Question about SvO2, VSD, PCW Sat

[HillaryC, RN, CRNA]

Hi everyone,

Yesterday I was in a CCRN review course, and there were a couple of things mentioned during the CV lecture that left me confused. We were going over a few practice questions, and one had to do with VSD and sats in the RA, PA, LA -- basically, how can you tell there's a VSD. The instructor mentioned the idea of a "step up", but she was saying you'd compare sats in the RV vs. a Pulm. Cap. Wedge Sat (not pressure), even though the question didn't say Wedge sat, it said sat in the PA. She also said she thought there was a typo in the question, which I'm not sure about. One choice was comparing RA and PA, which I thought was the answer, because normally, sat wouldn't change between the RA and the PA, right? She was saying a wedge sat looks forward and can tell oxygenation on the L side of the heart. I understand that's how a wedge pressure works, but if you draw blood with the balloon up, you're still just getting blood from a pulmonary capillary, aren't you? the blood has a ways to go before it is oxygenated, doesn't it?

This instructor is very knowledgeable about CV topics, so I don't like to doubt her, but she did make a mistake on one other part of the lecture (she insisted that first two letters in a pacer stood for chamber sensed then chamber paced). I just think since she was saying there was a typo and reading PA as PCW sat (which I'm pretty sure you don't have to know about for the CCRN), that maybe she was reading too much into the question. I also could swear that I had this question on the practice CD and read the rationale correctly.

Am I right about this? Shouldn't sat normally stay the same between the RA and the PA?

Thanks,

Hillary

[rcpals]

The reason PAC blood is different than RA blood is this. RA blood just represents the sum of all venous blood returning to the heart. Blood from the pulmonary artery is "mixed Venous". In order to be mixed venous it must go through 2 heart valves to get mixed with blood coming from the coronary sinus etc... PA has a higher Saturation (around 70%)

than RA.

Simple, but basically correct

[HillaryC, RN, CRNA]

The reason PAC blood is different than RA blood is this. RA blood just represents the sum of all venous blood returning to the heart. Blood from the pulmonary artery is "mixed Venous". In order to be mixed venous it must go through 2 heart valves to get mixed with blood coming from the coronary sinus etc... PA has a higher Saturation (around 70%)

than RA.

Simple, but basically correct

Ok, I guess that makes sense. Obviously I still have a lot to learn about this :) Blood from the coronary sinus is deoxygenated, right? Can you give me examples of what sats in the RA, RV, and PA would be in a normal person? A textbook SvO2 would be 75%, right?

I found the original question. The first two choices are obviously wrong:

A diagnosis of ventricular septal rupture could best be confirmed by:

(A) CXR, to check for an alteration in cardiac silhouette

(B) CO measurements

© Oxygen saturation levels in the PA and the left atrium

(D) Oxygen saturation levels in the PA and the right atrium

So, which would you choose, C or D? What threw me off and led me to think the instructor might be wrong is that (1) she said D was a typo and should have said RV (not RA), and (2) she started talking about a PCW sat, which is not in the question, and not part of CCRN curriculum, as far as I know.

Thanks so much! I'm trying to read up on SvO2 and all this stuff. I don't know enough about cardiac anatomy!

Hillary

[rcpals]

Hi Hilary,

This is how I believe it works. If you have a Ventricular Septal Defect (VSD) / rupture, the high pressure in the LV will push a little (or alot depending on the size of the VSD / Rupture) oxygenated blood into the RV through the VSD. This will increase Svo2 in the RV and PA because freshly oxygenated blood from the LV will be mixing with the purely venous blood of the RV. The larger the hole the closer RV and PA Svo2 will approximate arterial Sao2. So if post anteroseptal MI the patients Svo2 suddenly rises we might suspect VSD / rupture. I hope this helps. I believer the PCW sat she is talking about is just Svo2. It's not actually a wedged sat. The PAC is just floating in the PA.

The correct answer is D if you replace RA with RV. ( I believe this is accurate.) Anyone else??? I'm sure some CCRN's and Cardiothoracic RNs can help out on this.

Jeff RCP