Vignette: This 75 yo patient was admitted for dizziness and abdominal pain. How will you interpret this ECG strip?
a. Junctional rhythm with premature atrial complexes and premature ventricular complexes
b. Junctional rhythm with conducted PAC’s
c. Sinus bradycardia conducted normally and with aberrancy, junctional escape beats or escape capture bigeminy
d. Junctional rhythm with echo beats
Image 1 - 2 Leads
You must have thought that this is junctional rhythm with premature atrial complexes (PAC) and premature ventricular complexes (PVC). Actually that is a partly correct and partly wrong interpretation. If you look at junctional rhythms, look for sinus P waves. In strips with 2 leads, it is difficult to identify if the P waves are indeed sinus in origin because we need aVR to support our claims. If it is a sinus P wave, P wave will be upright (positive) in lead II and inverted (negative) in aVR. If junctional rhythms do not have P waves anywhere then there is sinus arrest or pause but if there is (as in this case) then we measure the P to P or atrial rate.
The P waves were marked with asterisk (Image 2). The red asterisk marked the obvious P waves and the blue asterisk marked the not so obvious P waves.
Image 3 – Full disclosure
We can say that the P waves are sinus in origin (Image 3) because it upright in leads II (blue arrows), III and aVF and inverted in aVR (red arrows). Thus, this is sinus bradycardia (extreme) with a rate of about 33 bpm. Those are sinus beats conducted normally (narrow QRS) and with aberrancy or right bundle branch (RBBB) morphology. Those beats are NOT PAC's and PVC’s. You were deceived by the QRS (junctional beats) before it. Those beats are not premature because it was the primary intrinsic sinus rhythm.
Why was it conducted with a narrow and wide QRS?
If you measure the RP interval or the R wave of the junctional beat and beginning of the P wave, you will notice that the RP interval is longer for sinus complexes with narrow QRS and the RP interval is slightly shorter for sinus complexes with wide complexes (RBBB). This is because the right bundle branch has not recovered yet (refractory period) when the sinus impulse arrived early (short RP interval). Thus, the impulse will be blocked in the right bundle creating a right bundle branch block pattern. If the sinus impulse arrived later (longer RP interval), then the right bundle branch has recovered which will allow normal conduction of both bundle branches creating a narrow QRS.
The Junctional Escape
The AV junction also had a pacemaker function. So, it will wait for a supraventricular impulse but if it did not detect a supraventricular impulse at its set time, it will escape and fire. In this case, it fired at a rate of about 40 bpm. What is seen on the surface ECG is that after the sinus beat a junctional beat can be seen after about 36 small squares or 42 bpm.
C. Sinus bradycardia conducted normally and with aberrancy with junctional escape beats. The other name for this is escape-capture bigeminy. This is because of the junctional escape and sinus capture of the ventricles in a bigeminal pattern.
What happened to the patient?
This patient was symptomatic (with dizziness) and a pacemaker was implanted.
Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura Publishing Co.