How the ECG is tested on Step 1

Step 1 rarely asks you to over-read a 12-lead tracing; it asks you to connect an ECG finding to a mechanism or a diagnosis. That means two skills: a systematic approach so you never miss a step, and quick recognition of a small set of classic patterns. Vignettes pair a short history with an ECG description or image and ask for the rhythm, the localized artery, the electrolyte, or the next step.

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Every wave is a depolarization or repolarization event. If you know what tissue makes each deflection, most abnormalities become mechanistic rather than memorized.

Waves, intervals, and what each represents

One cardiac cycle produces a P wave, a QRS complex, and a T wave on a background of standardized ECG paper. The paper scale is what makes intervals measurable: at 25 mm/s, each small box is 0.04 s and each large box (5 small boxes) is 0.20 s.

P QRS T PR 120–200 ms QRS <120 ms QT interval 1 small box = 0.04 s
The ECG complex. P = atrial depolarization; QRS = ventricular depolarization; T = ventricular repolarization. The PR interval spans atrial depolarization plus the AV nodal delay; the QT interval covers ventricular depolarization through repolarization.
  • P wave — atrial depolarization (SA node → atria).
  • PR interval — start of P to start of QRS; includes the AV node delay (normal 120–200 ms).
  • QRS complex — ventricular depolarization via the His–Purkinje system (normal <120 ms).
  • T wave — ventricular repolarization; the QT interval tracks total ventricular electrical activity.
error Common trap

Atrial repolarization does occur, but it is hidden within the much larger QRS — so there is no separate wave for it.

A systematic approach

Read every ECG the same way. A fixed sequence prevents the classic error of spotting one dramatic finding and missing a second.

StepWhat to checkNormal / key value
1. RateAtrial and ventricular rate60–100 bpm
2. RhythmRegular? P before every QRS?Sinus: upright P in II, one per QRS
3. AxisNet QRS in I and aVF−30° to +90°
4. IntervalsPR, QRS, QTPR 120–200 ms; QRS <120 ms
5. MorphologyST segment, T waves, Q waves, chamber sizeIsoelectric ST; no pathologic Q

Rate → Rhythm → Axis → Intervals → Morphology. Same order, every ECG.

Rate and rhythm

For a regular rhythm, the fastest rate estimate is the large-box method: find an R wave on a heavy line and count the large boxes to the next R wave.

300150100756050Count big boxes between two R waves: 300, 150, 100, 75, 60, 50…
The 300 method. The number of large boxes between consecutive R waves maps to the rate: 1 box = 300, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50 bpm.
  • Sinus rhythm: an upright P wave in lead II precedes every QRS, at 60–100 bpm.
  • Irregularly irregular with no discrete P waves suggests atrial fibrillation.
  • For irregular rhythms, count QRS complexes on a 6-second strip and multiply by 10.

Axis in two leads

The QRS axis is the mean direction of ventricular depolarization. You can place it in a quadrant using just leads I and aVF.

Lead I + aVF + Normal Left Right Extreme Quadrant method I + , aVF + → normal I + , aVF − → left axis I − , aVF + → right axis I − , aVF − → extreme (“leaving each other” = left; “reaching toward” = normal)
Axis by quadrant. Read the net QRS deflection in leads I and aVF: both positive = normal axis; I positive and aVF negative = left axis deviation; I negative and aVF positive = right axis deviation.
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If leads I and aVF are both mostly upright, the axis is normal — the two vectors are “reaching toward” each other. If they point away from each other, there is axis deviation.

High-yield patterns

A short list of patterns accounts for most Step 1 ECG questions. Learn the finding and the one mechanism or diagnosis it signals.

ECG findingThink
ST-segment elevationSTEMI — localize by leads (see the MI guide)
Peaked (tented) T wavesHyperkalemia
Short PR + delta waveWolff–Parkinson–White (pre-excitation)
Prolonged QTRisk of torsades de pointes
Prolonging PR then dropped QRSMobitz I (Wenckebach) AV block
No relationship between P waves and QRSThird-degree (complete) AV block
Irregularly irregular, no P wavesAtrial fibrillation

Pattern → diagnosis. These recur across Step 1 cardiology and pharmacology questions.

functions Worked example — calculate the rate

On a standard ECG, consecutive R waves are separated by 4 large boxes. What is the heart rate, and is it normal?

  1. Each large box is 0.20 s, so 4 large boxes = 0.80 s between beats.
  2. Rate = 300 ÷ (number of large boxes) = 300 ÷ 4.
  3. Compare to the normal range of 60–100 bpm.

Answer. 300 ÷ 4 = 75 bpm — a normal sinus rate (assuming an upright P wave precedes each QRS in lead II).

How to study it — and the traps to avoid

ECG mastery on Step 1 comes from doing the same systematic read until it is automatic, then layering on the pattern list.

  1. Anchor each wave to its tissue: P = atria, QRS = ventricles, T = ventricular repolarization.
  2. Memorize the box scale (0.04 s small, 0.20 s large) and the 300-method sequence.
  3. Use leads I and aVF for a 5-second axis read.
  4. Drill the high-yield patterns and their single associations until recognition is instant.
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ST-elevation localization links directly to myocardial infarction; peaked T waves connect to electrolyte and acid–base physiology.