"Cardiac resynchronization therapy in heart failure: Indications and choice of system", section on 'Alternative approaches'.) Procedure and lead placement The technique for surgical LV lead placement includes a mini-thoracotomy approach in the fourth or fifth intercostal space anterior to the midaxillary line. Typically, single lung ventilation is performed to allow dissection into the pericardial space for implantation of the epicardial LV lead [81]. It is important that the lead be placed at a basal https://www.uptodate.com/contents/cardiac-resynchronization-therapy-in-heart-failure-system-implantation-and-programming/print 12/37 7/6/23, 10:50 AM Cardiac resynchronization therapy in heart failure: System implantation and programming - UpToDate lateral location. The lead is fixed directly to the cardiac tissue using a specifically designed instrument and tested in the same manner as leads placed via a transvenous approach. A key advantage of surgical epicardial lead placement is that lead placement is not confined to the anatomic branches of the LV venous circulation as is the case with transvenous placement. (See 'General principles' above.) Programming In nearly all cases, the programming of epicardial CRT systems is the same as coronary sinus systems. (See 'Approach to programming' above.) Complications Epicardial lead placement entails an increased risk of adverse events and provides no greater benefit compared with transvenous placement. The risk of surgical lead placement is variable; some centers report complication rates similar to transvenous lead placement, while others report a much higher mortality rate than expected with transvenous lead placement: In a study of 30 patients who underwent cardiac surgery for isolated LV lead placement via a mini-thoracotomy following failed transvenous placement, the epicardial lead placement was successful in all patients and perioperative mortality was zero. However, this study did not report electrical lead testing data, rate of bleeding, or infectious complication rate [82]. In a series of 42 patients with "stand-alone" LV epicardial lead placement using a mini- thoracotomy approach, the mean length of stay was 3.4 days, and the 30-day adverse event rate was 17.5 percent; the adverse events included 4.8 percent mortality, 7.5 percent LV lead noncapture, and 5 percent infection [83]. A study that compared patients with surgically placed epicardial LV leads with transvenously placed leads found no difference over a mean five years of follow-up in which the overall clinical response rate to CRT pacing was 65 percent [84]. In this study, major adverse events occurred in 5 percent of patients undergoing stand-alone LV lead placement and 14 percent of patients with concomitant LV lead placement. HIS BUNDLE PACING Components of His bundle pacing systems The goal of His bundle pacing is to capture the His-Purkinje system to improve ventricular synchrony. His bundle pacing (HBP) either "directly" (ie, selectively) stimulates the His-Purkinje system or results in para-Hisian (ie, nonselective) stimulation that captures both the His-Purkinje system and adjacent RV myocardium [85]. The use of His bundle pacing in patients in who cannot undergo coronary sinus lead placement is https://www.uptodate.com/contents/cardiac-resynchronization-therapy-in-heart-failure-system-implantation-and-programming/print 13/37 7/6/23, 10:50 AM Cardiac resynchronization therapy in heart failure: System implantation and programming - UpToDate discussed separately. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system", section on 'Alternative approaches'.) In a HBP system, the RV lead is placed directly into the proximal septum at the location of the bundle of His. If indicated, an RA lead is placed in a manner similar to coronary sinus systems. (See 'System overview' above.) Lead placement His bundle leads are placed in the septum and are guided by pacing maneuvers to confirm His capture. After establishing anesthesia and transvenous access similar to coronary sinus leads, placement of a His bundle lead consists of the following: A pacing lead is advanced through a delivery system to a site on the atrial side of the tricuspid valve annulus or beyond the annulus in the anteroseptal RV. Next, the His bundle pacing site is evaluated using electrograms recorded from the candidate lead [4,86]. Published criteria suggest that the His Bundle pacing should result in a QRS duration less than 120 ms and ideally include a 20 to 40 ms isoelectric segment between the pacing stimulus and the QRS complex. Variants of this HBP technique include the use of a diagnostic electrophysiology catheter as a fluoroscopic reference, injection of intravenous contrast, or the use of an electroanatomic mapping system [86]. Placement of a His pacing system is successful in approximately 50 to 80 percent of attempts, though experience with this approach continues to develop: In a meta-analysis of retrospective studies, HBP for CRT had an average implant success rate of 80 percent and was associated with mean paced QRS duration 122.9 12.0 ms [4]. Compared with baseline values, there were improvements in LVEF, LV volumes, and HF symptoms. The most common complication was an increase in capture thresholds during follow-up. In the His-SYNC pilot trial that included 41 patients with HF and left bundle branch block (LBBB), patients assigned to HBP and biventricular CRT (ie, CRT with a coronary sinus lead) had similar New York Heart Association class and Kansas City Cardiomyopathy Questionnaire scores during follow-up [87]. However, many patients (48 percent) randomized to HBP crossed over to biventricular CRT due to inability to correct the native LBBB with HBP, and many patients (26 percent) randomized to biventricular CRT crossed over to HBP due to anatomic difficulties, which limits the external validity of the results. Programming Programming is guided by the underlying indication for the pacemaker. During evaluation and programming, it is important to distinguish between septal capture, nonselective His capture, and His bundle capture during threshold testing. However, studies https://www.uptodate.com/contents/cardiac-resynchronization-therapy-in-heart-failure-system-implantation-and-programming/print 14/37 7/6/23, 10:50 AM Cardiac resynchronization therapy in heart failure: System implantation and programming - UpToDate have shown that programming the pacemaker outputs to achieve selective His bundle capture does not provide better outcomes compared with nonselective capture [88]. Complications Limited comparative data exist on the complications associated with CRT among patients undergoing biventricular CRT versus either left bundle pacing (LBP) or HBP. Uncommon but unique complications of conduction system pacing include [89,90]: Atrial oversensing. Septal perforation. Tricuspid regurgitation. Coronary artery injury to septal perforator branches of the left anterior descending artery. High pacing thresholds. Uncertainty about the ability to safely extract HBP and LBP leads without injury to septal arteries or the septum or without creation of retained intramyocardial fragments. LEFT BUNDLE AREA PACING Components of left bundle area pacing systems The goal of left bundle area pacing (LBAP) is to provide CRT by stimulating the native cardiac conduction system to partially overcome intrinsic bundle branch block or interventricular conduction delay. The indications for LBAP pacing systems are discussed separately. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system", section on 'Choice of CRT system'.) LBAP systems are composed of an RV lead that paces in the area of the left bundle and, if indicated, a standard RA lead. Lead placement LBAP leads are placed in the proximal RV septum and are tested to confirm left bundle branch capture. Left bundle lead placement also requires 12-lead procedural ECG recording and a delivery sheath or catheter and should only be implanted by experienced operators and in accordance with published techniques, criteria, and applicable equipment [91]. The delivery system is first advanced into the RV over a guidewire. A transvenous pacing lead with either an extendable-retractable or fixed screw is then advanced through the delivery system to the appropriate location on the RV septum. https://www.uptodate.com/contents/cardiac-resynchronization-therapy-in-heart-failure-system-implantation-and-programming/print 15/37 7/6/23, 10:50 AM Cardiac resynchronization therapy in heart failure: System implantation and programming - UpToDate The candidate location is selected beginning approximately 1 to 1.5 cm distal (apical) and inferior of the location of the His bundle. Unipolar pacing from the initial candidate endocardial location allows recording of the baseline impedance, 12-lead ECG of the paced QRS morphology, and LV activation time (LVAT) as measured from the pacing stimulus to the peak depolarization in leads V5 to V6. Initial pacing prior to screw fixation is expected to yield an overall left bundle branch block (LBBB)-pattern QRS complex with characteristic "w" pattern in lead V1. The lead is then advanced into the septum with periodic pacing to evaluate for the presence of left bundle capture. Contrast venography and impedance measurements can help identify the depth in the septum and whether the lead has penetrated into the LV cavity ( image 3). The final location is accepted in accordance with published standards, but generally involving evidence of left bundle capture as assessed by QRS duration and morphology in V1, and LVAT 80 ms in V5 or V6. In a prospective study of 63 patients with HF and complete LBBB, CRT was successfully achieved in 97 percent with LBP, resulting in QRS narrowing to mean 118 12 ms associated with improvements in LVEF, volumes, and HF symptoms. Normalization of LVEF 50 percent occurred in 75 percent of patients [5]. Programming Programming is guided by the underlying indication for the pacemaker. During evaluation and programming, it is important to distinguish between septal capture and left bundle capture [92]. Complications The complications of LBAP leads are similar to those of His bundle pacing (HBP) leads (see 'Complications' above), but LBAP usually has better pacing thresholds and lower risk of atrial oversensing. Because LBAP pacing leads are placed more distally and deeper in the septum, the risks of septal perforation are higher than those observed with HBP leads [93]. 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: Arrhythmias in adults" and "Society guideline links: Cardiac implantable electronic devices".) https://www.uptodate.com/contents/cardiac-resynchronization-therapy-in-heart-failure-system-implantation-and-programming/print 16/37 7/6/23, 10:50 AM Cardiac resynchronization therapy in heart failure: System implantation and programming - UpToDate 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: Cardiac resynchronization therapy (The Basics)") SUMMARY AND RECOMMENDATIONS Types of cardiac resynchronization pacing: Traditional resynchronization systems with a coronary sinus lead Most cardiac resynchronization therapy (CRT) implantations are performed via a transvenous approach in which leads are placed in the right atrium (RA), right ventricle (RV), and coronary sinus (ie, left ventricular [LV] pacing lead) ( image 1). (See 'Traditional crt with a coronary sinus lead' above.) The coronary sinus lead is typically placed at the site of greatest electromechanical delay while avoiding phrenic nerve stimulation. (See 'General principles' above.) Surgical placement of an epicardial lead Epicardial CRT systems are composed of transvenous leads in the RA and RV and a lead placed directly on the lateral LV epicardium via a thoracotomy. (See 'Surgical placement of an epicardial lead' above.) The indications for epicardial lead placement are discussed elsewhere. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system", section on 'Alternative approaches'.) https://www.uptodate.com/contents/cardiac-resynchronization-therapy-in-heart-failure-system-implantation-and-programming/print 17/37 7/6/23, 10:50 AM Cardiac resynchronization therapy in heart failure: System implantation and programming - UpToDate His bundle pacing The goal of His bundle pacing (HBP) is to enable pacing and capture of the His-Purkinje system, which can improve synchrony and ventricular activation. His bundle leads are placed in the septum and are guided by pacing maneuvers to confirm His capture. (See 'Components of His bundle pacing systems' above.) The use of HBP in patients in whom a coronary sinus lead cannot be placed is discussed separately. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system", section on 'Alternative approaches'.) Left bundle area pacing The goal of left bundle area pacing (LBAP) is to provide CRT by stimulating the native cardiac conduction system to partially overcome intrinsic bundle branch block or interventricular conduction delay. LBAP systems are composed of an RV lead that paces in the area of the left bundle and, if indicated, a standard RA lead. (See 'Components of left bundle area pacing systems' above.) The indications for LBAP pacing systems are discussed separately. (See "Cardiac resynchronization therapy in heart failure: Indications and choice of system", section on 'Choice of CRT system'.) Initial program settings In general, we optimize device settings for the best clinical response and lowest battery use. The optimal program settings at the time of implant have not been fully defined and may be patient specific. Initial programming includes (see 'Initial settings' above): Program the atrioventricular delay to optimize resynchronization For patients who are in sinus rhythm, the pacing mode is typically programmed DDD, DDDR, or VDD depending on the need for atrial pacing. For patients in sinus rhythm and with intact atrioventricular (AV) conduction, the AV delay should be set to a value that minimizes the QRS duration. Increase the frequency of pacing In patients with relatively high resting heart rate or continuous atrial fibrillation, we increase the lower rate limit to target 100 percent CRT pacing. Choose the optimal site of pacing We pace the LV at a site with the greatest electrical delay (measured by the Q-LV interval) ( figure 1) that does not result in a wider QRS morphology, left bundle branch block (LBBB) morphology, or apical QRS morphology. The site of pacing should not stimulate the phrenic nerve. https://www.uptodate.com/contents/cardiac-resynchronization-therapy-in-heart-failure-system-implantation-and-programming/print 18/37 7/6/23, 10:50 AM Cardiac resynchronization therapy in heart failure: System implantation and programming - UpToDate Minimize power consumption We program CRT devices to promote battery longevity by choosing pacing sites with the lowest possible electrical thresholds, lowest impedance measurements, and shortest pulse width. Ancillary settings Other pacing settings are available, but routine selection of one method over another is controversial. (See 'Other parameters for optimization' above.) CRT nonresponse Most experts define nonresponse to CRT as lack of improvement in LV ejection fraction (LVEF) or New York Heart Association class. (See 'Identification of and approach to nonresponders' above.) Initial approach to nonresponders For most patients with HF with reduced ejection fraction who do not initially respond to CRT, we evaluate for causes of nonresponse and adjust settings as appropriate. In most patients, the initial approach to nonresponse is to change pacing parameters based on the surface ECG. (See 'Identification of and approach to nonresponders' above.) Additional programming If a response to CRT is not achieved with optimization of the surface ECG, other parameters may be adjusted on an individual basis (eg, fusion pacing, multisite pacing, echocardiographic optimization). (See 'Other parameters for optimization' above.) ACKNOWLEDGMENTS The UpToDate editorial staff acknowledges Leslie A Saxon, MD, Teresa DeMarco, MD, Wilson Colucci, MD, and Daniel J Cantillon, MD, FACC, HRS, who contributed to earlier versions of this topic review. Use of UpToDate is subject to the Terms of Use. REFERENCES 1. Leclercq C, Kass DA. 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A beginner's guide to permanent left bundle branch pacing. Heart Rhythm 2019; 16:1791. 92. Wu S, Chen X, Wang S, et al. Evaluation of the Criteria to Distinguish Left Bundle Branch Pacing From Left Ventricular Septal Pacing. JACC Clin Electrophysiol 2021; 7:1166. 93. Ponnusamy SS, Basil W, Vijayaraman P. Electrophysiological characteristics of septal perforation during left bundle branch pacing. Heart Rhythm 2022; 19:728. Topic 3500 Version 31.0 https://www.uptodate.com/contents/cardiac-resynchronization-therapy-in-heart-failure-system-implantation-and-programming/print 27/37 7/6/23, 10:50 AM Cardiac resynchronization therapy in heart failure: System implantation and programming - UpToDate GRAPHICS Chest radiographs of CRT-P and CRT-D Representative chest radiographs of (A) CRT-P and (B) CRT-D systems. The arrows in (B) denote the presence of a larger pulse generator and associated shocking coil on the right ventricular pacing lead required for