[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical skills](https://durgical.com/en/clinical-skills/) › [The ECG](https://durgical.com/en/clinical-skills/areas/ecg/) › Broad-complex tachycardia, VF and the arrest rhythms

# Broad-complex tachycardia, VF and the arrest rhythms

- ECG tracing

Dangerous fast rhythms that start in the ventricles, and the four rhythms of cardiac arrest; recognising them fast decides whether to shock.

Updated Nov 13, 2026

## Key points

- A broad-complex tachycardia (QRS 120 ms or more, rate over 100/min) is VT until proven otherwise, especially after infarction or with heart failure.
- Findings that prove VT: AV dissociation, capture beats and fusion beats; strongly suggestive: very broad QRS, extreme axis and precordial concordance.
- VF: chaotic, irregular deflections with no identifiable QRS, P or T waves; fine VF can look like asystole.
- Shockable arrest rhythms: VF and pulseless VT. Non-shockable: PEA and asystole, where CPR, adrenaline and the reversible causes (4 H and 4 T) matter most.
- Torsades de pointes: polymorphic VT twisting around the baseline with a long QT; treat with magnesium and correct potassium.

On this page

- The basics Key points
- What it is
- How to recognise it
- Why it happens
- Step by step
- The four cardiac arrest rhythms.
- Next step
- Memory aid
- Elsewhere in DurgiPedia

- Clinical Differential and pitfalls

- Study Test yourself

- Connections Connections

## What it is

A broad-complex tachycardia has a rate over 100/min and a QRS of 120 ms or more. Most are ventricular tachycardia (VT), above all in a patient with previous infarction or a cardiomyopathy; the rest are supraventricular rhythms conducted with a bundle branch block, aberrancy or an accessory pathway. VT can be monomorphic (every complex the same, usually from reentry around a scar) or polymorphic; torsades de pointes is the polymorphic VT of a long QT, whose complexes twist around the baseline. VT lasting 30 seconds or more, or causing collapse, is sustained; three or more beats lasting less is non-sustained. Ventricular fibrillation (VF) is chaotic electrical activity with no recognisable QRS and no output. Cardiac arrest has four rhythms: two shockable (VF and pulseless VT), and two non-shockable, pulseless electrical activity (PEA: organised complexes without a pulse) and asystole (no electrical activity). The rhythm check in an arrest takes seconds and decides the branch of the algorithm, so every clinician should be able to name these strips on sight and check the patient and the leads before believing them.

## How to recognise it

- Monomorphic VT: regular, rate usually 120–250/min, broad identical QRS complexes, often with no visible P waves.
- AV dissociation: P waves at their own slower rate, marching through the QRS complexes without a fixed relation; it proves a ventricular origin.
- Capture beat: a normal narrow QRS in the middle of the tachycardia when a sinus beat gets through; fusion beat: a hybrid between a narrow and a broad complex.
- Other pointers to VT: QRS over 160 ms, extreme axis (between −90° and ±180°, «northwest»), and all precordial QRS complexes pointing the same way (positive or negative concordance).
- Polymorphic VT: QRS shape and axis change from beat to beat; torsades de pointes adds a long QT on the beats before it.
- VF: irregular, chaotic waves of varying amplitude and shape with no QRS complexes; coarse VF has large waves, fine VF small ones.
- Asystole: an almost flat line, sometimes with P waves alone (ventricular standstill); confirm it in two leads with the gain turned up.
- PEA: any organised electrical rhythm (narrow or broad complexes) in a patient with no palpable central pulse.

## Why it happens

- Monomorphic VT is usually reentry around scar from an old infarction, or in a dilated, hypertrophic or Chagas cardiomyopathy.
- Activation spreads muscle to muscle instead of through the fast His–Purkinje system, which is why the QRS is broad and bizarre.
- VT from the ventricular outflow tracts can occur in structurally normal hearts and has a left bundle branch block shape with an inferior axis.
- Torsades follows early afterdepolarisations in a long QT: drugs that prolong QT, low potassium or magnesium, bradycardia or inherited channel disease.
- VF is the breakdown of organised activation into many chaotic wavelets; the ventricles quiver without pumping, and after some minutes it decays to asystole.
- PEA is electrical activity without enough mechanical output: hypoxia, hypovolaemia, hypo- or hyperkalaemia and other metabolic causes, hypothermia, tension pneumothorax, tamponade, toxins and thrombosis (coronary or pulmonary).

## Step by step

- Look at the patient and feel for a central pulse first — the same strip means cardioversion with a pulse and defibrillation without one.
- Check leads, gain and a second lead — a disconnected electrode, low gain or movement artefact can mimic asystole, VF or VT.
- Measure the QRS width — 120 ms or more with a rate over 100/min is a broad-complex tachycardia.
- Decide regular or irregular — regular broad is usually monomorphic VT; irregular broad suggests polymorphic VT or pre-excited AF.
- Look for P waves marching independently, capture and fusion beats — any of them confirms VT.
- Check the axis and the precordial leads — extreme axis or concordance favours VT.
- In an arrest, name the rhythm in a short pause — shockable (VF, pulseless VT) or non-shockable (PEA, asystole), then restart compressions at once.
- After return of circulation, record a 12-lead ECG — look for infarction, long QT, Brugada pattern, hyperkalaemia or pre-excitation.

## The four cardiac arrest rhythms.

| Rhythm | What the strip shows | Shockable? | Key point |
| --- | --- | --- | --- |
| [Ventricular fibrillation](https://durgical.com/en/conditions/cardiac-arrest/) | Chaotic waves, no QRS, varying amplitude | Yes | Fine VF can look like asystole: check gain and a second lead |
| [Pulseless VT](https://durgical.com/en/conditions/ventricular-tachycardia/) | Regular broad complexes, no pulse | Yes | Same strip with a pulse needs synchronised cardioversion instead |
| Pulseless electrical activity | Organised complexes, no palpable pulse | No | Search for the 4 H and 4 T while CPR continues |
| Asystole | Flat line, sometimes P waves only | No | Confirm leads and gain; P waves alone may respond to pacing |

## Differential and pitfalls

- SVT with bundle branch block or aberrancy can look like VT; age over 35 or known heart disease make VT far more likely. If unsure, treat as VT.
- Pre-excited AF is broad and irregular with changing QRS shapes; avoid AV-node blockers and get expert help.
- Hyperkalaemia can widen the QRS into a sine wave; check potassium and treat it as part of the emergency.
- Sodium-channel blocker poisoning (for example tricyclic antidepressants) gives a broad QRS and tachycardia.
- Movement, tremor or patient handling of a cable creates artefact that mimics VT or VF; normal complexes «marching through» and a conscious patient reveal it.
- Never give verapamil to a broad-complex tachycardia of unknown origin: in VT it can cause collapse.

## Next step

- No pulse: call for help, start high-quality CPR and follow the advanced life support algorithm; defibrillate VF and pulseless VT without delay.
- Shockable rhythm: give adrenaline and amiodarone (or lidocaine) as the algorithm sets after the shocks; non-shockable: adrenaline early and search for the 4 H and 4 T.
- VT with a pulse and shock, syncope, chest pain or heart failure: synchronised cardioversion with sedation under senior care.
- Stable VT: 12-lead ECG, monitoring, defibrillator pads in place and senior or cardiology help for amiodarone or procainamide.
- Torsades: magnesium sulfate, correct potassium, stop every QT-prolonging drug; overdrive pacing if it keeps recurring.
- Afterwards: electrolytes, troponin, ischaemia work-up and echocardiogram; cardiology assesses for an implantable cardioverter-defibrillator.

## Memory aid

4 H and 4 T: hypoxia, hypovolaemia, hypo- or hyperkalaemia, hypothermia; tension pneumothorax, tamponade, toxins, thrombosis.

## Test yourself

Think of the answer, then tap the question to see it.

## A 68-year-old with an old infarction: regular broad QRS at 170/min, P waves at 80/min marching through, and one narrow complex in the strip. Rhythm?

Monomorphic ventricular tachycardia: AV dissociation and a capture beat prove it. Check the pulse and stability; synchronised cardioversion if unstable, senior help for drugs if stable.

## During CPR the monitor shows chaotic irregular waves of varying size with no QRS complexes. What rhythm is it and what comes next?

Ventricular fibrillation, a shockable rhythm: defibrillate without delay and resume compressions immediately, following the advanced life support algorithm.

## The monitor shows regular narrow complexes at 90/min but the patient has no central pulse. What is it and what do you look for?

Pulseless electrical activity, non-shockable: continue CPR, give adrenaline early and search for the reversible causes (hypoxia, hypovolaemia, potassium, hypothermia, tension pneumothorax, tamponade, toxins, thrombosis).

## Which features of a polymorphic broad tachycardia point to torsades de pointes, and what is the first treatment?

Complexes that twist around the baseline, changing amplitude, with a long QT on the preceding beats. Give magnesium sulfate, correct potassium and stop QT-prolonging drugs.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-skills/broad-complex-tachycardia-vf-and-the-arrest-rhythms/)

## Elsewhere in DurgiPedia

- [ECG basics: waves, intervals and leads](https://durgical.com/en/physiology/ecg-basics-waves-intervals-and-leads/)
- [The cardiac electrical axis](https://durgical.com/en/physiology/the-cardiac-electrical-axis/)
- [The ECG in ischemia and infarction](https://durgical.com/en/physiology/the-ecg-in-ischemia-and-infarction/)
- [The ECG in potassium and calcium disorders](https://durgical.com/en/physiology/the-ecg-in-potassium-and-calcium-disorders/)
- [How arrhythmias arise](https://durgical.com/en/physiology/how-arrhythmias-arise/)
- [Arrhythmias and sudden cardiac death](https://durgical.com/en/general-pathology/arrhythmias-and-sudden-cardiac-death/)
- [Basic life support from the nursing side](https://durgical.com/en/nursing/basic-life-support-from-the-nursing-side/)
- [Cardiac monitoring at the bedside: leads, alarms and rhythm changes to report](https://durgical.com/en/nursing/cardiac-monitoring-at-the-bedside-leads-alarms-and-rhythm-changes-to-report/)

## Connections

## Related 11

- [Reading an ECG step by step](https://durgical.com/en/clinical-skills/reading-an-ecg-step-by-step/)
- [Narrow-complex tachycardia (SVT)](https://durgical.com/en/clinical-skills/narrow-complex-tachycardia/)
- [Atrial fibrillation and atrial flutter](https://durgical.com/en/clinical-skills/atrial-fibrillation-and-atrial-flutter/)
- [Right and left bundle branch block and fascicular blocks](https://durgical.com/en/clinical-skills/right-and-left-bundle-branch-block-and-fascicular-blocks/)
- [The long QT interval](https://durgical.com/en/clinical-skills/the-long-qt-interval/)
- [Hyperkalaemia and other electrolyte changes](https://durgical.com/en/clinical-skills/hyperkalaemia-and-other-electrolyte-changes/)
- [Brugada pattern and other sudden-death ECGs](https://durgical.com/en/clinical-skills/brugada-pattern-and-other-sudden-death-ecgs/)
- [Defibrillation and synchronised cardioversion](https://durgical.com/en/clinical-skills/defibrillation-and-synchronised-cardioversion/)
- [The emergency (ABCDE) station](https://durgical.com/en/clinical-skills/the-emergency-station/)
- [Transcutaneous pacing](https://durgical.com/en/clinical-skills/transcutaneous-pacing/)
- [Pericardiocentesis](https://durgical.com/en/clinical-skills/pericardiocentesis/)

## Related conditions 13

- [Ventricular tachycardia](https://durgical.com/en/conditions/ventricular-tachycardia/)
- [Cardiac arrest (VF, pulseless VT, asystole, PEA)](https://durgical.com/en/conditions/cardiac-arrest/)
- [Myocardial Infarction (Heart Attack)](https://durgical.com/en/conditions/myocardial-infarction/)
- [Long QT syndrome and torsades de pointes](https://durgical.com/en/conditions/long-qt-syndrome-and-torsades-de-pointes/)
- [Brugada syndrome](https://durgical.com/en/conditions/brugada-syndrome/)
- [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/)
- [Hypomagnesaemia](https://durgical.com/en/conditions/hypomagnesaemia/)
- [Cardiac tamponade](https://durgical.com/en/conditions/cardiac-tamponade/)
- [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/)
- [Pneumothorax](https://durgical.com/en/conditions/pneumothorax/)
- [Accidental hypothermia](https://durgical.com/en/conditions/accidental-hypothermia/)
- [Tricyclic antidepressant overdose](https://durgical.com/en/conditions/tricyclic-antidepressant-overdose/)

## See all 13

- [Chagas cardiomyopathy](https://durgical.com/en/conditions/chagas-cardiomyopathy/)

## In Pharmacology 7

- [Epinephrine](https://durgical.com/en/pharmacology/epinephrine/)
- [Amiodarone](https://durgical.com/en/pharmacology/amiodarone/)
- [Lidocaine](https://durgical.com/en/pharmacology/lidocaine/)
- [Procainamide](https://durgical.com/en/pharmacology/procainamide/)
- [Magnesium sulfate](https://durgical.com/en/pharmacology/magnesium-sulfate/)
- [Calcium gluconate](https://durgical.com/en/pharmacology/calcium-gluconate/)
- [Beta blockers](https://durgical.com/en/pharmacology/classes/beta-blockers/)

[← All clinical skills](https://durgical.com/en/clinical-skills/)

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: [Ventricular Tachycardia](https://www.ncbi.nlm.nih.gov/books/NBK532954/) (StatPearls (NCBI Bookshelf), 2026); [Ventricular Fibrillation](https://www.ncbi.nlm.nih.gov/books/NBK537120/) (StatPearls (NCBI Bookshelf), 2025); [Pulseless Electrical Activity](https://www.ncbi.nlm.nih.gov/books/NBK513349/) (StatPearls (NCBI Bookshelf), 2026); [Asystole](https://www.ncbi.nlm.nih.gov/books/NBK430866/) (StatPearls (NCBI Bookshelf), 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/clinical-skills/broad-complex-tachycardia-vf-and-the-arrest-rhythms/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-skills/broad-complex-tachycardia-vf-and-the-arrest-rhythms/