PassFinals
Cardiovascular

Supraventricular Tachycardia (SVT)

A re-entry circuit running through the atrioventricular (AV) node, either within it or looping via an accessory pathway, fires at a fixed rate and gives abrupt regular narrow-complex tachycardia.

In a nutshell

SVT here means a regular narrow-complex tachycardia, usually atrioventricular (AV) nodal re-entrant tachycardia (AVNRT) or orthodromic AV re-entrant tachycardia (AVRT). If unstable, give a synchronised shock at 70 to 120 J; if stable, modified Valsalva then adenosine 6 mg, 12 mg and 18 mg by rapid IV bolus.

Classic presentation

Abrupt-onset regular palpitations in a young adult, with a regular narrow-complex tachycardia and no visible P waves, and no life-threatening features.

Key points

  • Supraventricular describes a location, not a mechanism. Atrial flutter with 2:1 block and focal atrial tachycardia sit in the same algorithm box and behave differently.
  • Narrow means a QRS complex under 0.12 seconds. Anything broader goes down the broad-complex route and is treated as ventricular tachycardia until proved otherwise.
  • A slow adenosine push wastes the dose: its half-life is seconds, so the drug is gone before it reaches the AV node.
  • Adenosine is licensed for paroxysmal SVT including Wolff-Parkinson-White syndrome, so a delta wave alone does not bar it when the current rhythm is regular and narrow.
  • Adenosine can itself provoke atrial fibrillation. In a patient with a forward-conducting accessory pathway that is dangerous, which is why a defibrillator stays at the bedside.
  • Asthma contraindicates adenosine, and a beta blocker is no escape because acute asthma contraindicates IV esmolol too. Verapamil is the drug to reach for.
  • Vagal manoeuvres alone terminate 19 to 54% of episodes when done properly; the modified Valsalva raised conversion from 17% to 43% in the trial that introduced it.
  • The diagnostic value survives failure: a rhythm that slows then returns is atrial, one that stops is AV-node-dependent, and no change at all should make you check delivery.

First-line investigation

A 12-lead ECG during the tachycardia with a continuous rhythm strip running through treatment, because termination and transient AV block mean different things.

Management

Pulse, stability and the unstable branch

  • No pulse is cardiac arrest: start cardiopulmonary resuscitation (CPR) and follow the advanced life support (ALS) algorithm. With a pulse, use ABCDE (airway, breathing, circulation, disability, exposure), monitor, and record a 12-lead ECG.1,4
  • Five life-threatening features: shock, syncope with severe or ongoing hypotension, myocardial ischaemia, severe heart failure with pulmonary oedema, and immediately after return of spontaneous circulation (ROSC).1
  • Any one means synchronised cardioversion now, not adenosine. For SVT or atrial flutter, start at 70 to 120 J, increase stepwise, up to three attempts, with sedation or anaesthesia if conscious.4,1
  • Three failed shocks and still unstable: amiodarone 300 mg IV over 10 to 20 minutes, or procainamide 10 to 15 mg/kg (maximum 1 g) over 20 minutes, then shock again.4,1

Stable regular narrow-complex tachycardia

  • Modified Valsalva: semi-recumbent, strain hard enough to move the plunger of a 10 mL syringe, then flat with both legs passively raised, then sit back up.2,7,1
  • Carotid sinus massage is the alternative: one side only, up to 5 seconds, never after transient ischaemic attack or stroke or with a carotid bruit.2
  • If they fail, adenosine 6 mg rapid IV bolus, then 12 mg, then 18 mg. Stop escalating if high-grade AV block appears. Move on if it has not gone within 1 to 2 minutes.1,2,3
  • Give it through a large-bore cannula in a proximal vein such as the antecubital fossa, over 2 seconds, into the most proximal port, then flush immediately with 0.9% sodium chloride.5,3,2
  • Record a continuous ECG throughout, keep a defibrillator to hand, and warn the patient about flushing, breathlessness, chest tightness and a sense of dread lasting under a minute.3,5
  • Contraindicated in asthma or chronic obstructive lung disease with bronchospasm, long QT syndrome, severe hypotension, decompensated heart failure, sick sinus syndrome, and second- or third-degree AV block without a pacemaker.5,3
  • Dipyridamole multiplies its effect about fourfold and has caused asystole; theophylline blocks it. After a heart transplant the BNF sequence is 3 mg, then 6 mg, then 12 mg.5,3

If the rhythm persists or the diagnosis changes

  • Transient AV block without termination usually means atrial flutter or an atrial tachycardia. Treat the rhythm you have revealed; do not keep escalating adenosine.2,3
  • Next is IV verapamil 5 to 10 mg over 2 minutes, a further 5 mg after 5 to 10 minutes if needed, or an IV beta blocker. Then synchronised cardioversion.1,8
  • Never both: simultaneous IV beta blockade is a contraindication to IV verapamil and can cause profound AV block, bradycardia or asystole. Esmolol is barred for 48 hours after verapamil.8,9
  • Before either drug, exclude a pre-excited tachycardia, hypotension, impaired left ventricular function and conduction disease. If instability appears at any point, cardiovert.8,2,1

Pregnancy

  • No separate pregnancy pathway. Vagal manoeuvres first. The product information advises adenosine only when benefit outweighs risk; avoid verapamil in the first trimester unless essential. Involve obstetrics early.3,5,8,1

Disposition, driving and definitive treatment

  • Observe on monitoring long enough to catch early recurrence and drug effects, repeat the 12-lead ECG in sinus rhythm, and save the strips from before, during and after.2,6
  • If the rhythm was never captured, NICE matches the monitor to episode frequency: several a week, Holter up to 48 hours; every 1 to 2 weeks, external event recorder; rarer, implantable.6
  • After transient loss of consciousness (TLoC), the patient must not drive while waiting for specialist assessment.6
  • Driver and Vehicle Licensing Agency (DVLA) Group 1: at least 4 weeks off driving if the arrhythmia caused or could cause incapacity, and the DVLA must be told.10
  • Group 2: 3 months free of incapacitating arrhythmia and an ejection fraction of at least 40%. After ablation, Group 1 is 2 days and no notification.10
  • Refer recurrent or poorly tolerated episodes, syncope, diagnostic uncertainty or pre-excitation to cardiology, and discuss catheter ablation as definitive treatment for AVNRT or AVRT.2,6

Exam traps

  • A regular narrow tachycardia at about 150 per minute is often atrial flutter with 2:1 block. Adenosine will expose the flutter waves but will not stop it.
  • Three UK sources give three adenosine sequences: Resuscitation Council UK (RCUK) 6, 12, 18; the BNF 6, 12, 12; the product information 3, 6, 12. RCUK governs peri-arrest practice.
  • Never give an AV-nodal blocker in pre-excited atrial fibrillation, which is fast, irregular and broad. It pushes conduction down the accessory pathway and can cause ventricular fibrillation.
  • A normal post-conversion ECG does not exclude AVRT: a concealed accessory pathway conducts only backwards, so it never produces a delta wave.
  • Verapamil and an IV beta blocker are alternatives, not partners. Giving both, or esmolol within 48 hours of verapamil, can cause profound bradycardia or asystole.
  • Syncope alone is a worrying history, but the RCUK unstable criterion is narrower: syncope with severe or ongoing hypotension.
  • Sinus tachycardia is not on this pathway. Treat the pain, fever, bleeding or hypoxia driving it; do not normalise the rate with adenosine or a shock.

Illustrations

SVT on the ECGTwelve-lead ECG showing a regular narrow-complex tachycardia at around 180 per minute with no clearly visible P waves, a pattern compatible with AV nodal re-entrant tachycardia but not by itself proof of the exact circuit.Ewingdo, Wikimedia Commons · CC-BY-SA-4.0
Wolff-Parkinson-White pattern in sinus rhythmTwelve-lead ECG in sinus rhythm showing a short PR interval and delta wave from ventricular pre-excitation. A normal resting ECG does not exclude AVRT through a concealed accessory pathway.James Heilman, MD, Wikimedia Commons · CC-BY-SA-3.0

Key sources

  1. Resuscitation Council UK, Adult tachyarrhythmia algorithm V3 (updated March 2026) (RCUK Guidelines 2025, algorithm V3)Updated 1 Mar 2026
  2. 2019 ESC Guidelines for the management of patients with supraventricular tachycardia, European Heart Journal 2020;41(5):655 to 720 (doi:10.1093/eurheartj/ehz467)Published 31 Aug 2019
  3. Adenosine 3 mg/ml solution for injection, Summary of Product Characteristics (emc 11530)Updated 27 Nov 2025
  4. Resuscitation Council UK, Adult advanced life support Guidelines (Resuscitation Council UK Guidelines 2025)Published 27 Oct 2025
  5. BNF, Adenosine (BNF drug monograph)
  6. NICE, Transient loss of consciousness ('blackouts') in over 16s (CG109)Published 25 Aug 2010 | Updated 21 Nov 2023
  7. REVERT trial: postural modification to the standard Valsalva manoeuvre for emergency treatment of supraventricular tachycardias, Lancet 2015 (doi:10.1016/S0140-6736(15)61485-4)Published 24 Aug 2015
  8. Verapamil 2.5 mg/ml Solution for injection, Summary of Product Characteristics (emc 979)Updated 28 Oct 2025
  9. Esmolol hydrochloride 10 mg/ml solution for injection, Summary of Product Characteristics (emc 3057)Updated 5 Jan 2021
  10. DVLA, Assessing fitness to drive: cardiovascular disorders (DVLA Assessing fitness to drive)Updated 7 Nov 2025

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.