General Medical Sciences ECG practice questions
The ECG is a graphical recording of the heart's electrical activity and remains the fastest, cheapest way to diagnose arrhythmias, conduction blocks, ischemia, and electrolyte disturbances. A systematic approach — rate, rhythm, axis, intervals, and morphology — lets you extract this information reliably rather than pattern-matching in isolation. Mastery of ECG interpretation is tested heavily on medical licensing exams because it directly changes bedside management.
A 32-year-old woman undergoes a routine pre-operative ECG. It shows a regular rhythm at 108 beats per minute. Every QRS complex is preceded by an upright P wave in lead II, and the PR interval is constant at 160 ms. Which of the following best describes this rhythm?
A 68-year-old man has an ECG performed as part of an annual physical exam; he has no cardiac symptoms. The tracing shows progressive lengthening of the PR interval over three consecutive beats, followed by a P wave that is not followed by a QRS complex. The pattern then resets and repeats. Which of the following is the most likely diagnosis?
A 58-year-old man presents with 45 minutes of substernal chest pressure radiating to the jaw. His ECG shows ST-segment elevation in leads II, III, and aVF with reciprocal ST depression in leads I and aVL. Occlusion of which coronary artery is most likely responsible?
A 61-year-old man with end-stage renal disease misses two dialysis sessions and presents with generalized weakness. Serum potassium returns at 7.2 mEq/L. Which ECG finding is most characteristic of this electrolyte abnormality?
A 74-year-old woman is started on a fluoroquinolone antibiotic while already taking a Class IA antiarrhythmic for atrial fibrillation. Three days later she has a brief syncopal episode. Telemetry captures a polymorphic ventricular tachycardia in which the QRS amplitude appears to twist around the isoelectric baseline, preceded by a QTc of 540 ms. Which of the following best describes this arrhythmia?
ECG, answered
What is the fastest reliable way to estimate heart rate on an ECG?
For a regular rhythm, count the number of large boxes between two consecutive R waves and divide 300 by that number (300, 150, 100, 75, 60, 50 for 1-6 boxes). For an irregular rhythm, count the number of QRS complexes on a 6-second strip and multiply by 10.
How do I tell Mobitz I apart from Mobitz II on an exam question?
Look at the PR interval across several beats. If it progressively lengthens before a dropped QRS, it's Mobitz I (Wenckebach), which is usually benign. If the PR interval stays constant right up until a beat is suddenly dropped, it's Mobitz II, which carries a higher risk of progressing to complete heart block and often warrants pacing.
Why does inferior ST elevation show reciprocal changes in I and aVL?
Leads I and aVL view the heart from roughly the opposite direction to leads II, III, and aVF. When the inferior wall is injured and its leads show ST elevation, the electrically opposite leads record a mirror-image ST depression — this reciprocity actually strengthens the case for an acute infarct rather than pericarditis or another mimic.
Do I need to memorize exact QTc cutoffs for long QT syndrome?
You should know the general thresholds — a QTc over about 450 ms in men or 460 ms in women is considered prolonged, and values above 500 ms carry a meaningfully higher risk of torsades de pointes — but exam questions usually give you enough clinical context (drugs, electrolytes, symptoms) that you can reason to the diagnosis even if you don't recall the exact number.
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