The term normal sinus rhythm indicates depolarization by the sinus pacemaker at a standard rate (60-100 bpm) followed by normal atrial depolarization and atrioventricular (AV) conduction to the ventricles. This requires perfect coordination of the entire specialized conduction system so that every P wave is accompanied by a normal PR interval and then by a QRS complex. The structures of the specialized conduction system include the SA node, interatrial and internodal bundles, the AV junction (subdivided into the AV node and the bundle of His), the bundle branches, and the Purkinje fibers. Abnormalities of AV conduction can arise anywhere along this route, which produce the ECG findings that we will review in this chapter.
ELECTROPHYSIOLOGY OF NORMAL AV CONDUCTION
As we know, the surface ECG records most, but not all of the journey of the sinus impulse from the SA node to the ventricles. Recall that the SA nodal depolarization is too small to be visible on the surface ECG so the first evidence we have of the process is the P wave, which represents atrial depolarization. The QRS complex records ventricular depolarization as well as the early portion of repolarization. The PR interval, therefore, represents the time between the onset of atrial activation and that of the ventricular myocardium. The normal PR interval is 0.12 to 0.20 seconds, and should be measured in the lead that exhibits the longest interval from the beginning of the P wave to the beginning of the QRS complex, regardless of whether the QRS complex begins with a Q or an R wave (Figure 17-1).
The cardiac conduction system.
The PR interval is actually comprised of three separate elements: (1) conduction through the atria between the SA and AV nodes, (2) passage through the AV node itself, and (3) transmission through the His-Purkinje system to the ventricular muscle. As we will learn shortly, there are clinical implications whether a conduction abnormality resides above or below the AV node, something we cannot determine with precision on the surface ECG. To reveal these “hidden” components of AV conduction we need to utilize special techniques.
An electrophysiologic study with His bundle electrocardiography uses multipolar catheters that are inserted into the heart to record signals from multiple sites. The His bundle electrogram (HBE) records intracardiac signals from the low right atrium (A), His bundle (H), and right ventricle (V) (Figure 17-2). If we compare the timing of these intracardiac signals with those of the surface ECG, we can differentiate normal from abnormal AV conduction. The analysis includes an examination of the following items (Figure 17-3):