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EKG Interpretation Basics

Sep 9, 2025

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Overview

This lecture provides a systematic approach to interpreting 12-lead EKGs, covering waveform components, lead placement, measurement methods, and identification of key abnormalities important for diagnosing cardiac conditions.

EKG Waveform Fundamentals

  • P wave indicates atrial depolarization (SA node activity).
  • PR segment reflects AV node depolarization (signal paused at AV node).
  • PR interval spans from atrial to AV node depolarization (P wave + PR segment).
  • QRS complex represents ventricular depolarization (Q, R, S waves).
  • ST segment shows ventricles remain depolarized (no net movement).
  • T wave indicates ventricular repolarization.
  • QT interval is duration of ventricular depolarization and repolarization.

EKG Leads & Heart Regions

  • Leads II, III, aVF assess inferior heart (right/left ventricles).
  • Leads I, aVL, V5, V6 assess lateral left ventricle.
  • Leads V1, V2, aVR focus on right ventricle.
  • Leads V1–V4 assess anterior/septal heart.
  • 12 leads: 3 bipolar (I, II, III), 3 augmented (aVR, aVL, aVF), and 6 precordial (V1–V6).

EKG Paper Measurements

  • Large box: 5mm x 5mm (0.5mV, 0.2s).
  • Small box: 1mm x 1mm (0.1mV, 0.04s).
  • Amplitude = height (voltage); width = time.

Rate and Rhythm Analysis

  • Normal rate: 60–100 bpm; <60 bradycardia, >100 tachycardia.
  • Rate methods: R waves × 6 (10s strip), or 300 divided by boxes between R–R.
  • Rhythm: Regular if R–R intervals are consistent.
  • QRS width: Narrow (<0.12s, 3 boxes) or wide (>0.12s).
  • Sinus rhythm: P wave upright in II, inverted in aVR, each P followed by QRS.

Tachycardia/Bradycardia Differentials

  • Narrow regular tachycardia: Sinus tachycardia, atrial flutter, SVT.
  • Narrow irregular tachycardia: Afib, variable atrial flutter, multifocal atrial tachycardia.
  • Wide regular tachycardia: VT (ventricular tachycardia) until proven otherwise.
  • Wide irregular tachycardia: Polymorphic VT, afib w/ WPW, or bundle branch block.
  • Bradycardia: Sinus bradycardia, AV blocks, junctional, or ventricular rhythms.

ST Segment and Abnormalities

  • ST elevation: >1mm in most leads or >2mm in V2–V3, in 2 contiguous leads.
  • ST elevation ≠ always STEMI; consider pericarditis, early repolarization, etc.
  • ST depression: ≥0.5mm below baseline in 2 contiguous leads is significant for ischemia.
  • Types: Horizontal (most concerning), downsloping, upsloping.
  • Other: Reciprocal changes indicate STEMI; digoxin toxicity causes downsloping depression.

T Wave Abnormalities

  • T wave inversion (>1mm below baseline) can be normal in V1-V2/III, but concerning in aVL or V2–V3 (Wellens’ sign).
  • Hyperacute T waves: tall, broad, indicate early STEMI.
  • Biphasic T waves: Wellens’ A in V2–V3 (proximal LAD lesion); negative-positive = hyperkalemia.
  • Flat T waves: Possible ischemia or hypokalemia.
  • Peaked T waves: Hyperkalemia, hypermagnesemia, or De Winter’s (proximal LAD).

QRS Complex Abnormalities

  • Wide QRS (>0.12s): Think BBB, VT, WPW, hyperkalemia, paced rhythms, drug toxicity.
  • LBBB: M-shaped QRS in V5–V6, deep S in V1–V2.
  • RBBB: RSR’ in V1–V2, wide S in V5–V6.
  • Pathological Q waves: >0.04s width, >2mm deep, or >25% QRS height; abnormal in V1–V3.
  • Low voltage QRS: R waves in I+II+III <15mm, or V1–V3 <30mm; consider pericardial effusion, COPD, obesity, amyloidosis.
  • Poor R wave progression: Suggests anterior MI, RVH.

QT Interval Abnormalities

  • Prolonged QT: >460ms (females), >450ms (males). Risk for torsades de pointes.
  • Causes: Drugs (anti-arrhythmics/biotics/psychotics/depressants/emetics), low K/Mg/Ca, MI.
  • Short QT (<350ms): Hyperkalemia, hypermagnesemia, digoxin toxicity.

P Wave & PR Interval Abnormalities

  • Right atrial enlargement: P wave >2.5mm in II (or big positive in V1).
  • Left atrial enlargement: Bifid P wave in II; negative part in V1 >positive part.
  • Short PR (<0.12s): WPW (delta wave), PACs.
  • Prolonged PR (>0.20s): Heart block (1st, 2nd Mobitz I, 3rd degree).

Cardiac Axis Assessment

  • Lead I and aVF: Both positive = normal axis.
  • Lead I up, aVF down: Check II. II up = normal; II down = left axis deviation.
  • Lead I down, aVF up = right axis deviation.
  • Both down = extreme right axis deviation.
  • Left axis: LBBB, LVH, inferior MI, hyperkalemia.
  • Right axis: RBBB, RVH, anterior MI, VT.
  • Extreme right: Severe RVH, VT, obesity.

Key Terms & Definitions

  • P wave — Atrial depolarization.
  • QRS complex — Ventricular depolarization.
  • ST segment — Plateau phase after depolarization, before repolarization.
  • T wave — Ventricular repolarization.
  • QT interval — Time for ventricular depolarization/repolarization.
  • Bundle branch block (BBB) — Delay/blockage in cardiac conduction through bundle branches.
  • STEMI — ST-elevation myocardial infarction.
  • Axis deviation — The direction of the heart’s electrical activity.

Action Items / Next Steps

  • Practice 12-lead EKG interpretation using this systematic approach.
  • Review specific arrhythmias and pathology cases as recommended in the lecture.
  • Memorize criteria for key abnormalities (e.g., ST elevation/depression, Q waves, axis deviation).

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