Cardiac pacing systems are described by a three- or four-letter code. The first letter indicates the chamber in which pacing stimuli are delivered (atrium, A; ventricle, V; or both, D). The second letter indicates the chamber in which sensing of the intracardiac electrical signal is occurring (atrium, A; ventricle, V; or both, D). The third letter indicates the response of the device to a sensed signal (inhibition of pacing stimulus output, I; triggering [causing to occur] of stimulus output, T; or both, D). The fourth letter, R, indicates that the device is rate adaptive—that is, it uses one or more sensors to achieve increases and decreases in pacing rate to mimic normal physiologic responses to changes in metabolic need. Commonly used sensors are body motion sensors (e.g., accelerometers) and minute ventilation sensors; one or more sensors can be programmed to be used simultaneously (“blended” sensors).
The usual pacing system implanted in patients who do not have chronic atrial fibrillation (AF) is DDD(R), in which both sensing and pacing occur in both atria and ventricles; AAI(R) systems ( Fig. 7.1 ), which sense and pace only in the atrium, are still in use for patients with sinus node dysfunction and atrioventricular (AV) conduction, and there are systems that can switch between AAI(R) and DDD(R), or AAI(R) and VVI. VVI(R) systems ( Fig. 7.2 ), which sense and pace only in the ventricles, are generally reserved for patients with chronic atrial fibrillation or very old, infirm patients, although they may be used in some young patients with the rare need for backup pacing. Examples of standard dual-chamber pacemakers are shown in Figs. 7.1 , 7.3 , and 7.4 .
The base rate (lower rate limit, standby rate) of a pacing system is that programmed rate at which pacing will occur if there is no spontaneous cardiac depolarization. In devices programmed to rate responsiveness, the base rate is the lowest programmed rate at rest. The upper rate limit , which is either atrial (native P wave) based or sensor based, is the programmed maximum pacing rate that can occur. The maximum tracking rate is that rate at which ventricular pacing will be triggered by native P waves in a 1:1 relationship (atrial based); the maximum sensor-based rate is the highest programmed rate dictated by sensor input to the pulse generator. Whereas these rates are often programmed to be the same, the sensor-based rate can be programmed to exceed the tracking rate in response to exercise, thereby avoiding rapid ventricular paced rates triggered by supraventricular tachycardias.
The magnet rate (designated AOO, VOO, or DOO, as sensing, and therefore response to a sensed signal, do not occur; thus, the letter “O”—an asynchronous mode) is that nonprogrammable rate that occurs when a magnet is placed over the pulse generator. It varies with the manufacturer; several manufacturers set a constant magnet rate well above the expected spontaneous rate (e.g., 100 beats per minute) in order to allow myocardial depolarization (pacing) to be confirmed ( Fig. 7.5A ); other manufacturers set a rapid magnet rate for a specific number of cycles, followed by a slower rate (see Fig. 7.5B ). Because magnet placement eliminates sensing, pacing output occurs despite the existence of a spontaneous cardiac rhythm; repetitive atrial or ventricular beating is only very rarely a clinical consequence.
The programmed AV or PV intervals, independently programmable, define the interval between an atrial and ventricular stimulus or a sensed P wave (atrial electrogram) and the triggered ventricular stimulus, respectively. In DOO mode, the AV interval is generally shortened in order to usurp intact AV conduction and allow confirmation of ventricular pacing; some manufacturers design a lengthening of this interval after a specified number of cycles in order to assess native AV conduction (see Fig. 7.5B ).
After a sensed or paced event, an independently programmable refractory period ensues in each channel (atrial, ARP; and ventricular, VRP), during which the device will not respond to electrical signals. In DDD pacing systems, a programmable postventricular atrial refractory period (PVARP) is designed to prevent tracking of early P waves, which can be retrogradely conducted, thus avoiding “pacemaker-mediated tachycardia” and rapid paced ventricular rates.
Failure to capture, noncapture ( Fig. 7.6 ) indicates that a pacing stimulus output does not depolarize myocardial tissue. This can occur because of too low a programmed voltage output, an increase in myocardial stimulation threshold (such as occurs during hyperkalemia or flecainide treatment), pacing lead insulation break or fracture, lead dislodgement, or battery end of life; failure to capture may also be “functional” due to refractoriness of the myocardial tissue. Pacing system interrogation through manufacturer-specific programmers is often necessary to define the nature of the problem.
Undersensing ( Fig. 7.7 ) refers to failure to sense the intracardiac signal and is usually due to a poor signal rather than a pacing system failure; it can often be corrected by appropriate programming. Undersensing can also result from lead fracture or insulation break or lead dislodgment; interrogation will be necessary to confirm this diagnosis; if present, lead revision will be required.