Fast heart rhythms (SVT, VT and others)
Unstable means synchronised shock. Stable means read the ECG: narrow or broad, regular or irregular. Adenosine for regular narrow SVT, amiodarone for VT, and treat sinus tachycardia's cause, not the rate.
A 28-year-old woman has had a "racing heart" for an hour. She is anxious but talking. Pulse 186, regular, BP 112/74, no chest pain. The ECG shows a regular narrow complex tachycardia with no visible P waves.
What do you try first, and how do you give adenosine?
- 12-lead ECG and monitorRecord it before and during any treatment.
- Adverse features?Shock, fainting, ischaemic chest pain, heart failure.
- Unstable: synchronised DC shockUp to 3 attempts, with sedation, by a senior. Then amiodarone.
- Stable: read the ECGNarrow or broad (QRS 120 ms or more)? Regular or irregular?
- Regular narrow: vagal manoeuvre, then adenosineModified Valsalva first.
- Regular broad: treat as VTAmiodarone. Senior help.
- ElectrolytesPotassium, magnesium, calcium. Correct them.
- Sinus tachycardia?Treat the cause (pain, fever, sepsis, bleeding, PE), not the rate.
- Shock (systolic below 90, cold, confused)
- Collapse
- Chest pain or ischaemic ECG
- Heart failure
- Regular broad-complex tachycardia
- Very fast irregular broad complexes
- VT mistaken for SVT
- Pre-excited AF (WPW)
- Torsades de pointes
- Sinus tachycardia from sepsis, bleeding or PE treated as an arrhythmia
| Synchronised shock | Broad complex or AF 120–150 J biphasic, regular narrow or flutter 70–120 J, up to 3 attempts |
| Adenosine | 6 mg rapid IV push with flush, then 12 mg, then 18 mg |
| Stable VT | Amiodarone 300 mg IV over 10–60 minutes, then 900 mg over 24 hours |
| Torsades | Magnesium sulfate 2 g IV over 10 minutes |
| Potassium | Keep 4–5 mmol/L (mEq/L) |
Check every dose against a current drug reference and your hospital protocol.
On this page
First question: is the patient unstable?
- Synchronised DC cardioversion, up to 3 shocks, with sedation
- Then amiodarone 300 mg IV over 10–20 minutes, and repeat shock
- Read the ECG: narrow or broad, regular or irregular
- Treat accordingly
Second question: what is the rhythm?
- Usually SVT (AVNRT, AVRT) or atrial flutter
- Vagal manoeuvre, then adenosine
- Usually AF
- Rate control. See Atrial fibrillation
- Assume VT, especially with heart disease
- Amiodarone
- Senior help
- AF with bundle branch block, pre-excited AF, or polymorphic VT (torsades)
- Senior help
Regular narrow complex (SVT)
- Modified ValsalvaPatient semi-sitting, blows into a 10 mL syringe hard enough to move the plunger for 15 seconds, then lie flat immediately and lift the legs to 45 degrees for 15 seconds.
- Repeat onceIf the rhythm has not changed.
- Adenosine 6 mgRapid IV push into a large vein (antecubital), followed immediately by a 20 mL saline flush. Record the ECG during it.
- Then 12 mg, then 18 mgIf no effect after each dose.
- Still not convertedVerapamil (if no heart failure, not on a beta-blocker, certain it is SVT) or senior advice.
- Warn the patient before adenosine: a few seconds of chest tightness, flushing and a feeling of doom. It passes quickly.
- Adenosine can reveal atrial flutter (saw-tooth waves) without converting it. That is still useful.
- Avoid adenosine in severe asthma. Reduce the first dose to 3 mg if on dipyridamole, after heart transplant, or given through a central line.
Regular broad complex (VT until proven otherwise)
A regular broad-complex tachycardia is VT until proven otherwise, especially in someone with heart disease or a previous heart attack. Treating VT as SVT with verapamil can cause cardiac arrest.
A polymorphic VT where the QRS "twists" around the baseline, usually with a long QT. Causes: drugs (some antibiotics such as clarithromycin and fluoroquinolones, antipsychotics, ondansetron, methadone), low potassium or magnesium, congenital long QT.
- Magnesium sulfate 2 g IV over 10 minutes.
- Correct potassium.
- Stop QT-prolonging drugs.
- Pulseless: defibrillate (not synchronised).
Sinus tachycardia
A fast regular rhythm with normal P waves before every QRS, usually below 150 in adults, that speeds up and slows down gradually. It is a response, not the problem. Look for: pain, fever, sepsis, dehydration, bleeding, PE, anaemia, thyrotoxicosis, anxiety, alcohol withdrawal, salbutamol. Treat the cause. Do not give rate-slowing drugs.
Synchronised cardioversion
- Call for helpSenior and anaesthetist for sedation or anaesthesia.
- Pads onFront and side (apex), or front and back.
- Select SYNC modeCheck the markers appear on each QRS.
- EnergyBroad complex or AF 120–150 J biphasic. Regular narrow or atrial flutter 70–120 J.
- Shock up to 3 timesIncreasing energy if needed.
- Still unstableAmiodarone 300 mg IV over 10–20 minutes, then shock again.
Common mistakes
- Calling a regular broad-complex tachycardia "SVT with aberrancy" and giving verapamil.
- Giving adenosine slowly through a small hand vein without a flush.
- Treating sinus tachycardia from sepsis or bleeding with a beta-blocker.
- Forgetting to record the ECG during adenosine.
- Forgetting potassium and magnesium.
- Giving AV-node blockers for irregular, very fast broad complexes (pre-excited AF).
Frequently asked questions
What is the dose of adenosine for SVT?
6 mg by rapid IV push with a saline flush, then 12 mg, then 18 mg if needed, with ECG recording.
What is the modified Valsalva manoeuvre?
The patient blows into a 10 mL syringe hard enough to move the plunger for 15 seconds while semi-sitting, then lies flat immediately with the legs raised to 45 degrees for 15 seconds.
How is stable VT treated?
Amiodarone 300 mg IV over 10–60 minutes, then 900 mg over 24 hours, with correction of potassium and magnesium and senior help. Unstable VT needs synchronised cardioversion.
What energy is used for synchronised cardioversion?
120–150 J biphasic for broad-complex tachycardia and AF, and 70–120 J for regular narrow-complex tachycardia and atrial flutter, up to 3 attempts.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A patient has a regular narrow-complex tachycardia at 186/min with normal BP. What is the first treatment?
Answer: Modified Valsalva. SVT in a stable patient: vagal manoeuvres first, then adenosine 6 mg, 12 mg, 18 mg rapid IV.
- DC cardioversion: Stable patient.
- Digoxin: Not first.
- Amiodarone: Not first.
- Bisoprolol: Not first.
2. A patient with a previous MI has a regular broad-complex tachycardia at 170/min and BP 76/40. What is the next step?
Answer: Synchronised DC cardioversion. Unstable VT needs electrical cardioversion, then amiodarone.
- IV verapamil 5 mg: Dangerous in VT.
- Observe on a monitor: No.
- Oral amiodarone: Too slow.
- Adenosine: Not for unstable VT.
3. A patient has polymorphic VT twisting around the baseline. The previous ECG showed a long QT. Which drug is first?
Answer: IV magnesium. Torsades de pointes: magnesium, correct potassium, stop QT-prolonging drugs.
- Digoxin: No.
- Sotalol: Prolongs QT.
- Amiodarone: Prolongs QT.
- Adenosine: No.
4. A 60-year-old with previous MI has a regular broad-complex tachycardia at 160/min. He is alert with BP 125/80 and no chest pain or breathlessness. What is the treatment?
Answer: IV amiodarone. Broad-complex regular tachycardia without adverse features is treated as VT (especially with ischaemic heart disease): IV amiodarone 300 mg over 10–60 min, then infusion; correct K⁺ and Mg²⁺. Synchronised cardioversion if adverse features develop. Never give verapamil for a broad-complex tachycardia — it can cause collapse if VT. Torsades: IV magnesium. Long-term: ICD for secondary prevention.
- Synchronised DC shock immediately: DC shock is for adverse features.
- IV verapamil: Verapamil is dangerous in VT.
- Carotid sinus massage only: Carotid massage is ineffective in VT.
5. A woman taking clarithromycin and haloperidol collapses. ECG shows polymorphic VT with twisting QRS axis; between episodes QTc is 560 ms. After defibrillation, what is given?
Answer: IV magnesium sulfate. Torsades de pointes is polymorphic VT with long QT. Causes: drugs (macrolides, quinolones, antipsychotics, methadone, ondansetron, antiarrhythmics class Ia/III, antifungals), low K/Mg/Ca, congenital long QT, bradycardia. Treat: defibrillate if pulseless, IV magnesium sulfate 2 g, correct K⁺ (> 4.5) and Mg, stop QT drugs; overdrive pacing or isoprenaline for bradycardia-dependent torsades.
- IV amiodarone: Amiodarone prolongs the QT.
- IV sotalol: Sotalol prolongs the QT.
- IV procainamide: Procainamide prolongs the QT.
6. Class III antiarrhythmic drugs such as amiodarone and sotalol mainly work by:
Answer: Blocking potassium channels. Class III drugs block potassium channels, prolonging the action potential and the QT interval. Amiodarone also has class I, II and IV actions. Its side effects include thyroid dysfunction, lung fibrosis, liver injury, corneal deposits, photosensitivity and blue-grey skin.
- Blocking fast sodium channels: Class I.
- Blocking β-adrenergic receptors only: Class II.
- Blocking L-type calcium channels: Class IV.
- Opening potassium channels: Not their mechanism.
7. Which is a recognised adverse effect of long-term amiodarone?
Answer: Pulmonary fibrosis. Amiodarone (class III, but with actions of all classes) contains iodine and has a very long half-life. Adverse effects: pulmonary fibrosis, hypo- or hyperthyroidism, hepatotoxicity, corneal deposits, photosensitivity and blue-grey skin, peripheral neuropathy and QT prolongation. Monitor TFT, LFT and chest X-ray.
- Gingival hyperplasia: Phenytoin, ciclosporin, nifedipine.
- Red man syndrome: Vancomycin.
- Ototoxicity: Aminoglycosides, loop diuretics.
- Acute gout: Thiazides.
References and dose sources
- Resuscitation Council UK: adult tachycardia algorithm (2021)
- REVERT trial: modified Valsalva manoeuvre for SVT (Lancet 2015)
- Resus Council UK — ALS (tachycardia algorithm)
- NICE NG196 — atrial fibrillation
- Adenosine dosing: Drugs.com: Adenosine dosage (see also our adenosine dose page)
- Verapamil dosing: Drugs.com: Verapamil dosage (see also our verapamil dose page)
- Bisoprolol dosing: Drugs.com: Bisoprolol dosage (see also our bisoprolol dose page)
- Amiodarone dosing: Drugs.com: Amiodarone dosage (see also our amiodarone dose page)
- Magnesium sulfate dosing: Drugs.com: Magnesium sulfate dosage (see also our magnesium sulfate dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.