SVT: Pre-existing RBBB/LAHB

Report:

Tachycardia 102/min ? origin

Onset of supraventricular tachycardia 190/min

Right bundle branch block

Left anterior hemiblock

Anterior infarction, probably old

Comment:

Frontal plane axis of –90o and the monophasic R wave in V1 bode ill for the correct diagnosis of SVT, but the propitious timing of the recording allowed, in the second set of leads, recognition of identical QRS complexes in the two tachycardias. Had the ECG been taken a few seconds later, VT could have won the day unless previous tracings were available.

The nature of the initial mild tachycardia 102/min is uncertain; it could be PAT with 2:1 block. Disappointingly, its rate did not exactly double, but only came pretty close. The autonomous PAT with block is known to vary its rate, degree of block and even the shape of its P waves. The latter are often seen only in V1 – unfortunately not seen here at slower ventricular rate. There are some shallow deflections in lead 2 that could “pass” for atrial waves of the PAT. Applying Occam’s razor, one should diagnose PAT throughout.

In anterior infarcts, the LAHB often assumes a symmetrical up-down morphology in Lead 1, like here, of modest amplitude.

Fig 187a shows the patient’s sinus rhythm. It certainly wasn’t there in the original trace (I initially thought it was – the SVT ?VT engrossed my attention).