In this chapter we will cover three different, but very related, pathologic processes involving the SA node proper: sinus blocks, sinus pauses, and sinus arrest. In these three rhythms, the sinus pacemaker fires but the impulse does not make it to depolarize the atrial tissue. The result is that all three of these arrhythmic events do not form an initial P wave. In addition, since the atrial myocardium and the electrical conduction system are not depolarized, the rest of the heart also fails to depolarize. The result is that there are also no QRS complexes, ST segments, or T waves.
In the case of sinus blocks, the sinus node is completely blocked or obstructed from conducting the impulse. In the case of sinus pauses, the impulse propagation is delayed. Finally, sinus arrest is a combination of both complete block and delay.
These events are relatively uncommon in everyday practice. They are, however, very commonly seen when there is disease around the SA node proper. This condition is known as sick sinus syndrome. We will spend some time reviewing this syndrome at the end of this chapter. For now, let’s turn our attention to the events themselves.