Arrhythmias are disturbances of normal heart rhythm caused by disruption of the electrical impulses that coordinate myocardial contraction. They vary widely in cause, ECG appearance, urgency, and treatment.

This summary is intended for exam preparation and is based on Resuscitation Council UK guidance (2021). In real clinical practice, always follow full guidelines, escalate early, and involve senior clinicians.

Cardiac Arrest Rhythms

In a pulseless patient, there are four possible arrest rhythms, classified as shockable or non-shockable depending on whether defibrillation is effective.

Shockable Rhythms

• Ventricular tachycardia (VT)
• Ventricular fibrillation (VF)

Non-Shockable Rhythms

• Pulseless electrical activity (PEA)
  (Any organised electrical rhythm without a pulse, including sinus rhythm)
• Asystole
  (Absence of meaningful electrical activity)

Narrow Complex Tachycardia

A narrow complex tachycardia is defined by:

• Fast heart rate
• QRS duration < 0.12 seconds
  (≤ 3 small squares on a standard 25 mm/sec ECG)

Main Differentials

1. Sinus tachycardia
   • Normal P–QRS–T sequence
   • Physiological response (e.g. pain, sepsis, hypovolaemia)
   • Treat the underlying cause (could be due to thyroid storm – still a concern, a cardiac concerning cause)
2. Supraventricular tachycardia (SVT)
   • Very regular rhythm
   • Abrupt onset and offset
   • P waves often hidden within T waves
   • Managed with vagal manoeuvres → adenosine
     • blow into a white-tip syringe
     • cold water
     • holding breath
3. Atrial fibrillation (AF)
   • No visible P waves
   • Irregularly irregular ventricular rhythm
   • reduced cardiac output – can cause HF
   • mitral heart disease (mitral stenosis -mitral valve pushing against stenotic valve – left atrial hypertrophy – stretched conduction pathway)
   • non-cardiac causes – alcohol, thyroid issues, post-op sepsis,
   • cardioversion shock to sinus rhythm only done once anticoags
4. Atrial flutter
   • Saw-tooth flutter waves
   • Atrial rate ≈ 300 bpm
   • Often 2:1 conduction → ventricular rate ≈ 150 bpm

Distinguishing SVT from Sinus Tachycardia

Unstable Tachycardia (Any Type)

Patients with life-threatening features require urgent synchronised DC cardioversion:

• Syncope
• Myocardial ischaemia (e.g. chest pain)
• Shock
• Acute heart failure

If cardioversion fails, IV amiodarone is added.

Broad Complex Tachycardia

A broad complex tachycardia is defined by:

• QRS duration > 0.12 seconds
  (> 3 small squares)

Differential Diagnosis and Management

• Ventricular tachycardia / unknown cause → IV amiodarone
• Polymorphic VT (torsades de pointes) → IV magnesium
• AF with bundle branch block → Treat as AF
• SVT with bundle branch block → Treat as SVT

Unstable patients require synchronised DC cardioversion regardless of cause.

Atrial Flutter – Mechanism

Normally, atrial depolarisation occurs once per cycle. In atrial flutter, a re-entry circuit forms within an atrium due to an additional electrical pathway. The impulse continuously circulates without terminating.

• Atrial rate ≈ 300 bpm
• Narrow complex tachycardia
• Saw-tooth ECG appearance
• AV node limits ventricular conduction

Common conduction ratios:

• 2:1 → 150 bpm
• 3:1, 4:1, or variable

Management

• Same principles as atrial fibrillation
• Anticoagulation based on CHA₂DS₂-VASc
• Radiofrequency ablation can be curative

Prolonged QT Interval

The QT interval spans from the start of the QRS complex to the end of the T wave. The QTc corrects this for heart rate.

QTc is prolonged when:

• > 440 ms in men
• > 460 ms in women

Prolonged repolarisation increases the risk of after-depolarisations, which can trigger torsades de pointes.

Torsades de Pointes

• A polymorphic ventricular tachycardia
• QRS complexes appear to twist around the baseline
• May self-terminate or deteriorate into VT/VF

Causes of Prolonged QT

• Congenital long QT syndrome
• Medications:
  • Antipsychotics
  • Citalopram
  • Flecainide
  • Sotalol
  • Amiodarone
  • Macrolide antibiotics
• Electrolyte abnormalities:
  • Hypokalaemia
  • Hypomagnesaemia
  • Hypocalcaemia

Management

• Stop QT-prolonging drugs
• Correct electrolytes
• Beta blockers (excluding sotalol)
• Pacemaker or ICD if indicated

Acute Torsades Treatment

• Treat underlying cause
• IV magnesium (even if magnesium level is normal)
• Defibrillation if VT occurs

Ventricular Ectopics (PVCs)

Premature ventricular contractions arise from ectopic ventricular foci.

• Sensation of “missed” or “extra” beats
• Broad, abnormal QRS complexes
• Occur in healthy individuals but more common in heart disease

Bigeminy

• Every second beat is ectopic
• Normal beat → ectopic → normal → ectopic

Management

• Reassurance if infrequent and patient is well
• Specialist input if:
  • Structural heart disease
  • Syncope or chest pain
  • Family history of sudden cardiac death
• Beta blockers may help symptoms

Heart Block

First-Degree AV Block

• PR interval > 0.2 seconds
• Every P wave conducts
• Usually benign

Second-Degree AV Block

Some atrial impulses fail to conduct.

Mobitz Type 1 (Wenckebach)

• Progressive PR prolongation
• Eventually dropped QRS
• Cycle then resets

Mobitz Type 2

• Sudden dropped QRS complexes
• PR interval remains constant
• High risk of asystole

May occur in fixed ratios (e.g. 3:1).

2:1 block is difficult to classify and treated cautiously.

Third-Degree (Complete) Heart Block

• No relationship between P waves and QRS complexes
• High risk of asystole

Bradycardias

Bradycardia is typically defined as HR < 60 bpm.

It may be normal in athletes, but causes include:

• Beta blockers
• Heart block
• Sick sinus syndrome

Sick Sinus Syndrome

• SA node dysfunction
• Fibrotic degeneration
• Causes:
  • Sinus bradycardia
  • Sinus pauses
  • Sinus arrhythmia

Asystole Risk Factors

• Mobitz type 2 block
• Complete heart block
• Previous asystole
• Ventricular pauses > 3 seconds

Management of Unstable Bradycardia

• IV atropine (first line)
• Inotropes:
  • Isoprenaline
  • Adrenaline
• Temporary pacing
• Permanent pacemaker when available

Temporary Pacing Options

• Transcutaneous pacing (external pads)
• Transvenous pacing (catheter-based)

Atropine – Key Mechanism

• Antimuscarinic agent
• Blocks parasympathetic input
• Expected side effects:
  • Dilated pupils
  • Dry mouth
  • Urinary retention
  • Constipation

Last updated: January 2026