A heartbeat results from the transmission of an electrical signal sent throughout the heart muscle. Under normal circumstances, this impulse follows a specific pathway that leads to the most efficient contraction of the heart and allows for the delivery of essential oxygen and nutrients to the body. Interruptions in the normal conducting pathway can cause the heart to beat erratically and reduce the efficiency of the heart pump.
Electrical System
Cardiac muscle is unique in that it generates its own electrical impulse and transmits it throughout the heart to create a coordinated muscular contraction. Pacemaker cells located in the heart's right atrium generate the electrical signal and pass it throughout both the right and left atria. This impulse causes an atrial contraction that pushes blood into the right and left ventricles. Allowing time for the blood to move from the atria to the ventricles, the impulse pauses for 0.13 to 0.20 seconds. Continuing on, the impulse travels to and spreads throughout both ventricles simultaneously via bundle branches. This simultaneous transmission leads to a synchronized contraction of the heart's lower chambers. Because there is greater muscle mass in the ventricles compared to the atria, this contraction is much stronger and is the contraction associated with a heartbeat.
Normal sinus rhythm occurs when the electrical stimulus is conducted synchronously along the pathway from atria to ventricles. Interruptions in signal transmission alters the normal sinus rhythm and reduces the synchronicity of impulse conduction and muscle contraction.
Defining Heart Block
The Heart Rhythm Society describes a heart block as a general term referring to any disruption in the normal electrical conduction pathway of the heart. While blocks can occur at any point in the electrical system, the term heart block is most commonly used to indicate interruptions occurring between the atria and the ventricles.
AV Block
A node in the lower portion of the right atrium, called the atrioventricular (AV) node, is responsible for monitoring the slight pause that allows the ventricles to fill. A pause lasting longer than 0.20 seconds indicates a disruption in the communication between these chambers and is referred to as an AV block.
The National Heart Lung and Blood Institute identifies the three classes of AV blocks as first-, second- and third-degree. A first-degree AV block simply indicates a prolonged pause at the AV node. Unlike the other classes of AV block, first-degree blocks always follow the normal pattern of atrial then ventricular stimulation. Patients with first-degree AV blocks are usually asymptomatic and the block is generally left untreated.
In second- and third-degree AV blocks a disruption occurs in the atrial-ventricular contraction sequence. The American Heart Association states that in the event of a third-degree heart block where no communication occurs between the atria and ventricles, the ventricles will attempt to generate their own beat. This beat, however, is much too slow for the adequate delivery of oxygen and nutrients to the body.
Diagnosis
Disruptions in the electrical system are detected noninvasively by an electrocardiogram, or EKG. An EKG provides a recording of the electrical activity of the heart and indicates whether the impulse is being conducted through normal pathways or if abnormalities exist.
Treatment
In guidelines published by the American College of Cardiology, treatment for heart blocks vary. First-degree AV blocks generally go untreated, however, second- and third-degree blocks require the implantation of a pacemaker or anti-arrhythmic medications.
