Circ ECG Challenge Response! Regarding the 74 year old woman with SSS and a PM:
Diagnosis: normal sinus rhythm, premature atrial complexes in a bigeminal pattern, A sensed V paced, pseudofusion
The rhythm is irregular, with longer and shorter intervals which are equivalent to each other. Therefore the rhythm is regularly irregular. The average rate is 72 bpm. Each of the QRS complexes is preceded by a pacing stimulus. Therefore this is ventricular pacing. Before each QRS complex there is a P wave (+, *) with a stable PR interval. The mode of the pacemaker is A sense V paced or P wave synchronous ventricular pacing. The irregularly results from every other QRS complex which is slightly early or premature (v). The early QRS complex is preceded by a P wave (*) that has a morphology that is different from the P wave (+) of the other QRS complex. Therefore every other QRS complex is a premature atrial complex. The P wave of the QRS complex that is not premature (+) is positive in leads I, II, aVF, and V4-V6; therefore this is a normal sinus rhythm with premature atrial complexes in a bigeminal pattern. The sinus QRS complex has a normal duration (0.10 sec) with a terminal S wave in leads I and V5-V6 (←) and an R’ in V1 (→), resembling a right bundle branch block. However, it is not wide enough to be considered a full right bundle and is an intraventricular conduction delay to the right ventricle (although sometimes referred to as an incomplete right bundle branch bloc). The axis is physiologic left between 0° and -30° (positive in leads I and II and negative in lead aVF). The QT/QTc intervals are normal (400/440 msec). As this is not the morphology seen with a right ventricular pacemaker (in which there would be a left bundle branch block morphology) or a biventricular pacemaker (in which there would be a QS morphology or deep Q wave in lead I), the QRS complex is not the result of the pacing stimulus and is therefore the baseline QRS complex. There is a pacemaker stimulus which therefore does not capture. Therefore, this is termed pseudofusion and occurs because the intrinsic PR interval is the same as the AV delay of the pacemaker. The premature atrial complex is also preceded by a pacemaker stimulus. The QRS complex is slightly wider (0.12 sec) and has less of a right bundle branch block morphology with less of a terminal S wave in leads I and V5-V6. It does not have a morphology of either a right or biventricular paced complex. Therefore, there is still pseudofusion present but because the premature atrial complex is associated with a slightly longer time of AV nodal conduction (due to decrimental conduction) more of ventricular activation results from the pacemaker stimulus.
