Section I Premature atrial and AV junctional beats, paroxysmal supraventricular tachycardias, and AV junctional rhythms
The sinus node, or sinoatrial (SA) node, is the physiologic, intrinsic pacemaker of the heart and normally initiates each heartbeat. Pacemaker stimuli can, however, arise from other parts of the heart, such as the atria muscle, theatrioventricular (AV) junction, or the ventricles. The terms ectopy, ectopic pacemaker, and ectopic beat are used to describe these nonsinus heartbeats.
Ectopic beats are often premature; that is, they come before the next sinus beat is due. Thus atrial premature beats (APBs), AV junctional premature beats (JPBs), and ventricular premature beats (VPBs) may occur. Ectopic beats can also comeafter a pause in the normal rhythm, as in the case of AV junctional or ventricular escape beats. Ectopic beats originating in the AV junction (node) or atria are referred to as supraventricular (i.e., coming from above the ventricles).
In this chapter and the next, ectopic atrial and AV junctional (supraventricular) rhythms are described. Chapter 16 discusses ventricular ectopy.
ATRIAL ANDAV JUNCTIONAL (NODAL) PREMATURE BEATS
APBs[*] result from ectopic stimuli; that is, these beats arise from somewhere in either the left or right atrium but not in the SA node. The atria therefore are depolarized from an ectopic site. After an atrial or junctional depolarization, the stimulus may spread normally through the His-Purkinje system into the ventricles. For this reason, ventriculardepolarization (QRS complex) is generally not affected by APBs or JPBs.
APBs have the following major features (Figs. 14-1 to 14-3   ):
| |1. |The atrial depolarization is premature, occurring before the next normal P wave is due. |
| |2. |The QRS complex of the APB is often preceded by a visible P wave that usually has a slightly differentshape |
| | |and/or different PR interval from the P wave seen with normal sinus beats. The PR interval of the APB may be |
| | |either longer or shorter than the PR interval of the normal beats. In some cases, the P wave may be “buried” in |
| | |the T wave of the preceding beat. |
| |3. |After the APB, a slight pause generally occurs before the normal sinus beat resumes. This usually slight delay is|
| | |due to “resetting” of the SA node pacemaker by the premature atrial stimulus. This slight delay contrasts with |
| | |the longer, “compensatory” pause often (but not always) seen after VPBs ( Fig. 16-7 ). |
| |4. |The QRScomplex of the APB is usually identical or very similar to the QRS complex of the preceding beats. |
| | |Remember that with APBs the atrial pacemaker is in an ecto-pic location, but the ventricles are usually |
| | |depolarized in a normal way. This finding contrasts with VPBs, in which the QRS complex is usually very wide |
| | |because of abnormaldepolarization of the ventricles (see Chapter 16 ). |
| |5. |Occasionally, APBs result in aberrant ventricular conduction, so that the QRS complex is wider than normal. |
| | |Figure 14-3 shows an example of such beats causing delayed depolarization of the right ventricle. Thus these APBs|
| | |are associated with right bundle branchblock aberrancy. APBs with left bundle branch block aberrancy may also |
| | |occur. |
| |6. |Sometimes when an APB is very premature, the stimulus reaches the AV junction just after it has been stimulated |
| | |by the preceding normal beat. Because the AV junction,...