When activation is delayed, contraction becomes inefficient.
In left bundle branch block (LBBB), activation reaches much of the left ventricle late. Early-activated muscle may shorten while late-activated regions are still stretching. The ventricle can expend energy moving its own walls rather than ejecting blood efficiently. CRT aims to improve the coordination of activation and contraction.
Interactive activation schematic
Compare where and when activation occurs. The sequence plays automatically. Choose a pacing pattern, pause, or scrub through the sequence. The highlighted regions represent illustrative activation windows, not contraction or measured electrical propagation.
White solid: pacing leadBlue dashed: native conductionOrange ×: illustrative block
Activating nowNot highlighted
EarlierLater
Qualitative teaching model. Lead routes and the site of block are simplified; atrial leads are omitted. Dashed white lead segments pass behind the chambers in coronary veins, not through the left atrium. Not an implantation guide or validated electrophysiological or haemodynamic simulation.
How conventional CRT works
A generator coordinates right ventricular pacing with stimulation of the LV, usually through a lead in a coronary sinus tributary on the epicardial surface. An atrial lead is often used in sinus rhythm. The LV lead is not normally placed through the mitral valve into the LV cavity.
Lead location and programmed timing influence the resulting wavefronts. CRT does not simply increase the pulse rate, and simultaneous pacing does not guarantee perfect physiological activation.
What improvement can look like
Better coordination can improve effective stroke volume and reduce functional mitral regurgitation. Over months, some patients show reverse remodelling, smaller LV volumes and improved EF, alongside fewer symptoms and HF events.
Response varies with conduction pattern, scar, lead placement, rhythm and delivery of effective pacing. A patient may improve clinically without crossing an arbitrary EF threshold.
Who is most likely to benefit?
The strongest 2026 ESC indication is symptomatic HF despite optimal foundational treatment, LVEF ≤35%, sinus rhythm, LBBB and QRS ≥150 ms (I, A). In sinus rhythm with EF ≤35%, LBBB at 130–149 ms or non-LBBB at ≥150 ms has weaker support (IIa); non-LBBB at 130–149 ms has weaker support still (IIb). QRS <130 ms is not an indication without a separate high-grade AV-block pacing requirement.
Read ECG morphology and duration alongside symptoms, EF, rhythm, scar and the patient’s goals. An echo appearance of dyssynchrony alone does not replace electrical selection criteria.
Existing pacing, AF and timing of referral
A high burden of conventional RV pacing can itself contribute to LV dysfunction. Worsening HF with EF ≤35% and substantial RV pacing should prompt consideration of an upgrade. Patients requiring pacing for high-grade AV block need a separate pacing strategy assessment.
In AF, conducted beats and fusion can prevent effective biventricular capture. Assess actual effective pacing, not just a reassuring device counter; rate/rhythm treatment and sometimes AV-junction ablation are needed after specialist review.
Optimise medicines and reassess EF, but do not let the referral disappear during prolonged titration. The 2026 guidance permits parallel treatment initiation and CRT planning in selected symptomatic patients with EF ≤35%, LBBB and QRS ≥150 ms; reassess EF before implantation.
CRT-P or CRT-D?
CRT-P provides resynchronisation pacing. CRT-D adds an implantable defibrillator. The decision to add defibrillation requires its own assessment of ventricular arrhythmia risk, competing mortality, comorbidity and patient preference; the need for CRT does not automatically settle it.
Implantation begins follow-up
Discuss infection, bleeding, pneumothorax, lead displacement or failure, and phrenic nerve stimulation. Follow-up checks wound healing, capture thresholds, effective pacing, arrhythmias, symptoms and remodelling. Continue HF medicines and coordinate device-clinic and community HF reviews.
CSP belongs alongside CRT because both address activation, but the lead targets differ. The usual abbreviation is CSP. Conventional biventricular CRT stimulates ventricular muscle from different sites; CSP aims to engage the His–Purkinje system for rapid ventricular activation.
His-bundle pacing
A lead targets the His bundle near the atrioventricular conduction axis. It can recruit native conduction and sometimes correct bundle-branch block. Implantation can be technically demanding; capture thresholds, sensing and conduction disease beyond the pacing site can limit success.
Left bundle branch area pacing
A lead is advanced from the RV side into the interventricular septum towards the left conduction system. This can achieve rapid LV activation, often with favourable electrical parameters. Demonstrating conduction-system capture matters: septal myocardial pacing and confirmed left bundle capture are not synonymous.
Promising does not mean interchangeable evidence
The 2025 ESC pacing consensus supports selected uses of CSP. The 2026 HF guideline remains cautious about replacing conventional CRT routinely in HFrEF because long-term, patient-centred randomised evidence is insufficient. Specialist options may include CSP when a coronary-sinus lead cannot be placed or conventional pacing would be undesirable. Choice depends on indication, anatomy, operator experience and evidence.
Specific issues include septal perforation, lead positioning and capture verification, durability, and future lead management. A narrower paced QRS is a useful electrical observation, not by itself proof of superior long-term clinical outcomes.
Detect a changing trend. Then make a clinical decision.
FIRE1 / NORM — investigational
Under clinical investigation and not routinely available. The manufacturer states that the system is not approved for clinical use in any geography. This section describes research, not a standard-care implantation recommendation. Status checked 30 October 2026.
IVC monitoring aims to identify changes in venous congestion between appointments. It is a different task from resynchronisation or defibrillation: the sensor does not pace the heart or remove fluid.
What does FIRE1 measure?
A passive implanted sensor tracks IVC cross-sectional area and its respiratory variation. An external abdominal belt interrogates the sensor and transfers readings for review. Changes in sensor geometry alter its electrical resonance, allowing the system to infer changes in IVC dimensions.
This is not a direct measurement of pulmonary artery pressure, PCWP, cardiac output or total body water. IVC behaviour also depends on respiration, right-heart function and venous mechanics; trends require context.
What is the proposed care pathway?
Repeated home readings → a change from the patient’s baseline → clinical assessment → a treatment decision → reassessment. The intended benefit is earlier recognition and management of congestion, potentially before substantial symptoms or weight change.
That pathway needs patient engagement, dependable data transmission, staff to review trends and clear escalation arrangements. Remote monitoring moves work into a different setting; it does not eliminate community nursing or specialist workload.
What have the trials established so far?
FUTURE-HF was an early, single-arm feasibility study: 50 patients received implants, with successful technical performance and no device/procedure-related serious complications reported at the primary assessment. The study could assess feasibility and early safety, but could not prove an admission or survival benefit.
A 2026 pooled FUTURE-HF/FUTURE-HFII report followed 65 patients for 12 months. It reported strong agreement with CT-derived IVC area, high adherence and encouraging changes in congestion and clinical outcomes. Comparisons of events before and after implantation lack a randomised concurrent control: closer follow-up, medication changes and regression to the mean can contribute.
Randomised evaluation is needed to establish whether sensor-guided care improves outcomes beyond good usual care, and to define durability, complications, workload and cost-effectiveness. Small studies cannot exclude uncommon device risks.
Prevent an arrhythmic death. Assess the whole prognosis.
An ICD detects and treats dangerous ventricular rhythms. It can terminate ventricular tachycardia (VT) or ventricular fibrillation (VF), but does not directly reverse pump failure. An ICD alone does not provide the resynchronisation benefit of CRT.
Detection and treatment
The device senses intracardiac or subcutaneous electrical signals. Programmed rate zones, duration criteria and discrimination algorithms determine whether therapy is delivered. Selected VT can be terminated by antitachycardia pacing (ATP): a burst of paced beats intended to interrupt the circuit. Persistent rapid VT or VF may require a high-energy shock.
Not every fast rhythm needs a shock. Programming aims to avoid treatment of self-terminating arrhythmias, AF or oversensing while reliably treating dangerous rhythms.
The system must match the need
A transvenous ICD can provide ATP and bradycardia pacing. A conventional subcutaneous ICD avoids an intracardiac lead but does not provide chronic bradycardia pacing, CRT or conventional ATP. A CRT-D combines resynchronisation and defibrillation.
Consider anticipated pacing needs, vascular access, infection and lead risks, anatomy and patient preference. These are distinct trade-offs, not a simple ranking of devices.
Primary or secondary prevention?
Secondary prevention
Consider after survival of haemodynamically unstable VT/VF, once reversible causes have been assessed. The 2026 ESC recommendation requires anticipated survival beyond one year with good functional status; arrhythmias within 48 hours of MI need separate assessment. EF ≤35% is not a universal prerequisite for secondary prevention.
Primary prevention in systolic HF
For symptomatic NYHA II–III HF with EF ≤35% despite at least three months of optimal treatment, the 2026 ESC recommendation is stronger in ischaemic disease (I, B1) than non-ischaemic disease (IIa, B1). Expected meaningful survival exceeds one year. Routine primary-prevention implantation within 40 days of MI is not recommended.
EF is a starting point, not a complete arrhythmic-risk model. Scar, genotype, aetiology and previous rhythm events can change assessment; inherited cardiomyopathies require their own disease-specific pathways. Reassess after treatment and recovery rather than applying one threshold to every patient.
Discuss benefit, burden and future decisions
Explain appropriate and inappropriate therapies, painful shocks, anxiety, infection and lead complications. Consider competing risks from progressive HF, frailty and other illnesses. Device follow-up and remote monitoring require continuing engagement. Generator replacement is an opportunity to reassess goals and benefit, not an automatic repeat procedure.
What happens after a shock—and near the end of life?
Review symptoms, the stored electrograms, appropriateness of therapy and possible triggers such as ischaemia, electrolyte disturbance or worsening HF. Recurrent shocks or ongoing symptoms require urgent assessment. Medicines, reprogramming or ablation may reduce further arrhythmias and shocks.
Discuss possible future deactivation early and revisit it as goals change. Shock therapy can be disabled while needed bradycardia pacing or CRT continues. This should be an informed, documented discussion with the patient and appropriate clinical team.
Mechanical support buys time or provides longer-term assistance when native cardiac output is inadequate. Selection asks three questions: what is failing, what may recover, and what is the plan if recovery does not occur? Early advanced-HF or shock-team referral matters before irreversible organ injury develops.
Explore the supported circuit
Conceptual blood-flow pathways, not a cannulation guide or a validated haemodynamic simulation. Native circulation remains present to varying degrees.
LVAD: assist the left ventricle
Where does blood go?
A durable LVAD draws blood from the LV through an inflow cannula, usually at the apex, and pumps it through an outflow graft into the aorta. This can unload the LV and augment systemic flow. It does not replace the heart and has no oxygenator.
The RV must still deliver blood through the lungs to fill the LV. Severe RV dysfunction can therefore limit isolated LVAD support. Pump flow also depends on filling and the pressure against which the pump works; speed is not a simple independent cardiac-output dial.
When might it be used?
Consider specialist assessment for advanced HF despite medical and device treatment, recurrent admissions, inotrope dependence, severe exercise limitation or worsening end-organ function. Assess RV function, valve disease, renal/hepatic reversibility, infection, frailty and the ability to manage the device.
Possible intentions include bridge to transplant, bridge to candidacy, occasionally bridge to recovery, or longer-term destination therapy. Eligibility and access depend on the specialist programme and local commissioning; not every patient with a very low EF is suitable.
Living with an LVAD: support is a continuing treatment
The implanted pump connects through a driveline exiting the skin to an external controller and power supply. Education includes driveline care, power management, alarms and an emergency contact plan. Rehabilitation, nutrition, carers and psychological support are integral.
Important complications include driveline or deeper infection, bleeding (including gastrointestinal bleeding), stroke, pump thrombosis, haemolysis, RV failure and aortic regurgitation. Continuous-flow support can produce little palpable pulse; assess perfusion and use the centre’s device-specific assessment protocol.
Antithrombotic treatment is device- and patient-specific and usually includes vitamin K antagonist anticoagulation. Do not assume aspirin is mandatory for every device: ARIES-HM3 supports aspirin avoidance with continued VKA in HeartMate 3 recipients, reducing bleeding without increased thromboembolic events. Follow the implanting centre’s regimen.
VA-ECMO: temporary cardiopulmonary support
Drain vein → pump → oxygenator → artery
Venoarterial ECMO drains systemic venous blood, pumps it through a membrane oxygenator and returns it to the arterial circulation. It can support systemic perfusion and gas exchange in selected patients with refractory cardiogenic shock. It is intensive-care rescue support with frequent reassessment, not a durable community device.
Veno-venous ECMO returns oxygenated blood to the venous circulation. It supports gas exchange but relies on the native heart to drive systemic flow, so VV-ECMO alone does not provide circulatory support for isolated pump failure.
Perfusion can improve while LV loading worsens
Peripheral VA-ECMO returns blood retrogradely into the aorta and can increase LV afterload. A severely weak LV may fail to open the aortic valve, leading to distension, pulmonary oedema and intracardiac stasis. VA-ECMO does not automatically unload the LV.
Monitor native ejection, aortic-valve opening and LV size alongside perfusion. Selected patients need an additional unloading strategy. If recovering cardiac ejection delivers poorly oxygenated blood from diseased lungs, differential upper-body hypoxaemia can occur.
A bridge must lead somewhere
Set a plan for recovery, a decision after neurological/end-organ assessment, transition to durable support, or transplant assessment. Revisit it daily with the specialist team and family. Bleeding, thrombosis, stroke, infection, haemolysis and limb ischaemia can complicate ECMO. A circuit can maintain flow without reversing irreversible disease.
Neither device substitutes for treating the cause—for example revascularisation, arrhythmia treatment or management of myocarditis. The team must balance the likelihood of recovery or a viable next treatment against the burdens of continued support.