Rick Schreurs

9 General introduction INTRODUCTION The heart is the central organ of the cardiovascular system. Venous blood is collected in the right side of the heart and subsequently pumped to the lungs for oxygenation. The left side of the heart pumps the oxygenated blood to the systemic circulation. Each side of the heart consists of an atrium for collection of blood and a ventricle that ensures distribution to the pulmonary and systemic circulation. These chambers require coordinated activation and contraction in order to work optimally. The contraction is triggered by electrical stimuli in the heart ( Figure 1 ). The first electrical stimulus derives from the sinus node in the upper part of the right atrium (RA). The electrical wave front activates the RA and left atrium (LA) via Bachman’s Bundle and reaches the atrioventricular (AV) node via the internodal pathways where electrical propagation is slowed down compared to other cardiac tissue, causing a delay between atrial and ventricular activation. The ventricles are activated via the fast conduction His bundle and the left and right bundle branch towards the Purkinje fibers. From here the electrical impulses travel further through the slower conducting myocardial tissue in an endocardial-to-epicardial direction [1-3]. The result is a synchronous contraction of the right (RV) and left ventricle (LV), because all ventricular segments are activated in a short moment of time. Mainstem left bundle branch (LBB) Anterosuperior left bundle branch (LBB) Posteroinferior left bundle branch (LBB) Purkinje fibers Ventricular muscle Right bundle branch (RBB) Atrial muscle Internodal pathways Bundle of His AV-node Aorta Sinus node Superior caval vein Interatrial tract to left atrium (LA) (Bachman’s bundle) RV LV RA LA Figure 1. Conduction pathways through the heart. Total AV-block occurs at the level of the AV-node or Bundle of His (in red), while right (RBBB) and left bundle branch block (LBBB) are caused by damage to the RBB and LBB, respectively (in green). Adapted from [3]. 1