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Chapter 2: Cardiac arrhythmias 57

Alteration in atrioventricular
conduction

Prolonged AV transmission Occasional dropped beat No atrioventricular electrical
time, no dropped beats transmission

Progressive increase in PR Occasional dropped beat
Prolonged PR interval prior to dropped with no progressive PR P waves dissociated
interval from QRS complex
beat lengthening

Second degree heart block Second degree heart block
First degree heart block Third degree heart block
Mobitz I Wenckebach Mobitz II

Figure 2.10 Atrioventricular block.

Management β-blockeroverdoseandabenignformexistsintheyoung
No treatment is required. and athletes due to high vagal tone.

Pathophysiology
Second degree atrioventricular ThesiteofpathologyofMobitztypeIistheAVnodeitself
block – Mobitz type I (in contrast to Mobitz II where infra nodal pathology
(Wenckebach phenomenon)
is thought to be the cause). Complete AV block may
Definition develop, when a ventricular escape rhythm must occur
Second degree atrioventricular block is intermittent fail- for cardiac output to be maintained.
ure of AV transmission. In Mobitz type I (Wenckebach
phenomenon) the missed beat is preceded by a progres- Clinical features
sive lengthening of the PR interval. Patients are usually asymptomatic; however, an irregular
pulse is detected on examination.

Aetiology Investigations
Occurs most commonly in association with underlying The ECG reveals the progressive lengthening of the PR
acute coronary pathology such as post-MI. Wenckebach interval until a beat is missed after which the PR interval
phenomenon may result from digoxin, verapamil or returns to normal and the cycle recurs (see Fig. 2.12).

Rhythm strip Long PR
(II)

Figure 2.11 First degree atrioventricular block.