A 2:1 is not Mobitz II

Vignette: A 70 yo is admitted due to dizziness. What is your ECG interpretation?

Figure 1 - ECG case

The rhythm is sinus with an atrial rate of about 65 bpm and the ventricular rate of about 34 bpm. The PRI is 0.28 s and the QRS is 0.12 sec. There are 2 P wave for every 1 QRS (Figure 2).

Figure 2 - P waves marked with red arrows
Teaching points

1. This is not sinus bradycardia. The driving impulse is the sinoatrial node (SAN) and the (atrial) rate is about 65 bpm. So, we label this sinus rhythm and not sinus bradycardia. Yes, it is bradycardic but it is because only 1 sinus beat is conducted for every 2 sinus beats. Do not be misled by this.

2. This is not second degree AV block type II or Mobitz II. This is second degree AV block 2:1. During sinus rhythm, 2 to 1 AV block may manifest as 2 P waves followed by 1 QRS. This is a subtype of second degree AV block. A 2:1 AV block could either be second degree AV block type I (Mobitz I or Wenckebach) or type II (Mobitz II) . A long strip is needed to capture the mechanism or true nature of a 2:1 AV block. In telemetry floors, this is easily done by going back to beginning of the drop. Reviewing the beginning of the cycle showed 2:1 and 3:2 AV block pattern (Figure 3) . The 3:2 pattern clearly showed prolongation of the PR interval (0.28 sec/0.36 sec) before the dropped P wave. This means the 2:1 AV block is due to second degree AV block type I (Mobitz I or Wenckenach)

Figure 3 - Long lead II strip prior to the 2:1 AV block episode revealing a Mobitz I or Wenckebach pattern.
3. V1 is in the wrong place. V1 can tell if wide QRS rhythm has a right bundle branch block (RBBB) morphology or left bundle branch block (LBBB) morphology. In this case, V1 cannot tell what type of bundle branch block it is (QRS 0.12 sec) because V1 is in the wrong place. It was placed in the abdomen. After correction (Figure 4), it had a right bundle branch block pattern (qR in V1).

Figure 4 - Full disclosure of the case

4. This is ventriculophasic sinus arrhythmia. The PP interval is not regular. The PP interval encompassing a QRS is shorter compared to the PP interval not encompassing an QRs (Figure 5). This phenomenon is called ventriculophasic sinus arrhythmia. There are 2 proposed mechanism to explain the effect of the intervening QRS complex on the PP interval. One, the mechanical pressure of the ejected blood on the sinus node accelerates the sinus node discharge. Another is that the prolongation of the PP interval may be due to the vagal effect initiated by the carotid sinus reflex in response to systolic ejection.

Figure 5 - The strip showing a PP interval encompassing a R wave is shorter compared to a PP interval not encompassing a R wave.

5. In full disclosure, we can see fascicular block. In the full disclosure strip (Figure 4), there is left superior axis. To know how to measure the axis, please follow this link - http://ecg.utah.edu/lesson/2-1. There is qR in aVL and rS in II, III and aVF. So, there is left anterior fascicular block.

6. It is difficult to determine trifascicular block on surface ECG. The ECG has a combination of first degree AV block, right bundle branch block and left anterior fascicular block. A His-Bundle recording is needed for a definitive diagnosis of trifascicular block. This is bifascicular block.

Figure 6 - Electrical conduction system of the heart highlighting the Bundle of His (3), right bundle branch (10), left bundle branch (4) and branches which are left anterior fascicle (6) and left posterior fascicle (5).

Interpretation: Sinus rhythm, first degree AV block, second degree AV block 2:1 (due to a Wenckebach), left anterior fascicular block, right bundle branch block, (bifascicular block), ventriculophasic sinus arrhythmia.

Das and Zipes. 2012. Electrocardiography of arrhythmias : a comprehensive review. Elsevier PA
Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura Publishing Co.
Goldberger A. 2013. Goldberger’s Clinical Electrocardiography : A Simplified Approach 8Ed. Ph Elsevier
Issa Z, Miller J and Zipes D. 2012. Clinical Arrhythmology and Electrophysiology: A Comprehensive Review - A Companion to Braunwald’s Heart Disease 2nd Ed. PA Saunders
Surawicz B and Knilans TK. 2008. Chou’s Electrocardiography in Clinical Practice. 6th ed. PA. Saunders-Elseiver