ECG Rhythm

The rhythm is best analysed by looking at a rhythm strip. On a 12 lead ECG this is usually a 10 second recording from Lead II. Confirm or corroborate any findings in this lead by checking the other leads. A longer rhythm strip, recorded perhaps recorded at a slower speed, may be helpful.

A useful 7 step approach to rhythm analysis is described.

1. Rate —

• Tachycardia or bradycardia?
• Normal rate is 60-100/min.

2. Pattern of QRS complexes —

• Regular or irregular?
• If irregular is it regularly irregular or irregularly irregular?

3. QRS morphology —

• Narrow complex — sinus, atrial or junctional origin.
• Wide complex — ventricular origin, or supraventricular with aberrant conduction.

4. P waves —

• Absent — sinus arrest, atrial fibrillation
• Present — morphology and PR interval may suggest sinus, atrial, junctional or even retrograde from the ventricles.

5. Relationship between P waves and QRS complexes —

• AV association (may be difficult to distinguish from  isorhythmic dissociation)
• AV dissociation
  • complete — atrial and ventricular activity is always independent.
  • incomplete — intermittent capture.

6. Onset and termination —

• Abrupt — suggests re-entrant process.
• Gradual — suggests increased automaticity.

7. Response to vagal manoeuvres —

• Sinus tachycardia, ectopic atrial tachydysrhythmia — gradual slowing during the vagal manoeuvre, but resumes on cessation.
• AVNRT or AVRT — abrupt termination or no response.
• Atrial fibrillation and atrial flutter — gradual slowing during the manoeuvre.
• VT — no response.

Differential Diagnosis

Follow links below for examples of individual rhythms. 

Narrow Complex (Supraventricular) Tachycardias

Broad Complex Tachycardias

Regular

• Ventricular tachycardia
• Antidromic atrioventricular re-entry tachycardia (AVRT).
• Any regular supraventricular tachycardia with aberrant conduction — e.g. due to bundle branch block, rate-related aberrancy.

All regular BCTs should be considered to be VT until proven otherwise.

Irregular 

• Ventricular fibrillation
• Polymorphic VT
• Torsades de Pointes
• AF with Wolff-Parkinson-White syndrome
• Any irregular supraventricular tachycardia with aberrant conduction – e.g. due to bundle branch block, rate-related aberrancy.

Bradycardias

P waves present

Each P wave is followed by a QRS complex (= sinus node dysfunction)

• Sinus bradycardia
• Sinus node exit block
• Sinus pause / arrest

Not every P wave is followed by a QRS complex (= AV node dysfunction)

• AV block: 2nd degree, Mobitz I (Wenckebach)
• AV block: 2nd degree, Mobitz II
• AV block: 2nd degree, “fixed ratio blocks” (2:1, 3:1)
• AV block: 2nd degree, “high grade AV block”
• AV block: 3rd degree (complete heart block)

P waves absent

Narrow complexes

• Junctional escape rhythm

Broad complexes

• Ventricular escape rhythm

For escape rhythms to occur there must be a failure of sinus node impulse generation or transmission by the AV node. 

Further Reading