Sinoatrial and Atrioventricular Blocks

This section considers all the important disorders of impulse conduction that may occur within the cardiac conduction system. Heart block can occur anywhere in the specialized conduction system beginning with the sino-atrial connections, the AV junction, the bundle branches and their fascicles, and ending in the distal ventricular Purkinje fibers. Disorders of conduction may manifest as slowed conduction (1st degree), intermittent conduction failure (2nd degree), or complete conduction failure (3rd degree). In addition, 2nd degree heart block occurs in two varieties – Type I (Wenckebach) and Type II (Mobitz). In Type I block there is decremental conduction which means that conduction velocity progressively slows down until failure of conduction occurs. Type II block is all or none. The term exit block is used to identify conduction delay or failure immediately distal to a pacemaker site. Sino-atrial (SA) block is an exit block.

This section considers conduction disorders in the anatomical sequence that defines the cardiac conduction system.Sino-Atrial Exit Block (SA Block)Second Degree SA Block: This is the only degree of SA block that can be recognized on the surface ECG (i.e., intermittent conduction failure between the sinus node and the right atrium). There are two types, although because of sinus arrhythmia they may be hard to differentiate. Furthermore, the differentiation is electrocardiographically interesting but not clinically important.

Type I (SA Wenckebach): the following three rules represent the classic rules of Wenckebach,which were originally described for Type I AV block. The rules are the result of decremental conduction where the increment in conduction delay for each subsequent impulse gets smaller until conduction failure finally occurs. This declining increment results in the following findings:

1) PP intervals gradually shorten until a pause occurs (i.e., the blocked sinus impulse fails to reach the atria);

2) The pause duration is less than the two preceding PP intervals; and

3) The PP interval following the pause is greater than the PP interval just before the pause.

Differential Diagnosis: sinus arrhythmia without SA block.

Sino-Atrial Exit Block (Type I)

Type II SA Block: 1) PP intervals are fairly constant (unless sinus arrhythmia present) until conduction failure occurs. 2) The pause is approximately twice the basic PP interval.

Sino-Atrial Exit Block (Type II)

Atrio-Ventricular (AV) Block

Possible sites of AV block:

AV node (most common)

His bundle (uncommon)

Bundle branch and fascicular divisions (in presence of already existing complete bundle branch block)

1st Degree AV Block: The following are the ECG criteria:

1st Degree AV Block (PR = 280 ms)

1) PR interval > 0.20 sec,

2) All P-waves conduct to the ventricles.

2nd Degree AV Block

In “classic” Type I (Wenckebach) AV block, the PR interval gets longer (by shorter increments) until a nonconducted P-wave occurs. The RR interval of the pause is less than the two preceding RR intervals, and the RR interval after the pause is greater than the RR interval before the pause.These are the classic rules of Wenckebach (atypical forms can occur).

In Type II (Mobitz) AV block the PR intervals are constant until a nonconducted P-wave occurs.There must be two consecutive constant PR intervals to diagnose Type II AV block (i.e., if there is 2:1 AV block we can’t be sure if its type I or II). The RR interval of the pause is equal to the two preceding RR intervals. Type I AV block is almost always located in the AV node, which means that the QRS duration is usually narrow, unless there is preexisting bundle branch disease. Type II AV block is almost always located in the bundle branches, which means that the QRS duration is wide indicating complete block of one bundle; the nonconducted P-wave is blocked in the other bundle.

Complete (3rd Degree) AV Block

Usually see complete AV dissociation because the atria and ventricles are each controlled by separate pacemakers. Narrow QRS rhythm suggests a junctional escape focus for the ventricles with block above the pacemaker focus, usually in the AV node.Wide QRS rhythm suggests a ventricular escape focus (i.e., idioventricular rhythm). The location of the block may be in the AV junction or bilaterally in the bundle branches. The ECG below shows complete heart block with narrow QRS escape.

Complete Heart Block with Narrow QRS escape

AV Dissociation (independent rhythms in atria and ventricles):

This is not synonymous with 3rd degree AV block, although AV block is one of the causes. May be complete or incomplete. In complete AV dissociation the atria and ventricles are always independent of each other. In incomplete AV dissociation there is either intermittent atrial capture from the ventricular focus or ventricular capture from the atrial focus. There are three categories of AV dissociation (categories 1 and 2 are always incomplete AV dissociation).

F = fusion beat

Intraventricular Blocks

Right Bundle Branch Block (RBBB):

“Complete” RBBB has a QRS duration > 0.12s. Close examination of QRS complex in various leads reveals that the terminal forces (i.e., 2nd half of QRS) are oriented rightward and anteriorly because the right ventricle is depolarized after the left ventricle. This means the following:

1) Terminal R’-wave in lead V1 (usually see rSR’ complex) indicating late anterior forces;

2) Terminal S-waves in leads I, aVL, V6 indicating late rightward forces; and

3) Terminal R-wave in lead aVR indicating late rightward forces.

ECG Showing RBBB

The frontal plane QRS axis in RBBB should be in the normal range (i.e., - 30 to + 90 degrees). If left axis deviation is pre cent, think about left anterior fascicular block, and if right axis deviation is present, think about left posterior block in addition to the RBBB. “Incomplete” RBBB has QRS duration of 0.10 - 0.12second with the same terminal QRS features. This is often a normal variant. The “normal” ST-T-waves in RBBB should be oriented opposite to the direction of the terminal QRS forces; i.e., in leads with terminal R or R’ forces the ST-T should be negative or downwards; in leads with terminal S forces the ST-T should be positive or upwards. If the ST-T-waves are in the same direction as the terminal QRS forces, they should be labeled primary ST-T wave abnormalities.

Left Bundle Branch Block (LBBB)

“Complete LBBB” has a QRS duration > 0.12second. Close examination of QRS complex in various leads reveals that the terminal forces (i.e., 2nd half of QRS) are oriented leftward and posteriorly because the left ventricle is depolarized after the right ventricle.

1) Terminal S-waves in lead V1 indicating late posterior forces.

2) Terminal R-waves in lead I, aVL, V6 indicating late leftward forces; usually broad,monophasic R-waves are seen in these leads.

3) The “normal” ST-T-waves in LBBB should be oriented opposite to the direction of the terminal QRS forces; i.e., in leads with terminal R or R’ forces the ST-T should be downwards; in leads with terminal S forces the ST-T should be upwards. If the ST-T-waves
are in the same direction as the terminal QRS forces, they should be labelled primary ST-T wave abnormalities.

“Incomplete” LBBB looks like LBBB but QRS duration = 0.10 to 0.12second, with less ST-T change. This is often a progression of LVH.
Left Anterior Fascicular Block (LAFB) is the most common intraventricular conduction defect.

The following ECG features are seen:

1) Left axis deviation in frontal plane, usually -45 to -90 degrees;

2) rS complexes in leads II, III, aVF;

3) Small q-wave in leads I and/or aVL;

4) R-peak time in lead aVL > 0.04second, often with slurred R-wave downstroke;

5) QRS duration usually < 0.12second unless coexisting RBBB;

6) Usually see poor R progression in leads V1-V3 and deeper S-waves in leads V5 and V6; and

7) May mimic LVH voltage in lead aVL, and mask LVH voltage in leads V5 and V6.

Left Posterior Fascicular Block (LPFB) is a very rare intraventricular defect. The following ECG features are usually seen:

1) Right axis deviation in the frontal plane (usually > +100 degrees)

2) rS complex in lead I

3) qR complexes in leads II, III, aVF, with R in lead III > R in lead II

4) QRS duration usually < 0.12second unless there is coexisting RBBB Must first exclude (on clinical grounds) other causes of right axis deviation such as cor pulmonale, pulmonary heart disease, pulmonary hypertension, etc., because these conditions can result in the identical ECG picture.

Bifascicular Blocks

RBBB plus either LAFB (common) or LPFB (uncommon) Features of RBBB plus frontal plane features of the fascicular block (axis deviation, etc.).

Nonspecific Intraventricular Conduction Defects (IVCD) QRS duration > 0.10s indicating slowed conduction in the ventricles Criteria for specific bundle branch or fascicular blocks not met Causes of nonspecific IVCD’s include:
1) Ventricular hypertrophy (especially LVH)

2) Myocardial infarction (so called periinfarction blocks)

3) Drugs, especially class IA and IC antiarrhythmics (e.g., quinidine, flecainide)

4) Hyperkalemia

5) Myocarditis/dilated cardiomyopathy.