The problem CRT solves
In a healthy heart, the electrical signal spreads almost simultaneously across the left and right ventricles, so the two lower chambers squeeze together as a coordinated pump. In some patients with heart failure — particularly those with a left bundle branch block (LBBB) — that signal is delayed on the left side. The right ventricle contracts first, then the left side catches up, sometimes 150 milliseconds later. The walls of the left ventricle also contract out of sync with each other.
The result is mechanical dyssynchrony: the chamber wastes energy pushing parts of itself around instead of efficiently ejecting blood. Over time, this drives the heart muscle to weaken further, the ejection fraction to drop, and symptoms (breathlessness, fatigue, fluid retention) to worsen even on the best medications.
Cardiac resynchronization therapy (CRT) addresses this directly. By pacing the left ventricle and the right ventricle at the same time — or with a small, deliberate offset — CRT restores coordinated contraction. The chamber works as one pump again, and over weeks to months, the heart often remodels: it shrinks back toward a normal shape, the ejection fraction rises, and symptoms improve.
The major FDA-approved CRT-D platforms currently used in the United States are the Medtronic Cobalt HF / Crome HF (CRT-D), Abbott Gallant CRT-D, and Boston Scientific Resonate HF CRT-D.
Why CRT-D rather than CRT-P
The same hardware platform comes in two flavors. CRT-P is the resynchronization-only device — it paces the heart but cannot deliver shocks. CRT-D combines CRT with full ICD capability.
We choose CRT-D when:
- The ejection fraction is at or below 35% (the standard threshold for ICD therapy in heart failure with reduced EF).
- The patient is otherwise expected to live more than a year with a reasonable quality of life.
- There is no specific reason to avoid the defibrillator (very advanced age with limited life expectancy, severe non-cardiac illness, or a personal preference against shock therapy).
CRT-P is the right choice when resynchronization is clearly indicated but the long-term risk of sudden cardiac death is low, or when shocks would not be desired given the overall clinical picture. The implant procedure and benefit on heart failure are identical; the difference is whether the device also defends against sudden cardiac death.
Who tends to benefit most
The strongest responders share a few features: a left bundle branch block (not just any wide QRS), a QRS duration of at least 150 milliseconds, ejection fraction below 35%, and ongoing symptoms despite optimized medical therapy. Women tend to respond particularly well, as do patients whose cardiomyopathy is non-ischemic. Patients with a wide QRS due to right bundle branch block or nonspecific intraventricular conduction delay tend to benefit less, though they may still see improvement.
About two of three appropriately selected patients are clear responders — feeling better, with measurable improvement in EF and a reduction in hospitalizations. A small subset are super-responders whose ejection fraction normalizes. About a third see less benefit, which is one reason we re-evaluate at 3–6 months and adjust device settings if response is incomplete.
The third lead is what makes this hard
The technically demanding part of a CRT implant is placing the left-ventricular lead. We can’t put a lead directly inside the left ventricle because the bloodstream there is high-pressure and clot-prone. Instead, we feed the lead from the right atrium into the coronary sinus — a vein on the back of the heart — and then out into one of its branches that runs along the surface of the left ventricle. The lead paces the LV from the outside of the chamber, through the vein wall.
Anatomy varies. In a small percentage of patients, no suitable vein branch is available, and we either accept a less-than-ideal position or convert to an alternative strategy such as conduction system pacing (pacing the His bundle or left bundle area to recruit the heart’s own wiring).
Life with a CRT-D
Most patients begin to feel a difference in symptoms within weeks — first less breathlessness with activity, then better stamina, sometimes a noticeable change in how flat they can sleep at night. Heart-failure medications are usually continued and sometimes intensified, because the heart now tolerates them better. Remote monitoring follows the same pattern as any modern ICD: data sent from home, in-clinic checks once or twice a year, and an echo to track remodeling.
The device’s defibrillator behaves like a transvenous ICD — anti-tachycardia pacing for slower VT, shocks for fast or chaotic rhythms. Driving rules, MRI compatibility, and lifestyle considerations are the same as for any transvenous ICD and we go through them at the time of implant.
Manufacturer reference
For technical specifications, indications, and the latest official information on the CRT-D platforms referenced above, see the manufacturers’ product pages:
(External links — content is each manufacturer’s and may be technical.)