Cath lab cancelation after EMS activation

In the last 2 posts, I reviewed recent studies that looked at the decision to obtain a prehospital ECG, and a novel method to teach STEMI identification to novice ECG readers. This leads to the last installment in this trilogy: How often does EMS mistakenly activate the cath lab? For that matter, how good are emergency physicians?

Reference

(Side note: This is the second paper I have reviewed that has Jon Studnek as an author. He's a paramedic who also has a PhD, and is faculty at Carolinas Medical Center. He writes a lot, and I probably could fill all of my posts with reviews of his publications.)

AKA "Dr. Medic"

This study uses data from the Reperfusion of Acute Myocardial Infarction in Carolina Emergency Departments (RACE) program in North Carolina. This program has already been shown to improve time to PCI or lytics for patients with a STEMI,  as well as other process measures. While recent data on actual patient outcomes is mixed, there is no doubt that this ambitious collaboration has brought some order to the notoriously fragmented emergency health care system in the U.S.!

The investigators used data from the 14 hospitals in North Carolina that acted as the receiving centers for STEMI patients transferred for emergent percutaneous coronary intervention (PCI).They looked at  sub-groups of patients, broken down in three different ways:

• Patients who first presented to a PCI center, or to a non-PCI center;
• Patients who had the cath lab activated by EMS, or in the ED; and
• Patients who used EMS, or those who "walked in" to the ED.

Because of the size of the program, they were able to look at a total of almost 4000 catheterization lab activations. That's one of the main strengths of this study - they have a lot of data.

Another aspect of the study that gives it some "real-world" applicability is how they defined an "inappropriate" activation of the cath lab. While other authors have described a "clean cath" as an inappropriate activation, the authors acknowledge that there are many scenarios where PCI for presumed STEMI is appropriate, despite the 20/20 hindsight of a negative cath. Takotsubo cardiomyopathy, for example, often requires an emergent angiogram to clarify the diagnosis.

So, instead they defined an inappropriate cath lab activation "If catheterization was canceled because of ECG reinterpretation or if the patient was deemed not to be a candidate" for PCI. Clinical factors, such as age or DNR status, were used to determine candidacy.

The overall results, comparing paramedics and ED physicians were that 15% of activations were inappropriate:

They analyze the results further, breaking down the data into the subgroups described above. The group of interest is all the patients who were transported by EMS, and had their initial activation by EMS. In other words, none of these patients were "walk-ins," but it included both patients who were brought to PCI and to non-PCI centers (initially).

They compare these activations against all the patients who had cath lab activation performed by the ED physicians (both at PCI centers and non-PCI centers), with patients who either came in by EMS or car.

There are a couple different ways to analyze these results, but overall the physicans performed better that the medics. Well, 7+ years of training ought to pay off somewhere, and and incremental accuracy in ECG interpretation is a reasonable expectation.

However, you can't even conclude this from the data presented, since an activation may have been deemed "inappropriate" because of a patient's DNR code status, say, or severe comorbidities (e.g. sepsis, or terminal disease). Specifically, we don't have the break-down for ECG accuracy versus judging cath lab candidacy for the 2 groups - it may well be the case that medics are just as good as emergency physicians at reading ECGs, but the physicians are better at judging which patients actually warrant an emergent catheterization.

The last table emphasizes the point that, while this sort of study is great at generating statistically-significant results, there is a lot of "granularity" that is not accessible to us.

Clearly, not all EMS agencies or EDs are equal - some systems are better than others. In this table, note the range of appropriate activations:

There are few EMS agencies and EDs who are evidently did not generate a single inappropriate activation! However, a 100% appropriate activation rate may also suggest a system that is too restrictive, and is missing too many STEMIs.

On the other hand, it is concerning that some EDs, even at the big hospitals with cath labs, have a "false-positive" rate of 25%. Similarly, some EMS agency inappropriately activates the cath lab 1/3 of the time!

The Bottom Line

This isn't a study that you can use to change your clinical practice in the next shift. It isn't even very useful at changing practice at your EMS agency or ED. However, it points the way to doing the more practical research, by highlighting important aspects.

For example, how do paramedics at different agencies decide to activate the cath lab, and how do these methods correlate with accuracy? Could a closer look at the 65% - 100% range in appropriate activations suggest a "best practice" for EMS? Should we rely more on intensive continuing education for paramedics? Alternatively, should there be more emphasis on computerized and/or human algorithms for ECG interpretation?

Furthermore, since the "Not Cath Lab Candidate"group accounted for such a large proportion of the inappropriate activations (4.3%), might their be a better way to anticipate this exclusion? To a large degree, the cardiologist is the individual who is deciding the patient's candidacy for the cath lab, and it is often difficult for the emergency physician, let alone the paramedic, to anticipate their decision. I'm not sure that the accuracy of prehospital STEMI activation should be judged using such "soft criteria."

So, more research is called for, as usual. But this paper serves as a very useful guide for the future.

An Alternative Method of ECG Interpretation

Just when a paramedic student has started to feel somewhat confident about rhythm interpretation, she is introduced to the other 11 leads.

First off, the leads are organized even worse than the QWERTY keyboard. Inferior leads are the left of anterior, the lateral leads are in two different places, and aVR sits there all by itself, like a chump.

Then there are all the depressions and elevations, T waves flipping around, ischemia vs infarct. And then someone shows you how to pick up on a posterior MI by flipping the paper over. Madness, I tell you.

In particular, identifying a STEMI can be difficult, even if ST segment elevation is clearly seen. For example, the following ECGs all show ST segment elevation, but...

1. Not a STEMI

2. Not a STEMI either

3. Nope.

 But with a fairly undramatic ECG like:

4. Bingo - Occlusion of the proximal LAD

The first first three ECGs demonstrate 3 common cause of ST elevation that we see in EMS or the ED, so-called "mimics" of STEMI. Now, there are a host of rules and criteria to help you diagnose each of these mimics, but it's hard to learn all of these, and to feel confident about them.

Is there a simpler way to achieve ECG excellence? Some short-cut to Jedi-level ECG mastery other than slogging through hundreds of tracings?

Perhaps a training montage?

Well, no.

But there are a few different ways to develop pattern recognition, and switching up the methods can put things in perspective. Hartman and colleagues have helped the novice ECG student tremendously with a new, focused approach to ECG interpretation. While this does not replace experience, practice, and feedback on interpretations, it's a good alternative way to tackle ECGs.

Abstract. If you want a pdf, message me at Facebook.

The rule has 4 steps, and we'll tackle them in that order

1. Is there ST elevation in at least 2 related leads?

The first rule specifies a minimum amount of elevation: 1-2 mm in two anatomically related leads.

It doesn't take long before a paramedic student identifies their first patient with ST elevation. Okay, granted, it's usually not an actual STEMI that they find, since the majority of ST elevation found in the ED or by EMS is not a STEMI. Typically, ST elevation will be due to any number of "mimics," such as left bundle branch block (LBBB), left ventricular hypertrophy (LVH), early repolarization (ER), as well as a number of other conditions. Surprisingly, if you look at all the patients who come into the ED with ST elevation, only about 1 in 7 patients have a true STEMI!

On the other hand, if you don't have some ST elevation, the patient probably doesn't have a STEMI. (Yeah, we're going to miss a true posterior or a proximal left main. This rule is for the novice reader, okay?)

No ST elevation, so not a STEMI.

2. Is the QRS a normal height?

The heart, over a period of years, responds to hypertension by bulking up and adding muscle mass. This process results in LVH, which, in the long run, isn't good. It shows up on the ECG as deep S-waves in V1 and V2, and high R-waves in V5 and V6.

In the short term, though, it mainly serves to distract us, as it can produce ECG findings that can look a lot like a STEMI. If we look at ECG #1 above, we see ST elevations in leads V2 and V3. Could these represent a STEMI?

Likely no, for several reasons. Now, a lot of the reasons involve interpretation of subtle, qualitative signs - the morphology of the ST segments and T waves, "notching" of the J-point,  reciprocal changes, etc. it just doesn't "look" like a STEMI, but you need to read hundreds of ECGs to feel comfortable with those.

It is far simpler to count the big boxes. Rule #2 boils down 3 sub-steps:

• First, look at the S-waves in V1 and V2. Pick the deepest one, and count the big boxes.
• Next, look at the R-waves in V5 and V6. Pick the highest one, and count the big boxes.
• Last, add those two numbers. If it is over 7 big boxes, the ST elevation is probably due to LVH

7 big boxes equals 35 little boxes, or 35 mm. Count the small boxes if you prefer, or if the you're near the cutoff. Looking at ECG #1 as an example, and counting the little boxes, we find:

So, about 40 mm, or 8 big boxes, so likely not a STEMI.

3. Is the QRS a normal width?

Rule #3 is simple -  If the QRS is over 0.12 seconds long, don't call a STEMI.

Probably the most common cause of dramatic ST elevation is the LBBB, as in ECG #3 above. You can also see the same pattern if the the patient has a pacemaker.

Now, the experienced and sophisticated paramedic knows that there is a way to interpret the LBBB for signs of STEMI, but even the "simplified" rules for determining STEMI in LBBB are somewhat complicated. Many paramedics are familiar with the rule, but the new paramedic shouldn't be expected to make this call. If the patient has a pacemaker, it's even more unreliable to interpret the ECG.

4. Is there ST depression in at least 1 lead?


Rule #4 - if there is no ST depression, do not call a STEMI.

Most students have learned that you should look for reciprocal ST depression in a STEMI. Unfortunately, because of the non-intuitive, non-anatomic way that the ECG is arranged, it isn't clear which leads are "opposite" each other. And the patterns of depression can vary a lot, depending on which coronary artery is occluded. For example, an "inferior" STEMI may or may not have depressions in I and aVL; it depend on whether the culprit artery is the RCA or the obtuse marginal.

A much simpler criterion for reciprocal depression is any ST depression on the ECG. This would eliminate, for example, ECG #2 above. Although the computer interpretation was STEMI, it is a classic example of early repolarization, or possibly pericarditis (less likely, as the ECG did not evolve). Another example from my ED is this ECG:

27 y.o., prior dx of pericarditis

Just like ECG #2, there is diffuse ST elevation without any ST depression. Not a STEMI.

Applying the rule

Let's take another look at ECG #4:

Okay, going through the rules:

• Rule #1 - Over 1 mm of ST elevation is seen in both V1 and V2, which are anatomically contiguous.
• Rule #2 - The S-wave in V1 is about 1 big box deep, while the R-wave in V5 is 3 big boxes high. That's a total of 4, so the QRS height is normal.
• Rule #3 - The QRS looks narrow, about 0.100 seconds wide.
• Rule #4 - There are ST depressions in the lateral leads, most notably in V5.

So we see that this simple 4-step rule, intended to assist the novice paramedic, actually picks up a STEMI that the computer missed!

The Bottom Line

This elegant method of ECG interpretation, although intended for the student, can be very useful for the experienced paramedic as well.