Look at this ECG:
There are three main diagnostic possibilities:
- SVT with aberrant conduction due to bundle branch block
- SVT with aberrant conduction due to the Wolff-Parkinson-White syndrome
The most important distinction is whether the rhythm is ventricular (VT) or supraventricular (SVT with aberrancy), as this will significantly influence how you manage the patient. SVTs usually respond well to AV-nodal blocking drugs, whereas patients with VT may suffer precipitous haemodynamic deterioration if erroneously administered an AV-nodal blocking agent.
Unfortunately, the electrocardiographic differentiation of VT from SVT with aberrancy is not always possible.
There are several electrocardiographic features that increase the likelihood of VT:
- Absence of typical RBBB or LBBB morphology
- Extreme axis deviation (“northwest axis”)
- Very broad complexes (>160ms)
- AV dissociation (P and QRS complexes at different rates)
- Capture beats — occur when the sinoatrial node transiently ‘captures’ the ventricles, in the midst of AV dissociation, to produce a QRS complex of normal duration.
- Fusion beats — occur when a sinus and ventricular beat coincides to produce a hybrid complex.
- Positive or negative concordance throughout the chest leads, i.e. leads V1-6 show entirely positive (R) or entirely negative (QS) complexes, with no RS complexes seen.
- Brugada’s sign – The distance from the onset of the QRS complex to the nadir of the S-wave is > 100ms
- Josephson’s sign – Notching near the nadir of the S-wave
- RSR’ complexes with a taller left rabbit ear. This is the most specific finding in favour of VT. This is in contrast to RBBB, where the right rabbit ear is taller.
Examples of these ECG features are shown below:
The likelihood of VT is also increased with:
- Age > 35 (positive predictive value of 85%)
- Structural heart disease
- Ischaemic heart disease
- Previous MI
- Congestive heart failure
- Family history of sudden cardiac death (suggesting conditions such as HOCM, congenital long QT syndrome, Brugada syndrome or arrhythmogenic right ventricular dysplasia that are associated with episodes of VT)
The likelihood of SVT with aberrancy is increased if:
- Previous ECGs show a bundle branch block pattern with identical morphology to the broad complex tachycardia.
- Previous ECGs show evidence of WPW (short PR < 120ms, broad QRS, delta wave).
- The patient has a history of paroxysmal tachycardias that have been successfully terminated with adenosine or vagal manoeuvres.
There is no way to be 100% sure that the rhythm is SVT with aberrancy.
If in doubt, treat as VT!