Atrioventricular Block

Excerpt

The cardiac conduction system is formed by specialized fibers that initiate electrical impulse and conduct it through the heart's chambers. The human heart's dominant pacemaker is the sinus node, which generates an impulse to depolarize the atria and ventricles. The impulse the sinus node generates spreads to the atria and atrial depolarization, or contraction, occurs. This action is marked by the P wave on a surface electrocardiogram (ECG). After atrial depolarization, the wave of depolarization transfers to the ventricles via the atrioventricular (AV) conduction system. QRS marks ventricular depolarization on a surface ECG.

The AV conduction system consists of an AV node and specialized conduction fibers forming the His-Purkinjie system. The autonomic nervous system supplies the AV conduction system, particularly the AV node, resulting in variable conduction during various physiological states. AV conduction is evaluated by assessing the relationship between the P waves and QRS complexes. Typically, a P wave precedes each QRS complex by a fixed PR interval of 120 to 200 milliseconds. An AV block represents a delay or disturbance in the transmission of an impulse from the atria to the ventricles. This can be due to an anatomical or functional impairment in the heart's conduction system. An AV block can manifest as either temporary or permanent, and its severity determines its classification into three types: first-degree, second-degree, and third-degree AV block. Anatomically, an AV block is classified as either a supra (nodal) or intra/infra-His (infranodal) block. Infranodal blocks commonly present with a wide QRS complex, progress to the third-degree AV block, and require pacemakers.