Section 4 Heart block

First degree heart block

Second degree heart block – Mobitz type 1 or Wenckebach AV block

Second degree heart block – Mobitz type 2

Second degree heart block – 2:1 AV block

Second degree heart block – high grade

Third degree heart block – wide complex escape

Third degree heart block – narrow complex escape

Third degree heart block and atrial fibrillation

Case 44 A 73-year-old man with diabetes

First degree heart block

• A PR interval of greater than 200 ms (5 small squares). In practice a PR interval of 200-220 ms is of dubious clinical significance.

Features xof this ECG

• Sinus rhythm, 75 b.p.m., normal QRS axis

• Features of first degree heart block:

– long PR interval, 320 ms (Fig. 44.1)

• Probable old posteroinferior myocardial infarction

Fig. 44.1 Long PR interval.

Common causes of a long PR interval

→ Increased vagal tone

→ Idiopathic

→ Ischaemic heart disease

→ Rheumatic carditis

→ Digoxin toxicity

→ Electrolyte disturbances

Case 45 A 48-year-old man with 2 days of chest pain

Second degree heart block – Mobitz type 1 or Wenckebach AV block

• The PR interval becomes progressively longer until a P wave is not conducted. The pause that follows failed conduction is less than fully compensatory (i.e. less than two normal sinus intervals).

Features of this ECG

• Second degree heart block (3:2 and 4:3 AV conduction):

– constant atrial rate of 96 b.p.m.

– average ventricular rate of 66 b.p.m.

• Normal QRS axis

• Features of Wenckebach AV block:

– progressive lengthening of the PR interval followed by failure of AV conduction (Fig. 45.1)

• Features of acute inferior myocardial infarction (Fig. 45.2):

– ST elevation

– Q waves

Fig. 45.1 Rhythm strip.

Fig. 45.2 Lead III.

Clinical Note

This man had an acute inferior MI with transient second degree heart block. He did not require pacing. Wenckebach block canprogress to complete heart block; however, the escape rhythm usually arises in the proximal His bundle (narrow QRS) and is well tolerated.

Common causes of Wenckebach AV block

→ Inferior myocardial infarction

→ Drug intoxication (digoxin, beta blockers, calcium antagonists)

→ Heightened vagal tone (e.g. athlete)

Case 46 An 86-year-old lady with episodes of syncope

Second degree heart block – Mobitz type 2

• Most beats are conducted with a constant PR interval.

Occasionally there is an atrial contraction without a subsequent ventricular contraction, i.e. there is intermittent blocking of conduction either through the AV node or, more commonly, the His-Purkinje system. Disease of the His-Purkinje system is most often associated with a prolonged QRS duration.

Features of this ECG

• Sinus rhythm, 90 b.p.m., normal QRS axis

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Related

Related posts:

1. Hypertrophy patterns
2. Bundle branch block
3. Ventricular rhythms
4. Ischaemic heart disease

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