CRT stands for cardiac resynchronization therapy, a treatment for heart failure that uses a small implanted device to coordinate the beating of your heart’s lower chambers. In a healthy heart, both ventricles contract at the same time. In many people with heart failure, electrical signals travel through the heart unevenly, causing the two sides to pump out of sync. CRT corrects this by sending precisely timed electrical pulses to both ventricles simultaneously, restoring a coordinated squeeze that pumps blood more efficiently.
How CRT Works
Heart failure often comes with an electrical problem called a conduction delay, most commonly left bundle branch block. Normally, an electrical signal travels through a branching network of fibers so that the entire heart muscle contracts in one smooth motion. When part of that wiring is damaged or sluggish, one side of the heart contracts before the other. Some regions work harder while others barely contribute, wasting energy and reducing the amount of blood pushed out with each beat.
A CRT device is essentially a specialized pacemaker. Thin wires called leads are threaded through veins into the heart: one in the right ventricle, one positioned along the outside of the left ventricle via a vein on the heart’s surface, and typically one in the right upper chamber. The device, housed in a small metal case implanted under the skin near the collarbone, sends synchronized electrical impulses through these leads so both ventricles contract together. The result is better pumping strength, improved blood flow, and less strain on the heart muscle over time.
CRT-P vs. CRT-D
There are two versions of the device. A CRT-P (pacemaker) only provides the resynchronization pacing. A CRT-D (defibrillator) does the same pacing but also includes a built-in defibrillator that can deliver a shock if the heart develops a dangerous rhythm. In practice, most people who qualify for CRT already have a heart function low enough to warrant defibrillator protection, so CRT-D is the more commonly chosen option, even though guidelines don’t strictly mandate one over the other.
Who Qualifies for CRT
CRT isn’t for everyone with heart failure. The candidates who benefit most share a specific combination of features: a weakened heart (typically pumping 35% or less of its blood volume per beat, compared to the normal 55% or more), heart failure symptoms despite medication, and a widened QRS complex on an electrocardiogram, which signals that electrical conduction through the ventricles is delayed.
The width of that QRS signal matters a lot. People with a QRS duration above 150 milliseconds and left bundle branch block see the greatest improvement. Those with QRS between 120 and 149 milliseconds still benefit, but the gains are smaller. People without left bundle branch block respond less predictably. Some trials have also shown benefit in patients with slightly higher pumping function (up to 50%) who need frequent pacing due to a heart block, broadening the pool of people who might be helped.
What CRT Accomplishes
Large clinical trials have consistently shown that CRT reduces both deaths and hospitalizations in people with mild to moderate heart failure, reduced pumping function, and left bundle branch block. In one major trial, patients receiving CRT experienced a 26% reduction in a combined outcome of death, emergency heart failure visits, or worsening heart size, along with a 22% drop in the combined rate of death or hospitalization for heart failure. Many patients notice improved energy, less shortness of breath, and a better ability to handle daily activities within weeks to months of implantation.
Beyond symptom relief, CRT can actually reverse some of the damage heart failure has caused. The heart may physically shrink back toward a more normal size, a process called reverse remodeling. This structural improvement is one reason the benefits tend to grow over time rather than plateau.
Why CRT Doesn’t Work for Everyone
Roughly 10% to 30% of patients don’t respond meaningfully to CRT, depending on how “response” is defined. In one study of patients with clear guideline indications (left bundle branch block and wide QRS), about 10% were classified as non-responders. Two factors stood out as predictors: a very enlarged left ventricle (an internal diameter of 77 mm or more) and a fragmented electrical signal on the EKG. When neither factor was present, the chance of non-response dropped to around 4%. When both were present, the probability of non-response climbed to nearly 50%.
Other reasons CRT may underperform include scarring in the heart muscle from a prior heart attack (which pacing can’t fix), a lead that ends up in a suboptimal position, or the absence of true mechanical dyssynchrony despite what the EKG suggests. Doctors sometimes reprogram the device’s timing or reposition a lead to improve results.
The Implantation Procedure
CRT implantation is done in a cardiac catheterization lab or electrophysiology suite, usually under local anesthesia with sedation. The procedure typically takes one to three hours. A small incision is made below the collarbone, and the leads are guided through a vein into the heart using X-ray imaging. The trickiest part is positioning the left ventricular lead: it must be threaded through the coronary sinus, a vein that runs along the outside of the heart, to reach the left ventricle indirectly without puncturing it.
Once the leads are tested and secured, they’re connected to the pulse generator, which is tucked into a pocket created under the skin. The incision is closed, and you’re typically monitored for a few hours or overnight.
Recovery After Implantation
Recovery is relatively quick. You’ll need to keep your left arm still for about 12 hours after the procedure to help the leads settle into position. Most people return to normal daily activities within a few days. Driving and lifting heavy objects are typically off-limits for about a week. Your care team will schedule follow-up visits to check the device settings and fine-tune the pacing to get the best possible response.
Possible Complications
Complications occur in roughly 6% to 13% of cases. The most common issues include bruising or blood collection at the implant site, lead displacement (where a wire shifts out of position), infection around the device, and, rarely, a punctured lung from the insertion process. One complication somewhat unique to CRT is phrenic nerve stimulation, where the left ventricular lead inadvertently triggers the nerve that controls the diaphragm, causing an uncomfortable hiccup-like sensation with every heartbeat. This is reported in up to 30% of CRT patients to some degree, though it only causes significant problems in a smaller subset. Adjusting the pacing settings or, in some cases, repositioning the lead usually resolves it.
Newer Alternatives to Traditional CRT
A newer approach called conduction system pacing aims to achieve resynchronization by pacing the heart’s own electrical wiring directly, rather than stimulating the muscle from the outside. Two techniques fall under this umbrella: His bundle pacing, which targets the main electrical highway at the top of the ventricles, and left bundle branch area pacing, which stimulates the left branch of that highway deeper in the heart wall. Both have shown similar improvements in heart function compared to traditional CRT in observational studies and small randomized trials. Left bundle branch area pacing, in particular, can be combined with a conventional left ventricular lead for even greater electrical correction in patients who don’t fully respond to standard CRT alone. These approaches are still evolving but are increasingly used when traditional lead placement is difficult or produces inadequate results.