A 25 year apparently healthy boy with sudden onset right sided chest pain with this ECG

First ECG:
ECG Pulmonary Embolism
 Second ECG:
ECG Pulmonary Embolism

Thought 1: What is your diagnosis based on classic pattern found in this ECG?

Thought 2: Does his ECG suggest that he has had a "silent" inferior myocardial infarct?

Thought 3: What transient ECG abnormality you found in this case?


It's not every day you get to see a classic EKG finding for a very common medical condition. Most doctors have heard of S1Q3T3. That's the classic S1Q3T3 pattern that is:
  1. Deep S wave in Lead I: ≥1.5 mm
  2. Q wave in Lead III: ≥1.5 mm
  3. T wave inversion in Lead III
s1q3t3 pattern on ECG for pulmonary embolism
This finding is indicative of Right sided heart strain (Acute cor pulmonale) which can often be seen in patients with a pulmonary embolism.  If a young male with shortness of breath and this ECG presents on your boards, the answer is probably pulmonary embolism. Another important finding is "transient Incomplete RBBB" (present in 7 to 60% of cases), this also suggests acute cor pulmonale as electric conduction traverses down the right bundle. If we consider only  lead III then it could be a normal variant, despite Q wave and T wave inversion, its unlikely inferior myocardial infarction as explained in next section. Contrary to what we set out believing when we begin in medicine, most pathology does not fit neatly into a diagnostic box.  While we memorize as medical students that ST elevation = MI and S1Q3T3 = PE, the waters are much muddier and the diagnosis often less clear in real life. While the electrocardiogram (ECG) is not the most sensitive test for acute pulmonary embolism (PE), there are abnormalities that may help physicians make the diagnosis in the right clinical context.

Why Its not Myocardial infarction:

This isolated Q wave in lead III with a negative T wave is a common normal variant. Remember that lead III is "BIPOLAR", that is, it is the instantaneous difference in voltage recorded between the left leg and left arm. Therefore it has no special geographic significance in and of itself e.g.: it does not necessarily reflect the inferior wall of the heart.

Notice that when you record lead III, the positive pole of your galvanometer (or ECG recorder) is attached to the left leg and the negative pole is attached to the left arm. Therefore, lead III represents the instantaneous difference in voltage between these two points of the body.

III = aVF - aVL

The voltage recorded at aVL is being continuously subtracted from aVF.
Another way of representing lead III (remember your algebra?) is: III = aVF + (-aVL)

Putting it visually:
So an initial R wave in aVL which is quite common will translate into a Q wave in lead III. This Q wave can be very prominent at times but should not be misinterpreted as due to an inferior infarct. The same is true of the negative T wave in lead III. Notice in this case that a normal positive T wave in aVL is responsible for a negative T wave in lead III.

Beware of lead III! For diagnosing an inferior infarct look first to lead aVF which DOES HAVE true spatial significance and after that to leads III and II to which aVF contributes, but only indirectly.
Despite the Q wave and the negative T wave in lead III inferior MI is unlikely in this case and no further evaluation is needed for MI.

ECG changes in Pulmonary Embolism:

The ECG changes associated with acute pulmonary embolism may be seen in any condition that causes acute pulmonary hypertension, including hypoxia causing pulmonary hypoxic vasoconstriction.

Key ECG findings include:

  • Sinus tachycardia – the most common abnormality; seen in 44% of patients.
  • Complete or incomplete RBBB – associated with increased mortality; seen in 18% of patients.
  • Right ventricular strain pattern –  T wave inversions in the right precordial leads (V1-4) ± the inferior leads (II, III, aVF). This pattern is seen in up to 34% of patients and is associated with high pulmonary artery pressures.
  • Right axis deviation – seen in 16% of patients. Extreme right axis deviation may occur, with axis between zero and -90 degrees, giving the appearance of left axis deviation (“pseudo left axis”).
  • Dominant R wave in V1 – a manifestation of acute right ventricular dilatation.
  • Right atrial enlargement (P pulmonale) – peaked P wave in lead II > 2.5 mm in height. Seen in 9% of patients.
  • SI QIII TIII  pattern – deep S wave in lead I, Q wave in III, inverted T wave in III. This “classic” finding is neither sensitive nor specific for pulmonary embolism; found in only 20% of patients with PE.
  • Clockwise rotation – shift of the R/S transition point towards V6 with a persistent S wave in V6 (“pulmonary disease pattern”), implying rotation of the heart due to right ventricular dilatation.
  • Atrial tachyarrhythmias – AF, flutter, atrial tachycardia. Seen in 8% of patients.
  • Non-specific ST segment and T wave changes, including ST elevation and depression. Reported in up to 50% of patients with PE.
Simultaneous T wave inversions in the inferior (II, III, aVF) and right precordial leads (V1-4) is the most specific finding in favour of PE, with reported specificities of up to 99% in one study.

Compared to findings of Acute Coronary Syndrome:

While T wave inversions are commonly associated with acute coronary syndromes, there are several findings associated with pulmonary embolism that differentiate this diagnosis from ACS.
  • ACS is rarely associated with tachycardia
  • Both ACS and PE will present with elevated troponin
  • Ultrasonography may be useful in differentiating the two
  • Kosuge et al have shown that simultaneous inversion in III and V1 are diagnostically significant:
“negative T waves in leads III and V1 were observed in only 1% of patients with ACS compared with 88% of patients with Acute PE (p less than 0.001). The sensitivity, specificity, positive predictive value, and negative predictive value of this finding for the diagnosis of PE were 88%, 99%, 97%, and 95%, respectively. In conclusion, the presence of negative T waves in both leads III and V1 allows PE to be differentiated simply but accurately from ACS in patients with negative T waves in the precordial leads.”

Differential Diagnosis of S1Q3T3 Pattern:

The ECG changes described above are not unique to PE. A similar spectrum of ECG changes may be seen with any cause of acute or chronic cor pulmonale (i.e. any disease that causes right ventricular strain / hypertrophy due to hypoxic pulmonary vasoconstriction).

Acute cor pulmonale

  • Severe pneumonia
  • Exacerbation of COPD / asthma
  • Pneumothorax
  • Recent pneumonectomy
  • Upper airway obstruction

Chronic cor pulmonale

  • Chronic obstructive pulmonary disease
  • Recurrent small PEs
  • Cystic fibrosis
  • Interstitial lung disease
  • Severe kyphoscoliosis
  • Obstructive sleep apnoea


  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
  • Kosuge M, Kimura K, Ishikawa T, Ebina. Electrocardiographic differentiation between acute pulmonary embolism and acute coronary syndromes on the basis of negative T waves. Am J Cardiol. 2007 Mar 15;99(6):817-21. Epub 2007 Jan 30. PMID:17350373.