A sinoatrial node exit block is an interference of impulse conduction from the SA node. The SA node depolarizes normally, but the impulses are occasionally blocked before they can leave the node. As a result, there is an absence of P waves on the ECG. 

There are three components of this disease:

1. 1. The SA node generates a constant impulse, but the impulse cannot be seen on an ECG.
      • The presence of the SA node impulse is inferred from the presence of P waves.
      • Rate is presumed when normal P-P intervals are observed (see Figure 3.4).
   2. An area where the block occurs involves the perinodal cells.
      • This is inferred from the response by the P waves.
   3. An output wherein the P wave reflects the type of exit block.

Only second-degree type I and II SA node exit blocks can be diagnosed using a 12-lead ECG (see Figure 3).

In a first-degree SA node exit block, impulses exiting the SA node are slowed. The slowing of these impulses cannot be identified on an ECG.

Figure 3.3. Second-Degree, Type I (Wenckebach Type) Sinoatrial Nodal Exit Block

Second-Degree Type I Sinoatrial Nodal Exit Block

Figure 3.4. Second-Degree Type II Sinoatrial Exit Block

Second-Degree Type II Sinoatrial Nodal Exit Block

In a third-degree sinoatrial node exit block, the SA node’s impulses are completely blocked from reaching the right atrium. An escape rhythm may be maintained by a junctional rhythm. In some instances, a long pause may lead to cardiac arrest. As with the first-degree SA node exit block, diagnosis requires a sinus node electrode during an electrophysiological examination.