General Medical Sciences 12 Lead ECG practice questions
A 12-lead ECG records the heart's electrical activity from 12 different vectors (three limb leads, three augmented limb leads, and six precordial leads), producing a composite picture of rate, rhythm, axis, and regional myocardial activity. Because each lead 'looks' at the heart from a different angle, patterns of ST-segment, T-wave, and QRS change across specific lead groups let clinicians localize ischemia, infarction, and conduction disease to a particular coronary territory or conduction pathway. Interpreting a 12-lead ECG systematically (rate, rhythm, axis, intervals, morphology) is a core skill tested throughout general medicine and emergency medicine curricula.
A 22-year-old asymptomatic college athlete undergoes a routine ECG before joining the varsity swim team. The rhythm is regular, and the interval between successive R waves spans exactly 4 large boxes. Using the '300 method,' what is her heart rate?
A 58-year-old man presents with 45 minutes of crushing substernal chest pain radiating to the left arm. His ECG shows ST-segment elevation in leads V1 through V4 with reciprocal ST depression in leads II, III, and aVF. Which coronary artery is most likely occluded?
A 65-year-old woman presents with substernal chest pain and diaphoresis. Her ECG shows ST elevation in leads II, III, and aVF with reciprocal ST depression in leads I and aVL. Which additional step is most appropriate to evaluate for concomitant right ventricular involvement?
A 70-year-old man with chronic kidney disease on hemodialysis missed his last two dialysis sessions. His serum potassium returns at 7.2 mEq/L. Which ECG finding is most characteristic of this metabolic abnormality?
A 52-year-old man comes to the emergency department after an episode of substernal chest pain that resolved before arrival. He is now pain-free with normal vital signs, and his troponin is mildly elevated. His ECG shows deeply inverted, symmetric T waves in leads V2 and V3 with isoelectric ST segments and preserved R-wave progression. Which of the following is the most appropriate next step in management?
12 Lead ECG, answered
What is the fastest way to estimate heart rate from a 12-lead ECG?
For a regular rhythm, count the number of large boxes between two consecutive R waves and divide 300 by that number. For an irregular rhythm, count the number of QRS complexes on a 6-second strip and multiply by 10 instead, since box-counting assumes evenly spaced beats.
How can I remember which leads correspond to which coronary artery territory?
Group the leads by wall: V1-V2 are septal, V3-V4 are anterior (both usually LAD territory), I, aVL, V5-V6 are lateral (circumflex territory), and II, III, aVF are inferior (usually right coronary artery territory). Reciprocal ST depression in the opposite territory increases confidence that the changes are ischemic rather than a normal variant.
Why do reciprocal ST-segment changes matter when diagnosing a myocardial infarction?
Reciprocal changes are a mirror-image ST depression in leads opposite the infarct territory, and their presence makes an ischemic cause more likely and helps distinguish infarction from mimics such as pericarditis or benign early repolarization, which typically do not produce reciprocal depression.
Do I need to memorize exact voltage criteria for things like left ventricular hypertrophy or bundle branch block?
For most general medicine assessments, it is more useful to recognize the overall pattern (for example, broad or notched QRS complexes in bundle branch block, or high-voltage QRS complexes with strain-pattern ST-T changes in left ventricular hypertrophy) than to recall precise millivolt cutoffs, since those thresholds vary slightly between criteria sets such as Sokolow-Lyon and Cornell.
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