Cardiac Pacing and Pacemaker Rhythms

Pacemaker rhythms

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).

From Olshansky B, Chung M, Pogwizd S, Goldschlager N. Arrhythmia Essentials. Sudbury, MA: Jones & Bartlett Learning; 2012:241.

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 .

Figure 7.1

Normal atrial pacing (AAI or AAI[R]).

The 12-lead ECG with rhythm strips shows an atrial paced rhythm at a rate of about 92 bpm. The atrial pacing stimulus outputs are readily apparent, and they are followed by capture of the atria evident from the subsequent P wave. After ~ 242 ms, the ventricle activates normally through the AV node-His bundle and intraventricular conduction system, resulting in a normal narrow, normal-appearing QRS complex. This rhythm could represent a single-chambered atrial pacemaker or a dual-chamber pacemaker in which the intrinsic QRS activates the ventricle without the need for ventricular pacing.

Figure 7.2

Normal ventricular pacing (VVI or VVI[R]).

This 12-lead ECG tracing with rhythm strips shows a ventricular paced rhythm at a rate of 60 bpm. There is no preceding atrial activity and no preceding atrial stimulus outputs, indicating that this represents a single-channel pacemaker in a VVI or VVI(R) mode. The left bundle branch block pattern of the QRS with superior axis is consistent with pacing from the right ventricle apex. Note the 1:1 ventriculoatrial conduction best seen in leads II, III, and aVF, and the absence of visible pacing stimuli in some leads (e.g., II, III, aVL, aVF, and V 3 ). This is a common finding and explained by digital sampling techniques; significant confusion can be caused by the absence of visible pacing stimuli.

Figure 7.3

Dual-chamber pacing: Atrial and ventricular paced.

This 12-lead ECG tracing with rhythm strips shows consistent atrial and ventricular pacing at a rate of 71 bpm. The atrial pacing stimulus outputs are followed by a prolonged AV interval (of 140 ms), after which the ventricle is paced from the right ventricle apex (apparent from the left bundle branch block pattern and the superior axis). This represents dual-chamber DDD or DDD(R) pacing.

Figure 7.4

Dual-chamber pacing: Atrial sensed, ventricular paced.

This 12-lead ECG tracing with rhythm strips shows a ventricular paced rhythm, but each ventricular paced beat is preceded by a sinus P wave (sinus rate of 55 bpm). This represents a dual-chamber pacemaker with ventricular pacing in response to atrial sensing (P-synchronous pacing).

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.

Figure 7.5

Examples of two different magnet rates and AV intervals.

(A) This 12-lead ECG illustrates DOO function in use by several manufacturers. The rate of 100 bpm (magnet rate) is nonprogrammable, as is the short AV interval. The short AV interval is designed to disallow fusion QRS complexes by usurping native AV conduction, thus confirming ventricular capture. Atrial capture may not be discernible because of the short AV interval (e.g., V leads 4-6 in this ECG), so perusal of all 12 leads is mandatory. Several ECG machines will display arrows, as in this figure, or vertical lines signifying pacemaker stimulus outputs, which can be helpful in ascertaining that these outputs were in fact delivered; depending on sampling, however, such designations may themselves not be present. (B) This ECG displays simultaneously recorded 12 leads, run as a rhythm strip. The usefulness of recording all 12-leads as a rhythm strip allows identification of paced complexes in all ECG leads. In this manufacturer’s magnet mode, 3 AV outputs are delivered at 100 bpm and short AV interval, followed by outputs delivered at 85 bpm at the programmed AV interval, designed to evaluate native AV conduction. Had a regular 12-lead ECG been performed, the initial 3 beats at 100 bpm and short AV interval would have been missed, and ventricular capture not confirmed.

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.

Figure 7.6

Failure to capture (ventricle).

The 12-lead ECG shows an underlying sinus rhythm with complete heart block and a fascicular escape rhythm (right bundle branch block and left anterior fascicle block patterns at a rate of about 29 bpm). A VVI mode of function is present, evident from ventricular stimulus outputs that do not regularly follow sinus P waves. There is clear failure to capture with absence of paced QRS complexes. The second QRS complex could represent pseudofusion; “pseudofusion” describes the situation in which a pacemaker stimulus is superimposed on the native QRS complex but does not contribute to depolarization. Pseudofusion complexes can be seen with normally functioning pacemakers, and they differ from true fusion complexes, in which the intrinsic and paced depolarizations merge, leading to a QRS complex intermediate in morphology between native and paced ventricular beats.

(From Olshansky B, Chung M, Pogwizd S, Goldschlager N. Arrhythmia Essentials . Sudbury, MA: Jones & Bartlett Learning; 2012:247.)

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.

Figure 7.7

Undersensing (Failure to sense and failure to capture).

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Jan 30, 2019 | Posted by in CARDIOLOGY | Comments Off on Cardiac Pacing and Pacemaker Rhythms
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