ECG tutorial: Physiology of the conduction system
- Jordan M Prutkin, MD, MHS, FHRS
Jordan M Prutkin, MD, MHS, FHRS
- Associate Professor of Medicine, Division of Cardiology, Electrophysiology Section
- University of Washington
The cardiac conduction system is designed for electrical impulse creation and propagation. It allows for initiation of impulses in the atrium, slowed conduction in the atrioventricular (AV) node, and rapid propagation through the His-Purkinje system to allow synchronous contraction in the ventricles. Layers of redundancy occur, so that if one portion is damaged, there may be other areas that can compensate for the loss of function.
Cardiac cells have the inherent property of spontaneous depolarization, which creates the cardiac impulse. Cells within the sinus node have the fastest rate of spontaneous depolarization, and, therefore, the sinus node is the main pacemaker region of the heart. The AV node has the second fastest rate of spontaneous depolarization, which allows it to create an escape rhythm if the sinus node is diseased.
The sinus node (the most proximal part of the conduction system) exhibits the most automaticity and functions as the dominant pacemaker in normal circumstances. This structure generates a slow action potential, mediated by calcium currents, that exits the node and activates the atrial myocardium. (See "Myocardial action potential and action of antiarrhythmic drugs".) The atrial myocardium action potential has a rapid upstroke, mediated by sodium ions (figure 1) that help to quickly transmit the signal.
Several preferential tracts exist in the atria to more quickly spread electrical signals . In the right atrium, these include the crista terminalis and pectinate muscles. The Bachman bundle begins anterior to the superior vena cava and crosses the superior interatrial septum to facilitate right to left atrial conduction. A superior pulmonary bundle and septo-atrial bundle speed conduction in the left atrium.
As the atrium is depolarized, a "P" wave is transcribed on the surface electrocardiogram (figure 2). Since the sinus node is in the superior right atrium, the signal goes from superior to inferior, anterior to posterior, and right to left. The P wave is upright and slightly notched in all of the limb leads, with the exception of aVR which has a negative P wave. The precordial leads also show an upright P wave, although in leads V1 and V2 the P wave is usually biphasic; an initial positive followed by a negative deflection reflects depolarization of the right atrium (which is anterior) and then the left atrium (which is posterior).