I am sorry, but this site is only supported in an strict HTML compliant browser. The site may continue to function, but may not display properly.

If you are using Internet Explorer 6 or earlier, we recommend you update your browser to Intenet Explorer 8+ or try a compliant browser such as Firefox or Google Chrome.

6. ECG Conduction Abnormalities

Topics for Study:

  1. Introduction
  2. Sino-Atrial Exit Block
  3. Atrio-Ventricular (AV) Block
    1. 1st Degree AV Block
    2. Type I (Wenckebach) 2nd Degree AV Block
    3. Type II (Mobitz) 2nd Degree AV Block
    4. Complete (3rd Degree) AV Block
    5. AV Dissociation
  4. Intraventricular Blocks
    1. Right Bundle Branch Block
    2. Left Bundle Branch Block
    3. Left Anterior Fascicular Block
    4. Left Posterior Fascicular Block
    5. Bifascicular Blocks
    6. Nonspecific Intraventricular Block
    7. Wolff-Parkinson-White Preexcitation



image 01-23

This section considers all the important disorders of impulse conduction that may occur within the cardiac conduction system illustrated in the above diagram. 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; so lets begin . . .

Sino-Atrial Exit Block (SA Block):

2nd 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 3 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:

Differential Diagnosis: sinus arrhythmia without SA block. The following rhythm strip illustrates SA Wenckebach with a ladder diagram to show the progressive conduction delay between SA node and the atria. Note the similarity of this rhythm to marked sinus arrhythmia. (Remember, we cannot see SA events on the ECG, only the atrial response or P waves.)

image 03-18

Type II SA Block

image 03-19

Atrio-Ventricular (AV) Block

Possible sites of AV block:

1st Degree AV Block

PR interval > 0.20 sec; all P waves conduct to the ventricles.

image 07-05

2nd Degree AV Block

The diagram below illustrates the difference between Type I (or Wenckebach) and Type II AV block.

image 01-16

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.

Complete (3rd Degree) AV Block

AV Dissociation (independent rhythms in atria and ventricles)

Intraventricular Blocks

Right Bundle Branch Block (RBBB)

Left Bundle Branch Block (LBBB)

Left Anterior Fascicular Block (LAFB)... the most common intraventricular conduction defect

Left Posterior Fascicular Block (LPFB).... Very rare intraventricular defect!

Bifascicular Blocks

Nonspecific Intraventricular Conduction Defects (IVCD)

Wolff-Parkinson-White Preexcitation

Although not a true IVCD, this condition causes widening of QRS complex and, therefore, deserves to be considered here

QRS complex represents a fusion between two ventricular activation fronts:

ECG criteria include all of the following:

QRS morphology, including polarity of delta wave depends on the particular location of the accessory pathway as well as on the relative proportion of the QRS complex that is due to early ventricular activation (i.e., degree of fusion).

Delta waves, if negative in polarity, may mimic infarct Q waves and result in false positive diagnosis of myocardial infarction.

Test your knowledge on lessons 6!