Is this a STEMI or an optical illusion? - by Dr. Bojana U.

Thank you all for such interesting discussion!

Many things on this ECG are not what they appear to be. In that context, you can consider this tracing as an optical illusion!

First finding that stands out is inverted P waves in inferior leads and V3-V6. Second, PR interval seems to be very, very short. If you look at lead I only, you could say that PR interval is barely 60msec. So, this must be a junctional rhythm, right?

Next, if measured from TP line, there are no doubts about diffuse ST elevations in leads: II, III, aVF and V3-V6. STEs in inferior leads are 1mm high and almost horizontal shape in lead III; making very worrisome signs for chest discomfort patient. Actually, this patient was admitted to hospital as inferior wall STEMI!

What if I tell you that all of that is an illusion? PR interval on this ECG is normal, and there are actually NO ST segment elevations!!! Would you believe me??

In normal ECG, P waves are positive in leads I, II, III and aVF and negative in aVR.  They can be biphasic in V1, but are usually positive in the rest of the precordial leads. But, in this ECG P waves are NEGATIVE in inferior leads and in V3-V6.  This indicates retrograde conduction to the atria. The cardiac pacemaker must be anatomically lower than sinus node and the impulse from there continues in a backward direction through the atria. That means that the rhythm could be originate in the AV node (junctional rhythm) or in low atria (ectopic or low atrial rhythm).

Actually, the literature has shown many variety and has been confusing over the past years about precise anatomically location of junctional pacemakers. According to Bill Nelson it is generally believed that most pacemaker cells in the AV junction are in the perinodal cells or in the Hiss bundle, not in the AV node itself.

In junctional rhythm P and QRS relation could be: P wave preceding; superimposed on; or after QRS. If P wave precedes QRS complex these Ps are retrograde with a short PR interval (<120msec).

Picture 1. P and QRS relations in junctional rhythm

Heart rate in junctional rhythm is usually ≤60/min, but it can be 60-100/min (called accelerated junctional rhythm) or >100/min (called junctional tachycardia).
In our case, we should carefully observe PR interval in all leads.

Picture 2. PR interval measured in limb leads

Looking in lead I only, PR interval seems to be very short. But it is an optical illusion! We are used to see uniform and completely positive P wave in lead I. In this case the P wave is biphasic, meaning P1 component is isoelectric (slightly negative?) and P2 is positive deflection.

The measured PR interval is a little above 120 ms. This is shown in Picture 2 by drawing a vertical line in simultaneous leads from the onset of the P wave to the QRS complex.

So, we have an abnormal (retrograde) P wave and a NORMAL PR interval here. Per Chou, the old name for this rhythm is coronary sinus rhythm, but it has been replaced by the terms ectopic atrial rhythm (sometimes designated low or left atrial) or AV junctional rhythm. This is because inverted P waves in II, III and aVF with normal PR intervals can be elicited by stimulating sites other than the coronary sinus and because PR duration may be normal when  the junctional impulse conducted anterogradely is delayed .

All terms meaning the same: that the ectopic pacemaker is located in the low atrium, producing retrograde conduction through the atria and normal delay through the AV node. Actually, the pacemaker focus is within the atrial myocardium.

Picture 3. Frequency of atrial ectopic sites

Seventy-five percent of atrial ectopic foci are located in right atrium, while 25% is located in left one.
The exact place of ectopic focus in atria would determinate P wave morphology. In general it is impossible to locate ectopic focus without electrophysiology studies, but some findings could be helpful. Especially we can use P wave morphology to differentiating right atrial from left atrial foci (see the algorithm). Also, in general: the more deeply P waves are in inferior leads, the more inferiorly is located atrial ectopic focus. Low amplitude, biphasic or even positive P is for more superiorly located ectopic focus.

Picture 4. One of algorithms to identified site of ectopic atrial rhythm

The causes of this ectopic rhythm are many; and vary from completely benign to serious. Low atrial rhythm has been reported in acute amlodipine intoxication. A rare autosomal dominant disorder in four generations of a family with congenital heart diseases and low atrial rhythm has also been documented recently. It is often seen in pediatric population, especially with congenital heart diseases. Also, it has been reported in adult population with atrial septal defect. This rhythm is sometimes seen in inferior wall MI.

The next ‘illusion’ on this ECG is presence of STEs.

Picture 5. “STEs” in inferior leads and in V6 suggesting for STEMI

Bill Nelson was writing about an eponym- “Emery phenomenon”. It is named by Dr. James Emery who described this interesting phenomenon in his article from 1978. He reminds that for every atrial P wave there must be an atrial T wave. Normally, the sinus node P waves are positive in the limb leads, and the “T of the P” is a negative deflection; but unseen, because it occurs during the QRS complex. When atrial depolarization is ectopic and recorded as a negative event, the “T of the P” becomes positive, and can distort the end of QRS complex, simulating ST elevation. (From Nelson’s ECG site) -

Picture 6. Illustration of Emery phenomenon

This patient was admitted to hospital and MI was rule out by negative serial troponin’s level and by cardiac ECHO without LV wall motion abnormality.
After few hours, actually, this rhythm disappeared spontaneously. It was replaced with normal sinus rhythm. His symptoms resolved with BP control.

Picture 7. Moment of spontaneously termination of low atrial rhythm

Picture 8. Sinus beats clearly show no STEs (red lines). Slightly differences in QRS morphology during low atrial rhythm comparing to sinus rhythm are shown with green circles.

So, there is not truly STEs on this ECG; it is more like QRS distortion, QRS complex is simply shifted due to Emery phenomenon.

The cause of appearance of this rhythm to our patient remains unknown. The Amlodipine’s serum level in this case wasn’t measured.

Very important note! Ectopic atrial rhythm can be seen in truly STEMI. So, STEs in this rhythm could be caused from real deal ischemia! Keep that in your mind, please.

This ECG (picture 9.) is from another patient with proven inferior STEMI. But, in this other case you have some helpful findings strongly favoring STEMI. They are: size of T waves and notching QRS complexes in inferior leads; but more important is presence of reciprocal changes in lead aVL.

Picture 9. Ectopic atrial rhythm associated with true inferior wall STEMI in another patient.


  1. Do all ectopic atrial rythm exhibit this Emery phenomenon???
    Thanks for the brilliant posts ...they are wonderful.

    1. I will propose an answer for Vijay Balaji’s Question — namely, that the likelihood of an ectopic atrial rhythm (or a junctional rhythm with negative P waves preceding the QRS in lead II) — depends somewhat on the amplitude of this negative P wave. Just like T wave amplitude normally has some correlation to QRS amplitude (ie, in many cases, the larger the QRS — the larger we expect the T wave in that lead to be) — the deeper and wider the negative P wave in lead II — the more we might expect the T of this P wave to be larger (and therefore distort the early portion of the QRS). Otherwise — just like there may be “flat” T waves in some patients with even large amplitude QRS complexes — not all patients with even large negative P waves preceding the QRS will manifest distortion of the early ST segment in those leads. So, the “answer” is “Sometimes Yes — sometimes No”. That said — being AWARE of the Emery phenomenon should go a LONG way toward recognition of when ST-T wave elevation is NOT due to the negative P waves (as in Picture 9 in Uzelac Blog above).

    2. Thanks Ken for the explanation

  2. Very helpful discussion, and thoroughly illustrated! Thank you very much.