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Cardiac resynchronization therapy
View on Wikipediafrom Wikipedia
 | This article needs additional citations for verification. (August 2016) |
| Implanted cardiac resynchronization device | |
|---|---|
 | |
| ICD-9-CM | 00.51, 00.54 |
| MeSH | D058409 |
| eMedicine | 1839506-devices |
Cardiac resynchronisation therapy (CRT or CRT-P) is the insertion of electrodes in the left and right ventricles of the heart, as well as on occasion the right atrium, to treat heart failure by coordinating the function of the left and right ventricles via a pacemaker, a small device inserted into the anterior chest wall.[1]
CRT is indicated in patients with a low ejection fraction (typically <35%) indicating heart failure, where electrical activity has been compromised, with prolonged QRS duration to >120 ms.[2]
The insertion of electrodes into the ventricles is done under local anesthetic, with access to the ventricles most commonly via the subclavian vein, although access may be conferred from the axillary or cephalic veins. Right ventricular access is direct, while left ventricular access is conferred via the coronary sinus (CS).
CRT defibrillators (CRT-D) also incorporate the additional function of an implantable cardioverter-defibrillator (ICD), to quickly terminate an abnormally fast, life-threatening heart rhythm. CRT and CRT-D have become increasingly important therapeutic options for patients with moderate and severe heart failure.[3] CRT with pacemaker only is often termed "CRT-P" to help distinguish it from CRT with defibrillator (CRT-D).
Indications
[edit]The key indication for CRT is left bundle branch block (LBBB) of the heart, a cardiac abnormality leading to delayed left ventricular contraction. LBBB causes a QRS prolongation of ≥120 ms on the electrocardiogram, contributing to poor left ventricular coordination and reduced systolic function, thereby reduced ejection fraction (<35%). This reduction in ejection fraction is considered heart failure.[2]
Heart failure patients are generally considered if in New York Heart Association (NYHA) class II or III heart failure. Current National Institute for Health and Care Excellence (NICE) guidelines state that CRT-D device placement is inappropriate for class IV heart failure, but placement of CRT-P devices may be appropriate in certain circumstances.[4][5][6]
| NYHA Class | ||||
|---|---|---|---|---|
| QRS interval | I | II | III | IV |
| <120 milliseconds | ICD only if high risk of sudden cardiac death | Not indicated | ||
| 120–149 milliseconds without LBBB | ICD only | ICD only | ICD only | CRT-P |
| 120–149 milliseconds with LBBB | ICD only | CRT-D | CRT-P or CRT-D | CRT-P |
| ≥150 milliseconds with or without LBBB | CRT-D | CRT-D | CRT-P or CRT-D | CRT-P |
Method
[edit]
- Atrial lead at the right appendage
- Right ventricular lead at the apex
- Left ventricular lead through the coronary sinus.[7]
CRT requires the placement of an electrical device for biventricular pacing, along with placement of (at least) two pacing leads, to facilitate stable left ventricular and right ventricular pacing. For all elements, the first stage of the process is local anaesthetic followed by incision to allow for approach from the appropriate vein. From here, the leads and device can be inserted.[1]
Right ventricular lead placement
[edit]A venipuncture is made, and a guide wire inserted into the vein, where it is guided, with use of real time X-ray imaging, through to the right ventricle. The guide wire is then used to assist in the placement of the electrode lead, which travels through the venous system into the right ventricle where the electrode is embedded.[1]
Left ventricular lead placement
[edit]This is generally performed subsequent to RV lead placement, with the RV lead providing a backup in case of accidental damage to the electric fibers of the heart, causing an asystolic event. As with the RV lead, a guide wire is first inserted, allowing for the insertion of a multi-delivery catheter. The catheter is subsequently maneuvered to the opening of the coronary sinus in the right atrium. From here a contrast media is injected, allowing the surgical team to obtain a coronary sinus phlebogram to direct the placement of the lead into the most suitable coronary vein.[1]
Once the phlebogram has been obtained, the multi-delivery catheter is used to guide in the lead, from the chosen vein of entry, into the right atrium, through the coronary sinus and into the relevant cardiac vein.[1]
Left ventricular lead placement is the most complicated and potentially hazardous element of the operation, due to the significant variability of coronary venous structure. Alterations in heart structure, fatty deposits, valves and natural variations all cause additional complications in the process of cannulation.[1] However, this risk can be reduced using AI-based[8] preoperative visualization of LV venous anatomy using computer tomography (CT) imaging.
Device placement
[edit]The device is inserted in a subcutaneous pocket created by the surgeon, the choice of left or right side of the chest wall is determined mainly by the patient's preference or location of preexisting device. The device, similar to that of a traditional pacemaker, is generally no larger than a pocket watch and has inserts for the electrode leads.[1]
Benefits
[edit]Several studies have also shown that CRT can decrease mortality, reverse left ventricular remodeling, and improve quality of life, walking distance, and peak oxygen uptake (VO2 max).[9] A 2013 study showed that CRT improved the left ventricular ejection fraction (LVEF) by an average of 10.6% 12 months after placement.[10]
Complications
[edit]Key complications include:[2]
- Dissection or perforation of coronary sinus which can in turn cause pericardial effusion
- Inability to cannulate coronary sinus (approximately 5% of patients)
- Bleeding and pocket haematoma, each of which with an incidence of less than 1%[11]
- Myocardial perforation, pneumothorax and infection, all of which have an incidence of less than 1%
Technology
[edit]Several research papers[12][13] have proposed software platforms for planning and guiding the implantation of CRT devices. This research proposes using pre-operative images to characterize tissue and left ventricle activation to identify potential target regions for deploying the CRT leads.
References
[edit]- ^ a b c d e f g "Cardiac Resynchronization Therapy Technique: Approach Considerations, Placement of Pacing Leads, Programming of Device". emedicine.medscape.com. Retrieved 17 July 2018.
- ^ a b c Goldman, Lee (2012). Goldman-Cecil Medicine. USA: Elsevier Saunders. p. 379. ISBN 978-9996096563.
- ^ Choi, Anthony J.; Thomas, Sunu S.; Singh, Jagmeet P. (2016). "Cardiac Resynchronization Therapy and Implantable Cardioverter Defibrillator Therapy in Advanced Heart Failure". Heart Failure Clinics. 12 (3): 423–436. doi:10.1016/j.hfc.2016.03.010. ISSN 1551-7136. PMID 27371518.
- ^ a b "Implantable cardioverter defibrillators and cardiac resynchronisation therapy for arrhythmias and heart failure". National Institute for Health and Care Excellence. 25 June 2014. Retrieved 22 August 2022.
- ^ Normand, C; Linde, C; Singh, J; Dickstein, K (2018). "Indications for Cardiac Resynchronization Therapy: A Comparison of the Major International Guidelines". JACC: Heart Failure. 6 (4): 308–316. doi:10.1016/j.jchf.2018.01.022. ISSN 2213-1779. PMID 29598935.
- ^ Henin, M; Ragy, H; Mannion, J; David, S; Refila, B; Boles, U (2020). "Indications of Cardiac Resynchronization in Non-Left Bundle Branch Block: Clinical Review of Available Evidence". Cardiology Research. 11 (1): 1–8. doi:10.14740/cr989. ISSN 1923-2829. PMC 7011924. PMID 32095190.
- ^ Mima, Takahiro; Baba, Shiro; Yokoo, Noritaka; Kaichi, Shinji; Doi, Takahiro; Doi, Hiraku; Heike, Toshio (2010). "Effective cardiac resynchronization therapy for an adolescent patient with dilated cardiomyopathy seven years after mitral valve replacement and septal anterior ventricular exclusion". Journal of Cardiothoracic Surgery. 5 (1): 47. doi:10.1186/1749-8090-5-47. ISSN 1749-8090. PMC 2898667. PMID 20525228.
- ^ Sinitca, Aleksandr; Chmelevsky, Mikhail; Arduino, Chiara; Zubarev, Stepan; Shirshin, Aleksandr; Dokuchaev, Arsenii; Budanova, Margarita; Khamzin, Svyatoslav; Bazhutina, Anastasia; Rud, Sergei; Rainer, Werner (26 November 2023). "A Machine Learning-Based Approach for Automatic Coronary Sinus Vein Segmentation and Anatomy Reconstruction" (PDF). 2023 Computing in Cardiology Conference (CinC). Vol. 50. doi:10.22489/CinC.2023.220. ISBN 979-8-3503-8252-5 – via IEEE Xplore.
- ^ Leyva, Francisco; Nisam, Seah; Auricchio, Angelo (2014). "20 Years of Cardiac Resynchronization Therapy". Journal of the American College of Cardiology. 64 (10). Elsevier BV: 1047–1058. doi:10.1016/j.jacc.2014.06.1178. ISSN 0735-1097. PMID 25190241.
- ^ Brambatti, M.; Guerra, F.; Matassini, M. V.; Cipolletta, L.; Barbarossa, A.; Urbinati, A.; Marchesini, M.; Capucci, A. (5 February 2013). "Cardiac resynchronization therapy improves ejection fraction and cardiac remodelling regardless of patients' age". Europace. 15 (5). Oxford University Press (OUP): 704–710. doi:10.1093/europace/eus376. ISSN 1099-5129. PMID 23385052.
- ^ Auricchio, Angelo; Gasparini, Maurizio; Linde, Cecilia; Dobreanu, Dan; Cano, Óscar; Sterlinski, Maciej; Bogale, Nigussie; Stellbrink, Christoph; Refaat, Marwan M.; Blomström-Lundqvist, Carina; Lober, Christiane (September 2019). "Sex-Related Procedural Aspects and Complications in CRT Survey II". JACC: Clinical Electrophysiology. 5 (9): 1048–1058. doi:10.1016/j.jacep.2019.06.003. PMID 31537334. S2CID 201159537.
- ^ Mountney, P.; Behar, J. M.; Toth, D.; Panayiotou, M.; Reiml, S.; Jolly, M. P.; Karim, R.; Zhang, L.; Brost, A. (November 2017). "A Planning and Guidance Platform for Cardiac Resynchronization Therapy". IEEE Transactions on Medical Imaging. 36 (11): 2366–2375. doi:10.1109/TMI.2017.2720158. ISSN 0278-0062. PMID 28678701. S2CID 3871779.
- ^ Zhou, Weihua; Hou, Xiaofeng; Piccinelli, Marina; Tang, Xiangyang; Tang, Lijun; Cao, Kejiang; Garcia, Ernest V.; Zou, Jiangang; Chen, Ji (December 2014). "3D fusion of LV venous anatomy on fluoroscopy venograms with epicardial surface on SPECT myocardial perfusion images for guiding CRT LV lead placement". JACC: Cardiovascular Imaging. 7 (12): 1239–1248. doi:10.1016/j.jcmg.2014.09.002. ISSN 1876-7591. PMID 25440593.
Cardiac resynchronization therapy
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Introduction and Background
Definition and Purpose
Cardiac resynchronization therapy (CRT) is a pacemaker-based intervention designed to deliver simultaneous or near-simultaneous electrical impulses to the right and left ventricles, thereby restoring coordinated ventricular contraction in patients exhibiting ventricular dyssynchrony.[1] This therapy addresses the inefficient pumping caused by asynchronous contractions, particularly in cases of left bundle branch block or other conduction abnormalities that delay left ventricular activation.[6] The primary purpose of CRT is to enhance cardiac output, alleviate heart failure symptoms, and mitigate adverse ventricular remodeling in individuals with reduced ejection fraction and conduction delays.[2] By optimizing the timing of ventricular depolarization, CRT improves mechanical efficiency and reduces the energy expenditure associated with dyssynchronous heartbeats.[6] At its core, CRT involves a battery-powered pulse generator implanted subcutaneously, connected to multiple leads: typically one in the right ventricle, another in the left ventricle accessed via the coronary sinus, and often a third in the right atrium for atrioventricular synchrony.[1] Devices are categorized as CRT-P, which provides pacing functions only, or CRT-D, which incorporates defibrillator capabilities to manage life-threatening arrhythmias alongside resynchronization.[2] CRT targets patients with systolic heart failure characterized by a left ventricular ejection fraction (LVEF) of ≤35% and prolonged QRS duration on electrocardiogram, indicating significant conduction delay.[1] This population generally includes those with moderate to severe symptoms despite optimized medical therapy, focusing on restoring synchrony to support overall cardiac function.[2]History
The concept of cardiac resynchronization therapy (CRT) emerged in the late 1980s and early 1990s, driven by recognition of ventricular dyssynchrony as a key contributor to heart failure progression and initial animal studies showing hemodynamic improvements from biventricular pacing.[7] At the 7th World Symposium on Cardiac Pacing in 1983, de Teresa and colleagues first described CRT principles in four patients with heart block and dyssynchrony, marking an early clinical exploration.[8] By the early 1990s, small pilot studies tested dual-chamber pacing for symptomatic relief in heart failure, but it was the late 1990s when targeted biventricular pacing experiments demonstrated potential to correct left ventricular dyssynchrony in patients with prolonged QRS durations.[9] Key clinical milestones accelerated CRT's development in the early 2000s. The MUSTIC trial, published in 2001, was the first large randomized study, enrolling 67 patients with severe heart failure and showing significant gains in quality of life, 6-minute walk distance, and peak oxygen consumption after 12 weeks of biventricular pacing compared to atrial pacing alone.[10] That same year, the U.S. Food and Drug Administration (FDA) approved the first CRT pacemaker, Medtronic's InSync device on August 28, 2001, for patients with moderate-to-severe heart failure, enabling commercial transvenous implantation.[11] The CARE-HF trial in 2005 provided stronger evidence, randomizing 813 patients to CRT or medical therapy and reporting a 37% relative reduction in the primary endpoint of death or hospitalization for heart failure over 29 months, with sustained mortality benefits in long-term follow-up. During the 2000s, CRT transitioned from experimental to guideline-directed therapy, with early integration into implantable cardioverter-defibrillators (CRT-D) following FDA approval of the first such device on June 26, 2002, allowing simultaneous arrhythmia management and resynchronization for high-risk heart failure patients.[12] Trials like MADIT-CRT (2009) and RAFT (2011) broadened applications to asymptomatic or mildly symptomatic cases with left bundle branch block, reducing heart failure events by 41% and 25%, respectively.[4] Post-2020 advancements have included conduction system pacing techniques, such as His-bundle and left bundle branch area pacing, as physiological alternatives to traditional biventricular CRT, particularly for non-responders, with studies showing comparable or superior resynchronization rates and lower complication risks. In 2025, the Heart Rhythm Society issued a Clinical Consensus Statement on conduction system pacing, reinforcing its application in CRT for improved outcomes in select patients.[13][14] By 2025, CRT adoption has grown substantially, influenced by the 2022 AHA/ACC/HFSA guidelines, which upgraded recommendations to class I for patients with ejection fraction ≤35%, QRS ≥150 ms, and left bundle branch block on optimal medical therapy, emphasizing earlier intervention.[5] Worldwide, approximately 190,000 CRT devices are implanted annually as of 2023, due to refined patient selection and technological refinements.[15]Pathophysiology and Rationale
Cardiac Dyssynchrony
Cardiac dyssynchrony refers to the uncoordinated contraction of the cardiac ventricles, resulting in suboptimal mechanical efficiency during systole.[16] It encompasses electrical dyssynchrony, characterized by delayed conduction leading to a prolonged QRS duration exceeding 120 ms on surface electrocardiogram (ECG), and mechanical dyssynchrony, which manifests as asynchronous regional wall motion.[16] The primary types include interventricular dyssynchrony, involving delayed activation between the left and right ventricles, and intraventricular dyssynchrony, marked by regional delays within the left ventricle (LV), such as between septal and lateral walls.[17] This pattern is most commonly observed in patients with left bundle branch block (LBBB), where altered conduction pathways disrupt the normal sequence of ventricular depolarization.[18] The condition often arises from conduction abnormalities, including LBBB or right ventricular pacing, which induce asynchronous electrical activation.[19] These disruptions are frequently exacerbated in the context of ischemic or non-ischemic cardiomyopathy, where underlying myocardial pathology amplifies the mechanical discordance.[20] Assessment of cardiac dyssynchrony relies on multiple imaging modalities. On ECG, it is primarily evaluated using QRS duration as a surrogate marker, calculated as the time interval from the onset of the Q wave to the end of the S wave in the lead showing the widest complex:
A QRS duration greater than 120–150 ms indicates significant electrical dyssynchrony, particularly in LBBB morphology.[21] Echocardiography provides direct visualization of mechanical dyssynchrony through signs such as septal flash—an early inward septal motion followed by rebound—and apical rocking, an abnormal apical motion due to opposing wall displacements.[22] Cardiac magnetic resonance imaging (MRI) can also quantify dyssynchrony by assessing regional strain and timing differences, offering superior tissue characterization in complex cases.[23]
The hemodynamic consequences of dyssynchrony include inefficient ventricular pumping, as asynchronous contractions reduce stroke volume and cardiac output.[24] It also promotes increased mitral regurgitation through altered papillary muscle timing and annular distortion, further impairing forward flow.[25] Over time, these effects drive progressive adverse ventricular remodeling, characterized by dilation and fibrosis, independent of other heart failure risk factors.[26] This dyssynchrony contributes to the worsening progression of heart failure by amplifying systolic dysfunction and morbidity.[26]
Role in Heart Failure
Cardiac dyssynchrony plays a central role in the progression of heart failure with reduced ejection fraction (HFrEF) by inducing asynchronous ventricular contraction, which impairs overall cardiac efficiency. This discoordinate activation leads to suboptimal timing of regional contractions, resulting in inefficient blood ejection and a substantial reduction in stroke volume, alongside an increase in end-systolic volume due to prolonged contraction in late-activated regions.[27][24] In non-ischemic HFrEF, this mechanical inefficiency further promotes regional stress, contributing to myocardial fibrosis and scarring through altered stretch and perfusion patterns in delayed segments.[27][28] Atrioventricular dyssynchrony, often due to prolonged PR intervals, can compound these effects by impairing diastolic filling and overall hemodynamics.[29] Dyssynchrony is particularly prevalent in moderate to advanced HFrEF, affecting approximately 20-30% of patients with New York Heart Association (NYHA) class II-IV symptoms, where it exacerbates systolic dysfunction and correlates with left ventricular ejection fraction (LVEF) below 35% and QRS duration exceeding 130 ms.[30] These features contribute significantly to heart failure hospitalizations, with dyssynchronous patients experiencing higher rates of acute decompensation due to worsened pump function compared to those with isolated systolic impairment.[31][32] Observational studies demonstrate that dyssynchrony independently predicts adverse outcomes, distinguishing it from other heart failure etiologies like pure systolic dysfunction by correlating with elevated mortality risks—for instance, annual mortality rates of up to approximately 20% in untreated advanced cases with marked electrical delay.[33][34] In cohorts with LVEF <35% and prolonged QRS, dyssynchrony amplifies prognostic worsening, with higher all-cause mortality and event rates observed versus synchronized counterparts.[35][36] The rationale for cardiac resynchronization therapy (CRT) in this context lies in its ability to address this reversible dyssynchronous component, restoring coordinated contraction in a manner complementary to pharmacotherapy, which primarily targets neurohormonal activation but cannot correct electrical-mechanical mismatches.[27][24] By specifically mitigating the hemodynamic penalties of dyssynchrony, CRT interrupts the vicious cycle of remodeling and decompensation unique to this subgroup of heart failure.[37]Indications and Patient Selection
Clinical Criteria
Cardiac resynchronization therapy (CRT) candidacy is determined by evidence-based criteria emphasizing patients with symptomatic heart failure and electrical dyssynchrony who remain inadequately managed despite optimal medical therapy. Core indications include New York Heart Association (NYHA) functional class II-IV symptoms, left ventricular ejection fraction (LVEF) ≤35%, and prolonged QRS duration on electrocardiogram, particularly in sinus rhythm. Specifically, a Class I recommendation applies to patients with left bundle branch block (LBBB) and QRS duration ≥150 ms (Class IIa for QRS 120-149 ms), while a Class IIa recommendation is given for non-LBBB morphology with QRS ≥150 ms.[38][5] Patients must have failed optimized guideline-directed medical therapy (GDMT), including beta-blockers, renin-angiotensin-aldosterone system inhibitors, and sodium-glucose cotransporter-2 inhibitors, typically for at least 3 months unless contraindicated. CRT is applicable to both ischemic and non-ischemic etiologies of heart failure, though non-ischemic cardiomyopathy often shows greater reverse remodeling response. In patients with dilated cardiomyopathy, a common non-ischemic etiology, CRT is recommended for those with LVEF ≤35%, LBBB morphology, and QRS duration ≥150 ms as part of guideline-directed therapy for heart failure with reduced ejection fraction.[5] Reversible causes of heart failure, such as acute ischemia or uncontrolled hypertension, must be excluded prior to implantation to ensure chronicity of the condition. In patients with atrial fibrillation, CRT may be considered (Class IIa) if atrioventricular node ablation can achieve ≥40% biventricular pacing.[5][38][39] Response predictors enhance patient selection to maximize benefits. Electrical dyssynchrony assessed by QRS morphology and duration remains the primary criterion per current guidelines; while historical echocardiographic markers of mechanical dyssynchrony (e.g., septal-to-posterior wall motion delay) showed correlations with outcomes, they are not recommended for routine selection due to limited reproducibility and trial evidence. Comorbidities like advanced renal dysfunction (e.g., estimated glomerular filtration rate <30 mL/min/1.73 m²) may diminish candidacy due to higher procedural risks and lower response rates.[5][40] In clinical trials, CRT recipients are typically aged 60-80 years, reflecting the peak incidence of advanced heart failure, with a 60-70% male predominance due to higher rates of ischemic disease in men. These demographics underscore the need for equitable access, as women often exhibit superior response profiles despite underrepresentation in studies.[41][42]Guidelines and Contraindications
Cardiac resynchronization therapy (CRT) is guided by major international recommendations from organizations such as the American Heart Association (AHA), American College of Cardiology (ACC), Heart Failure Society of America (HFSA), and European Society of Cardiology (ESC), which outline evidence-based criteria for patient selection to maximize benefits in heart failure management. The 2022 AHA/ACC/HFSA Guideline for the Management of Heart Failure provides a Class I recommendation (strong evidence, benefit >>> risk) for CRT in patients with left ventricular ejection fraction (LVEF) ≤35%, New York Heart Association (NYHA) class II, III, or ambulatory IV symptoms, left bundle branch block (LBBB) morphology, and QRS duration ≥150 ms on guideline-directed medical therapy (GDMT).[5] Similarly, the 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy recommend CRT as Class I for symptomatic heart failure (NYHA II-III) with LVEF ≤35%, LBBB, and QRS ≥150 ms despite optimal medical therapy, with Class IIa considerations for QRS 120-149 ms in LBBB or non-LBBB with QRS ≥150 ms in NYHA III-IV. The 2025 ACC/AHA Appropriate Use Criteria (AUC) for implantable cardioverter-defibrillators, CRT, and pacing update these frameworks by integrating conduction system pacing (e.g., His bundle or left bundle branch area pacing) scenarios, rating traditional biventricular CRT as appropriate (score 7-9) for LVEF ≤35% with LBBB and QRS ≥150 ms across NYHA II-III/ambulatory IV, while noting emerging roles for physiologic pacing alternatives in select cases.[43] Guideline recommendations are stratified by class to reflect the strength of evidence and clinical applicability: Class I indicates procedures that are effective, with benefit far outweighing risk; Class IIa suggests reasonable use with moderate benefit over risk; Class IIb implies possible benefit but with greater uncertainty; and Class III denotes no benefit or potential harm, where CRT is not recommended.[5] For instance, CRT is Class III (no benefit) for QRS <120 ms regardless of morphology, based on trials like RE-THIN-Q and ECHO-CRT showing lack of efficacy.[5] The choice between CRT with pacemaker (CRT-P) and CRT with defibrillator (CRT-D) depends on the need for secondary prevention of sudden cardiac death, with CRT-D preferred in patients with LVEF ≤30-35% and ischemic cardiomyopathy (Class I), while CRT-P suffices for non-ischemic cases without ventricular arrhythmias (Class IIa).[43]| Scenario | CRT-P Appropriateness (Score 1-9) | CRT-D Appropriateness (Score 1-9) | Key Criteria |
|---|---|---|---|
| LVEF ≤35%, NYHA II-III, LBBB, QRS ≥150 ms, sinus rhythm | 7-9 (Appropriate) | 7-9 (Appropriate) | On GDMT; ischemic or non-ischemic etiology[43] |
| LVEF ≤35%, NYHA III-IV, non-LBBB, QRS 130-149 ms, sinus rhythm | 4-6 (May be appropriate) | 4-6 (May be appropriate) | Limited benefit; consider alternatives like conduction system pacing[43] |
| LVEF ≤35%, NYHA II, LBBB, QRS 120-149 ms, sinus rhythm | 4-6 (May be appropriate) | 7-9 (Appropriate if ICD indicated) | Expected survival >1 year; GDMT optimized[5] |
| LVEF 36-50%, AV block, NYHA II-IV | 7-9 (Appropriate) | 4-6 (May be appropriate) | For pacing indication; BLOCK-HF trial support[5] |
CRT Upgrades in High RV Pacing and Low Ejection Fraction
In patients with existing pacemakers or ICDs who develop heart failure symptoms, reduced ejection fraction (LVEF ≤35%), wide paced QRS (≥150 ms), and high right ventricular (RV) pacing burden (≥20%), upgrading to CRT-D (adding a left ventricular lead for biventricular pacing) can provide significant benefits beyond continuing ICD therapy alone. The BUDAPEST CRT Upgrade trial (2023), a randomized multicenter study of 360 patients (mean age 73, mean LVEF 25%, mean RV pacing ~86%), demonstrated that CRT-D upgrade markedly reduced the primary composite endpoint of heart failure hospitalization, all-cause mortality, or lack of left ventricular reverse remodeling (32.4% vs 78.9% at ~1 year; adjusted OR 0.11, p<0.001).[45] Key secondary benefits included:- Lower composite of HF hospitalization and mortality (adjusted HR 0.28, p<0.001).
- Reverse remodeling: LVEF increase of +9.76% and LV end-diastolic volume reduction of -39 mL at 12 months (both p<0.001).
- Fewer serious ventricular arrhythmias.
Implantation Procedure
Device Types
Cardiac resynchronization therapy (CRT) devices are implantable systems designed to deliver biventricular pacing to restore synchronous ventricular contraction in patients with heart failure and dyssynchrony. The primary types include CRT pacemakers (CRT-P) and CRT defibrillators (CRT-D), which differ in their capabilities, size, and indications based on patient arrhythmia risk.[3][1] CRT-P devices provide resynchronization through pacing alone, without defibrillation capability, making them suitable for heart failure patients who require synchronization but have low risk of life-threatening ventricular arrhythmias. These devices feature a smaller pulse generator, typically with a volume of 20-40 cc, powered by lithium-based batteries that offer a projected longevity of 7-12 years depending on pacing demands and device model. The pulse generator includes algorithms for optimizing atrioventricular (AV) and interventricular (VV) delays to enhance resynchronization efficiency.[3][29][47] In contrast, CRT-D devices integrate resynchronization pacing with implantable cardioverter-defibrillator (ICD) functionality to detect and terminate ventricular tachycardia or fibrillation via shocks or antitachycardia pacing. These are indicated for primary prevention in patients with ischemic heart failure and reduced ejection fraction (≤35%), particularly those with prior ventricular arrhythmias or high sudden cardiac death risk. CRT-D pulse generators are larger, typically 40-70 cc, to accommodate high-voltage capacitors for shock delivery, and their battery life is shorter, generally 5-8 years, due to the additional energy demands. Like CRT-P, they incorporate AV/VV optimization algorithms within the pulse generator.[3][48][29] Device selection is guided by clinical guidelines, prioritizing CRT-D for patients with elevated arrhythmia risk, such as those with New York Heart Association class II-IV symptoms, left bundle branch block, and QRS duration ≥150 ms, while CRT-P suffices for lower-risk profiles. Emerging options include leadless CRT systems, such as ultrasound-based endocardial pacing (e.g., WiSE-CRT), which received FDA approval in April 2025 and is available in limited release as of late 2025 but remains non-standard and reserved for cases where conventional implantation fails; investigational hybrid approaches combining subcutaneous or extravascular ICDs with pacing for CRT, developed since the early 2020s, aim to reduce lead-related complications but are not yet widely available. Additionally, as of 2025, devices supporting left bundle branch area pacing (LBBAP) integrated with CRT, such as BIOTRONIK's Acticor/Rivacor Sky CRT-D, offer alternatives for improved implantation success and stability in select patients.[48][29][49][50][51][52]Lead Placement
In cardiac resynchronization therapy (CRT), the right ventricular (RV) lead is typically positioned at the RV apex or along the interventricular septum via transvenous access through the subclavian or axillary vein, where it serves to sense and pace the RV myocardium.[38] This placement achieves high procedural success rates exceeding 95%, with active fixation mechanisms enhancing stability and reducing dislodgement risks, particularly when septal positioning is selected to promote more physiological activation patterns.[38] Fluoroscopic guidance in multiple projections ensures precise positioning while minimizing complications such as perforation, which occurs in 0.6-5.2% of cases and is higher with apical placement (odds ratio 3.37).[38] The left ventricular (LV) lead is advanced retrogradely through the coronary sinus (CS) ostium into a posterolateral or lateral cardiac vein tributary, targeting the region of latest mechanical or electrical activation to optimize resynchronization.[53] Quadripolar leads are preferred for this placement due to their ability to allow multipolar pacing configurations, which help avoid phrenic nerve stimulation affecting 5-10% of implants and facilitate adjustments for suboptimal initial positioning.[38] Success rates for CS cannulation and LV lead deployment range from 90-95%, though challenges such as CS dissection (0.7-2.1% incidence) or venous tortuosity contribute to a 10-15% failure rate, often necessitating alternative approaches like His bundle pacing.[38][54] An atrial lead is optionally placed in the right atrial appendage for patients in sinus rhythm to maintain atrioventricular (AV) synchrony, accessed transvenously via the cephalic or axillary vein with active or passive fixation.[53] This positioning yields success rates above 95-98%, with the cephalic vein approach reducing complications compared to subclavian access, though lateral wall placement carries a higher risk of phrenic nerve capture.[38] These leads connect to biventricular pacemakers or implantable cardioverter-defibrillators to deliver synchronized pacing.[53] Fluoroscopy remains the standard imaging modality for real-time guidance during lead placement, enabling multi-angle visualization to confirm electrode contact and electrical thresholds.[38] Pre-procedural computed tomography (CT) angiography is increasingly utilized for detailed coronary venous mapping, with AI-enhanced algorithms introduced since 2023 improving segmentation accuracy and predicting optimal LV lead sites by integrating anatomical and scar data from prior imaging.[55][56] This targeted approach enhances procedural efficiency and response rates in complex anatomies.[57]Advanced Techniques for LV Lead Stabilization
In challenging cases where the LV lead experiences repeated intraoperative dislocations (≥2 times) despite trials of different lead models or configurations from at least two manufacturers, the loop technique serves as a safe and effective alternative to aborting the procedure or resorting to more invasive methods like epicardial leads. The loop technique is performed transvenously under fluoroscopic guidance without additional incisions or hardware:- Advance a PTCA-style guidewire into the target coronary vein (e.g., lateral or posterolateral branch).
- Position the LV lead tip distally in the desired location.
- Partially retract the guidewire while keeping the lead tip stable.
- Re-advance the guidewire to form a deliberate loop or curve in the lead body, anchoring it against vessel walls, often using an adjacent branch for mechanical stability.
- Test pacing parameters (threshold, impedance, sensing) and confirm no phrenic stimulation.
- Remove the guidewire after stability is achieved and confirm final position.
Surgical Technique
The implantation of a cardiac resynchronization therapy (CRT) device is a minimally invasive procedure performed in an electrophysiology or catheterization laboratory under continuous fluoroscopic guidance to ensure precise lead positioning.[1] Patients undergo preoperative preparation, including fasting for at least 6 hours, evaluation of coagulation status, and adjustment of anticoagulants such as withholding rivaroxaban for 24-48 hours to minimize bleeding risks.[60] Antibiotic prophylaxis with intravenous cefazolin (typically 2 g) is administered 30-60 minutes prior to incision to prevent device-related infections.[60] Local anesthesia, using 2% lidocaine, is applied at the venous access and device pocket sites, supplemented by conscious sedation with agents like midazolam (1-6 mg) or fentanyl (25-100 μg) to maintain patient comfort without general anesthesia.[61][60] Venous access is established using a sheath (9-11 Fr depending on device type) inserted into the cephalic, axillary, or subclavian vein, with the axillary approach preferred under fluoroscopic guidance at the confluence of the second and third ribs to reduce complications such as pneumothorax (incidence approximately 0.66%).[1][60] A venogram with 10-15 mL of contrast may be performed if access is challenging, confirming vein patency.[61] A subcutaneous or submuscular pocket is created in the left prepectoral region for the pulse generator, with subpectoral placement favored for CRT-defibrillators to lower erosion risk; meticulous hemostasis is ensured using electrocautery.[60] Leads are inserted sequentially, starting with the right ventricular lead advanced through the tricuspid valve to the apex or septum, followed by the right atrial lead (if required) into the appendage, and finally the left ventricular lead via coronary sinus cannulation using specialized catheters or guidewires for ostium localization.[61][1] Fluoroscopy guides advancement, with coronary sinus venography (using balloon occlusion and contrast) to visualize posterolateral branches for optimal left ventricular lead deployment in the proximal or mid-segment.[61] Each lead undergoes intra-procedural testing: pacing capture thresholds are verified to be below 2 V at 0.5 ms pulse width, sensing amplitudes exceed 5 mV for R-waves, and absence of phrenic nerve or diaphragmatic stimulation is confirmed by adjusting vectors if needed.[29][61] Leads are secured to the pectoralis fascia, the sheath is split to avoid dislodgement, and the device is connected for initial programming to optimize atrioventricular and interventricular delays.[61] The incision is closed in layers with absorbable sutures, and a sterile dressing is applied.[1] The procedure typically lasts 1-3 hours, depending on complexity, and may allow for same-day discharge in uncomplicated cases with adequate post-procedure monitoring.[62] The standard transvenous endocardial approach via the coronary sinus is used in over 95% of cases; epicardial placement, involving limited thoracotomy, is reserved for the rare instances (about 5%) where transvenous access fails due to anatomical issues.[61] Emerging trends include robotic assistance to enhance precision in lead manipulation and reduce radiation exposure, as demonstrated in initial case reports of minimally invasive robotic CRT-defibrillator implants without sternotomy.[63]Mechanism of Action
Electrical Resynchronization
Cardiac resynchronization therapy (CRT) achieves electrical resynchronization primarily through biventricular pacing, which coordinates the activation of the left and right ventricles to correct conduction delays associated with ventricular dyssynchrony. In patients with prolonged QRS duration, typically due to left bundle branch block, biventricular pacing restores synchronous ventricular activation by delivering simultaneous or sequentially timed impulses to both ventricles via leads in the right ventricle and a coronary sinus branch targeting the left ventricle. This intervention shortens the QRS duration by an average of 20-40 ms, as observed in clinical studies evaluating acute hemodynamic responses, thereby promoting more uniform electrical wavefront propagation across the myocardium.[64] Optimization of the atrioventricular (AV) delay is crucial to ensure effective atrial contribution to ventricular filling without allowing intrinsic conduction to interfere. Programmed AV delays of 80-120 ms are commonly targeted, balancing preload enhancement with prevention of fusion beats that could undermine resynchronization; this range has been validated in echocardiographic optimization protocols where shorter delays maximize left ventricular filling time. Pacing is typically delivered in DDD-biventricular (BiV) mode for patients in sinus rhythm, enabling atrial tracking and dual-chamber support while providing biventricular stimulation. Advanced algorithms, such as SmartDelay, automate adjustments by dynamically optimizing the AV delay and interventricular (VV) interval, often applying a left ventricular offset of 0-80 ms to fine-tune synchrony based on intrinsic conduction intervals.[65][66] Effective electrical resynchronization is quantified by the degree of QRS narrowing, calculated as , where is the baseline duration and is the paced duration; a reduction of ≥20 ms correlates with improved outcomes.[67] This narrowing reflects reduced intraventricular conduction delays and has been associated with decreased dispersion of repolarization, as evidenced by lower rates of new-onset ventricular arrhythmias in implantable cardioverter-defibrillator interrogations following CRT activation. By minimizing heterogeneous repolarization times, CRT lowers the substrate for reentrant arrhythmias, enhancing electrical stability without increasing proarrhythmic risk in responsive patients.[68]Mechanical and Hemodynamic Effects
Cardiac resynchronization therapy (CRT) reverses mechanical dyssynchrony, particularly septal dyssynchrony, by restoring coordinated contraction patterns in the left ventricle (LV).[69] This reversal is evident through echocardiographic measures such as septal flash and apical rocking, which diminish post-implantation, leading to improved interventricular and intraventricular synchrony.[70] Studies using speckle-tracking echocardiography demonstrate that CRT significantly enhances LV twist mechanics, with peak twist increasing immediately after pacing due to better electromechanical activation.[71] Similarly, radial and circumferential strain patterns normalize, reducing regional disparities in shortening and stretch, which contributes to overall mechanical efficiency.[72] Hemodynamically, CRT augments LV contractility and output by increasing the maximum rate of LV pressure rise (dP/dtmax) by approximately 18% at rest, reflecting enhanced systolic performance.[73] Stroke volume rises by 10-20% in responders, driven by optimized ejection without excessive metabolic demand.[74] Additionally, pulmonary capillary wedge pressure decreases substantially, from baseline levels around 16 mmHg to about 10 mmHg chronically, alleviating left-sided filling pressures and reducing pulmonary congestion.[75] A key long-term mechanical benefit of CRT is reverse remodeling, which halts and reverses LV dilation by decreasing end-systolic volume. This is quantified as the change in LV end-systatic volume (ΔLVESV), defined by the equation:
where a negative value indicates improvement, and a reduction of ≥15% at 6 months is a standard threshold for significant reverse remodeling associated with better outcomes.[76]
The effects of CRT differ temporally, with acute hemodynamic improvements—such as immediate boosts in stroke volume and dP/dtmax—occurring within hours of activation due to resynchronized contraction.[72] Chronically, over 6-12 months, CRT promotes structural changes including fibrosis regression in the LV lateral wall, evidenced by reduced expression of fibrogenic markers like p38 MAPK and TNF-α, further supporting sustained mechanical recovery.[72]