Basic principles of pacing
The aim of this chapter is to give sufﬁcient background and information about cardiac pacemakers to allow interpretation of electrocardiograms (ECGs) and telemetry strips of normal pacemaker behavior. For more in-depth information, such as would be necessary for programming pacemakers, a standard pacing text should be consulted. Several of these are listed in the bibliography. Most italicized terms are deﬁned in the glossary at the end of the chapter.
The pertinent anatomy for cardiac pacing includes the sinoatrial (SA) node, the atrioventricular (AV) node, and the His-Purkinje system (Figure 1.1). The SA node is located at the superior aspect of the crista terminalis (not pictured), near the junction with the superior vena cava. It is normally the dominant pacemaker in the heart, because its rate of depolarization exceeds that of other areas that normally possess properties of automaticity, such as the more inferior areas of the crista terminalis and the His-Purkinje system. The SA node can, in turn, be suppressed by an even faster rhythm, such as an atrial tachycardia, or pacing by an implanted pacemaker.
Figure 1.1 Schematic of conduction system anatomy
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The AV node is normally the only electrical connection between the atria and the ventricles. Electrical activation proceeds from the right atrium, through the AV node to the His-Purkinje system, and then to the ventricles. The His-Purkinje system comprises myocardial cells that are specialized for rapid conduction. Its anatomic components are (in order of activation) the His bundle, the bundle branches (right and left) and the Purkinje ﬁbers. The HisPurkinje system delivers the electrical impulse rapidly from the AV node to widely dispersed areas of the left and right ventricular endocardium, making activation nearly simultaneous throughout the ventricles. This rapid conduction, and simultaneous activation of the right and left ventricles, results in the narrow QRS complex seen on a normal ECG. If an impulse is transmitted throughout the ventricle without using the His-Purkinje system, it takes longer for the ventricles to be activated, and hence the QRS complex is wider. An example would be a premature ventricular contraction (PVC). Another would be ventricular pacing, because the pacemaker lead is usually not positioned so as to activate initially the His-Purkinje system. (Furthermore, in this case the ventricles are activated sequentially rather than simultaneously.) The components of the surface ECG reﬂect the cardiac chambers and conducting system (Figure 1.2). The activation of the atria creates the P wave on the surface ECG. Electrical conduction through the AV node to the HisPurkinje system is relatively slow, so there is normally a 120–200 millisecond (ms) delay between the start of atrial activation and the start of ventricular activation. The delay between the onset of the surface P wave and the onset of the QRS complex is due mostly to conduction through the AV node, with some contribution from intraatrial conduction, and His-Purkinje system conduction. The activation of the ventricles creates the QRS complex. As noted above, the AV node and His-Purkinje system are normally the only electrical connection between the atria and the ventricles. Failure of electrical conduction through the AV node and/or His-Purkinje system results
H B P
Figure 1.2 The conducting system is reﬂected in the normal QRS complex. SN =
sinus node, His = His bundle, BB = bundle branches, P = Purkinje ﬁbers.
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Basic principles of pacing
References: 2 Hesselson A. Simpliﬁed interpretations of pacemaker ECGs. Oxford: Blackwell
New York: Futura Publishing Co., 2000.
Philadelphia: W.B. Saunders, 2000.
6 Love CJ. Handbook of cardiac pacing. Georgetown, Texas: Landes Bioscience, 1998.
Volume 1: Cardiac pacing: a case approach. New York: Futura Publishing Co., 1998.
New York: Futura Publishing Co., 1998.
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