Normal cardiac conduction

Normal cardiac electrical conduction

• The sinoatrial (SA) node generates electrical impulses that activate the atria and begin the cardiac electrical cycle

  • The SA node is primarily innervated by the right vagus nerve

  • The SA node is typically perfused by the right coronary (RCA, 55-60%) & left circumflex arteries (LCx, 40-45%) in most people

  • The SA node is dense with adrenergic and cholinergic nerve terminals that modulate atrial rate.

  • Impulses from the SA node activate the atrium through 3 intra-atria pathways (anterior, middle, and posterior intranodal tracts)

• Impulses reaching the atrioventricular (AV) node are slowed (refractory period) to modulate atrial impulse conduction to the ventricles

  • The AV node is primarily innervated by the left vagus nerve

  • The AV node is typically perfused by the right coronary (RCA, 85-90%) & left circumflex arteries (LCx, 10-15%) in most people.

• Conduction continues through the bundle of His and into the ventricles. The bundle of His is perfused by the anterior and posterior descending arteries making it less vulnerable to mild ischemia

  • The bundle of His divides into the right and left bundle branches at the top of the inter-ventricular septum

• The right bundle branch continues to the apex of the right ventricle and base of the anterior papillary muscle. The proximal right bundle is small (considered a bundle branch or a fascicle

• The anatomy of the left bundle branch is more variable and may not always bifurcate into left anterior and left posterior fascicular branches

• The "trifascicular system" refers to the RBB (considered a fascicle) and left anterior and posterior fascicles

  • The left anterior fascicle (LAF) extends to the anterosuperior papillary muscle

  • The left posterior fascicle (LPF) extends to the posterioinferior papillary muscle

  • Normal activation of the LV spreads simultaneously from two above sites near the papillary muscles. The impulses normally travel in opposite directions and cancel each other out on the surface ECG.

• Purkinge fibers connect to the ends of the bundle branches and weave through the endocardium to simultaneously activate the right and left ventricular endocardium.

• Purkinje fibers seem to be more resistant to ischemia than other myocardial fibers.