Internodal System

Orderly contractions of the atria and ventricles are regulated by the transmission of electrical impulses that pass through modified cardiac muscle cells (the cardiac conduction system) interposed within the contractile myocardium. This intrinsic conduction system is composed of specialized subpopulations of cells that spontaneously generate electrical activity (pacemaker cells) or preferentially conduct this activity throughout the heart. Following an initiating activation (or depolarization) within the myocardium, this electrical excitation spreads throughout the heart in a rapid and highly coordinated fashion. This system of cells also functionally controls the timing of the transfer of activity between the atrial and ventricular chambers. Interestingly, a common global architecture is present in mammals, with significant interspecies differences existing at the histological level (1,2).

Discoveries relating to the intrinsic conduction system within the heart are relatively recent. Gaskell, an electrophysi-ologist, coined the phrase heart block in 1882, and Johannes E. von Purkinje first described the ventricular conduction system

From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

in 1845. Importantly, Gaskell also related the presence of a slow ventricular rate to disassociation with the atria (3). The discovery of the bundle of His is attributed to its namesake, Wilhelm His Jr. (4). He described the presence in the heart of a conduction pathway from the atrioventricular node through the cardiac skeleton; the pathway eventually connected to the ventricles.

Tawara in 1906 verified the existence of the bundle of His (5). Because of the difficulty in distinguishing the atrioventricular nodal tissue from the surrounding tissue, he defined the beginning of the bundle of His as the point at which these specialized atrioventricular nodal cells enter the central fibrous body (which delineates the atria from the ventricles). Tawara is also credited with being the first to identify clearly the specialized conduction tissues (modified myocytes) that span from the atrial septum to the ventricular apex, including the right and left bundle branches and Purkinje fibers.

A thorough understanding of the anatomy and function of the cardiac conduction system is important for those designing cardiovascular devices and procedures. surgical interventions (heart valve replacements/repair, repair of septal defects, coronary bypass grafting, congenital heart repair, and so forth) are commonly associated with temporary or permanent heart block because of damage to the conduction system or disruption of its anterior tract anterior tract

Internodal Tracts

Fig. 1. The conduction system of the heart. Normal excitation originates in the sinoatrial (SA) node, then propagates through both atria (internodal tracts shown as dashed lines). The atrial depolarization spreads to the atrioventricular (AV) node, passes through the bundle of His (not labeled), and then to the Purkinje fibers, which make up the left and right bundle branches; subsequently, all ventricular muscle becomes activated

Fig. 1. The conduction system of the heart. Normal excitation originates in the sinoatrial (SA) node, then propagates through both atria (internodal tracts shown as dashed lines). The atrial depolarization spreads to the atrioventricular (AV) node, passes through the bundle of His (not labeled), and then to the Purkinje fibers, which make up the left and right bundle branches; subsequently, all ventricular muscle becomes activated blood supply (6-10). When designing corrective procedures or devices, the designer needs to consider means to avoid/correct damage to cellular structures of the conduction system. For example, advances in surgical techniques for the repair of ventricular septal defects have reduced the incidence of complete atrioventricular block from 16% in the 1950s to less than 1% currently (11,12). In addition, for those patients with abnormal conduction systems, many rhythm control devices such as pacemakers and defibrillators aim to return the patient to a normal rhythm and contraction sequence (13-21). Research is even investigating repair/replacement of the intrinsic conduction system using gene therapies (22).

A final example illustrating why an understanding of the heart's conduction system is critical to the design of devices and procedures is cardiac ablation systems. These systems purposely modify the heart to: (1) destroy portions of the conduction system (e.g., atrioventricular nodal ablation in patients with permanent atrial fibrillation); (2) eliminate aberrant pathways (e.g., accessory pathway ablation in Wolff-Parkinson-White syndrome); or (3) destroy inappropriate substrate behavior (e.g., ablation of ectopic foci or reentrant pathways in ventricular tachycardias, Cox's Maze ablation for atrial fibrillation, etc.) (23-26).

This chapter provides basic information on the cardiac conduction system to enhance one's foundation for future research and/or reading on this topic. The information in this chapter is not comprehensive and this should not be used to make decisions relating to patient care.

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