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:
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 MI:
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:
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.
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
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