7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate Official reprint from UpToDate www.uptodate.com 2023 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Modes of cardiac pacing: Nomenclature and selection : Mark S Link, MD : N A Mark Estes, III, MD : Susan B Yeon, MD, JD, FACC All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Jun 2023. This topic last updated: Dec 07, 2022. INTRODUCTION Once it has been established that bradycardia or a conduction disorder warrants permanent pacing, the most appropriate pacing mode for the patient must be selected. The choice depends upon the specific abnormality that is present, since a wide range of pacemaker functions have been developed to accommodate specific clinical needs ( table 1). (See "Permanent cardiac pacing: Overview of devices and indications".) To facilitate the use and understanding of pacemakers, a standardized classification code has been developed. Most patients can be managed with one of two or three common modes (AAI, VVI, or DDD), with or without rate responsiveness. Contemporary pacemakers are versatile and capable of the most commonly used pacing modes and basic functions (ie, mode switching and rate responsiveness). Some advanced features are available in selected devices. Pacemaker nomenclature and the clinical application of common pacing modes and functions will be reviewed here. NOMENCLATURE Five position code A three-letter code describing the basic function of the various pacing systems was first proposed in 1974 by a combined task force from the American Heart Association and the American College of Cardiology and subsequently updated by a committee from the North American Society of Pacing and Electrophysiology (NASPE) and the British Pacing https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 1/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate and Electrophysiology Group (BPEG). The code, which has five positions, is designated the NBG code for pacing nomenclature ( table 2) [1]. The code is generic and does not describe specific or unique functional characteristics for each device. When a code includes only three or four characters, it can be assumed that the positions not mentioned are "O" or absent. Position I The first position reflects the chamber(s) paced. "A" indicates the atrium, "V" indicates the ventricle, and "D" means dual chamber (ie, both the atrium and the ventricle). Position II The second position refers to the chamber(s) sensed. The letters are the same as those for the first position: "A" for atrium, "V" for ventricle, "D" for dual. An additional option "O" indicates an absence of sensing. Programmed in this mode, a device will pace automatically at a specified rate, ignoring any intrinsic rhythm. (See 'Asynchronous pacing' below.) Manufacturers sometimes use "S" in the first and second positions to indicate that the device is capable of pacing only a single cardiac chamber. Once the device is implanted and connected to a lead in either the atrium or the ventricle, "S" should be changed to "A" or "V" in the clinical record to reflect the chamber in which pacing and sensing are occurring. Position III The third position refers to how the pacemaker responds to a sensed event. "I" indicates that a sensed event inhibits the output pulse and causes the pacemaker to recycle for one or more timing cycles. "T" indicates that an output pulse is triggered in response to a sensed event. "D" indicates that there are dual modes of response. This designation is restricted to dual- chamber systems. An event sensed in the atrium inhibits the atrial output but triggers a ventricular output. There is a programmable delay between the sensed atrial event and the triggered ventricular output to mimic the normal PR interval. If the ventricular lead senses a native ventricular signal during the programmed delay, it will inhibit the ventricular output. "O" indicates no response to sensed input; it is most commonly used in conjunction with an "O" in the second position. Position IV The fourth position reflects rate modulation, also referred to as rate responsive or rate adaptive pacing. (See 'Rate responsiveness' below.) "R" in the fourth position indicates that the pacemaker has rate modulation and incorporates a sensor to adjust its programmed paced heart rate in response to patient https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 2/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate activity. From a practical standpoint, "R" is the only indicator commonly used in the fourth position. "O" indicates that rate modulation is either unavailable or disabled. "O" is often omitted from the fourth position (ie, DDD is the same as DDDO). Position V The fifth position is rarely ever utilized but specifies the location or absence of multisite pacing, defined as stimulation sites in both atria, both ventricles, more than one stimulation site in any single chamber, or a combination of these. The fifth position of the code is rarely used. "O" means no multisite pacing "A" indicates multisite pacing in the atrium or atria "V" indicates multisite pacing in the ventricle or ventricles "D" indicates dual multisite pacing in both atrium and ventricle The most common application of multisite pacing is biventricular pacing for the management of heart failure. This issue is discussed in detail separately. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system".) ADDITIONAL FEATURES In addition to the above basic pacing modes, modern pacemakers have additional features to improve performance in a variety of specific clinical settings. Mode switching and rate responsiveness are available in all contemporary pacemakers. Some features are available in select devices and can be utilized as specific clinical situations demand. Mode switching In dual-chamber pacing systems (DDD/DDDR or less commonly, VDD/VDDR), the ventricle will be paced following every sensed atrial event, up to a programmed maximum ventricular rate. If the patient develops a paroxysmal atrial tachyarrhythmia (eg, atrial fibrillation [AF]), the ventricle would then be paced at this maximum programmed rate for the duration of the arrhythmia, which is obviously undesirable. Mode switching refers to automatic reprogramming of a pacemaker to a mode that no longer tracks the intrinsic atrial rate, usually VVI, DDI, or DVI with or without rate responsiveness. When the sensed atrial rate again falls below the mode switching cutoff and the device assumes that a physiologic rhythm has been restored (ie, with termination of the arrhythmia), the pacing mode automatically reverts to the original programming. https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 3/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate All contemporary dual-chamber pacemakers have mode switching capabilities. This feature can be activated or disabled, depending upon the clinical situation. Rate responsiveness As described above, rate responsiveness, also referred to as rate modulation or rate adaptation, refers to the ability of a pacemaker to adjust its programmed paced rate based upon patient activity. A variety of sensors have been designed to determine when a patient is physically active (eg, vibration, minute ventilation, change in right ventricular impedance). The range of heart rates, the pace of acceleration and deceleration, and the degree of activity required to initiate this response are all programmable in rate-adaptive pacing modes. Modes to minimize ventricular pacing Right ventricular (RV) pacing causes the right ventricle to contract before the left ventricle (LV), and causes the septum to contract before the lateral wall of the LV, simulating the effects of left bundle branch block. This phenomenon is referred to as ventricular dyssynchrony or asynchrony. Whether due to RV pacing or intrinsic conduction abnormalities, dyssynchrony can cause or exacerbate heart failure in some patients and increase the frequency of AF. (See "Overview of pacemakers in heart failure" and "The role of pacemakers in the prevention of atrial fibrillation".) Native atrioventricular (AV) conduction is hemodynamically preferable to RV pacing. With an increased understanding of the detrimental effects of RV pacing, efforts have been made to develop pacing modes that minimize ventricular pacing [2-7]. Examples of novel pacing strategies for this purpose include the following: Ventricular avoidance pacing algorithms A dual-chamber device can be programmed to pace AAI (allowing native conduction), but if specific criteria are met that signify a loss of AV conduction, the pacemaker will automatically switch to DDD pacing for some period of time until the algorithm once again determines the presence of intrinsic AV conduction. This approach has been associated with a markedly lower rate of frequency of ventricular pacing compared with conventional dual-chamber pacing (9 versus 99 percent and 4 versus 74 percent in two studies) [3,4,7]. AV search hysteresis Algorithms exist that will prolong the programmed AV delay in a dual-chamber device to allow native conduction when present. The mechanism and frequency with which the algorithm allows AV prolongation to determine the presence of intrinsic AV conduction and the degree to which the AV delay can be extended are variable depending on manufacturer and model [5]. If native conduction with a long PR or AR is present, the device will allow this to continue until the allowed interval is exceeded and there is no intrinsic QRS. This will generally reset the algorithm to the original programmed AV interval. https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 4/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate Biventricular pacing In some patients, ventricular dyssynchrony is unavoidable due to intrinsic conduction disease or inevitable ventricular pacing. If such a patient also has heart failure and LV dysfunction, synchrony may be restored with biventricular pacing, or cardiac resynchronization therapy (CRT), which improves outcomes in selected patients. Cardiac resynchronization therapy is discussed in detail elsewhere. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system".) Patients with LV dysfunction and a mildly reduced ejection fraction (EF; eg, 36 to 49 percent) who have an indication for permanent pacing have traditionally received a standard dual-chamber pacemaker. However, given the potential for ventricular pacing (especially RV apical pacing) to cause or exacerbate LV dysfunction, it is believed that patients might have better outcomes if cardiac resynchronization therapy (ie, biventricular pacing) was the initially implanted device. One trial, BLOCK-HF, demonstrated that in patients with AV block and LV systolic dysfunction (LVEF <50 percent) with NYHA class I, II, or III heart failure, biventricular pacing was superior to conventional RV pacing for the primary outcome of time to death from any cause, urgent IV therapy for heart failure, or 15 percent or greater increase in LV end-systolic volume index [8]. In a post hoc analysis of the BLOCK-HF trial, patients in the biventricular pacing arm were more likely to have improvement in NYHA functional status and in a clinical composite score incorporating various clinical outcomes [9]. Additional randomized clinical trials are needed to assess conventional pacing versus biventricular stimulation in patients with mildly reduced LV function. His bundle pacing His bundle pacing is an alternative approach to reduce the risk of dyssynchrony, as discussed separately. (See "Permanent cardiac pacing: Overview of devices and indications", section on 'His bundle pacing'.) PACING MODES In selecting the ideal pacing mode, the patient's overall physical condition, associated medical problems, exercise capacity, left ventricular function, and chronotropic response to exercise must be considered along with the underlying rhythm disturbance. Some of the various ventricular and atrial pacing systems available and their NBG codes are shown in the table ( table 3). Single-chamber pacing Early pacemakers were designed to sense and pace in a single chamber. Ventricular pacing can prevent ventricular bradyarrhythmias or asystole of any etiology. Atrial pacing can be used in patients with isolated sinus node dysfunction (SND) and intact AV conduction. https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 5/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate VVI or VVIR pacing Ventricular demand pacing (ventricle paced, ventricle sensed, and pacemaker inhibited in response to a sensed beat) remains the most commonly used pacing mode. Advantages of ventricular demand pacing include the requirement for only a single lead and the ability to protect the patient from dangerous bradycardias of any etiology. However, ventricular demand pacing cannot maintain AV synchrony, and lack of AV synchrony can result in pacemaker syndrome. (See 'Pacemaker syndrome' below.) Virtually all devices currently in use are capable of VVIR pacing. VVIR pacing is primarily indicated in patients with chronic atrial fibrillation with a slow ventricular response. By contrast, in a patient with normal sinus rhythm, VVIR pacing should not be used as an excuse to forego attempts at placing an atrial lead. If sinus node function is intact, dual-chamber (DDD) pacing preserves AV synchrony and maintains the patient's natural heart rate response to activity. This approach is optimal and should be used whenever possible. (See 'Physiologic pacing' below.) AAI or AAIR pacing Atrial demand pacing (atrium paced, atrium sensed, and pacemaker inhibited in response to sensed atrial beat) is appropriate for patients with SND who have intact AV nodal function. Patients with symptomatic sinus bradycardia or sinus pauses, but with an intact ability to accelerate their heart rate with exertion, can be programmed in an AAI mode. Those who cannot adequately accelerate their heart rate should have rate responsive capability available (ie, AAIR). As with ventricular demand pacemakers, these devices have the benefit of requiring only a single lead. However, unlike ventricular single-chamber pacemakers, they will not protect patients from ventricular bradyarrhythmias due to AV conduction block. Due to this limitation, atrial demand pacemakers are infrequently used. Many clinicians are concerned that a patient who already has sinus node disease will later develop AV conduction disease. Although it would be uncommon for AV block to develop precipitously and result in a catastrophic event, gradual development of AV conduction system disease may require upgrade of the pacemaker to a dual- chamber device. Pacemaker upgrade can be technically more difficult than original placement of a dual-chamber pacemaker, and the second procedure obviously entails additional cost and patient risk. However, if the patient with SND is assessed carefully and does not have AV node disease at the time of pacemaker implant, the occurrence of clinically significant AV nodal disease is very low (less than 2 percent per year) [10]. Assessment prior to use of an AAI system should include incremental atrial pacing at the time of pacemaker implant. Although criteria vary among https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 6/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate institutions and implanting clinicians, the adult patient should be capable of 1:1 AV nodal conduction to rates of 120 to 140 beats/minute. Dual-chamber pacing DDD or DDDR pacing The dual-chamber (DDD) pacing system provides physiologic pacing (see 'Physiologic pacing' below), with sensing and pacing capabilities in both the atrium and the ventricle. The pacemaker will be totally inhibited in the presence of sinus rhythm with normal AV conduction if the sinus rate is faster than the programmed lower rate of the pacemaker and the intrinsic AV conduction is faster than the programmed AV interval. If there is sinus bradycardia but normal AV conduction with the intrinsic QRS occurring before the end of the programmed AV interval, there will be atrial pacing with a native QRS complex following each paced atrial beat. Both the atrium and ventricle will be paced if there is sinus bradycardia and delayed or absent AV conduction. The ventricle will be paced synchronously with the atrium if there is normal sinus rhythm with delayed or absent AV conduction. As a result, there are four different rhythms that can be seen with normal pacemaker function ( waveform 1): Normal sinus rhythm Atrial pacing, normally conducted to the ventricle with a native QRS AV sequential pacing Atrial sensing and ventricular pacing The DDD pacing mode is appropriate for patients with AV block who have normal sinus node function. DDD pacing is also considered by some to be the mode of choice in carotid sinus hypersensitivity with symptomatic cardioinhibition. However, most patients should receive a pacemaker capable of DDDR pacing, even if rate response is not initially programmed "on." The ideal patient for DDDR pacing is one with combined sinus nodal and AV nodal dysfunction in whom DDDR pacing would restore rate responsiveness and AV synchrony. DDDR pacing is also appropriate for patients with SND and normal AV conduction. As noted above, many practitioners are not comfortable with AAIR pacing. Use of DDDR pacing mode with an algorithm that will minimize ventricular pacing is often preferred. https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 7/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate DDI or DDIR pacing In the DDI pacing mode, there is atrial sensing and pacing, and ventricular sensing and pacing; however, the pacemaker will not track intrinsic atrial activity. When there is a sensed native atrial rate, the pacemaker will inhibit both atrial and ventricular output, thereby allowing native conduction to the ventricle. If AV block develops, ventricular pacing will occur at a programmed rate, but will not be synchronized with the atrium. As an example, if a device is programmed DDI at 50 beats per minute, and the patient has sinus rhythm at 60 with 1:1 AV conduction, the device will be fully inhibited. If AV block develops, the pacemaker will pace the ventricle at 50 beats per minute. If sinus bradycardia develops, the pacemaker will pace the atrium and ventricle synchronously at 50 beats per minute. In the DDI mode, if the sinus rate is below the programmed rate, the pacemaker will pace the atrium and ventricle sequentially. There are few, if any, advantages of DDI or DDIR pacing at this time. At one time, this mode was helpful for the patient with atrial tachyarrhythmias. Since DDI does not "track," this mode would alleviate the concern of fast ventricular rates in response to the atrial tachyarrhythmia. However, this has become much less important since essentially all dual-chamber devices now have mode switching capability. (See 'Mode switching' above.) Less common modes VDD and DVI mode remain programmable options in most pacemakers but are rarely used. VDD pacing VDD pacing (ventricle paced, atrium and ventricle sensed, and either inhibition or tracking of the pacemaker in response to a sensed beat) may be appropriate for the patient with normal sinus node function and conduction disease of the AV node. Dual-chamber (two lead) VDD pacing systems have largely been supplanted by DDD pacemakers. However, a single-lead VDD pacing system, now available for many years, has increased interest in the use of VDD as the initial pacing mode in patients with AV block but normal sinus node function [11-13]. In these systems, atrial sensing is accomplished from "floating" sensing electrodes on the atrial portion of the ventricular pacing lead. One limitation to the use of a single-lead VDD pacemaker is that patients with initially normal SA node function may develop SND. This would then require a second procedure to place an atrial lead capable of pacing in order to maintain AV synchrony and chronotropic competence. However, this is an infrequent occurrence [14,15]. DVI pacing DVI pacemakers (atrium and ventricle paced, ventricular sensing only, and inhibition of pacemaker in response to sensed ventricular beat) are now of historical interest only. DVI pacing is, by definition, limited by the absence of atrial sensing, which prevents the https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 8/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate restoration of rate responsiveness in the chronotropically competent patient. In addition, lack of atrial sensing may lead to competitive atrial pacing and initiation of atrial rhythm disturbances. Asynchronous pacing Pacemakers may be programmed to pace at a fixed rate, without attempting to sense or react to native cardiac activity. These modes are referred to as asynchronous pacing. AOO, VOO, or DOO mode In these modes, the atrium, ventricle, or both are paced, but the pacemaker has no sensing capability and hence there is no sensing response of the pacemaker. Asynchronous pacing modes are rarely used long-term. These modes, however, may be temporarily necessary for patients who are undergoing a surgical procedure, especially if the patient is pacemaker-dependent. Electrocautery could be sensed by the pacemaker and misinterpreted as native cardiac activity, thereby inhibiting pacing output. This could produce significant bradycardia or asystole in a pacemaker-dependent patient. Thus, prior to surgery, the pacemaker could be reprogrammed to an asynchronous mode that turns off its sensing capability. After surgery, the pacemaker should be reprogrammed to its prior mode. Alternatively, a magnet placed over the pacemaker will deactivate its ability to sense and, while left in place, will result in asynchronous pacing. Although this approach has been used for many years there are some concerns that are likely more theoretical than real. Pacing in an asynchronous mode can be associated with competition between the native and the paced rhythms, with the possibility that a paced impulse will occur during a native T wave (or the vulnerable period). To reduce this risk, asynchronous pacing could be programmed to a relatively higher rate ( 80 beats/minute). PHYSIOLOGIC PACING Physiologic pacing is a term that has been used to describe pacing systems that most closely approximate normal cardiac behavior. It most commonly refers to systems that maintain AV synchrony (eg, AAI or DDD systems, in contrast to VVI systems), but has also been applied to rate responsive pacemakers. (See 'AAI or AAIR pacing' above and 'Dual-chamber pacing' above and 'Rate responsiveness' above.) Potential advantages Physiologic pacing has several potential hemodynamic and clinical advantages compared with VVI pacing [16]. These include: Reduced incidence of atrial fibrillation (AF) The incidence of atrial tachyarrhythmias, particularly AF, is reduced by physiologic compared with VVI pacing. (See "The role of https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 9/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate pacemakers in the prevention of atrial fibrillation".) Reduced incidence of thromboembolic events A lower rate of thromboembolic events is suggested in the meta-analysis of physiologic pacing discussed below, and may be secondary to the lower incidence of AF [17]. Improved hemodynamics The maintenance of AV synchrony and "atrial kick" is hemodynamically favorable and physiologic pacing improves cardiac output, arterial pressure, and coronary blood flow [18-21]. The magnitude of these improvements is small, and their clinical significance is not clear but, in some studies, physiologic pacing has resulted in a lower incidence of heart failure [22]. (See "Hemodynamic consequences of atrial fibrillation and cardioversion to sinus rhythm", section on 'Atrial systole'.) Avoidance of pacemaker syndrome VVI pacing is associated with the development of "pacemaker syndrome." This syndrome is due to AV dyssynchrony or retrograde ventricular-to-atrial conduction. It is prevented by physiologic pacing [23,24]. (See 'Pacemaker syndrome' below.) Effects on outcomes Several trials and a meta-analysis have compared physiologic and VVI pacing [17,24-29]. In the aggregate, these reports demonstrate that across the spectrum of patients with bradycardic indications for pacemakers, physiologic pacing does not improve survival or the incidence of heart failure, but does reduce the incidence of AF and may reduce the incidence of stroke. The meta-analysis included data from five randomized trials [17]: A Danish trial of 225 patients with sinus node dysfunction (SND) and normal AV conduction [22,25,30]. The MOST trial of 2010 patients with SND, 20 percent of whom also had AV conduction disease [26,31]. The CTOPP trial of 2568 patients with both SND and AV conduction disease (42 percent with SND) [27,29,32,33]. The PASE trial of 407 patients, 43 percent with SND [24,34]. The UKPACE trial of 2021 older adult patients, all of whom had AV conduction disease [28]. The meta-analysis included 7231 patients and over 35,000 patient-years of follow-up. The average patient age was 76. Most patients randomly assigned to physiologic pacing received a https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 10/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate dual-chamber (DDD) pacemaker, while all of those in the Danish trial and some in CTOPP received an AAI pacemaker. The following findings were noted: There was no difference in the incidence of heart failure or in all-cause mortality between physiologic and VVI pacing (31 versus 33 percent all-cause mortality). Physiologic pacing significantly reduced the incidence of AF (17 versus 22 percent). Physiologic pacing appeared to reduce the incidence of stroke (5.2 versus 6.3 percent). However, the authors suggested cautious interpretation of this finding, due to borderline statistical significance (95% CI 0.67-0.99, p = 0.035) and the lack of adjustment for multiple hypothesis testing. Subgroup analysis suggested that physiologic pacing may be more beneficial in patients with SND than those with AV block. Among patients with SND, physiologic pacing appeared to reduce the combined endpoint of stroke or cardiovascular death. However, the authors suggested caution in the interpretation of this subgroup analysis, because of heterogeneity in the populations of the included trials (ie, different percentages of patients with SND). Issues specifically related to physiologic pacing and SND are discussed separately. There was no additional advantage in several other subgroups that are often considered to derive a greater benefit from physiologic pacing (eg, those with left ventricular dysfunction or heart failure). Patients appear to prefer physiologic pacing, suggested by relatively high crossover rates from VVI to dual-chamber pacing in the two trials in which this was easy to do. (See 'Patient preference' below.) The results of the meta-analysis are broadly consistent with those of the individual trials. However, due to some differences in the patient populations (eg, SND versus AV block, older adult patients), and the pacemakers used (eg, AAI versus DDD), some points from individual studies are worth consideration: As in the subgroup analysis, results from the Danish trial suggested a greater benefit of physiologic pacing in patients with SND than in those with AV block, which we now understand to be a function of maintaining intrinsic AV conduction in the SND group [30]. In adults with advanced age, the rates of AF and stroke are high regardless of pacing mode. The value of physiologic pacing may be less significant in this group, particularly those with AV block [28,35]. This was best illustrated in the UKPACE trial of 2021 older adult https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 11/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate patients (average age 80), all of whom had AV block [28]. In this population, physiologic pacing did not reduce the rates of mortality, AF, or thromboembolism. Pacemaker syndrome Pacemaker syndrome is a phenomenon associated with the loss of AV synchrony and is seen most commonly with single-chamber VVI pacing. It is defined as the adverse hemodynamics associated with a normally functioning pacing system, resulting in overt symptoms or limitation of the patient's ability to achieve optimal functional status [23]. The development of the pacemaker syndrome with VVI pacing may require upgrade from a VVI pacemaker to a dual-chamber system in some patients. Symptoms most commonly include general malaise, easy fatigability, dyspnea, orthopnea, cough, dizziness, atypical chest discomfort, and a sensation of throat fullness and, less commonly, may result in pre-syncope or syncope. Physical examination may reveal hypotension, rales, increased jugular venous pressure with cannon A waves, peripheral edema, and murmurs of tricuspid and/or mitral regurgitation [23,36]. Patient preference Although no consistent mortality benefit has been identified in randomized clinical trials with physiologic pacing, patients seem to prefer physiologic pacing as illustrated by the following observations: In a double-blind crossover study of different pacing modes, 86 percent of patients preferred physiologic pacing [37]. In PASE and MOST, quality of life scores were higher in patients with SND randomly assigned to physiologic pacing [24,26]. In the clinical trials comparing physiologic and VVI pacing, when crossing over from VVI to physiologic pacing was easy, up to 38 percent of patients chose to cross over. In PASE and MOST, all patients received dual-chamber pacemakers, and randomization to physiologic or VVI pacing occurred after implantation [24,26]. Thus, crossover required only device reprogramming (as opposed to a second procedure). The crossover rates in these two trials were 26 and 38 percent (compared with less than 5 percent in the other trials). The high crossover rate in MOST led to questions about the validity of the results. However, in a later study, the results of intention-to-treat and on-treatment analyses were similar [31]. As in the original report from MOST [26], the on-treatment analysis showed no difference in the primary endpoint between the two pacing modes, but there was a significant reduction in the incidence of AF with physiologic pacing. Based upon the reduced incidence of AF and patient preference, we suggest that physiologic pacing should be used in most patients who require a pacemaker. https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 12/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate MODE SELECTION ALGORITHMS A number of guidelines and algorithms are available for determining the appropriate pacing mode for patients with sinus node disease and AV node disease [38]. The indications and contraindications for the various types of pacing modes are listed in the accompanying tables ( table 4) [38]. In this listing, chronotropically competent refers to the ability of a patient to achieve an appropriate heart rate for a given physiologic activity. Several algorithms are also available: General algorithms for all bradycardic indications ( algorithm 1 and figure 1) An algorithm for sick sinus syndrome ( algorithm 2) An algorithm for AV block ( algorithm 3) SOCIETY GUIDELINE LINKS Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Cardiac implantable electronic devices".) INFORMATION FOR PATIENTS UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." th th The Basics patient education pieces are written in plain language, at the 5 to 6 grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more th th sophisticated, and more detailed. These articles are written at the 10 to 12 grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon. Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.) Basics topic (see "Patient education: Pacemakers (The Basics)") Beyond the Basics topic (see "Patient education: Pacemakers (Beyond the Basics)") https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 13/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate SUMMARY AND RECOMMENDATIONS Nomenclature For permanent cardiac pacing, the five position NBG code indicates the chamber(s) paced, the chamber(s) sensed, the response to sensing, presence or absence of rate modulation, and location or absence of multisite pacing ( table 2). (See 'Nomenclature' above.) Selection of pacing mode In selecting a pacing mode, the patient's overall physical condition, associated medical problems, exercise capacity, left ventricular (LV) function, and chronotropic response to exercise must be considered along with the underlying rhythm disturbance. Some commonly used pacing modes are shown in the table ( table 3). (See 'Pacing modes' above.) Most patients with a standard bradycardic indication for pacing can be managed with one of three common pacing modes (with or without rate responsive pacing): AAI(R), VVI(R), or DDD(R). At the time of implantation, one should consider how many leads will be necessary and which additional features, if any, will be of potential value. Choice of single- or dual-chamber pacemaker The most important choice in most patients with a bradycardic indication for pacing is whether to place a single- or dual- chamber pacemaker (see "Permanent cardiac pacing: Overview of devices and indications" and 'Pacing modes' above). The choice varies with the clinical setting: Atrial fibrillation In patients with chronic atrial fibrillation (AF) who require a pacemaker due to slow ventricular response, we recommend a single-chamber ventricular pacemaker (VVI or VVIR) (Grade 1B). (See 'VVI or VVIR pacing' above.) Sinus rhythm In patients in sinus rhythm with conditions that could be managed with either a single- or a dual-chamber pacemaker (eg, atrioventricular [AV] block, sinus node dysfunction [SND]), we suggest a dual-chamber pacemaker (Grade 2B). (See 'Physiologic pacing' above and 'Dual-chamber pacing' above.) Exception for advanced age or difficult two lead implantation A single- chamber pacemaker is a reasonable alternative in patients who are adults with advanced age or whose anatomy and physical condition make the implantation of two leads more difficult than usual. In such cases, the additional costs and risks of a dual-chamber physiologic pacemaker may outweigh the potential benefits of the reduced risk of AF and patient preference. VVI(R) pacing will be effective in all such patients. (See 'VVI or VVIR pacing' above.) https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 14/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate Possible exception for SND with intact AV conduction An AAI(R) pacemaker is a reasonable alternative to VVI(R) pacing in the subset of patients with SND in whom AV conduction is intact and meets intra-implant testing criteria (see "Sinus node dysfunction: Treatment"). However, effective AAIR pacing is more commonly accomplished with a dual-chamber pacemaker with a ventricular pacing avoidance option. (See 'AAI or AAIR pacing' above and 'Modes to minimize ventricular pacing' above.) Additional features The following additional features are appropriate for selected patients: Rate responsiveness This feature can be programmed for patients who are active, but not chronotropically competent. (See 'Rate responsiveness' above.) Mode switching This feature can be programmed for patients with paroxysmal atrial arrhythmias. (See 'Mode switching' above.) Ventricular pacing avoidance Algorithms to avoid ventricular pacing are appropriate for most patients with PR prolongation or type II AV block. Some parameter to minimize ventricular pacing is available in most contemporary devices. (See 'Modes to minimize ventricular pacing' above.) ACKNOWLEDGMENT The UpToDate editorial staff acknowledges David L Hayes, MD, who contributed to earlier versions of this topic review. Use of UpToDate is subject to the Terms of Use. REFERENCES 1. Bernstein AD, Daubert JC, Fletcher RD, et al. The revised NASPE/BPEG generic code for antibradycardia, adaptive-rate, and multisite pacing. North American Society of Pacing and Electrophysiology/British Pacing and Electrophysiology Group. Pacing Clin Electrophysiol 2002; 25:260. 2. Sweeney MO, Prinzen FW. A new paradigm for physiologic ventricular pacing. J Am Coll Cardiol 2006; 47:282. https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print 15/31 7/6/23, 10:54 AM Modes of cardiac pacing: Nomenclature and selection - UpToDate 3. Sweeney MO, Bank AJ, Nsah E, et al. Minimizing ventricular pacing to reduce atrial fibrillation in sinus-node disease. N Engl J Med 2007; 357:1000. 4. Sweeney MO, Ellenbogen KA, Casavant D, et al. Multicenter, prospective, randomized safety and efficacy study of a new atrial-based managed ventricular pacing mode (MVP) in dual chamber ICDs. J Cardiovasc Electrophysiol 2005; 16:811. 5. Olshansky B, Day JD, Moore S, et al. Is dual-chamber programming inferior to single- chamber programming in an implantable cardioverter-defibrillator? Results of the INTRINSIC RV (Inhibition of Unnecessary RV Pacing With AVSH in ICDs) study. Circulation 2007; 115:9. 6. Ricci RP, Botto GL, B n zet JM, et al. Association between ventricular pacing and persistent atrial fibrillation in patients indicated to elective pacemaker replacement: Results of the Prefer for Elective Replacement MVP (PreFER MVP) randomized study. Heart Rhythm 2015; 12:2239.