Heart block

Abnormal conduction between the atria and ventricles, termed AV conduction blocks, are common conditions in internal medicine. They range from mild asymptomatic findings to emergent, life threatening conditions. In 1964, Lev published the anatomic basis for atrioventricular blocks in the American Journal of Medicine, and still today, the disease that carries his name is responsible for nearly 50% of cases. The common nomenclature of AV block is documented as:

1st degree - slowed conduction between atria and ventricles with maintained synchrony

2nd degree - impaire conduction with intermittent loss of synchrony (separated into type I and II)

3rd degree - complete loss of synchrony between atria and ventricles. 

For the most part, conduction block are a disease of ageing or ischemia. Lev's disease, is used to describe the gradual fibrotic changes that occur with ageing to the cardiac conduction system, which lead to impaired functioning. Myocardial ischemia is the next most common cause, and can be acute or chronic. One study (Circulation, 1978) examined the natural history of heart block in patients following acute MI, and found that AV block is common. Twenty-two percent of patients developed second degree heart block type II. The most common type of blocks were LBBB and RBBB with anterior fascicular block. Patients who developed high grade AV block post MI had a significantly increased mortality (47% vs 23%), which was often directly related to the arrhythmia development and hemodynamic compromise. Other less common causes for AV block include:

Infection - endocarditis (aortic root abscess leading to AV pathway dysfunction), diptheria, scarlet fever, tuberculosis (with pericardial invovlement) mumps, lyme, toxoplasmosis

Inflammatory conditions - rheumatoid arthritis, lupus

Miscellaneous - sarcoidosis and amyloidosis (from infiltration)

Cardiomyopathy - hypertrophic cardiomyopathy, myocarditis'

Genetic - inherited forms of AV block exist, but are uncommon

Medications - remember to take a detailed history to identify agents that will impact the conduction system. 

Today, our patient was taking donepezil (an acetylcholinesterase inhibitor), which may have contributed to the development of bradycardia and heart block. There are multiple case reports of high grade AV block and even torsades des pointes with this class of medications. Donepezil is thought to be more associated with SA node dysfunction, causing sinus bradycardia more than high grade AV block, but both have been documented.  

After treating possible underlying conditions and removing offending medications, the next step is to consider pacing and pacemaker insertion.Determining treatment for the large majority of patients, which have ischemic or age related conduction disease, is based on symptoms and risk of worsening conduction delay.  

Generally accepted reasons for pacemaker insertion in AV block include:

1. Complete heart block, 

2. Symptomatic second degree heart block

3. Second degree block type II with consecutive dropped atrial contractions

4. Second degree block with previous wide complex on ECG

5. Exercise induced high grade blocks

Patients with bifascicular and trifascicular block may also require a pacemaker in the setting of syncope, given it may be due to the development of transient third degree heart block. More detailed instructions can be found in the AHA guidelines for pacemaker insertion linked below.

AHA pacemaker guidelines