Understanding Atrioventricular Block
Atrioventricular blocks (AV) are conduction delays or a complete block of impulses from the atria into the ventricles. AV block may be due to increased vagal tone that may be elicited during sleep, athletic training, pain or stimulation of the carotid sinus. Damage of the conduction system secondary to hereditary fibrosis or sclerosis of the cardiac skeleton are known as idiopathic progressive cardiac conduction disease. Ischemic heart disease causes 40% of AV blocks.1 AV blocks are also seen in cardiomyopathies, myocarditis, congenital heart diseases and familial diseases. A plasma potassium concentration above 6.3 mEq/L may also cause AV block. They may be iatrogenic, from medications such as Verapamil, Diltiazem, Amiodarone and Adenosine, or from cardiac surgeries and catheter ablations for arrhythmias.
AV blocks are further classified according to the degree of blockage and include first degree AV block, second degree AV block and third degree AV block. We discuss each of these arrhythmias in this article.
First Degree Atrioventricular 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.
Figure 1. First degree atrioventricular block. The P waves are buried within the T waves. The PR interval is 280 milliseconds (about 7 small squares). Common causes: increased vagal tone, athletic training, inferior myocardial infarction, mitral valve surgery, myocarditis from Lyme disease, and AV nodal blocking drugs.
Photo contributed by Edward Burns of Free open access meducation #FOAMed - http://lifeinthefastlane.com/ecg-library/basics/first-degree-heart-block/
Second Degree Atrioventricular 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 (also known as 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.
Figure 2. Second Degree AV Block – Mobitz type I. There is a clustering of the first four QRS complexes. Notice the prolongation of the PR interval (1st PR interval 240 milliseconds, 2nd and 3rd PR interval is between 320 to 360 milliseconds and 4th PR interval is 360 milliseconds) followed by the absence of the QRS complex. The Wenckebach pattern here is 5:4 P wave:QRS complexes.
Photo contributed by: Dr Harry Patterson, FACEM of Free open access education #FOAMed – http://lifeinthefastlane.com/ecg-library/basics/wenckebach/
Mobitz type I is a benign condition that rarely causes hemodynamic instability; asymptomatic patients need no further treatment. Symptomatic patients will require a pacemaker.
The Mobitz type II AV block is secondary to a disease involving the His-Purkinje system, in which there is a failure to conduct impulses from the atria into the ventricles. A block occurs after the AV node within the bundle of His, or within both bundle branches. The His-Purkinje system is an all-or-none conduction system, therefore in Mobitz type II there are no changes in the PR interval, even after the non-conducted P wave. Because of this, Mobitz type 2 has a higher risk of complete heart block compared to Mobitz type I.
Figure 3. Second degree AV block Mobitz type II. The P waves are at a constant rate. There is no prolongation of the PR interval. The RR intervals before the dropped beats are constant.
Photo contributed by Edward Burns of Free open access meducation #FOAMed - http://lifeinthefastlane.com/ecg-library/basics/mobitz-2/
Third Degree Atrioventricular 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 that 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. It is necessary for the patient to have a pacemaker inserted.
Figure 4. Third degree AV block (complete heart block). Notice that the P waves occur every 920 milliseconds and the RR interval is every 1240 milliseconds exhibiting a dissociation in impulse rates.
Photo contributed by Edward Burns of Free open access meducation #FOAMed - http://lifeinthefastlane.com/ecg-library/basics/complete-heart-block/
- Zoob M, Smith KS. The etiology of complete heart-block. Br Med K 1963; 2:1149.