The normal cardiac impulse originates in the sinus node, a crescent-shaped structure approximately 9 to 15 mm long that is located at the juncture of the superior vena cava and right atrium (Figure 1-1).1 After the electrical impulse exits the sinus node, it proceeds to activate the right and left atria. Activation of the atria is responsible for the P wave recorded on the electrocardiogram (ECG) (Figure 1-1, orange color). Activation of the normal human atria takes approximately 90 to 100 ms, the right atrium being activated within approximately 65 ms.2 The last area to be activated is the left atrial appendage, although atrial tissue near the left inferior pulmonary vein can also be activated very late. It should be noted that as the spread of atrial activation occurs, some sections of the right and left atria are activated at the same time.2
Schematic of the electrocardiogram and cardiac conduction system. The pink color represents the electrical system, orange activation of the atria, and green activation of the ventricles. (See text for details.)
A controversy has existed for decades concerning the existence of specialized internodal pathways for conduction between the sinoatrial (SA) node and the atrioventricular (AV) node. The essence of this controversy is whether preferential atrial conduction between the SA and AV nodes occurs over specialized pathways or whether the activation wavefront proceeds through nonspecialized or ordinary atrial myocardium.1,3 Most data strongly suggest that no specialized pathways of conduction exist. The observed preferential conduction of atrial impulses along various anatomical routes can be explained adequately by various factors, such as the complex anatomy of the right atrium, which includes multiple “holes” such as the orifices of the inferior and superior venae cavae and coronary sinus ostium, as well as the orientation of atrial fibers in longitudinal and perpendicular directions, with conduction being faster in the longitudinal direction.4
The PR interval in the ECG encompasses activation in the atria, AV node, His-Purkinje system, and ventricles (Figure 1-1). The AV node is a complex structure that includes the compact node and its posterior extensions.5 The AV node was initially considered to be subdivided into 3 functional zones: the AN, N, and NH zones.6-8 The N zone appears to be the most common area where block occurs in the AV node. However, a more recent microelectrode mapping of the AV node suggests that it includes 6 different cell types.9 Slow conduction through the AV node is multifactorial, in part related to the complex arrangement of nodal cells and connective tissue, reduced electrical cellular coupling, and action potentials that are dependent on the slow inward calcium current.10 The impulses exit from the AV node into the ventricular specialized conduction system that has more ...