The sinus node can be affected by physiologic and pathologic mechanisms. There are three manifestations of sinus node dysfunction: the first is inappropriate sinus bradycardia. Sinus bradycardia, per se, as we have previously learned, may be a normal finding in the presence of normal increased vagal tone, for example in highly trained athletes. It may be pathologic, however, when the sinus bradycardia can be shown to be related to problems with central nervous system perfusion or heart failure. These patients can often be shown to have chronotropic incompetence with, for example, exercise testing.
Other types of sinus node dysfunction are sinus arrest and sino-atrial (or SA) block.
Both sinus arrest and SA block exhibit sudden unexpected pauses with missing P waves. In sinus arrest there is no relationship between the pause and the duration of the normal P-P interval, whereas the pause in SA block, since the sinus node is firing, but the impulse does not conduct to the atrial chambers, is an exact multiple of the normal P-P interval. The following animation illustrates what we have just described.