Should you activate the cardiac cath lab?

EMS is called to a walk-in medical clinical for a 52 year old male with a chief complaint of palpitations.

The nurse advises the treating paramedic that the patient underwent mitral valve repair and aortic valve replacement about 2 weeks earlier. After the procedure he required synchronized cardioversion for atrial fibrillation.

Today he showed up at the clinic because he felt anxious and felt like his heart was racing. He denies chest discomfort. He is not nauseated and he has not vomited. There is no shortness of breath.

EMS assesses the patient’s vital signs and captures a 12-lead ECG.

would you activate wm

Should paramedics call a “Code STEMI” from the field?

This dilemma plays out hundreds (perhaps thousands) of times every single day in the United States and all over the world. Untold millions of dollars have been spent on 12-lead ECG monitors and various “solutions” to transmit the ECG to the hospital in an effort to reduce first medical contact-to-balloon times while minimizing false positive cardiac cath lab activations.

In the early days of prehospital 12-lead ECGs much of the education focused on atypical presentations and anginal equivalents. We encouraged paramedics to “cast a wide net” and warned them that their index of suspicion needed to be higher for diabetics, females, and the elderly.

As a consequence we perform a lot of 12-lead ECGs on patients whose pre-test probability of STEMI is relatively low. Unfortunately, many of these patients have abnormal baseline ECGs, often with features that can easily mimic acute STEMI.

Here are the reasons you shouldn’t call a Code STEMI for this patient.

Clinical correlation is weak

When the patient presents with typical signs and symptoms of an acute coronary syndrome, even subtle ECG abnormalities are taken more seriously. Conversely, when the presentation is weak, the ECG evidence must be compelling.

The heart rate is > 130

Tachycardia is known to be one of the factors associated with false positive cardiac cath lab activation. It’s also worth remembering that the number one determinant of myocardial oxygen demand is heart rate. You want to make sure you’re not dealing with demand-side ischemia.

Atrial flutter is present

This is best seen in leads II and III.

When flutter waves are superimposed on top of ST-segments it can fool the computerized interpretive algorithm into giving the ***ACUTE MI SUSPECTED*** message.

flutter waves

There is probable arm lead reversal

If you look at the ECG the QRS complex is negatively deflected in lead I and positively deflected in lead V6. This should raise a red flag because they are both lateral leads with similar vectors. Be sure to troubleshoot any data quality problems prior to transmitting an ECG or announcing a Code STEMI.

The S-waves are very deep in the anterior leads

This is the most important finding on this 12-lead ECG from a prehospital perspective. Let’s look at the interpretive statement.

interpretive statement markup

Remember, the ***ACUTE MI SUSPECTED*** message is a subordinate message, meaning that it is “triggered” by one of the messages below it. So keep reading and look for words like “acute” or “injury”.  “Age undetermined” can’t trigger the message so in this case it was triggered by the statement “anterior injury pattern.”

Look at the anterior leads with your own eyes.

leads V2 and V3

Now you see what the computer sees! ST-segment elevation is present in leads V2 and V3.

But does it represent STEMI?

Almost certainly not! Why? Because the S-waves are very deep! 35 mm in lead V2 and off the bottom of the ECG paper in lead V3! If you take nothing else away from this blog post take this lesson away.

You must take ST-elevation with a grain of salt when the S-waves are deep.

This is important because several studies have shown that LVH is the most common STEMI mimic.

That’s not to say that patients with LVH do not experience acute STEMI! LAD occlusion tends to attenuate the voltage of the S-waves in the anterior leads for reasons that are not completely understood.

This is one of many fascinating insights into electrocardiography I owe to Stephen Smith, M.D. (@SmithECGBlog). LAD occlusion rarely meets the voltage criteria for left ventricular hypertrophy in the precordial leads.

There is a general pattern of T-wave discordance

In the last blog post we discussed the QRS/T angle and how a general pattern of T-wave discordance usually indicates a secondary ST/T-wave abnormality (as opposed to the primary ST/T-wave changes of ischemia/injury). It should always make you stop and think!

In this particular case the treating paramedic transmitted the ECG to the Emergency Department and discussed the situation with the attending physician who advised that he did not believe that a Code STEMI was warranted based on the information that was presented.

However, because the patient was experiencing SVT, the treating paramedic did give 6 mg of adenosine while en route to the hospital.

adenosine flutter waves wm

Two points here.

First, the PRINT button is your friend! Even when adenosine is unsuccessful it can sometimes prove to be diagnostic (as in this case).

Second, consider that 2:1 atrial fibrillation is a real possibility when the large block method indicates that the heart rate is near 150 bpm!

While it’s fascinating to see underlying flutter waves during the administration of adenosine a careful examination of the 12-lead ECG would have revealed the diagnosis.


I like to think in terms of a risk barometer when I treat a possible STEMI patient. This is one occasion where the fire service taught me something that turns out to be useful in patient care!

stemi barometer

It’s also the reason I quote Tomas Garcia, M.D. and his advice to “consider the company it keeps” when analyzing an ECG abnormality. When true STEMI is present the evidence tends to stack up. Conversely, when a STEMI mimic is present (as in this case) the evidence tends to point the other direction.

As Stephen L. Goldberg, M.D. explains in his blog post at the Journal of Invasive Cardiology website:

“[W]e are asked to perform an angiogram on someone with an uncompelling story, with abnormal ECGs not meeting guideline recommendations — such as minimal ST elevations in the face of Q waves in someone with non-cardiac symptoms…it may be that the difference in false positive activations which vary between 10% and 60% is due to differing thresholds for referring these non-guideline recommended cases for emergent caths…it seems reasonable as a quality assessment measure for hospitals to address how many “unnecessary” cath lab activations occur. This could lead to greater discrimination and perhaps enhance quality, rather than lead to missed STEMI activations, as so many fear would happen with a higher threshold.”

It’s appropriate to consider the anginal equivalents and I agree with “casting a wide net” but prehospital activation of the cardiac cath lab is for clear-cut STEMI.


Avery S, Kotch N, Kroman A, White R, Wassmer P. Technical Factors Involved in False Positive ECG STEMI Diagnoses. Diagnostic and Interventional Cardiology. July 13, 2014.

McCabe J, Armstrong E, Kulkarni A et al. Prevalence and Factors Associated With False-Positive ST-Segment Elevation Myocardial Infarction Diagnoses at Primary Percutaneous Coronary Intervention–Capable Centers. Arch Intern Med. 2012;172(11). doi:10.1001/archinternmed.2012.945.

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)


About author

Tom Bouthillet
Tom Bouthillet - 25 posts

Tom Bouthillet (@tbouthillet) is Editor-in-Chief of (@ECGTraining) and Fire Captain/Paramedic in South Carolina where he is the Emergency Cardiac Care Program Manager and the STEMI and CARES Site Coordinator of his fire department.

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