This part of Simplified in Practice explains the Mobitz type I second-degree AV block also called as Wenckebach phenomenon along with its clinical importance.

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The PR interval is gradually lengthened resulting in a non-conducted P wave

1. The PR interval is the longest just before the dropped beat.
2. Immediately after a dropped beat, the PR interval is the smallest.

Other features

1. The P-P interval remains relatively constant throughout the cycle.
2. The greatest increase in PR interval duration occurs typically between the first and second beats of the cycle.
3. The RR interval gradually shortens with each beat of the cycle.
4. The Wenckebach pattern keeps repeating in P: QRS groups with ratios of 3:2, 4:3, or 5:4.

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Mechanism

1. Mobitz I is often caused by a reversible conduction block at the AV node.
2. Malfunctioning AV nodal cells gradually exhaust until they are unable to conduct an impulse. The His-Purkinje system cells, on the other hand, have a tendency to fail quickly and unexpectedly (i.e. producing a Mobitz II block)

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The Wenckebach phenomenon causes

1. Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone
2. Increased vagal tone (e.g. athletes)
3. Inferior MI
4. Myocarditis
5. Following cardiac surgery (mitral valve repair, Tetralogy of Fallot repair)

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Clinical importance

AV blockages on ECG in a simple manner:

*p wave = wife*

*qrs complex = husband*

*Pacer = counselling*

Sinus Rhythm (Normal)

• The wife (p wave) is waiting for her husband at home (qrs)
• Every night, the spouse (qrs) arrives on schedule

AV block of the first degree

• At home, the wife (p wave) is waiting
• The spouse (qrs) arrives late every night, but he does so consistently and at the same time each night

Type I (Weinkebach) 2nd degree block

• At home, the wife (p wave) is waiting
• Every night, the spouse (qrs) arrives later and later till one night he does not arrive at all

(Note: to notice this trend, the spouse (qrs) must return home for at least two evenings in a succession)

AV block type II, 2nd degree

• At home, the wife (p wave) is waiting
• Occasionally, the spouse (qrs) returns home, and occasionally, he does not. When he does return home, he always does so at the same time

(Note that this is often more significant than type I (Weinkebach) and may need therapy) (pacer).

AV block of the 3rd degree

1. The wife (p wave) is no longer at home waiting for her husband.
2. She and her husband (qrs) are now on different schedules, have no connection, and no longer communicate. Each partner has his or her own timetable.

(It’s worth noting that this typically necessitates counselling in the form of a temporary or permanent pacemaker.)

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This is the next part of our series- Simplified in Practice where we break down ECG interpretation for various conditions. To read the earlier parts of the series, click here: Simplified in Practice: ECG basics & how to interpret,  Simplified in Practice: How to detect myocardial ischaemia?, Simplified in Practice: Anterior Myocardial Infarction- Clinical nuggets on ECG characteristics, Simplified in Practice: What are the ECG changes when inferior STEMI starts?, Simplified in Practice: ECG features of posterior MI Simplified in Practice: ECG nuggets in old STEMI (LV aneurysm, Simplified in practice: ECG Nuggets for atrial flutter,  Simplified in practice: Understanding ECG features of ectopic heartbeats & Simplified in Practice: ECG nuggets for atrioventricular block of the first degree

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Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

The author is a practising super specialist from New Delhi.