Review of Factors Associated With False-Positive Emergency Medical Services Triage for Percutaneous Coronary Intervention

I found an interesting article while surfing the internet this morning.

Yamamoto Swan, Pamela BA, Nighswonger, Beverly RN, Boswell, Gregory L RN, & Stratton, Samuel J. MD, MPH. (2009). Factors Associated With False-Positive Emergency Medical Services Triage for Percutaneous Coronary Intervention. Western Journal of Emergency Medicine, 10(4),

This is a retrospective analysis of 12-lead cases from Orange County California Emergency Medical Services between February 2006 and June 2007.

For those of you who are not aware, in Southern California they use computerized interpretive algorithms to diagnose STEMI in the field. They’ve taken a lot of flack about this from the EMS intelligentsia who interpret it (wrongly) as evidence that fire-based EMS is somehow inferior.

The truth is far more complicated than that.

In the system studied they used three different types of 12-lead monitors.

There were 548 patients who were triaged from the field for primary PCI at a STEMI Receiving Center.

19 cases were excluded from the study for various reasons.

393 patients (74.3%) had PCI with significant coronary lesions found.

The remaining 136 (25.7%) were considered false positives, which included 121 patients (22.9%) who were determined by the ED physician to have no need for PCI, and 15 patients (2.8%) with no culprit artery.

False positive cases were associated with the following variables:

  • A specific brand of one of three monitors used in the system
  • Sinus tachycardia
  • Missing lead recording on 12-lead printout
  • Atrial fibrillation
  • Female gender
  • Poor ECG baseline

A discussion ensues during which the authors make this important statement:

“Poor ECG baseline and failure to record all 12 leads for machine algorithm interpretation are false-positive associated variables that can be addressed by improved quality in field acquisition of 12-leads.”

It can’t be said often enough! That’s why I’m always harping on achieving excellent data quality!

The authors continue:

“Variables more difficult to address are sinus tachycardia and atrial fibrillation, which had a tendency to be wrongly interpreted by machine algorithm as acute MI.”

It would be interesting to know if they are including atrial flutter in with atrial fibrillation. Either way the message is clear. The specificity of the computerized interpretive algorithms is highest when a tachycardia is not present.

Then the authors make this interesting statement:

“An unexpected finding was the association of one type of 12-lead machine with false-positive triage. Once this was re-validated by repeat data analysis, we advised the device manufacturer of the findings. Adjustments and changes to the algorithm for the device have been made and follow-up study is in progress. The type of monitor associated with false-positive 12-leads is not identified in this paper because the oversight Institutional Review Committee for the study requires that a written release from the manufacturer be obtained and such a release was declined.”

A few points here.

First, why in the world would the Institutional Review Committee for the study require a written release from the manufacturer? Research is research and outcomes are outcomes. It’s difficult to escape the conclusion that the IRC was afraid of getting sued.

Second, shame on the device manufacturer for not giving permission for the results to be published. They should just be happy that valuable feedback was given back to the company by the researchers so they can make improvements to their algorithm.

Third, it doesn’t take a rocket scientist to figure out which manufacturer’s 12-lead monitor was associated with a higher rate of false positives!

Let’s think about it. Two of the three use the GE-Marquette 12SL interpretive algorithm (ZOLL and Physio-Control). One of the three uses their own algorithm. Does it really take a college level Introduction to Logic class to connect the dots?

The authors of course admit to some limitations, including this one which I found interesting:

“A more subtle limitation is that our definition of false-positive triage does not take into account patients who were determined by the receiving physicians to lack evidence for an acute STEMI MI, when in fact such an MI was present and PCI could have been a benefit.”

To be honest, I was just amazed that so many activations were canceled by the ED physicians! They acted as gatekeepers, which is extremely important considering the high number of false positive activations triggered by the paramedics in the system.

The fact that only 2.8% of patients who were cathed had no culprit artery is extremely impressive to me. I’m not even convinced that a canceled STEMI Alert (or whatever they call it in Southern California) should be called a “false positive”.

They also state:

“While left bundle branch block was analyzed within the study population 12-leads, there was not an association of this finding with false-positive triage; on the other hand the study was limited in that we did not test for false-positive association with left ventricular hypertrophy, pericarditis, left ventricular aneurysm, and early repolarization.”

This is in startling contrast to the study by Larson et al. that showed almost half of patients with LBBB had no culprit artery! Who knows, maybe the ED physicians in Southern California use Sgarbossa’s Criteria. On the other hand, the authors admit they didn’t study false negatives, so it’s entirely possible they just aren’t cathing the LBBBs the way used to in Minnesota.

I say “used to” because it was Dr. Smith et al. that came up with excessive discordance as a marker of acute STEMI in LBBB.

Overall, a very interesting and worthwhile article. This is exactly the type of research that needs to be happening right now!


  • Frankly Tom, I think you should write for paramedic textbooks a "Tips of the trade" section on how to get excellent quality EKGs and troubleshooting. Everything I learned about looking at which leads are affected are from this website site – the textbooks are still very lacking on how to do the skill well. Even Physio-Control's new online course with Tim Phalen boils down to "prep, keep still and put a blanket over the leads" but don't really go in depth on what to do when things go wrong. As an intern I would throw the 12-lead on and agonize when V6 and aVL would have a baseline wander and I would be like WHYYYYYYY???? (And how do I fix this…as I tear out the tape and stick down every single lead!!)

  • SoCal Medic says:

    I agree, I think the bulk of the false positives that I see in the system that I work in comes down to simple lead placement and prepping the patient. Expose the chest, palpate the costals, place the leads on the extremities over the muscle, not the bone, organization of wires, the simple basics of it all. My old Training chief used to tell us the most dynamic of problems is nothing more than simple ones put together, if you can not get the simplest of ones right, then the dynamic one will just compound itself because of it.

  • Tom B says:

    MIFL -Thanks for the feedback! A contact of mine with ties to the Society of Chest Pain Centers took an interest in my STEMI Alert training PowerPoint, so maybe my pointers on achieving excellent data quality will find its way into their programs. We'll see! As an educator, I've always tried to pass along what I've learned through the school of hard knocks. Any time I've ever learned something and thought, "Why didn't they tell me this before?" I've made a mental note and I often pass it along to my students (especially when it comes to ACS/12 lead ECG).That's actually how I developed the insight that the reason EMS 12-lead ECG training is weak is because very few people in EMS are truly qualified to teach it! That's not a slam against anyone. It's just that the majority of the current educators didn't come up through EMS when 12-leads were a priority.It's scandalous that new paramedics still aren't being trained in 12-lead interpretation in school. I have a lot of respect for Tim Phalen, and I enjoyed his online program, but it should be viewed as an introduction to STEMI recognition. It's an excellent starting point, but with budgetary constraints, it's often viewed as the endpoint.Continuous quality feedback mechanisms will prove to be the most effective tool for paramedic 12-lead education, in my opinion. That's why we started the "12-lead case of the month" in my department. Adults learn best in short doses, and the message needs to be repeated until it becomes part of the permanent knowledge base.Ideally, they won't even realize how much they're learning. I'm even thinking about sneaking in axis deviation and bundle branch recognition. They were taught this in their initial 12-lead training (taught by yours truly) but it's generally not something you retain after a single class.Tom

  • Tom B says:

    SoCal Medic – Your old Training Chief is right on target! The good news is, if we can train paramedics to routinely capture 12-lead ECGs for the right patients, with the first set of vital signs, and achieve excellent data quality a high percentage of the time, we can solve a huge percentage of the "false positive" problem!The next thing to do is figure out how many STEMIs are being missed (the "false negatives") and then determine whether or not the problem is a weakness with the computerized algorithms or a weakness with the actual criteria (1 mm of ST-elevation in 2 or more contiguous leads, 2 mm in lead V2 or V3).I suspect the latter! In the vast majority of cases I've reviewed, if the ECG shows STEMI, but the interpretive algorithm shows "ST/T wave abnormality" it's because technically, the criteria is not met.Sometimes this is due to hyperacute T-waves (without ST-elevation) and sometimes it's due to QRS complexes with low voltage where ST-elevation is present, but it's not > 1 mm in 2 contiguous leads. However, a well trained electrocardiographer can still spot the STEMI, especially if reciprocal changes are present, however subtle.Again, is this a weakness in the interpretive algorithm?Tom

  • Shaggy says:

    Wow what a contrast. SoCal Medic stated a while back they have to rely on the computerized rhythm to diagnose a STEMI, while here they keep trying to bang into our head to pretty much ignore it. So what do you guys think? Should we rely on it or use it as a guideline to prompt for further alanysis? Do you think this system workds well down there compared to elsewhere where reliance on it is not advocated?

  • Tom B says:

    Shaggy – I think the computerized interpretation is a tool like any other. It's useful, but it has to be well understood.In Minnesota they use interpetive statements with paramedic over-read and they're doing wonderfully.There are a few take-home messages here, in my opinion.1.) You must capture a 12-lead ECG with excellent data quality, because poor data quality confounds the interpretive algorithm. I think this part is huge.2.) You need to be educated enough to confirm the computerized interpretive statement. If it says ***ACUTE MI SUSPECTED*** and further down it says "Anterior injury pattern" you need to be able to look at the ECG and see ST-elevation in the anterior leads.3.) We need to be especially careful in the setting of a tachycardia.So is the computerized interpretive statement a substitute for paramedic STEMI recognition? Absolutely not! Paramedics also need to be able to identify acute STEMI.So why use computerized interpretive statements at all? Because when the patient has signs and symptoms of ACS, the data quality is excellent, the interpretive statement says ***ACUTE MI SUSPECTED***, and the paramedic agrees with the computerized interpretive statement, then there is a very high likelihood that the patient is experiencing acute STEMI, especially when a tachycardia is not present.Keep in mind, there are two issues here. One is calling the STEMI Alert from the field. The other is bypassing the local non-PCI hospital for a STEMI center.Each system needs to figure out how it's going to handle marginal cases, and there is no "one size fits all" solution. Age, risk factors, and contraindications to thrombolytic therapy all need to be taken into consideration.Keep in mind that approximately 50%of STEMI patients self-report to non-PCI hospitals, so there needs to be a strategy for interhospital transfer anyway, and there's still a role for thrombolytics depending on the transfer time.Tom

  • Shaggy says:

    Wow, that response should have been an educational post all on its own!

  • Tom B says:

    Shaggy -This is why I like having a blog as opposed to say, writing a book. Web 2.0 technology allows multi-lateral coversations with a variety of health care professionals.It's a far superior medium, in my opinion.Tom

  • Anonymous says:

    I enjoyed reading your recent thoughts on our STEMI paper, Tom. We collected data on atrial fibrillation separately from flutter. Paramedic STEMI training here is in the works now, and a false negatives study is already under way. Best,Pam Swan

  • Tom B says:

    Hi, Pam! Thank you so much for leaving a comment! I really appreciate it.I was curious about atrial flutter because I've seen it cause a "false positive" interpretive statement of ***ACUTE MI SUSPECTED*** with the LP12.For some reason, I've never seen atrial fibrillation trigger a "false positive" interpretive statement.Of course, the LP12 uses the GE-Marquette 12SL interpretive algorithm, which was not the only algorithm used in the study.I was really hoping to find out if sinus tachycardia and atrial fibrillation are associated with "false positives" for both interpretive algorithms.It's also interesting to me that "false negatives" are being studied as well! I'm looking forward to reading your next paper on the topic.Just out of curiosity, can you tell me the total number of "false positives" that were caused by poor data quality or missing leads?It's obviously a big problem. I'm just wondering how big!Tom

  • WFDFireMedic says:

    Nice article; My feeling is that relying on the computer in the monitor is a mistake. It misdiagnoses the rhythm 48% of the time, it is not sensitive enough to properly separate the STEMIs from the NSTEMIs. Secondly, as a Firefighter / Paramedic who has worked in several combined systems; subjectively, the Fire Service does not have the proper mission focus on the complex issues currently facing emergency medical response. Firefighting is the primary focus, even though the majority of responses (70-80%) made by the organization are medical in nature. Not criticizing, I love doing both, simply and observation, firefighters like fighting fire, no sin in that.

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