In a second-degree, Mobitz Type II AV block, atrial impulses traveling through the AV node or bundle of His are interrupted so that every second, third, or fourth impulse is blocked1. The PR interval remains unchanged with each cardiac cycle until an atrial impulse fails to the reach the ventricles2. The blocked impulse causes this “lost” contraction and missing QRS complex. A Mobitz Type II block can produce a slow ventricular rate despite having a normal sinus rhythm coming from the SA node1.
A Mobitz type II block differs from Mobitz Type I because there is no progressive prolongation of the PR interval before a QRS is skipped. There are many possible causes, but Mobitz type II is rarely seen in individuals without an underlying cardiac condition2. Common causes include:
- Myocardial ischemia/infarction: The AV node is perfused by the right coronary artery in 90% of individuals so an anterior MI may manifest as a Mobitz Type II due to septal ischemia1, 3.
- Cardiac surgery: AV node conduction is affected in approximately 25% of CABG patients¹. Other operations close to the septum such as valve repairs may interfere with AV node conduction3.
- Cardiac sarcoidosis: Inflammation and scarring of the myocardium may interfere with cardiac conduction2.
- Hyperkalemia: High potassium levels can suppress SA node impulses and interfere with AV node conduction3.
- Medications: Beta-blockers, calcium channel blockers, digoxin (digitalis toxicity), and amiodarone alter conduction through the AV which may lead to a block2.
“When some QRSs don’t get through, now you have a Mobitz II”
Clinical presentation will vary depending on the heart rate, frequency of non-conducted P waves, and presence of comorbidities2. The most feature of a Mobitz type II block is an irregular pulse2. Bradycardia or an increase in non-conducted beats will reduce cardiac output because less blood is ejected by the ventricles. An individual may experience symptoms related to decreased cardiac output– lightheadedness, fatigue, dizziness, or syncope2.
Second-degree, Mobitz type II is a potentially lethal arrhythmia and requires immediate treatment. Unstable patients may need pharmacologic therapy with atropine or chronotrope infusion (dopamine or epinephrine) for treatment of hypotension and bradycardia1. Symptomatic individuals should also receive temporary pacing in DVI or DD mode to increase heart rate and cardiac output4. An individual should be monitored for progression to a complete (third degree) heart block.
- University of Maryland Medical Center Office of Clinical Practice and Professional Development. (2014). Introduction to cardiac rhythm interpretation (6th ed.).
- Sauer, W. H. (2018). Second degree atrioventricular block: Mobitz type II. Retrieved from https://www-uptodate-com.proxy-su.researchport.umd.edu/contents/second-degree-atrioventricular-block-mobitz-type-ii?search=second%20degree%20av%20block&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1
- Burns, E. (2018). AV block: Second degree, Mobitz II. Retrieved from https://lifeinthefastlane.com/ecg-library/basics/mobitz-2/
- Bojar, R. M. (2016). Manual of perioperative care in adult cardiac surgery (5th ed.). West Sussex, UK: Wiley-Blackwell