🩺

ECG Interpretation Guide

Jul 10, 2025

Overview

This lecture covers a systematic approach to ECG interpretation, focusing on reading ECGs step-by-step for clinical assessment of rate, rhythm, intervals, axis, and identifying common cardiac pathologies.

Steps in ECG Interpretation

  • Determine the heart rate: tachycardia (>100 bpm), normal (60–100 bpm), or bradycardia (<60 bpm).
  • Assess the heart rhythm: classify QRS as narrow/wide and regular/irregular.
  • Evaluate the axis: determine if normal, left, right, or extreme right axis deviation.
  • Analyze intervals: focus on PR and QT intervals for AV blocks and risk of torsades de pointes.
  • Inspect P-waves for atrial enlargement.
  • Examine QRS complex for bundle branch blocks or ventricular hypertrophy.
  • Assess ST segment and T-waves for evidence of ischemia or infarction.

Heart Rate Calculation Methods

  • Count R-waves in a 10-second rhythm strip, multiply by 6.
  • Count boxes between R-waves, divide 300 by that number.

Tachycardia Rhythms

  • Narrow, regular: sinus tachycardia (upright P in lead II), atrial flutter (sawtooth pattern), PSVT (no visible P waves).
  • Narrow, irregular: atrial fibrillation (no obvious P waves, fibrillatory baseline), atrial flutter with variable block, multifocal atrial tachycardia (≥3 different P-wave morphologies).
  • Wide, regular: monomorphic ventricular tachycardia.
  • Wide, irregular: polymorphic VT (torsades de pointes), ventricular fibrillation.

Bradycardia Rhythms

  • Sinus bradycardia: normal PR interval, every P followed by QRS.
  • First-degree AV block: prolonged PR interval (>200 ms), no dropped QRS.
  • Second-degree Mobitz I (Wenckebach): PR gets longer until QRS is dropped.
  • Second-degree Mobitz II: constant prolonged PR, intermittent QRS drops.
  • Third-degree AV block: complete dissociation between P and QRS.

Axis Determination

  • Both Lead I and aVF positive: normal axis.
  • Lead I positive, aVF negative, Lead II negative: left axis deviation (LBBB, LVH, left anterior fascicular block).
  • Lead I negative, aVF positive: right axis deviation (RBBB, RVH, left posterior fascicular block).
  • Both Lead I and aVF negative: extreme right axis deviation (common in monomorphic VT).

PR and QT Intervals

  • PR >200 ms: AV blocks.
  • PR <160 ms with delta wave and wide QRS: Wolff-Parkinson-White syndrome.
  • QT >½ R-R interval or >500 ms: risk for torsades de pointes.
  • Causes of prolonged QT: antiarrhythmics, macrolides, certain antidepressants, antipsychotics, hypokalemia, hypomagnesemia, hypocalcemia.

Atrial Enlargement

  • Left atrial enlargement: "bifid" P in lead II, large terminal V1 P.
  • Right atrial enlargement: tall P in lead II (≥2.5 mm), large initial V1 P.
  • Common causes: pulmonary hypertension, valve disease.

QRS Analysis: Bundle Branch Blocks & Hypertrophy

  • LBBB: deep S in V1/V2 and notched R in V5/V6.
  • RBBB: rSR′ in V1/V2, wide slurred S in V5/V6.
  • LVH: deep S in V1/V2 + tall R in V5/V6 >35 mm.
  • RVH: tall R in V1/V2, deep S in V6.

ST Segment & T-wave Changes

  • ST depression (horizontal/downsloping ≥1 mm): myocardial ischemia.
  • T-wave inversion: possible ischemia.
  • Troponin positive + ST depression/T-inversion: NSTEMI; normal troponin: unstable angina.
  • ST elevation (≥1 mm limb, ≥2 mm precordial, two contiguous leads): myocardial infarction.
  • Convex ("frown") ST elevation: MI; concave ("smile"): benign or pericarditis.
  • Reciprocal changes help localize STEMI (anterior: V1–V4; lateral: I, aVL, V5–V6; inferior: II, III, aVF; posterior: V7–V9).
  • Diffuse ST elevation + PR depression: pericarditis.
  • Hyperkalemia: peaked T-waves, possible ST elevation.

Key Terms & Definitions

  • QRS Complex — Part of ECG representing ventricular depolarization.
  • STEMI — ST-Elevation Myocardial Infarction.
  • NSTEMI — Non-ST-Elevation Myocardial Infarction.
  • PR Interval — Time from onset of P wave to start of QRS; indicates AV conduction.
  • QT Interval — Time from start of QRS to end of T-wave; reflects ventricular depolarization and repolarization.
  • Bundle Branch Block — Delay/block in conduction through right or left bundle branches.

Action Items / Next Steps

  • Practice ECG rate and rhythm calculations from sample strips.
  • Memorize criteria for axis deviation and bundle branch blocks.
  • Review causes of ST segment and T-wave changes.
  • Check recommended readings on AV block types and acute coronary syndromes.

Full transcript