ECG Atlas- Section 3

Conduction Abnormalities




Heart Block

Generally this term refers to interruption of nerve conduction within the electrical pathways of the heart. Nerve conduction can be interrupted at any level of the electrical pathways. It is quite common in the AV node and nerves below the AV node (His bundle, left & right bundle branches, Purkinje fibers), but it is uncommon within the SA node or in the body of the atrium.

First-Degree AV Block

PR interval of greater than 0.2 second

Due to delay in AV node conduction

Differential Diagnosis of prolong PR interval:
    Increase vagal tone
    Hyperkalemia
    Digitalis
    Ca and Beta Blockers
    Ischaemic heart disease (especially Inferior MI)
    Old age


Case 1

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Second-Degree Heart Block

Intermittent blocking of conduction through AV node or nerve pathways below.

Further Classified into 2 types, Mobitz I and II. ( I find this really confusing, so I rather think about where in the conduction system block occurs.)

Mobitz type 1(Wenckebach)- Level of block is usually at AV node. There is progressive lengthening of the PR interval until the p wave is not conducted. The QRS complex is narrow; suggesting the level of block is the AV node.

Mobitz type 2- Level of block is below the AV node. Therefore PR interval is not prolong and remain constant. The width of QRS complex will usually indicates how far down the conduction is diseased. (ie. If block is very close to AV node QRS will be narrow, if QRS complex is wide then the block is further down the conduction system)


Case 1

Summary of heart block at AV junction


Figure 1.9 from 150 Practise ECGs: Interpretation and Review





Third-Degree, Complete Heart Block

Nothing gets through the AV node. Unrelated p waves and QRS complex (ie AV dissociation)

When block is at the level of the AV node, the takeover pacemaker is just below the node (His bundle), the QRS complex is therefore normal.

When block develops in the infranodal conduction system, the takeover pacemaker is in the body of ventricule, the QRS is wide, and the rate is slow. This will be an indication for pacemaker therapy.


Case 1

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Right Bundle Branch Block

When the right bundle branch is blocked, septum and left ventricle are activated normally. Current then spreads from left to the right ventricle, which is depolarised late.

Features of RBBB:
   -Wide QRS duration of 120 ms or more
   -RSR pattern in V1
   -Slurring of S wave in left side leads (V4-6, I and aVL)

Differential Diagnosis of RBBB:
    Normal variant
    ASD, Tetralogy of Fallot's
    Acute, massive Pulmonary embolus
    Cor pulmonale


Case 1

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Left Bundle Branch Block

Sequence of ventricular depolarisation is almost the opposite to RBBB.

Features of LBBB:
   -Wide QRS complex in left side leads (I, aVL and V6), 120 ms or more
   -Loss of septal q wave in left side leads (I, aVL and V6)

Differential Diagnosis of LBBB:
    Occaionally a normal variant
    Ischaemic heart disease
    Hypertension
    Cardiomyopathy
    Calcific aortic stenosis
    Idiopathic fibrosis
    Following cardiac surgery


Case 1

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Left Anterior and Posterior Fascicular Block

The left bundle divides into anterior and posterior branches.

Left anterior fascicular block is more common than posterior block because of its size and location.

Features of LAFB:
   -Left axis deviation more than -30°
   -r wave in all inferior leads (II, III and aVF)
   -Absence of other causes of left axis deviation

Features of LPFB:
   -Usually in significant left ventricular disease
   -Right axis deviation between 90°-120°
   -Initial q wave in leads II, III, aVF
   -T wave inversion in inferior leads
   -Absence of other causes of right axis deviation


Case 1





Bifascicular Block

RBBB plus block of one of the two branches of the left bundle. (ie. RBBB+LAFB or RBBB+LPFB)

Diagnosis is simple: RBBB plus features of LAFB/LPFB


Case 1





Trifascicular Block

A combination of bifascicular block and first-degree AV Block.


Case 1

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