What is heart block?
The normal electrical conductive pathway through the heart originates at the sinoatrial (SA) node in the right atrium, propagates through the atria (right to left) but also downward, inferiorly toward the ventricles. At the junction between the atria and the ventricles is the atrioventricular (AV) node, and from there, the conduction wave travels throughout the ventricles via the Bundle of His which splits into two main tracts which further subdivide into the Purkinje fibers. Thus, the time it takes for this wave of conduction to travel, although very brief, is enough to skew the contractions of the atria and ventricles, effecting a sequential pumping action that engenders a vector force outward from atria to ventricles and from ventricles to the rest of the body.
The very nature of electrical depolarization includes an excitatory phase and an refractory phase, so that a dominant impulse is not hampered by competing impulses. The automaticity of the SA node makes its impulse the dominant impulse.
Atrioventricular Heart Block (AV Block)
A block can occur at any point in the conductive pathway due to congenital abnormalities or heart disease (infarction). The blocks typically interfere with the atrial and ventricular contraction coordination. A complete block at the AV node would mean the ventricles would use their own automaticity to contract, as a “rescue” mechanism. Incomplete blocks result in varying degrees of heart pump inefficiency, leading to symptoms from dizziness and light-headedness to syncope and/or death.
Causes of Heart Block:
- Structural abnormality in the SA or AV node or the conductive pathways associated with them
- Drugs (digoxin, beta blockers, and some calcium channel blockers)
- Vagal inhibitory influences (autonomic nervous system)
- Myocardial infarction
- Cardiomyopathy and carditis
Delayed vs. Intermittent vs. Complete AV Block
The severity of an AV block, clinically, is based on the extent of the block:
- First degree AV block: not a true block, but delayed conduction from atrium to the ventricles without interruption in atrioventricular conduction
- Second degree AV block: intermittent block of atrial conduction to the ventricles, often in patterns such as 2:1, 3:1, etc
- Third degree AV block: complete block of signal from the atria to the ventricles
Signs and symptoms are along a continuum based on the degree of heart block, from rare if any symptoms to severe disturbances in cardiac output associated with other arrhythmias and sudden cardiac death.