DIAGNOSIS: Paroxysmal Supraventricular Tachycardia [PSVT] with aberrancy.

EXPLANATION: There is a regular tachycardia present, rate 227, with a QRS duration of 120 msec.

How can one differentiate those tachycardias which are vertricular in origin from those which originate above the ventricle and are conducted with BBB, i.e. aberrancy?

It is generally useless to look for P waves in the midst of all this QRS-T activity. The P waves are simply to difficult to see. The exception would be if A-V dissociation is present. Then P waves may occasionally be seen on the EKG out of rhythm with the QRS complexes; fusion or capture beats may be seen as other evidence of dissociation. However, P waves are much more likely to be recognized clinically by observation of the jugular venous pulsations known as canon A waves. If A-V dissociation is present, then the tachycardia is ventricular, but only 50% of ventricular tachycardias have A-V dissociation; the other 50% have retrograde conduction to the atria and thus one to one conduction. Thus the absence of AV dissociation does not prove the supraventricular origin of the tachycardia.

What about axis?

If the axis is negative in leads I and aVF, i.e. no man's land, this is a point in favor of V-tach. However, a normal axis does not ""rule out"; ventricular origin. Likewise, concordance in the precordial leads [all complexes either positive or negative] makes V-tach likely, but its absence does not help to differentiate.

What about the clinical setting?

If the age is over 50 years or there is a history of coronary disease and the patient has never had a tachyarrythmia, there is a 90% likelihood that the rhythm is ventricular. The converse does not prove supraventricular origin. Likewise the stability of the vital signs is useless. Patients with ventricular tachycardia may have normal BP whereas those with PSVT may be extremely hypotensive and unstable.

There is one useful observation; the morphology of the QRS in V1. When the QRS is predominantly negative in V1, a slick downstroke to an early intrisicoid deflection (the major change in polarity of the QRS having a duration of 60 msec or less), either with an rS or just a Q wave, this is 90% specific for LBBB and aberrancy. Likewise, when the QRS is predominantly positive in V1, an rsR' is 90% specific for RBBB.

Generally, unless there is clear evidence for PSVT, one should assume that ventricular tachycardia is present and treat the patient accordingly.

In this case, that of a 29 year old clinically stable female with pal