. Here's the situation: Where I work, we draw mixed venous samples from the Swan (as opposed to continuous SvO2 monitoring). Our intensivist ordered for a sample to be drawn from the proximal port and the distal port. When he got the results he said "What the heck? This lady's PA sat is lower than her venous sat?" And then went on to say that he thought she might have some shunting going on because of those numbers.

What I don't understand is this--wouldn't it be normal for the blood to be LESS oxygenated in the PA normally? There isn't any gas exchange going on in the PA, correct? The blood hasn't reached the lungs quite yet. So why would he expect it to be higher in the PA??

His next move was a 2D echo with bubble study to see if there was actually any shunting (there wasn't). So then I guess he came to the conclusion that her sats were due to her pneumonia.

Just to give you background, this lady came in SOB, hypoxic, got a Swan, has high PA pressures (like 40's-50's), normal CVP, high wedge (around the 20's). First day she was admitted she ended up getting more and more bradycardic on us, ended up getting intubated as well. Thed mixed venous samples were taken before she was intubated. Also, the 2D echo showed moderate mitral regurg.

[sicushells, RN]

#1 You're comparing the sample drawn from the distal port (the mixed venous) to that drawn from the proximal port (a venous gas) right?

#2 A normal pO2 on a venous gas is around 40mm Hg (http://www.emedmag.com/html/pre/fea/features/038120044.asp) normal SvO2 is 60-75 mmHg, so venous should be lower based on the normal values. When you think about it, even though there is no gas exchange occuring in the PA, the venous blood in the PA is mixed with blood that has undergone gas exchange in the lungs, thus the term "mixed venous".

I hope that helped explain a little better about what values you are looking for with mixed/venous gases.

www.pacep.org is a good reference for understanding pulmonary art. catheters.

[CABG patch kid, BSN, RN]

To sicushells: yes, we were comparing venous (proximal) to mixed venous (distal).

Okay, now I'm really confused because I just found a study that compared ScvO2 (sample from proximal port in RA) and SvO2 (sample from distal port in PA) and the study consistently showed that SvO2 is lower than ScvO2, time and time again. There are other studies to back this up. The explanation I finally found is that the PA contains blood that is mixed with blood from the SVC, IVC and coronary sinus (hence the name mixed venous, it is not mixed with arterial blood). Since the O2 consumption of the myocardium is so high, the blood returned from the coronary sinus will have a very low O2 sat, which makes SvO2 lower than ScvO2.

Now I guess I'll still have to ask around at work because either the doctor was really tired and thought there was something abnormal when it really wasn't, or maybe I'm remembering things wrong and it was really higher in the PA. Or, maybe there is something that I'm totally missing and I need to go pick the brain of our pulmonologist.

[kdavis308]

Your initial thoughts were correct the SVO2 is lower than the ScVO2. Where people see this reversed is drawing from the tip to fast pulling a small amount of blood from within the lung. You will then get a falsely high reading. On avg. expect the SvO2 to be about 5 less than the ScVO2. By the way, more than one study has demonstrated an ScVO2 is adequate alone.

[PostOpPrincess, BSN, RN]

Did you get a full pulmonary profile, btw?

Would be interesting to see it all.

[dr who]

It should be expected that a venous oxygen saturation will be lower in the pulmonary artery than in the SVC. Recall that the ScVO2 is lower than the SVO2. Why is this? Recall that the coronary sinus empties blood from the heart into the right atrium. This coronary venous blood has a venous oxygen saturation LOWER than that returning from the SVC, lower than that returning from the IVC, because the heart extracts oxygen more so than other tissues (CSO2sat is usually around 20-40%; whereas SVCO2sat is usually around 70s%).

Thus, the blood from the coronary sinus dilutes the blood from the body, and after mixing and going into the pulmonary artery, lowers your PA catheter Saturations.

Your intensivist friend should have not been surprised by this finding. In fact, should there have been left-to-right shunting of blood through the atrial septum, that should only have added oxygenated blood to the mix and increased the PA cath Saturations. Perhaps, your intensivist friend was alarmed that the SvO2 was higher than the SvcO2 and was looking for a left-to-right shunt. That would explain thing well also, but then you would have been the one who missed what he/she said.

OR, perhaps he was concerned about the magnitude of difference and was wondering why the SvcO2 was so high, thinking that a left-to-right atrial shunt was sending a jet of oxygenated blood close to the oximeter, thus giving interest about a shunt.

Dr. Who