Case B5. Second Degree AV Block: Mobitz Type I or Wenckebach Block. St Emlyn’s ECG Library

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History

This ECG was recorded from a 49 year old man admitted to the coronary care unit following an inferior myocardial infarction.

ECG

Rhythm: Sinus rhythm is present wth P waves recorded at a rate of 75 per minute. Not every P wave is followed by a QRS complex and dropped beats occur; second degree AV (heart) block is present. The PR interval shows progressive prolongation with each successive beat until the dropped beat occurs. A type of second degree AV block often known as Wenckebach block and sometimes classified as Mobitz type I AV block is present. Dropped beats are well seen in the rhythm strip and in leads I, H and III and in addition each precordial lead contains a dropped beat. After the dropped beat atrioventricular conducdon is restored and the process is repeated. The shortest PR interval occurs with the conducted beat following a dropped beat and this may be normal or prolonged: in this case it is prolonged at .22 seconds. Morphology: There are pathological Q waves in leads II, III and aVF and in lead V4. Lead V3 has a small R wave before the negative wave which by definition is therefore an S wave. The Q waves in V5 and V6 are less than .04 seconds in duradon and cannot therefore be considered pathological. There are raised ST segments typical of acute myocardial infarction in leads II, in and aVF. T wave inversion is present in leads III and aVF and the T waves are flat in the lateral precordial leads. The appearances are of recent inferior infarction. The total QRS complex width exceeds .12 seconds and the QRS complex in lead VI shows an rSR” pattern. This is the pattern of right bundle branch block.

Comment

Second degree AV .block is due to an increasing delay in AV conduction with each successive complex until ultimately failure of conduction occurs. The site of the block is usually relatively high in the junctional tissues, near or within the AV node, although the precise site of the block cannot be determined from the surface ECG. This type of AV block is a relatively common finding after inferior infarction when ischaemia affects the conducting tissue. Wenckebach block following inferior infarction does not usually require treatment and progression to more advanced degrees of AV block is unusual. However in this case there is evidence of more widespread conduction disturbance as right bundle branch block is also present.

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