Venous Oxygen Saturation (Nursing)

00:01 Our next concept is mixed venous oxygen saturation.

00:06 In my practice, it took me more than two years to completely understand this.

00:10 So if you don't get it right off the bat, it's okay, just go back and keep going over the slides until you finally get it.

00:19 Mixed venous oxygenation, the normal is about 60%-80%.

00:23 What this means is, it's the percentage of oxygen that is returning to the heart.

00:28 So the lungs put oxygen into the blood, and they attach to the hemoglobin, and then the heart ejects it out to the body, the body then uses it, but it doesn't use all of it, we only want it to use about 20%-40%.

00:43 So that means that 60%-80% is returning to the heart.

00:46 So the lungs doesn't have to pump so much oxygen back into the heart and back into the blood, it only needs to do about 20%.

00:55 So again, think about mixed venous oxygenation is just the oxygen that's left over after the body uses it.

01:01 Now we call this a mixed venous oxygen saturation because we're getting oxygen returning from the brain, the arms, the coronary sinuses or the coronary veins, that gut, the legs, everywhere, it's gets all mixed up into that pulmonary artery.

01:19 And then we draw an oxygenation from it.

01:22 Now, if we have a central line, and we draw an oxygenation from that, that's called an ScvO2.

01:30 So you get a snapshot of what's happening but you may not see everything in the body.

01:35 So a mixed venous is a little bit better than an ScvO2.

01:40 What are the determinants of an SvO2? That's oxygenation, hemoglobin, cardiac output and metabolic demand.

01:49 So we've discussed the SvO2 determinants.

01:51 Now let's break those down further so we can understand them a little bit more clearly.

01:57 We break the determinants into two sections, oxygen delivery and oxygen consumption.

02:02 We're going to focus on oxygen delivery right now.

02:05 When we look at oxygen delivery, I want you to think of three things: lungs, blood and heart.

02:11 We're learning all of these things so if we have a low SvO2, we can try to analyze and find out what's the cause of my low SvO2.

02:19 First off, we're going to look at lungs, oxygenation.

02:22 The whole goal of the lungs is to get oxygen into the bloodstream.

02:27 So how do we know if we're getting enough oxygen into the bloodstream? One thing we can do is look up at our pulse ox.

02:37 And see if we are almost to 100%.

02:40 We like our oxygen saturation to be between 92% and 100%.

02:46 But that doesn't give us a clear understanding of what the lungs are completely doing.

02:50 It's just a snapshot.

02:52 So there's another test that we can perform in order to get a clearer picture of the lungs to make sure they're doing everything that they're supposed to be doing.

02:59 That test is an arterial blood gas analysis.

03:02 When you get an ABG, there's two things I want you to look at when you're trying to determine why my SvO2 is low.

03:10 First off is PaO2.

03:11 And then we look at SaO2.

03:14 With PaO2, this is just how many oxygen molecules are diffused in the bloodstream.

03:20 Think about a soda bottle or a coke can, when you open that up, What comes out? Carbonation comes out, that CO2 that's diffused in the soda.

03:33 It's the same thing with oxygen and blood.

03:37 When we're looking at PaO2, it's how many molecules of oxygen are diffused in that bloodstream.

03:43 We'd like it to be about 80 to 100, anything greater than 100 and we're starting to get into hyperoxia anemia.

03:50 So we want between 80 and 100.

03:52 If we're less than 80, then we're not getting enough oxygen into the bloodstream.

03:57 Next, we're going to look at SaO2.

03:59 This is how many of your hemoglobin are fully saturated with oxygen.

04:04 This correlates with your pulse ox, so we want it to be about 92%-100%.

04:09 If we're under 92%, then we don't have enough oxygen that to test your hemoglobin.

04:16 So think about it.

04:17 If our SvO2 we want it to be normals of 60%-80%.

04:22 And our pulse ox, we want it to be 92%-100%.

04:26 If we start with a pulse ox that's already low, meaning let's say our pulse ox is about 88% or our SaO2 is about 88%.

04:35 Then the body uses it, it's going to be low when it gets back to that right so the heart and when we take a sample of it.

04:41 It may be, you know, the body may be pulling 20% of that away.

04:45 So for starting at 88%, we're going to finish at 68%.

04:50 If we're going to start at let's say 75% oxygen saturation, which is a big deal.

04:57 We're going to go down to 55%.

05:01 So when you identified that your oxygenation may be the issue, what can we do about it? Well, we can increase the amount of oxygen that they're breathing in, maybe by nasal cannula or increasing the FiO2 if they're intubated.

05:16 Maybe we need to increase the respiratory rate, maybe we need to increase the amount of ventilation, or the PEEP, the positive end-expiratory pressure.

05:24 All of these things can affect how many oxygen molecules get into the bloodstream.

05:30 The second part we look at, remember lungs, blood, heart, the second thing we're going to look at is blood.

05:36 So, if your lungs are getting enough oxygen into the bloodstream, the part of the blood that's carries that away is the hemoglobin.

05:44 We need to make sure that we have enough hemoglobin to carry the oxygen to the tissues.

05:51 So we may have an oxygen saturation of 100%.

05:54 But if you've got a hemoglobin that's really low, the body's just going to extract all of that and your SvO2 is going to be low.

06:01 So what causes hemoglobin to be too low? Things such as bleeding, hemodilution and anemia.

06:08 So we want to look at also on your ABG, you should have a hemoglobin level.

06:13 So let's make sure our hemoglobin, we have enough oxygen carrying capacity, meaning we have enough hemoglobin to support the metabolic demands.

06:21 Now the third part we're going to look at with oxygen delivery is heart.

06:25 Remember, lungs, blood and heart.

06:27 If our lungs are getting enough oxygen into the bloodstream, and our blood has enough hemoglobin or oxygen carrying capacity to take that oxygen to where it needs to go, we need to have a strong enough left ventricle to get that oxygen to perfuse every part of the body.

06:43 We're going to look at cardiac output and cardiac index to determine that.

06:48 If our cardiac output or index is too low, then we're not basically perfusing the body and getting enough oxygen to it, which means our SvO2 is going to be too low.

06:58 Now remember, the determinants of cardiac output are heart rate and stroke volume.

07:03 Now, when we break down stroke volume, the determinants of that is preload, afterload and contractility.

07:10 So we need to make sure if our SvO2 is too low, and we say, "Okay, I've got enough oxygen of my ABG and I've got enough hemoglobin, the last part of the oxygen delivery, it could be my heart's not strong enough." So we need to figure out what's going on with the heart that we're not getting enough cardiac output or in the flow of that left ventricle.

07:32 We're going to dive into that a little bit later so we can get an understanding.

07:35 So I'm not going to talk all about it right now.

07:39 But if we determine that our oxygen, our hemoglobin and our cardiac output is good, meaning if we see that our lungs, our blood and our heart are doing what they're supposed to do, then that means our oxygen delivery is great.

07:52 So the only thing left to determine why our SvO2 is low is our metabolic demand, meaning our body is consuming too much oxygen.

08:00 We see this in shivering, fever, anxiety, pain, increased muscle activity, and increased work of breathing.

08:11 All of these increase the amount of oxygen that we're going to be consuming, which means the amount of oxygen returning to the right side of the heart is going to be low.

08:19 Now, in our patients that are really, really sick in their critical care environment, many times there's multiple things attributing to a low SvO2.

08:28 So what I'm saying is you can't look at one thing and go, "Okay, it has to be the lungs or it has to be low hemoglobin." You could have low oxygenation and low hemoglobin or you could have low oxygenation and a borderline cardiac output and an increased metabolic demand that are all three causing your SvO2 to be low.

08:49 So we have to look at all these all the time when our SvO2 is low to determine what are the causes of our SvO2 being low.

08:59 Remember, our SvO2 normal is 60%-80%.

09:03 Now this can fluctuate when we increase patient activity, or we such as turning, bathing, suctioning, your SvO2 may drop and that's okay.

09:16 It needs to return to normal within a few seconds.

09:19 If it doesn't, if it slowly recovers, meaning let's say we were at 60% when the patient was just supine, then we turned them, we bathe them and we brought them back and now it's dropped to 40% and it's taking minutes to get back up to a normal SvO2.

09:34 This is indication of cardiopulmonary issues, you've got a weak heart, or you've got bad lungs or a combination of both that are not able to allow that patient to recover quickly.

09:46 Just keep that in mind.

09:49 Let's break these down, low SvO2.

09:52 What can cause a low SvO2? My hemoglobin is too low, my SaO2 is too low, or my cardiac output is not high enough to meet tissue needs.

10:03 Remember, lungs, blood, heart.

10:06 My lungs aren't doing their job so my SaO2 is too low.

10:10 I've got anemia or I've got bleeding or hemodilution, so my hemoglobin is too low.

10:15 So I don't have enough oxygen carrying capacity, or my heart isn't strong enough that left ventricle is not doing what it's supposed to.

10:22 And my cardiac output is too low not meeting my tissue demands.

10:26 If those are okay, then remember my oxygen consumption is too much.

10:31 I had a patient one time that was intubated and he was sedated, and everything was fine on the oxygen delivery side.

10:40 But my SvO2 remained in the 40s.

10:43 Well, what we found out was the patients were breathing and he was a little bit agitated, and his muscles were just pretty rigid, causing his oxygen consumption to be way too high.

10:54 So we ended up having to paralyze this patient, which greatly increased my SvO2.

11:01 So this patient could recover more quickly.

11:04 Now let's look at the causes of a high SvO2.

11:07 What would cause your mixed venous oxygenation to be higher than 80%? The first cause could be the initial stages of sepsis.

11:15 Remember, we have a bacterial infection that's causing mass vasodilation.

11:19 And to over compensate for that, our left ventricle begins to contract harder, so our cardiac output goes way high.

11:26 Because of this, our SvO2 will increase greater than 80%.

11:31 The second cause could be an intracardiac shunt.

11:34 This is a hole in the septum that allows oxygenated blood to not go out to the body, but rather go back over to the right side.

11:42 This is your atrial septal defects, your ventricular septal defects or your patent foramen ovales.

11:48 So you got a hole and the oxygenated blood goes back over and then your pulmonary artery catheter reads that oxygenated blood so you'll have SvO2 that are 85%, 95% and that's because you've just got oxygenated blood before it goes back into the lungs.

12:03 The last cause of this, though rare is cyanide poisoning, which can cause an elevated SvO2.