CardiologyTier 2Topic

Cardiac resynchronisation therapy (know indications for)

What CRT is

Pacing both ventricles to correct electrical dyssynchrony in HFrEF
Hardware: RA lead + RV lead + LV lead via the coronary sinus into a lateral/posterolateral epicardial vein
- CRT-P (pacemaker) vs CRT-D (with defibrillator)
The only pacing therapy that improves mortality in heart failure
Corrects the electrical substrate; it is not a substitute for GDMT

LBBB -> septum activates early, lateral wall late
-> Interventricular (RV before LV) and intraventricular (septum vs lateral wall) dyssynchrony
-> septal "flash"/bounce: septum contracts against a relaxed lateral wall, wasted work
-> effective dec stroke volume, inc mitral regurgitation (papillary muscle dyssynchrony), inc wall stress
-> inc myocardial O2 consumption for less output

Pre-excites the late-activating lateral wall -> coordinated contraction
-> inc dP/dt, inc SV, dec functional MR, dec LVEDV
-> reverse remodelling over 3-6 months (the substrate of the mortality benefit)
Reverse remodelling, not acute haemodynamics, predicts outcome

Selecting patients - it is an ECG decision, not an echo one

QRS duration and morphology on 12-lead ECG
LVEF by TTE (or CMR/MUGA if poor windows)
NYHA class on optimal medical therapy for >=3 months
Echo dyssynchrony indices (TDI, speckle tracking) were tested in PROSPECT and failed to predict response - do not select on them

Better response	Worse response
True LBBB morphology	RBBB or non-specific IVCD
QRS >=150 ms	QRS 130-149 ms, and no benefit <130 ms
Non-ischaemic aetiology	Extensive posterolateral scar (LV lead over scar)
Female sex	Male
Sinus rhythm	AF with incomplete biventricular capture
Less LV dilatation	Severe dilatation, advanced disease

Indications - LVEF <=35%, symptomatic, on optimal GDMT >=3 months, expected survival >1 yr

QRS	Morphology	Class
>=150 ms	LBBB	I
>=150 ms	non-LBBB	IIa
130-149 ms	LBBB	I (IIa in some documents)
130-149 ms	non-LBBB	IIb
<130 ms	any	III - harm (EchoCRT: inc mortality)

HFrEF requiring ventricular pacing for high-grade AV block -> CRT rather than RV pacing (includes AF)
Upgrade from pacemaker/ICD to CRT if EF <=35% with high RV pacing burden and worsening HF

CRT is reasonable if QRS criteria met, but requires near-100% biventricular capture
If capture inadequate: AV node ablation ("pace and ablate")
- APAF-CRT: dec mortality vs rate control in narrow-QRS AF with HF
Rate-control drugs alone rarely achieve adequate capture

CRT-D if the patient also meets primary-prevention ICD criteria (ischaemic aetiology, younger, good survival expectancy)
CRT-P preferred in: non-ischaemic aetiology with good expected reverse remodelling, elderly, frailty, significant comorbidity
- DANISH: no mortality benefit from ICD in non-ischaemic cardiomyopathy

Conduction system pacing (CSP) - where it sits in 2026

Left bundle branch area pacing (LBBAP) - lead screwed deep into interventricular septum to capture the conduction system
LEFT-BUNDLE-CRT (2026): LBBAP did NOT meet non-inferiority vs biventricular CRT in typical LBBB, though both achieved high response rates
LECART: fewer device-related reinterventions with LBBAP
-> BiV pacing remains the default CRT strategy in LBBB; LBBAP is the preferred CSP option when the coronary sinus lead fails, the anatomy is unsuitable, or an AV-block patient needs physiological pacing
His-bundle pacing: fallback if LBBAP fails (high thresholds, lead revision rate)

1. Confirm BiV pacing >98% - ectopy, AF, fusion, loss of capture

2. Reprogram AV/VV delays (echo-guided or device algorithm)

3. Reposition or add an LV lead (avoid apical position and scar)

4. Re-check GDMT is truly maximal

5. Consider CSP upgrade, or advanced HF referral (transplant/LVAD)

Trial evidence
- CARE-HF - CRT-P vs medical therapy: dec mortality
- COMPANION - CRT-P and CRT-D vs medical therapy: dec death/hospitalisation
- MADIT-CRT and REVERSE - mild symptoms (NYHA I-II): dec HF events, reverse remodelling; benefit confined to QRS >=150 ms and LBBB
- EchoCRT - narrow QRS with echo dyssynchrony: harm
- BLOCK-HF - CRT better than RV pacing when pacing needed with EF <=50%
- APAF-CRT - AV node ablation + CRT in AF: dec mortality
~60% gain a modest but sustained (6-12 months or longer) symptomatic improvement
Association with AF: CRT reduces AF burden through reverse remodelling, but AF undermines capture

Reverse remodelling peaks at 3-6 months; reassess EF before deciding on non-response
Super-responders (~10%): EF normalises
- This is HFimpEF - continue all GDMT and the device; withdrawal causes relapse (TRED-HF principle)
Non-responders: worse prognosis than never having met criteria

Implant: coronary sinus dissection/perforation, tamponade, pneumothorax, LV lead failure to place (~5%)
Phrenic nerve stimulation - diaphragmatic twitching; reprogramme the pacing vector or reposition
Lead dislodgement (LV lead the most prone), infection (device pocket infection -> full system extraction)
Loss of biventricular capture - the commonest reversible cause of deterioration
Inappropriate ICD shocks (CRT-D)
Device-related: generator change every 5-8 yr, venous stenosis/occlusion
Driving restrictions apply after implant and after any ICD therapy

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