Supraventricular Tachycardia

Supraventricular tachycardia (SVT) refers to regular narrow complex tachycardias originating above the bundle of His, usually AV nodal reentrant tachycardia (AVNRT, about 60%), AV reentrant tachycardia (AVRT, about 30%) and atrial tachycardia (10%).

Prevalence 2.25 per 1000; women affected twice as often as men; onset commonly in young adulthood.

Mechanism: reentry circuit, triggered by premature atrial or ventricular beats; triggers include caffeine, alcohol, stress, exercise, pregnancy.

Usually benign but disabling; rarely causes tachycardia mediated cardiomyopathy.

• Abrupt onset and offset of palpitations (heart rate 140 to 250 beats per minute), dyspnoea, chest tightness, lightheadedness, neck pulsations ("frog sign"), polyuria (atrial natriuretic peptide release), syncope (rare).
• Haemodynamic instability (hypotension, chest pain, heart failure) in few.
• Regular rapid pulse.

• 12 lead ECG during tachycardia: narrow QRS regular, rate 150 to 250, P waves retrograde or hidden; short RP interval (AVNRT), long RP (atypical AVNRT, atrial tachycardia).
• Baseline ECG for pre excitation (delta wave).
• Adenosine response: abrupt termination suggests AVNRT or AVRT; transient AV block with persistent atrial activity suggests atrial tachycardia or flutter.
• Ambulatory monitoring for infrequent episodes.
• Echocardiography, thyroid tests.
• Electrophysiology study for diagnosis and ablation.

Acute

• Unstable (hypotension, chest pain, heart failure, altered mentation): synchronised cardioversion 50 to 100 J.
• Stable: vagal manoeuvres (modified Valsalva with legs raised: strain for 15 seconds at 40 mmHg then supine leg lift, converts 43%), adenosine 6 mg rapid IV push followed by 20 mL saline flush; if no response, 12 mg, then 12 mg (use 3 mg via central line; reduce with dipyridamole and carbamazepine; avoid in asthma and in pre excited AF).
• Diltiazem 0.25 mg/kg IV over 2 minutes (or verapamil 5 to 10 mg IV) or metoprolol 5 mg IV in adenosine failures with preserved ejection fraction.
• Pregnancy: vagal manoeuvres, adenosine; metoprolol for prevention.

Long term

• Catheter ablation is first line curative therapy (slow pathway modification for AVNRT, success over 95%, AV block risk under 1%; accessory pathway ablation for AVRT).
• Pharmacological: beta blocker (metoprolol 25 to 100 mg twice daily), diltiazem 120 to 360 mg or verapamil 120 to 480 mg daily; flecainide 50 to 150 mg twice daily or propafenone in structurally normal hearts (with AV nodal blocker); sotalol.
• "Pill in the pocket" approach for infrequent episodes (diltiazem 120 mg with metoprolol 50 mg, or flecainide 200 to 300 mg) in selected patients.
• Avoid triggers (caffeine, alcohol, stimulants), teach vagal manoeuvres.
• Restrictions on driving and hazardous occupations depend on frequency and haemodynamic effect.