You’ve heard all these fancy terms, mixed venous blood gas, ScvO2, SvO2, thrown around the ICU all the time. Here, I explain what they are. Regardless of whether you’re a nurse, respiratory therapist, medical student, resident, or even a fellow, these terms may sometimes be quite confusing as everyone talks about them like, “duh, you’re supposed to know this”. If you have to determine whether you’re going to measure SvO2 vs. ScVO2, it all comes down to the catheter type.

You’re here because you don’t know this. Or perhaps you don’t know that you don’t know this. Or perhaps you need a refresher course. I am not going to get into the deep dive complexities of venous oxygen levels as that could be a complete chapter in a textbook but feel free to ask questions below and either I or our community will try to answer the questions. 

SvO2:

– The meaning of SvO2 is venous oxygen saturation 
– taken from a Swan Ganz catheter also called a PA or pulmonary artery catheter. Given then tools normally found in the ICU, one cannot measure this value without these catheters. 
– also called a mixed venous oxygen saturation. 
– this is a combination of venous oxygen from both the SVC and IVC

ScvO2

– The meaning of ScvO2 is central venous oxygen saturation
– taken from a central line that terminates in the Superior Vena Cava or right atrium. That central line could be placed in the internal jugular, subclavian, or axillary vein.
– it’s easy to confuse the two because the PA is more “central” but think about it as getting this value from the “central line”

Correlation between ScvO2 & SvO2

ScvO2 is generally 5-6% higher (sometimes more) as this blood comes from the brain and upper extremities which generally consume less O2 than the organs and lower extremities. This is more pronounced in shock states. There is also some data where this is shown to be the opposite. The cited data states the former.

Normal Range SvO2 and ScVO2

Generally speaking, the normal ScvO2 and SvO2 is above 70%. Remember that these values are dependent on two main components, oxygen delivery (DO2) and oxygen consumption (VO2).
The normal range of SvO2 and ScvO2 is 70-75%. Now this comes with many caveats as many patients are different. Their underlying pathologies for which they’re in the ICU could lead to derangements in this value where we accept a lower number and just figure that the patient “lives there”.

What causes decreased ScvO2 & SvO2?

When considering oxygen delivery you need to think about the actual concentration of O2 in the blood, the PaO2.
You also need to think about what transports blood, the hemoglobin.
Lastly, you need to think about the cardiac output which is what makes the blood circulate around the body. An issue in any of these three will cause a drop in the ScvO2 or SvO2.
When looking at consumption of O2 in the tissues, the VO2, you can potentially think through these as you are managing your patient. Shivering, pain, hyperthermia, stress, and an increased work of breathing could all increase your VO2 and therefore lead to a decrease in your SvO2 or ScvO2.

Monitoring of ScvO2 and SvO2

Monitoring of these two values is usually done via the actual catheter being hooked up to the monitor or the swan box. If I’m being quite frank, I personally place the line and my awesome nursing staff makes this happen. Nonetheless, with a PA catheter you can trend the SvO2. There are certain types of central venous catheters which will allow you to trend the ScvO2. I personally do not have experience with the latter.

Does ScvO2 work for fluid responsiveness?

Now, we learned as a static measurement, ScvO2 does not work to predict fluid responsiveness. But what if one were to study the delta ScvO2? As in, obtain the baseline ScvO2 of a patient, provide a fluid bolus, and then assess the ScvO2 after that? Thankfully, Khalil et al. performed that exact investigation to see if it was worth something. Hat tip to that team.

Whenever one of these studies on fluid responsiveness are conducted, the first thing I wonder is what did they compare it to. As in, what was defined as the gold standard, of sorts. Here we have to remember the definition of fluid responsiveness. It is NOT seeing an increase in the blood pressure after giving a bolus of fluids. It is providing an increase in cardiac output/index depending on how you express your measures, or an increase in stroke volume. If you need to learn more about fluid responsiveness, I have a dedicated post HERE.

How was fluid responsiveness defined?

Here, they defined fluid responsiveness as an increase in cardiac output (CO) greater than or equal to 15% after a 500cc fluid bolus. Every patient had a method of measuring thermodilution which I am assuming means that they placed a pulmonary artery (Swan-Ganz) catheter in all these patients in the emergency department. Obtaining consent must have been interesting when enrolling these patients of which they were able to enroll 88 of them.

Interesting findings included that 30.7% enrolled with shock were not fluid responsive. ScvO2 increased in patients who were fluid responsive and actually went down in patients who were not fluid responsive. They calculated the delta ScvO2 from prior to volume expansion to after volume expansion and they found this to be associated with fluid responsiveness. Great win! They calculated the diagnostic performance of ∆ScvO2 and the area under the ROC was 0.84 which is excellent for predicting fluid responsiveness.

What ScvO2 value should we be looking for to predict fluid responsiveness?

The authors state that the “best cut-off value found was 4%”. What this means is that if a patients ScvO2 at baseline is 64% and then after a 500cc bolus it increases to 72%, the patient is fluid responsive and may benefit from more. The authors used their requisite wording and stated “that studies with larger populations are required”. I emailed the authors to find out what types of catheters they used for obtaining the PA and whether they placed a secondary catheter to obtain the ScvO2 in the SVC.

The article where I have obtained some of this information is FREE! A hat tip to the authors. I have also posted many articles on resuscitation and fluid responsiveness HERE.
-EJ

Systematic Review and Meta-analysis of using ScVO2 for Fluid responsiveness

To support the data above, a systematic review and meta-analysis was published in May of 2023 looking at using ScVO2 for fluid responsiveness. Hat tip to the authors. Read these data for yourself as it is free for you to download. They analyzed 240 patients in five observational trials. ΔScvO2 exhibited excellent performance for defining fluid responsiveness. The area under the receiver operating characteristic curve (AUHSROC) was 0.86. Gold standard type tests have an AUC of 0.9 or greater, but this is still excellent. The mean cutoff value for ΔScvO2 was 4%. If the ΔScvO2 is 4% or greater, the patient is defined as being fluid responsive. Some caveats include the fact that these patients were all on mechanical ventilation. The largest is that 4% falls within the margin of error of some blood gas analyzers. Will this change your practice?

Citations for SvO2 and ScVO2:

Rivers EP, McIntyre L, Morro DC, Rivers KK. Early and innovative interventions for severe sepsis and septic shock: taking advantage of a window of opportunity. CMAJ. 2005 Oct 25;173(9):1054-65. doi: 10.1503/cmaj.050632. PMID: 16247103; PMCID: PMC1266331.
Website with Article
Link to FULL FREE PDF
Reinhart Paper Link

Khalil MH, Sekma A, Zhani W, Zorgati A, Ben Soltane H, Nouira S; GREAT Network. Variation in central venous oxygen saturation to assess volume responsiveness in hemodynamically unstable patients under mechanical ventilation: a prospective cohort study. Crit Care. 2021 Jul 13;25(1):245. doi: 10.1186/s13054-021-03683-6. PMID: 34256822.
Link to Article
Link to FULL FREE PDF

Pan, J., Sun, Y., Xu, Z. et al. Variation in central venous oxygen saturation to evaluate fluid responsiveness: a systematic review and meta-analysis. Crit Care 27, 203 (2023). https://doi.org/10.1186/s13054-023-04480-z
Link to Article
Link to FULL FREE PDF

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