1

What are the different types of pacemakers?

In modern practice, cardiac pacing can be provided in numerous ways and by a range of devices, including transcutaneous pacing pads; temporary external pacing devices connected to transvenous or epicardial wires; and highly sophisticated, multifunctional, programmable, permanently implanted pulse generators with permanent intracardiac or epicardial leads. Indications for cardiac pacing are generally guided by two main factors: symptomatic arrhythmias and conduction abnormalities ( Box 7-1 ).

Pacing impulses can be delivered to a single chamber (atrium or ventricle), dual chambers (atrium and ventricle), or multiple chambers (e.g., biventricular pacing for cardiac resynchronization therapy [CRT]). Bipolar leads (cathode and anode present on the lead itself) have predominantly been used since the late 1990s. The advantage of bipolar leads (compared with unipolar leads where the pulse generator functions as anode) is that the distance between the anode and cathode is much smaller, and this confers a reduced susceptibility to electromagnetic interference (EMI). Typically, pacemaker leads are placed in the right atrial appendage, in the right ventricle, or in both in a dual-chamber device ( Figure 7-1 ).

Chest x-ray appearance of a pacemaker.

2

How does a pacemaker work?

In an asynchronous mode of programming, pacing impulses are delivered to the chamber where a lead resides at a set rate regardless of spontaneous electrical activity. In a sensing mode, electrical activity can be detected in the chamber where a lead resides, and depending on device programming, what is sensed can cause either inhibition or triggering of pacing in that chamber. For example, in a sensing mode of programming, if a spontaneous depolarization is sensed, the device inhibits itself from delivering a pacing stimulus, and it looks for a subsequent depolarization during the next preset time interval. If no spontaneous depolarization of the chamber is sensed within the programmed limits, the device delivers a pacing stimulus and looks for a subsequent depolarization during the next preset time interval. Regardless of the device used or the configuration of the leads, pacemaker function always must be programmed to the needs of the individual patient.

3

What do the pacemaker codes represent?

The current North American Society of Pacing and Electrophysiology/British Pacing and Electrophysiology Group (NASPE/BPEG) generic five-position code for antibradycardia, adaptive-rate, and multisite pacing is presented in Table 7-1 .

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  Position I: The first letter specifies the chamber or chambers being paced.

  
  
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  Position II: The second letter specifies the chamber or chambers where sensing takes place.

  
  
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  Position III: The third letter specifies the response to sensed events.

  
  
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  Position IV: The fourth letter specifies the presence or absence of potential rate modulation.

  
  
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  Position V: The fifth position specifies the location or absence of multisite pacing (when present).

It is important to understand what these letters imply so that one knows what pacing behavior to expect from a patient’s device. In addition to basic sensing and pacing functionality, modern devices may include additional features such as mode switching and rate responsiveness that allow for improved performance.

Perhaps the easiest way to understand these codes is by starting with the second letter, the chamber where sensing takes place, because that is really where the activity starts. For example, in a theoretical VAT mode, a sensed atrial depolarization triggers ventricular pacing. Such a mode might be useful for patients after ablation of the atrioventricular (AV) node and would be physiologic. However, the risk of such a mode would be tracking of a fast atrial rate, resulting in a fast ventricular rate.

Commonly employed modes include the following:

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  AAI mode —ensures adequate heart rate in a patient with symptomatic sinus bradycardia and normal AV conduction. In AAI mode, if no atrial depolarization occurs within a preset time interval, the device provides atrial pacing at a preset rate, while a sensed spontaneous atrial depolarization inhibits the device from pacing the atrium.

  
  
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  VVI mode —ensures adequate ventricular rate (e.g., in a patient with atrial fibrillation and a slow ventricular response or in a patient with impaired AV conduction). In VVI mode, if no ventricular depolarization occurs within a preset time interval, the device provides ventricular pacing at a preset rate, while a sensed ventricular depolarization inhibits the device from pacing the ventricle. When an external pacing box is in use with temporary pacing leads (e.g., after cardiac surgery), VVI mode is preferred to the default DDD mode in patients with a ventricular lead only.

  
  
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  DDD mode —ensures that each spontaneous atrial depolarization is followed by a ventricular depolarization because it enables sensing and subsequent inhibition or triggering of pacing of both the atrium and the ventricle. DDD mode is a very versatile and physiologic mode that is commonly programmed for various clinical scenarios.

Asynchronous modes (e.g., AOO, VOO, DOO) are most often used for emergency situations (e.g., acute high-degree AV conduction block, asystole) or in an environment (e.g., operating room) where EMI (e.g., electrocautery) can cause inhibition of pacing based on sensed intrinsic electrical activity, especially in patients who are pacemaker dependent. A risk of an asynchronous mode of pacing is competition with a spontaneous underlying rhythm and the potential to deliver a pacing impulse at a vulnerable portion of the cardiac cycle, leading to an arrhythmia (e.g., ventricular fibrillation from an “R on T” phenomenon).

4

Explain physiologic pacing.

AV synchrony optimizes ventricular filling and, ultimately, cardiac output. Dual-chamber pacing modes can sense and trigger or inhibit pacing in one or both chambers. Such a mode of pacing (i.e., AAI, DDD) is “physiologic” because it ensures that the atrial rate is the same as the ventricular rate and that an atrial systole immediately precedes every ventricular systole. This pacing can be especially important in patients with ischemic cardiomyopathy or diastolic dysfunction, where an atrial kick and AV synchrony improve cardiac output. In addition to improved hemodynamics, physiologic pacing modes minimize AV valvular insufficiency that occurs with isolated ventricular pacing and retrograde atrial depolarization. These modes may reduce the incidence of atrial tachyarrhythmias, reducing potential thromboembolic events from stasis in a noncontracting atrium.

5

What is mode switching?

Mode switching refers to an automatic reprogramming of a pacemaker to prevent tracking of a sudden rapid atrial rate to the ventricle. Once sinus rhythm has been restored, the pacemaker should revert quickly to the original programming. Mode switching refers to an automatic, temporary reprogramming of the device to prevent symptoms resulting from a rapidly paced ventricular rate.

6

Describe rate-adaptive pacing

The ability to increase the heart rate when needed is crucial to optimal systemic perfusion. Rate modulation (also called rate adaptation) is a functionality incorporated into most modern pacemakers that allows the device to increase the paced heart rate automatically when needed to meet metabolic demands. The fourth letter of the NASPE/BPEG code designates the presence or absence of rate modulation. This functionality is accomplished according to a computerized algorithm in proportion to the change in certain monitored physiologic variables. The most common sensor currently employed is a piezoelectric accelerometer to sense vibrations and accelerations secondary to bodily motion. Sensors detecting a change in thoracic impedance ostensibly secondary to increased minute ventilation are also available, and a blended sensor detecting both accelerations and changes in thoracic impedance are used in certain devices. Preoperatively, it may be beneficial to disable the rate-adaptive pacing function for some patients to avoid unnecessarily rapid rates of pacing that may be diagnostically confusing in the perioperative period.