Pacemaker Codes

No discussion of pacemakers can take place without an understanding of the generic pacemaker code, which has been published by the North American Society of Pacing and Electrophysiology (NASPE) and British Pacing and Electrophysiology Group (BPEG). This code (NBG ^† was

1417

Figure 35-1 Pacemaker with one quadripolar lead that provides atrial and ventricular sensing and ventricular pacing. This chest x-ray film shows a number of features of a modern pacing system. The generator is located in the left pectoral region. The single lead enters the subclavian vein under the clavicle but superficial to the first rib (a common site for lead problems, although no problem is demonstrated here). In this device, there are two electrodes in the right atrium that can provide sensing to detect intrinsic atrial activity. The ventricular portion of the lead shows the classic bipolar pattern, with a ring electrode just proximal to the tip electrode, and these electrodes can be used for sensing intrinsic ventricular activity and for depolarizing the ventricle. This is a ventricular pacing system with pacing in the triggered and inhibited mode (VDD), and this configuration is placed into patients with a functioning sinus node but a nonfunctioning atrioventricular node. This system cannot be used to depolarize the atrium. Because the surface electrocardiogram often demonstrates ventricular pacing that tracks the atrial activity, inspection of the surface electrocardiogram typically produces an erroneous diagnosis of a dual chamber (DDD) pacemaker.

The first two positions of this code (i.e., chamber paced and chamber sensed) seem relatively straightforward. Although early pacemakers provided only ventricular support, current models can provide pacing in the atria and ventricles, and these devices can also be programmed to determine intrinsic activity in these chambers as well. The code does not describe the array of diagnostic data that can be accumulated by these devices.

Probably the most confusing aspect of the NBG code is the third position (i.e., response to sensed event). Most pacemakers are programmed either to the DDD (dual chamber pacing and sensing, both triggered and inhibited mode) or VVI mode (for single chamber, ventricular pacing in the inhibited mode). Two other modes frequently found are VDD (ventricular pacing with atrial tracking) and DDI (dual chamber pacing and sensing, but inhibited mode only). In the United States, it is uncommon to find an atrial-only pacemaker (AAI), but these devices are indeed implanted in patients with sinus node disease in other countries. The third position describes the following behavior:

D (dual): For DDD pacemakers, atrial pacing will take place in the "inhibited" mode; i.e., the pacing device will emit an atrial pulse if no sensed atrial event (or intervening ventricular event, since any ventricular event will reset atrial timing) takes place within the appropriate timeframe. In DDD or VDD devices, once an atrial event has occurred (whether native or paced), the pacing device will ensure that a ventricular event follows (up to the Upper Tracking Rate [see glossary]). Dual response pacemakers are used to provide atrioventricular (AV) synchrony.

I (inhibited): The pacer will emit a pulse to the appropriate chamber unless it detects intrinsic electrical activity. In DDI mode, AV synchrony is provided only when the pacemaker paces the atrial chamber. If intrinsic atrial activity is present, then no AV synchrony is provided by the pacemaker.

---

1418

T (triggered): The pacing device will emit a pulse only in response to a sensed event. Triggered mode is used when the device is being tested.

The VDD and DDI modes deserve further comment. VDD pacing is used for the patient with AV nodal dysfunction but intact and appropriate sinus node behavior. VDD pacing is accomplished with a single lead that incorporates atrial sensing electrodes and ventricular conductors that can pace and sense (see Fig. 35-1 ). A VDD device has no atrial pacing capability. As a result, in a patient who depends on atrial contraction to augment cardiac output, events that produce VVI pacing (e.g., sinus rate below the programmed rate, battery depletion) or asynchronous ventricular pacing (e.g., magnet placement in many devices, electromagnetic interference) can lead to deteriorating hemodynamics.^{[9] [10]}

DDI is rarely used as a primary mode of pacing. DDI pacing is indicated for the patient who has a dual-chamber pacemaker and has episodes of paroxysmal atrial dysrhythmia (e.g., paroxysmal atrial fibrillation). DDI pacing prevents high ventricular rates (i.e.; pacing at the upper tracking rate) that could result from attempted tracking of the atrial arrhythmia, and it provides AV synchrony only when the atrium is paced. Some DDD pacemakers are programmed to enter the DDI mode on the detection of high (programmable) atrial rates (called Mode Switch, Automatic Mode Switch, or Atrial Tachy Response, depending on manufacturer). For the DDD pacemaker that has switched to DDI, perturbations (e.g., such as atrial rates greater than 400/min), electromagnetic interference from the ESU or magnet placement and removal) can cause the pacemaker to transiently revert back to the DDD mode, with resultant appearance of AV pacing or ventricular pacing at the upper tracking rate (see "Pacemaker Magnets").

Rate modulation (the fourth position) also remains a poorly understood concept. Because some patients cannot increase their heart rate in response to increased oxygen demand (chronotropic incompetence), pacemaker manufacturers have devised a number of mechanisms to detect "patient exercise," such as sensors that detect vibration, respiration, and pressure ( Table 35-2 ). As the sensor detects "exercise," it increases the pacing rate (termed "sensor indicated rate"). As the exercise tapers, this sensor indicated rate returns to the programmed LRL. The sensitivity of these sensors to their exercise signals and the rates of change in pacing are programmable features in current generators.

With the 2002 revision of the NBG, the fifth column now describes multisite pacing functionality (it had been used to describe antitachycardia function, but this scheme has been abandoned and a generic defibrillator code has been established). Atrial multisite pacing is being investigated as a means to prevent atrial fibrillation, ^[11] and ventricular multisite pacing is an acceptable treatment for pacing the patient with a dilated cardiomyopathy.^{[12] [13] [14]}