Editors

PekkaRaatikainen

Differential Diagnosis of Broad Complex Tachycardia

Broad complex tachycardia must always be presumed to originate from the ventricles unless reliably shown otherwise.
Ventricular tachycardia (VT) must not be misdiagnosed as supraventricular tachycardia (SVT) - a misdiagnosis the other way round is not as dangerous Supraventricular Tachycardia (SVT).
After emergency treatment, a patient with broad complex tachycardia must be referred for a specialist assessment. The referral letter should include a detailed medical history and a copy of an ECG recording taken both during the tachycardia and during normal rhythm.

In practice, ventricular tachycardia always has wide QRS complexes, whereas the QRS complexes of an atrial arrhythmia are usually narrow.
- Over 80% of all broad complex tachycardias originate from the ventricles.
- A ventricular rhythm with a rate of less than 100/min during the acute phase of an infarction is known as an “idioventricular rhythm”. It is usually benign and requires no treatment.
Wide QRS complexes in supraventricular tachycardias (SVT, atrial flutter, atrial fibrillation) are caused by permanent bundle branch block (LBBB, RBBB), aberrant conduction or, more rarely, WPW syndrome (delta wave).
Aberrant conduction refers to functional bundle branch block (usually RBBB) caused by the fast heart rate. As the rate slows down, the QRS complexes resume their normal morphology.
It is important to realise that it is possible for VT to last for several hours and not cause haemodynamic instability.

Broad complex tachycardia in an elderly patient with a diseased heart is almost invariably of ventricular origin (almost 100% probability), whereas tachycardia having the shape of a bundle branch block in a young, otherwise healthy patient is usually SVT with aberrant conduction.
Vagal stimulation (carotid massage or Valsalva manoeuvre) may slow the heart rate or stop an episode of SVT, but it will not affect VT except in a few isolated cases Supraventricular Tachycardia (SVT).
Adenosine may be used in specialist health care for the diagnosis and treatment of broad complex tachycardia in haemodynamically stable patients.

The diagnosis of broad complex tachycardia can in most cases be confirmed by a systematic analysis of a 12-lead ECG.
- When deciding on the diagnosis, the ECG findings must always be scrutinised against the patient's history and clinical signs.
- An ECG recorded during the arrhythmia should always be compared with an ECG taken during the patient's normal rhythm, which may reveal matters significant for diagnosis (e.g. bundle branch block, old infarction, delta wave).
Monomorphic broad complex tachycardia
- The most common cause is a survived myocardial infarction or some other severe heart disease.
- Broad complex tachycardia of atrial origin always has the morphology of a bundle branch block.
- Many algorithms have also been developed for the differential diagnosis of monomorphic broad complex tachycardia but their use is in practice limited to specialist care.
Polymorphic broad complex tachycardia
- In a patient with a healthy heart, very fast (> 200/min) broad complex tachycardia with variation in the RR interval is suggestive of WPW syndrome and atrial fibrillation, but the patient may also have an underlying inherited ion channel disorder.
- In patients with a heart disease, polymorphic tachycardia is in most cases caused by ischaemia or deterioration of heart failure.
Table T1 presents a summary of ECG changes important for the differential diagnosis of broad complex tachycardia. See also picture 1.

	Ventricular tachycardia	Broad complex SVT
History and clinical findings	Elderly patient Heart disease (myocardial infarction, cardiac insufficiency)	Young patient Structurally a healthy heart
QRS duration and axis	Usually > 160 ms Abnormal frontal plane axis (over -45°) The QRS “direction” in all the precordial leads is consistent (concordance).	Usually 120-140 ms Normal frontal plane axis or slight left axis deviation No QRS concordance
QRS shape	Changes according to the arrhythmia mechanism (differs from typical bundle branch block*) Fusion beats (intermediate between a normal beat and a VT beat). Capture beats, resembling normal QRS complexes, among the VT	Typical RBBB or LBBB
Other ECG features	VA dissociation is diagnostic for VT, but is missing in about half the cases.

^* In some ventricular tachycardias that originate from the conduction pathways the morphology of the QRS complexes may resemble that of typical bundle branch block
References
Alzand BS, Crijns HJ. Diagnostic criteria of broad QRS complex tachycardia: decades of evolution. Europace 2011;13(4):465-72. [PubMed]