A 67-year-old man presents to the emergency department after fainting. He was outside tending to his lawn for several hours in the heat, when he stood up suddenly from pulling weeds and fainted. He denies any preceding symptoms and returned to baseline within 1 minute. The patient is not aware of any medical problems and just started seeing a primary care physician last week. He recently used a friend's prescription for ondansetron for nausea. His temperature is 99.3°F (37.4°C), blood pressure is 142/88 mmHg, pulse is 107/min, respirations are 14/min, and oxygen saturation is 99% on room air. Physical exam reveals intact cranial nerves, normal strength and sensation, and a stable gait. His abdomen is soft and nontender. An ECG is performed as seen in Figure A. Which of the following is the most likely diagnosis based on this patient’s ECG?
Previous myocardial ischemia
Acute myocardial infarction
Intermittent torsades des pointes
Pulmonary embolism
Previous myocardial ischemia
This patient is presenting after syncope, likely secondary to dehydration and orthostatic hypotension given his outdoor gardening in the heat and symptoms when standing up rapidly. The patient’s ECG reveals deep, pathologic Q waves, which in this case are an incidental finding indicative of a previous myocardial infarction or a previous ischemic event.

The Q wave is a negative deflection just prior to the R wave. Q waves are a normal finding when they are small and are seen on most ECGs. However, pathologic Q waves are indicative of myocardial ischemia. Pathologic Q waves are generally diagnosed when they are >2 mm deep, >1 mm wide, >25% of the QRS complex height, or are seen in leads V1-V3. The interpretation of Q waves depends on other findings on ECG and the patient's symptoms. Deep Q waves in the setting of ST elevation indicate an acute myocardial infarction. On the other hand, pathologic Q waves found incidentally indicate a previous myocardial infarction or previous ischemic event.

Rovai et al. review Q waves. They discuss how Q waves can be used to predict the location and size of a myocardial infarction as correlated with MRI. They recommend testing such as cardiac MRI to assess for previous infarction in the appropriate patient population.

Figure/Illustration A is an ECG demonstrating pathologic Q waves (red arrows). Note the deep and wide Q waves; this raises concern for a past ischemic event.

Incorrect Answers:
Answer A: Acute myocardial infarctions would present with ST elevation in a vascular distribution on ECG with possible reciprocal depressions. While T wave inversions may be indicative of new or acute ischemia, deep pathologic Q waves without ST elevation are indicative of previous infarction.

Answer B: Hypokalemia would cause U waves on ECG, which appear as a small, "second T wave" after the initial T wave. Note they are not U-shaped despite their name. Management is centered on repletion of potassium and often magnesium as well.

Answer C: Intermittent torsades de pointes can cause syncope and should be suspected in a patient with syncope in the setting of a prolonged QT interval. This patient’s QT interval is not particularly prolonged, and his syncope seems to be secondary to orthostatic hypotension. Management is centered on cessation of QT-prolonging medications and magnesium administration. Note that while ondansetron does prolong the QT interval, it is unlikely to cause torsades given the patient took it a week ago.

Answer E: Pulmonary emboli would present on ECG with sinus tachycardia in addition to pleuritic chest pain and hypoxia. Note that the ECG is a nonspecific test for pulmonary emboli, and CT angiography is the preferred confirmatory test for this condition. Syncope is a possible presentation of larger pulmonary emboli (though the patient would also be hemodynamically unstable).

Bullet Summary:
Pathologic Q waves are indicative of a previous myocardial infarction.