Identifying changes in the ST segment is key to detecting myocardial ischemia. In this video from our Exercise Stress ECG Essentials course, we'll examine patient ECGs in order to learn how to identify baseline segments, know which leads to look at, and understand why the "three-in-a-row" rule is important when analyzing stress ECGs.
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This course covers the essentials of exercise stress testing—what it is, when to do it, and how to do it. Along the way, we'll discuss issues such as which mode of exercise to use (e.g., treadmill versus cycle ergometer), when it’s safe to continue or when to stop the test, how to deal with ECG artifacts, and how to describe the test results.
[00:00:00] So, in terms of deciding whether the test is positive for myocardial ischemia, the ST segment is really the key thing that we need to look at. So, to review what that means, the ST segment is this portion, here and it's defined as the end of the QRS, right here, which is also known as the J point, the end of the QRS or the J point. We start there and we go to the beginning of the T wave. Now, in this case, it's very easy to see where the T wave starts. We find sometimes the T wave kind of blurs into the
[00:00:30] ST segment and it's a little bit harder to figure out. So, our ST segment is right, here. If the test is positive for myocardial ischemia, we're going to have, typically, a depression of the ST segment. If the test is negative for myocardial ischemia, the ST segment is going to remain where it is. That's referred to as on baseline. Another way to say that is isoelectric. So, isoelectric or baseline means there's no change, no significant change. So here, we have our ST segment. The baseline we'll deal with in a bit
[00:01:00] but this is the baseline, here and our ST segment is basically on the baseline. Here's our baseline and here, our ST segment is significantly below the level of our baseline. Also, see here, that our T wave is probably somewhere in, here and here. It's probably biphasic T wave and it's a little difficult to see, actually, where the T wave begins. Here, it's clear where the T wave starts, though it's unusual, that little bump on the T wave. ST segment elevation is somewhat rare
[00:01:30] during a stress test but this is very serious. Here's our baseline and here's our ST segment. T wave is here somewhere. Again, it's hard to say exactly where the T wave begins. This ST segment is supposed to be down here, so this ST segment is significantly elevated. And this is usually a sign of a very serious occlusion of a very major coronary artery, for example, 99% occlusion of the left main is what gave us this, right here, during stress. In terms of the baseline, what’s typically used for exercise stress testing
[00:02:00] is the PR segment. So, let's review what that is. Here's our P wave and it's very easy to see where the P wave ends. That's not always the case. Where the P wave ends is where the PR segment begins. Where the QRS begins, which is right here, in this case, is where our PR segment ends. So basically, this is our PR segment, right here and this is our baseline, and this should be at roughly the same level as our ST segment. Here's our ST segment again. Isoelectric is another term for an ST segment that's on baseline.
[00:02:30] We have drawn a line, here, to show you where the baseline is in the PR segments and we can see that the ST segments are significantly depressed, at least, here. Notice, here, it looks different and this is because we almost always are going to have some motion artifact, when the patient is walking on the treadmill. So, we'll deal with this issue later but you're typically going to see that the amount of ST segment change is going to vary from QRS to QRS, based on the patient moving on the treadmill.
[00:03:00] Here's our baseline and here's our ST segment elevated above baseline. Someone once said that reading a technically poor echocardiogram is like looking for a polar bear in a snowstorm. We have a similar problem sometimes, with reading an EKG, with a lot of artifact. Here, we have probably our baseline, ST segment, baseline, very different looking ST segment, baseline, ST segment. Hard to see what's going on, little hard to even see the P waves. This is because the patient is moving around.
[00:03:30] Typically, your obese patients, there's going to be more flesh moving around, you're going to have even more artifact. It's almost always the problem with all patients though, trying to read through the artifact. One of the tricks we could use is look at different leads. Almost always you'll find a couple of leads where it's easier to see what's going on. Now, we have to keep in mind the idea of contiguous leads looking for ST depression, for example, if we think there's an ST segment depression in lead 1, well, we're going to go ahead and look in AVL,
[00:04:00] right? If we think there's something going on in V5, we're going to look at V6 and V4. So, looking at neighboring leads can be helpful. Here, we have the same strip we had before. A little tough to see what's going on with things in terms of the ST segment and even the rhythm. Same patient, same condition, neighboring lead. A lot easier to see the ST segment, which is probably depressed consistently. Here's our baseline ST segment. Baseline ST segment seems to be depressed
[00:04:30] all of the time, and we can see our rhythm better, we can see our P waves a lot better. Maybe here, we can't really tell if it's depressed. Here, yes, it does seem to consistently be ST segment depression. Many people use the three in a row rule to help sweat this out. It's kind of an unofficial thing but it seems to work well. If we have significant ST segment depression in three in a row or more than three in a row of our QRSTs then we're willing to believe, typically, that it's real as opposed to artifactual. If it's not there
[00:05:00] consistently, which we're defining here as three in a row or more, then it probably is just artifact. When we have our maximal exercise strip, which is automatically run by the stress machine, the patient is moving maximally and we tend to have a lot of times maximal artifact. Very difficult to read. One minute after maximal, patient is probably still walking on the treadmill but at a slower speed, there's no hill, there's a lot less motion. Same patient, one minute into recovery. If the ST
[00:05:30] segment depression is real, if there really is significant myocardial ischemia, the ST segment changes are probably still going to be here one minute later. And here, this strip is a lot cleaner, a lot easier to read. So, the ST segment is really our key. We can often rely on using multiple leads but we should use leads that are next to each other. We have a situation, lead two we want to sort out, let's look at two, three in AVF. We're not sure about V1, let's look at V1, V2, that sort of thing. We want to try to read through
[00:06:00] the snowstorm and find the polar bear by sorting out the real ST segment changes from the artifact.