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In 400 BCE, Hippocrates observed that persons who suffered severe attacks of swooning without any manifest cause died suddenly. The artificial electronic pacemaker has been one of the great technological advances in cardiology, able to provide a remedy for many of the conditions that were recognized by Hippocrates more than two millennia ago. Until the development of the pacemaker, there were few options for patients with conduction system failure. Today’s sophisticated devices are used to treat not only bradyarrhythmias and heart block, but also have antitachycardia functions and play an increasing role in the treatment of heart failure.

Up until now we have discussed ECG findings that record the heart’s native electrical system (Figure 21-1). Artificial electronic pacemakers either integrate with, or completely replace natural conduction, requiring an entirely new set of terms and abbreviations. But fear not, because we’re going to take this slowly and there’s a table of abbreviations with definitions at the end of the chapter to help you.

Figure 21-1.

Conduction system of the heart.


There are three basic components of any pacemaker (1) a generator (battery or power supply), (2) one or more electrodes (wires or leads), and (3) circuitry (computer or logic) (Figure 21-2). The generator is a battery-based power supply, sealed within a stainless steel or titanium case that is typically placed in a subcutaneous pocket below the clavicle. Also encased is the computer microprocessor, which can be programmed with a variety of functions. The generator is connected to the myocardium via one or more electrodes that are inserted transvenously into the right atrium, right ventricle, or both chambers. A third electrode may be placed through the coronary sinus and extended into an epicardial coronary vein to depolarize the left ventricle. Leadless pacemakers are a recent development where an entire self-contained system is implanted within the myocardial chamber. The electrode input and circuitry of the pacemaker that is devoted to a particular chamber of the heart is called its channel (eg, atrial or ventricular channel).

Figure 21-2.

Dual-chamber pacemaker. The atrial lead (pink) is typically inserted in the right atrial appendage. The ventricular lead (orange) is typically placed in the right ventricular apex or septum.

A single-chamber pacemaker is one with an electrode in only one chamber, either the right atrium or right ventricle. A dual-chamber pacemaker contains leads in both the right atrium and right ventricle. A biventricular pacemaker includes leads to activate both the right and left ventricles. If a dual-chamber pacemaker also has a left ventricular electrode, the device is termed a dual-chamber biventricular pacemaker.

The Pacemaker ECG Complex

A pacemaker complex looks different than ...

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