Identifying Acute STEMI in the Presence of Paced Rhythm

EMS responds to a local coffee shop for a 77-year-old male who has reportedly fainted. While en route the call is upgraded to an unconscious patient and an engine company is dispatched for backup.

On arrival the patient is conscious but lethargic. He appears acutely ill. The skin is pale and diaphoretic. His past medical history includes hypertension, dyslipidemia, myocardial infarction, a coronary artery bypass graft and heart failure. His current medications are aspirin, metoprolol, atorvastatin, lisinopril and furosemide.

Upon further questioning he admits to chest discomfort. His OPQRST assessment is as follows:

≫ Onset: 15 minutes prior to EMS arrival;

≫ Provoke: Nothing makes the pain feel better or worse;

≫ Quality: Dull;

≫ Radiate: The pain does not radiate;

≫ Severity: 7/10; and

≫ Time: No previous episodes.

His vital signs are a heart rate of 74, respiratory rate of 22, blood pressure of 116/67, oxygen saturation of 88% on room air and a temperature of 98.4 degrees F.

The patient is relocated to the ambulance and undressed from the waist up. A well-healed surgical scar is noted from previous open-heart surgery as well as an implantable medical device in the upper-left chest. Other notable findings include jugular venous distention and breath sounds that reveal crackles in the lung bases.

Figure 1: Initial rhythm

STEMI initial rhythm

Figure 2: Ventricular paced rhythm

Ventricular paced rhythm


The patient is placed on oxygen via nasal cannula at 4 Lpm. The cardiac monitor is attached–the 12-lead ECG shows ventricular paced rhythm. (See Figures 1 and 2, above.)


There are those who say it’s impossible to identify acute ST elevation myocardial infarction (STEMI) in the presence of paced rhythm. Some paramedics will even argue that performing a 12-lead ECG on a patient with a ventricular paced rhythm is a waste of time.

In paced rhythm, much like left bundle branch block, the ST segments and T wave should be deflected opposite the majority of the QRS complex.

Leads with a mostly positive QRS complex (see Figure 3A, below) are expected to show ST-segment depression and leads with a mostly negative QRS complex (see Figure 3B, below) are expected to show STsegment elevation.

Figure 3: Appropriate ST-segment and T-wave discordance

Appropriate ST-segment and T-wave discordance

A group of investigators came up with a scoring system to identify acute myocardial infarction in the presence of left bundle branch block, known as Sgarbossa’s criteria.(See Figure 4, below.) The criteria included:

  1. Concordant ST-segment elevation (ST-segment elevation in a lead with a positive QRS) ≥ 1 mm.
  2. ST-segment depression in leads V1, V2, or V3.
  3. Discordant ST-segment elevation (STsegment elevation in a lead with a negative QRS) ≥ 5 mm.

Of these, the first criterion has stood the test of time and is considered to be the most reliable.

Other researchers proposed a modification to Sgarbossa’s third criterion that takes into account the depth of the S wave.2 Specifically, they’ve shown that ST-segment elevation ≥ ¼ the depth of the S wave is both sensitive and specific for identifying acute STEMI in the presence of left bundle branch block.

Figure 4: Sgarbossa’s criteria

Sgarbossa's criteria

Figure 5: Concordance (ST-segment elevation â‰¥ 1 mm)

Concordance (ST-segment elevation ≥ 1 mm)


In this case we see concordant ST-segment elevation in leads I, aVL, V2, V5 and V6. (See Figure 5, above.) We also see excessively discordant ST-segment elevation (ST-segment elevation ≥ ¼ the depth of the S wave) in leads V3 and V4. (See Figure 6, below.)

Figure 6: Excessive discordance (ST-segment elevation ≥ ¼ depth of S wave)

Excessive discordance (ST-segment elevation ≥ ¼ depth of S wave)

When we mark these leads we can see a familiar pattern that is consistent with left anterior descending (LAD) occlusion. (See Figure 7, below.)

Figure 7: LAD occlusion pattern

LAD occlusion pattern

Figure 8: Reciprocal change in lead III

Reciprocal change in lead III


When ST-segment elevation from LAD occlusion “crosses over” to the high lateral leads, we should look for reciprocal changes in the inferior leads. (See Figure 8, above.)

The QRS is negative in lead III so we would normally expect ST-segment elevation. Instead, we see ST-segment depression. Therefore, we can assume this represents a reciprocal change.


The 12-lead ECG was transmitted to the hospital and the cardiac cath lab was activated. The patient went into cardiac arrest upon arrival at the ED and was not successfully resuscitated.


1. Sgarbossa E, Pinski S, Barbagelata A, et al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block. N Engl J Med. 1996;334(8):481—487.

2. Smith S, Dodd K, Henry T, et al. Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule. Ann Emerg Med. 2012;60(6):766—776.

EMS 12-Lead

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