NORMAL ELECTRICAL ACTIVITY

P Wave

A complete electrocardiographic cycle, intervals, and segments are shown in Figure 34-1 . Under normal circumstances, the sinoatrial node has the most rapid spontaneous depolarization rate and is therefore the dominant cardiac pacemaker ( Table 34-1 ). From the sinoatrial node, the impulse spreads through the right and left atria. Specialized tracts conduct the impulse to the atrioventricular node, but they are not essential. The three selective inputs to the atrioventricular node are the right superior "fast" pathway, the right superior "slow" pathway, and a left-sided pathway that is less well characterized. On the ECG, depolarization of the atria is represented by the P wave. The initial depolarization involves primarily the right atrium and occurs predominantly in an anterior, inferior, and leftward direction. Subsequently, it proceeds to the left atrium, located in a more posterior position.