SVT with Pre-Existing Left Bundle Branch Block

Report:

Supraventricular tachycardia 200/min[! XE "Supraventricular tachycardia:LBBB:QRS0.16\"" !]

Northwest axis +230o

Small voltage in frontal leads

Left bundle branch block

Comment:

This is a difficult trace to diagnose with certainty: the main feature favouring supraventricular origin is the morphology typical of LBBB (sharp downstroke, delayed ascent) in V1. The bizarre axis would normally favour VT. The negative concordant precordial pattern used to favour VT once (not much any more7) and still conveys a bizarre impression. The QRS complex duration, less than 0.16” in LBBB morphology, also favours SVT.

The decisive test is to look at the QRS morphology in sinus rhythm, 45 minutes later: there is a very similar LBBB there (Fig 7a). The interesting thing is the axis, approximately +250o, again almost the same as during the tachycardia. This is an extreme axis, either right or left; its only proven association is with congestive cardiomyopathy, usually of the idiopathic variety8. This patient had a left ventricular ejection fraction of 25% with normal coronary angiogram. Maybe she had “cardiac asthma”.

The patient was readmitted with PAT with block (Fig 7b) and its unblocked variant (Fig 7c). This may be a hazard when a digoxin-toxic patient is electrically cardioverted into irreversible VF for what’s mistakenly believed to be a VT.