Second-degree heart block, or second-degree atrioventricular (AV) block, refers to a disorder of the cardiac conduction system in which some atrial impulses are not properly conducted to the ventricles.
Some P waves are not followed by a QRS.
Second-degree AV block is composed of 2 types: Mobitz I or Wenckebach block, and Mobitz II.
The Mobitz I second-degree AV block is characterized by a progressive prolongation of the PR interval, which results in a progressive shortening of the R-R interval, as shown in the image below.
2nd Degree Mobitz Type 1
Ultimately, the atrial impulse fails to conduct, a QRS complex is not generated, and there is no ventricular contraction.
The Mobitz II 2nd degree AV block is characterized by an unexpected nonconducted atrial impulse, as shown in the image below. Thus, the PR and R-R intervals between conducted beats are constant.
Notice preserved R - R interval
Mobitz I (Wenckebach) block
- No specific therapy is required, unless the patient is symptomatic.
- Patients with suspected myocardial ischemia should be treated with an appropriate anti-ischemic regimen.
- AV nodal blocking agents (including beta-blockade) should be avoided.
- Symptomatic patients should be treated with atropine and transcutaneous pacing. However, atropine should be administered with caution in patients with suspected myocardial ischemia, as ventricular dysrhythmias can occur in this situation.
Mobitz II block
- As with type I block, AV nodal agents should be avoided, and an anti-ischemic regimen should be instituted if ischemia is suspected.
- Transcutaneous pacing pads should be applied to all patients, including asymptomatic patients, as patients with Mobitz II second-degree AV block have a propensity to progress to complete heart block. The transcutaneous pacemaker should be tested to ensure capture. If capture is not able to be achieved, then insertion of a transvenous pacemaker is indicated, even in asymptomatic patients.
- Urgent cardiology consult is indicated for patients who have symptomatic type II block and for those asymptomatic patients who are unable to achieve capture with transcutaneous pacing.