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CRAT: ECG Interpretation: Leads, Axis, Intervals, and Infarction Patterns

CRAT · Work through your exam, one topic at a time.

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ECG Interpretation: Leads, Axis, Intervals, and Infarction Patterns

The ECG Interpretation domain tests your ability to read a 12-lead ECG: lead placement and quality, axis, intervals, chamber enlargement, conduction blocks, ischemia and infarction patterns, and drug or electrolyte effects. Items often give a short clinical stem plus lead findings, so localization matters as much as the pattern name.

Placement, axis, intervals

  • V1 sits at the 4th intercostal space, right sternal border; V2 at the 4th ICS left; V4 at the 5th ICS midclavicular line; V3 between V2 and V4; V5 at the anterior axillary line; V6 at the midaxillary line, same level as V4.
  • Lead I inverted with a negative P wave suggests left arm/right arm reversal.
  • Axis: lead I and aVF both positive is normal; I positive with aVF negative and lead II negative is left axis deviation; I negative with aVF positive is right axis deviation.
  • Normal intervals: PR 0.12–0.20 s, QRS <0.12 s, QTc <0.44 s in men and <0.46 s in women.

Ischemia, infarction, and localization

  • ST elevation in two contiguous leads defines a STEMI pattern; thresholds in V2–V3 vary with age and sex.
  • Inferior: II, III, aVF. Lateral: I, aVL, V5, V6. Anterior: V1–V4. Reciprocal ST depression supports the diagnosis.
  • Posterior infarction shows ST depression and tall R waves in V1–V3. Right ventricular infarction shows elevation in V1 and V4R; nitrates can cause dangerous hypotension.
  • Pathologic Q waves, T-wave inversion, and hyperacute T waves complete the evolution.

Chamber and conduction findings

  • Left ventricular hypertrophy by Sokolow-Lyon: S in V1 plus R in V5 or V6 >35 mm.
  • LBBB: QRS ≥0.12 s, broad notched R in I and V6, deep S in V1. RBBB: rSR′ in V1 and wide slurred S in I and V6.
  • P pulmonale: peaked P >2.5 mm in II. P mitrale: broad, notched P >0.12 s in II.
  • Hyperkalemia: peaked T waves and widening QRS. Hypokalemia: flat T waves and U waves.

Worked example

A patient with chest pain has 2 mm ST elevation in II, III, and aVF with ST depression in I and aVL. Three contiguous inferior leads plus reciprocal lateral depression indicate an inferior STEMI, most often from right coronary artery occlusion. The next checks are a right-sided lead for V4R elevation and blood pressure before any nitrate, because right ventricular involvement is preload-dependent.

Traps

  • Limb lead reversal producing a false inferior infarct or false right axis.
  • Ignoring aVR, where ST elevation can signal left main disease.
  • Applying one ST threshold to V2–V3 regardless of age or sex.
  • Measuring QT in a lead with a prominent U wave or an unclear T-wave end.
  • Calling LVH-related ST depression acute ischemia without clinical context.