The AV Blocks

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The AV Blocks


The term AV block is used when there is delayed or failed conduction of impulses from the atria to the ventricles. AV blocks are classified according to the location of the block and the severity of the conduction abnormality. The following classification will be used in this course:

  1. First Degree AV Block
  2. Second Degree AV Block
  3. Third Degree AV Block
First Degree AV Block

The measurement of conduction time between the atria and ventricles is represented by the PR interval on electrocardiograms (ECG). This component includes the intra-atrial conduction, represented by the P wave, and the conduction from the AV node into the His-Purkinje system. Prolongation of the PR interval of more than 200 milliseconds is considered to be a first-degree AV block. These can be due to structural abnormalities within the AV node, an increase in vagal tone, and drugs that slow conduction such as digoxin, beta-blockers. and calcium channel inhibitors. It is important to note that in first degree AV block, no actual block occurs.


From the picture above, P waves are buried within the T waves. The PR interval is prolonged, about 280 milliseconds (about 7 small squares).

CHARACTERISTICS

1) P waves are present and upright

2) Regular but prolonged PR interval

3) All QRS complex are present and the width are normal

4) Regular RR interval

Second Degree AV Block

Second-degree AV blocks are occasional non-conducted P waves with prolonged RR intervals. There are two types under this classification. Mobitz type I (Wenckebach) occurs when there is an intermittent conduction block within the AV node that results in a failure to conduct an impulse from the atria into the ventricles. The impaired nodal conduction is progressive to the point that there is a total block. This causes an absent impulse into the ventricles, reflected by the disappearance of the QRS complex in the ECG. 

2nd Degree Type 1 (Mobitz 1)


Mobitz type I. First PR interval 240 milliseconds, Second and third PR interval is between 320 to 360 milliseconds and fourth PR interval is 360 milliseconds) followed by the absence of the QRS complex.

CHARACTERISTICS

1) All P waves are present and upright

2) Has progressively lengthening PR interval

3) Has absent of QRS complex after the longest PR interval

4) This pattern as in No: 3 continues in a cycle

5) QRS width are normal

2nd Degree Type 2 (Mobitz 2)


CHARACTERISTICS

1) All P waves are present and upright

2) PR interval are regular (constant)

3) QRS width are normal

4) There are absence of QRS complex

Third Degree AV Block

         A complete failure of the AV node to conduct any impulses from the atria to the ventricles is the main feature of third-degree AV block. There is AV dissociation and escape rhythms that may be junctional or ventricular, which represent perfusing rhythms. This is due to AV nodal disease or a disease involving the His-Purkinje system caused by coronary artery disease, enhanced vagal tone, a congenital disorder, underlying structural heart disease such as myocardial infarction, hypertrophy, inflammation or infiltration, Lyme disease, post-cardiac surgery, cardiomyopathies, rheumatologic diseases, autoimmune diseases, amyloidosis, sarcoidosis, or muscular dystrophy. At any time, the patient may suffer ventricular standstill that may result in sudden cardiac death. Pacemaker insertion is necessary to provide needed perfusion.


CHARACTERISTICS

1) No relationship between P waves and QRS complexes

2) No consistent PR intervals




IN SUMMARY
1st Degree AV Bolck = Prolongation of the PR interval
2nd Degree AV Block Type 1 = Progressive lengthening of the PR interval (Long, longer then drop of QRS)
2nd Degree AV Block Type 2 = Normal PR intervals then QRS dropped
3rd Degree AV Block = No relationship between P waves and QRS complexes



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