Question: This tracing shows (choose one from the list below)
a) atrial bigeminy with aberrant conduction
b) ventricular bigeminy
|Answer: a) atrial bigeminy with aberrant conduction.|
Discussion: Every other QRS is wide, different looking from the sinus beats and, at a quick glance, the tracing seems to show ventricular bigeminy. However, careful observation reveals a prematurely occuring P wave (↓) in front of these wide QRSs and this is an examle of atrial bigeminy with aberrant conduction. Why does aberrant conduction ever occur? It is simply because the two bundle branches (BBs) have different length of refractory periods. If an impulse occurs when one BB is still refractory while the other has recovered from the refractory period, the impulse will conduct thru the recovered BB, bypasssing the refractory BB: aberrant conduction results. Of course, if an impulse occurs at a time when both BBs have recovered from the refractory period, it will conduct normally, while if both BBs are refractory, it won't conduct at all (non-conducted PAC). This was well diagrammed and explained in the Medscape ECG of the week posted on 3/7/2012.