CRT-D Overview

A CRT-D is a CRT system with a defibrillator. The generator can sense, pace, run ATP, deliver shocks, and pace the left ventricle to resynchronize a dyssynchronous failing heart. It’s the appropriate device when the patient meets both resynchronization criteria (HFrEF + LBBB or wide QRS with appropriate substrate) and defibrillator criteria (primary or secondary prevention).

How the system works

• Three leads: RA in the appendage, RV ICD lead in the apex or septum, LV lead in a lateral or posterolateral coronary vein via the coronary sinus.
• Pacing the LV and RV simultaneously (or with a programmable offset) narrows the QRS and improves mechanical synchrony.
• The ICD function operates the same as a transvenous ICD — VT/VF zones with ATP and shock.
• Modern LV leads are quadripolar — four electrodes along the LV lead provide multiple programmable vectors to manage phrenic capture and threshold.

Types / Variants

CRT-D vs CRT-P

Platforms

• Medtronic Cobalt HF / Crome HF with Attain Performa quadripolar LV lead.
• Abbott Quadra Assura / Quadra Allure with Quartet quadripolar LV lead.
• Boston Scientific Resonate / Inogen / Vigilant with Acuity X4 quadripolar LV lead.

Indications & candidate selection

Strongest indication:

• LVEF <=35%, NYHA II-IV on optimal GDMT.
• Sinus rhythm with LBBB and QRS >=150 ms — the highest-response substrate.
• Life expectancy >1 year with reasonable functional status.

Reasonable in selected patients:

• LBBB with QRS 130-149 ms.
• Non-LBBB wide QRS (response is less predictable).
• High RV pacing burden expected (>40%) in HFrEF — CRT prevents pacing-induced cardiomyopathy.
• AF with planned AV node ablation to ensure 100% BiV pacing.

Less likely to respond:

• QRS <130 ms.
• Extensive posterolateral scar on imaging — the LV lead pacing site sits in dead tissue.
• Severe RV dysfunction independent of LV.

Key programming considerations

• BiV pacing percentage: target >=90% — ideally 95-100%. Below 90%, benefit collapses.
• AV delay: optimise to maximise LV preload without truncation; echo-guided in some centers, algorithm-driven in most.
• VV offset: programmable timing between LV and RV pacing; some platforms auto-optimise (e.g., AdaptivCRT, SyncAV).
• LV vector: choose the vector with adequate capture threshold and no phrenic capture; use the quadripolar lead to avoid revision.
• MultiPoint Pacing: when available, pace from two LV electrodes per beat to broaden the activation wavefront — consider in non-responders.
• ICD zone programming: same as TV-ICD — long detection times, SVT discriminators on in the VT zone.

What to know in the lab

• Coronary sinus cannulation is the rate-limiting step — multiple sheath shapes available; CS angiography defines the target vein.
• Target a lateral or posterolateral vein at the basal-mid level; apical-only positions undermine response.
• Avoid the great cardiac vein (anterior) when possible — apical anterior pacing rarely resynchronizes.
• Confirm LV capture threshold and screen for phrenic capture at maximum output on every vector before exiting the lab.
• Document a 12-lead in BiV pacing — paced QRS should be narrower than intrinsic by >=20 ms; if not, revisit lead position.

Common issues & troubleshooting

• Low BiV pacing percentage — PVCs, AF, sinus tachycardia outpacing the upper rate, or programmed AV delay too long. Address the rhythm first.
• Phrenic nerve capture — switch LV pacing vector; if all vectors capture, lower output if margin allows or revise lead.
• Rising LV capture threshold — switch vectors; check for lead microdislodgement on fluoroscopy.
• CRT non-responder despite >90% BiV — reassess substrate (scar burden, QRS morphology, AV optimisation, MPP enablement).
• Inappropriate shocks — long detection times, SVT discriminators, optimise rate control in AF patients.

Manufacturer reference

• Boston Scientific CRT-D
• Medtronic ICDs (includes CRT-D platforms)
• Abbott ICDs (includes CRT-D platforms)