Understanding the ECG


Updated: 03 September 2023

Notes from The ECG Made Easy (7ed) by John R. Hampton

Main Features

The Normal ECG


  • PR Interval: 200ms (5 small squares)
  • QRS Complex: 120ms (3 small squares)
  • QT Interval: 450ms


  • Sinus arrythmia
  • Supraventricular extrasystoles are always normal

The Cardiac Axis

  • Normal axis: QRS upward in I, II, III. Still normal if QRS is downward in III
  • Minor degrees of axis deviation are normal

QRS Complex

  • Small Q wave in I, VL, and V6
  • RSR pattern in V1 normal if less than 120ms
  • R wave smaller than S in V1
  • R wave less than 25mm in V6
  • R + S less than 35mm in V1

ST Segment

  • Should be isoelectric

Isoelectric: “having or involving no net electric charge or difference in electrical potential”

T Wave

  • May be inverted in III, VR, V1, V2, V3 (in some people)

Conduction Problems

First Degree Block

  • One P wave per QRS complex
  • PR interval greater than 200ms

Second Drgree Block

  • Movitz type 2: occasional nonconducted beats
  • Wenkebach: progressive lengethening of PR interval, then a non-conducted P wave, and then repeats
  • 2:1 (or 3:1) block: two or three P waves per WRS complex with normal P wave rate

Third Degree (complete) Block

  • No relationship between P wave and QRS complex
  • Wide QRS complex
  • QRS complex rate less than 50/min
  • Sometimes narrow WRS complex rate (50-60/min)

Right Bundle Branch Block

  • QRS duration greater than 120ms
  • RSR pattern
  • Usually dominant R wave in V1
  • Inverted T wave in V1 and sometimes V2, V3
  • Deep and wide S wave in V6

Left Anterior Hemiblock

  • Marked left axis deviation - deep S wave in II and III with slightly wide QRS

Left Bundle Branch Block

  • QRS complex greater than 120ms
  • M pattern in V6 and sometimes V4, V5
  • No septal Q waves
  • Inverted T wave in I, Vl, V5, V6, and sometimes V4

Bifascicular Block

  • Left Anterior Hemiblock + Right Bundle Branch Block

Supraventricular Rhythms

Common Rhythms

  • Sinus rhythm
  • Atrial extrasystoles
  • Junction (AV nodal) extrasystoles
  • Atrial tachycardia
  • Atrial flutter
  • Junctional (AV nodal) tachycardia
  • Junctional (AV nodal) escape
  • Atrial fibrillation

Rhythm Abnormalities

  • Extrasystoles: single early beats supressing the next sinus beat
  • Escape beats: absence of sinus beat followed by late single beat
  • Tachycardias
  • Bradycardias

Characteristics of Supraventricular Rhythms

In general

  • Narrow QRS (less than 120ms)
  • Same QRS as in sinus rhythm
  • Normal T wave

May have a wide QRS if there is a bundle branch block or Wolfff-Parkinson-White syndrome

  • Sinus rhythm
    • one P wave per QRS
    • P-P interval varies with respiration
  • Atrial or junctional (AV nodal) extrasystoles
    • early QRS complex
    • no P wave, or abnormally shaped (atrial) P wave
    • narrow and normal QRS complex
    • normal T wave
    • nexgt P wave is ‘reset’
  • Atrial tachycardia
    • P wave rate 300/min
    • abnormal P wave, usually short PR interval
    • Usually more than one P wave per WRS complex, P wave rate sometimes 200-240/min with 2:1 block
  • Atrial flutter
    • P wave rate 300/min
    • sawtoothed pattern
    • 2:1, 3:1, 4:1 block
    • block increased by carotid sinsuss pressure
  • Junctional (AV nodal re-entry) tachycardia
    • commonly, but inappropriately, called SVT
    • no P waves
    • rate usually 150-180/min
    • carrotid sinus pressure may cause reversion to sinus rhythm
  • Atrial fibrillation
    • this si the most irregular rhythm
    • QRS complex over 160/min but can be slower
    • no P wave identifiable, completeley irregular baseline
  • Escape ryhthms
  • bradycardias, atrial fibrillation odes not occur as an escape rhythm

Ventricular Rhythms

Common Ventricular rhythms

  • Ventricular extrasystoles
  • Accelerated idioventricular rhythm
  • Ventricular escape
  • Ventricular fibrillation

Rhythm Abnormalities

  • Extrasystoles: single early beats supressing the next sinus beat
  • Escape beats: absence of sinus beat followed by a late single beat
  • Tachcardias
  • Bradycardias

Characteristics of Ventricular Rhythms

In general

  • Wide QRS complexes (greater than 120ms)

  • different QRS complex from those seen in sinus

  • change of axis compared to sinus

  • abnormal T waves

  • Ventricular extrasystoles

    • early QRS complex
    • no P wave
    • QRS complex wide
    • abnormally shaped QRS
    • abnormally shaped T wave
    • next P wave is on time
  • Accelerated idioventricular rhythm

  • Ventricular escape

    • Single beats or complete heart block
  • Ventricular tachycardia

    • no P waves
    • QRS complex rate greater than 160/min
    • accelerated idioventricular rhythm: like ventricular tachycardia, QRS rate less than 120/min
  • Ventricular fibrillation

    • look at the patient not the ECG
    • ECG is totally disorganized, patient will have lost consciousness

Myocardial Infraction

Sequence of ECG changes

  1. Normal ECG
  2. Raised ST segments
  3. Appearence of Q waves
  4. Normalization of ST segments
  5. Inversion of T waves

Site of infraction

  • Anterior infraction: changes in leads V3, V4, often also V2, V5
  • Inferior infraction: changes in leads III anf VF


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