CARDIAC PACEMAKERS



Cardiac pacemakers are devices either im­planted permanently or inserted temporarily, consisting of a pulse generator and an electrode that is either placed transvenously into the right ventricle and/or atrium or sutured directly into the epicardium at the time of surgery. Small elec­trical impulses, generated by the pulse generator and delivered via the electrode catheter, depolar­ize local cells to threshold potential and cause the entire chamber to depolarize. Pacemakers are widely used for treating bradyarrhythmias but can also be useful for treatment of some tachyarrhyth­mias.

A five-position pacemaker code has been de­veloped to describe the pacing modalities avail­able in any particular pacemaker. The first three letters of the code are used commonly and the last two are optional. The first letter is the chamber paced (V = ventricle, A = atrium, and D = atrium and ventricle). The second letter is the chamber in which sensing occurs (V = ventricle, A = atrium, D = atrium and ventricle, and 0 = none). The third letter indicates the mode of response: sensed spontaneous activity inhibiting pacemaker output (I), trigger discharge into the refractory period (T), or trigger ventricular pacing in response to a sensed atrial event as well as in­hibition of ventricular pacing during a sensed ventricular event (D). The fourth letter codes pro-grammability of the pacemaker, and the fifth codes antitachyarrhythmia functions. Some ex­amples are illustrated in Table 8-6. The main ad­vantages of dual-chamber pacing modes are the preservation of AV synchrony and/or the ability to increase ventricular paced rate with an increase in atrial rate. Safeguards are built into the VDD and DDD modes so that the ventricular response cannot exceed a predetermined upper rate should an atrial tachyarrhythmia occur. A problem unique to dual-chamber pacing modes (DDD or VDD) that trigger ventricular depolarization from sensed atrial activity is pacemaker-mediated tachy­cardia. During pacemaker-mediated tachycardia, the pacemaker senses a retrograde P wave con­ducted after a paced ventricular beat or a pre­mature ventricular complex and triggers a sub­sequent ventricular depolarization after the programmed AV delay. This paced ventricular complex can again conduct retrogradely to the atrium, creating a sensed P wave that generates another paced ventricular complex. If this contin­ues, a sustained “reciprocating” tachycardia that utilizes the pacemaker as the antegrade limb may occur. Extensive programmability of the newer pacemaker models, particularly that of atrial reor (3) abnormal discharge rate. Malfunction may be intermittent. Many pacemakers alter their dis­charge rate when the battery approaches deple­tion.