Atrial fibrillation with aberrant ventricular Conduction ( mistaken for premature ventricular complex)

Answer to ECG challenge 

Premature atiral impulses are normally conducted to the ventricles with norrow QRS complexes very similar to a normal sinus impulse . If the impulse is too premature , it may find one of the bundle branches still refractory from the previous impulse and will be conducted with a wide QRS complex . these premature atrial impulses that are followed by wide QRS complexes are aberrantly conducted impulses , which can be mistaken for premature ventricular complex . Aberrantly conducted atrial impulses usually right bundle branch block configuration with rSR pattern in V1 because the right bundle branch has a longer refractory period than the left bundle branch in most individual .  

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Consider the case illustrated in Figure below  (lead V1) with intermittent runs of what looks like ventricular tachycardia.Note that the basic rhythm is irregularly irregular indicating atrial fibrillation.  The wide QRS complexes are examples of tachycardia-dependent LBBB aberration, not runs of ventricular tachycardia.  Note the morphology of the wide beats.  Although there is no initial “thin” r-wave, the downstroke of the S wave is very rapid .

message 

In medicine there is not any clear black and white border due to that with respect of general rule that abberations are often RBBB like pattern , but ,in this case we have LBBB pattern .