First-Degree Heart Block

Also called first-degree AV block is a disease of the electrical conduction system of the heart in which the PR interval is lengthened beyond 0.20 seconds.
1st-degree-block-60
1st-degree-block-60

This lengthening of the PR interval is caused by a delay in the electrical impulse from the atria to the ventricles through the AV node. Normally and in the case of ACLS, first-degree heart block is of no consequence unless it involves myocardial infarction or an electrolyte imbalance. Although first-degree heart block is not clinically significant for ACLS, recognition of the major AV blocks is important because treatment decisions are based on the type of block present.

Second-Degree Heart Block (Type 1)

Also called Mobitz 1 or Wenckebach is a disease of the electrical conduction system of the heart in which the has progressive prolongation until finally the atrial impulse is completely blocked and does not produce a QRS electrical impulse.
Once the p-wave is blocked and no QRS is generated, the cycle begins again with the prolongation of the PR interval.
One of the main identifying characteristics of second degree heart block type 1 is that the atrial rhythm will be regular.
2nd Degree Block Type 1 Rhythm Strip with dropped QRS
2nd Degree Block Type 1 Rhythm Strip with dropped QRS

In the above image, notice that the p-waves are regular, the PR-interval progressively gets longer until a QRS is dropped and only the p-wave is present. Although second degree heart block type-1 is not clinically significant for ACLS, recognition of the major AV blocks is important because treatment decisions are based on the type of block present.

Second-Degree (AV) Heart Block (Type 2)

Also called Mobitz II or Hay is a disease of the electrical conduction system of the heart. Second-degree AV block (Type 2) is almost always a disease of the distal conduction system located in the ventricular portion of the myocardium.
2nd Degree Block Type 2
2nd Degree Block Type 2


This rhythm can be recognized by the following characteristics:
  1. non-conducted p-waves (electrical impulse conducts through the AV node but complete conduction through the ventricles is blocked, thus no QRS)
  2. P-waves are not preceded by PR prolongation as with second-degree AV block (Type 1)
  3. fixed PR interval
  4. The QRS complex will likely be wide

Second-degree AV block (Type 2) should be treated with immediate transcutaneous pacing or transvenous pacing because there is risk that electrical impulses will not be able to reach the ventricles and produce ventricular contraction. Atropine may be attempted if immediate TCP is not available or time is needed to initiate TCP. Atropine should not be relied upon and in the case of myocardial ischemia it should be avoided.

Complete Heart Block

Third-degree AV block or complete heart block is the most clinically significant AV block. Complete heart block occurs when the electrical impulse generated in the SA node in the atrium is not conducted to the ventricles. When the atrial impulse is blocked, an accessory pacemaker in the ventricles will typically activate a ventricular contraction. This accessory pacemaker impulse is called an escape rhythm. Because two independent electrical impulses occur (SA node impulse & accessory pacemaker impulse), there is no apparent relationship between the P waves and QRS complexes on an ECG. Characteristics that can be seen on an ECG include:
  1. P waves with a regular P to P interval
  2. QRS complexes with a regular R to R interval
  3. The PR interval will appear variable because there is no relationship between the P waves and the QRS Complexes
3rd Degree Block Diagram ECG Rhythm Strip
3rd Degree Block Diagram ECG Rhythm Strip

In the image above note that the p-waves are independent of the QRS complexes. Also note the 4th QRS complex (impulse) looks different from the others. This is because it is from a different accessory pacemaker in the ventricle than the other QRS complexes.

Common Causes

The most common cause of complete block is coronary ischemia and myocardial infarction. Reduced blood flow or complete loss of blood flow to the myocardium damages the conduction system of the heart, and this results in an inability to conduct impulses from the atrium to the ventricles. Those with third-degree AV block typically experience bradycardia, hypotension, and in some cases hemodynamic instability. The treatment for unstable third-degree AV block in ACLS is transcutaneous pacing.