CRT stands for cardiac resynchronization therapy, a device implanted in the chest that helps the two lower chambers of the heart beat in sync again. It’s used for people with heart failure whose heart’s electrical system has gone out of rhythm, causing the left and right ventricles to contract at slightly different times. That mistiming reduces the amount of blood the heart pumps with each beat and worsens heart failure symptoms like fatigue and shortness of breath.
How CRT Works
In a healthy heart, an electrical signal travels through both ventricles almost simultaneously, so they squeeze together in a coordinated way. In many people with heart failure, that signal gets delayed on one side, usually the left. This is called a left bundle branch block. When one ventricle contracts before the other, blood sloshes inefficiently between the chambers instead of being pumped out to the body. The result is less blood ejected per beat and more strain on an already weakened heart.
A CRT device fixes this by sending timed electrical pulses to both ventricles through thin wires called leads. By stimulating both sides at once (or nearly so), the device restores coordinated contraction and improves the heart’s pumping efficiency. This is sometimes called biventricular pacing, because both ventricles are being paced rather than just one.
Who Qualifies for CRT
CRT isn’t for everyone with heart failure. Guidelines from major cardiology organizations recommend it when three conditions overlap: the heart’s pumping strength (ejection fraction) is 35% or lower, the electrical signal through the ventricles is delayed (shown by a wide QRS complex on an ECG, typically 150 milliseconds or more), and the person still has symptoms despite being on heart failure medications. Those symptoms are classified using a scale from Class II (mild limitation during normal activity) through Class IV (symptoms at rest).
Patients with a left bundle branch block pattern on their ECG tend to benefit the most. For people whose ECG shows a different type of conduction delay with a QRS between 120 and 150 milliseconds, the evidence is weaker, and guidelines generally consider CRT appropriate only in more advanced cases.
CRT-P vs. CRT-D
There are two types of CRT devices. A CRT-P is a pacemaker only. It synchronizes the heartbeat but does nothing if a dangerous heart rhythm occurs. A CRT-D combines the pacing function with a built-in defibrillator that can deliver a shock to reset the heart if it detects a life-threatening arrhythmia like ventricular fibrillation.
When someone already qualifies for a defibrillator based on their arrhythmia risk, a CRT-D is the standard choice. But the added defibrillator comes with trade-offs: higher complication rates, a greater risk of infection, shorter battery life requiring more frequent replacements, higher cost, and the possibility of inappropriate shocks, which can cause significant psychological distress. For older patients or those with multiple other serious health conditions, the survival benefit of the defibrillator component shrinks considerably. In those cases, a CRT-P alone may be the better option, and current guidelines emphasize shared decision-making between the patient and their care team.
What the Procedure Looks Like
Implanting a CRT device is a minimally invasive procedure, typically done under local anesthesia with sedation. The surgeon makes a small incision near the collarbone, usually on the left side, and threads three thin leads through a vein into the heart. One lead goes into the right atrium (upper chamber), one into the right ventricle, and the third is guided through the coronary sinus, a vein that wraps around the outside of the heart, to reach the left ventricle. Placing that third lead on the mid-lateral wall of the left ventricle is the trickiest part of the procedure and a key factor in how well the device works.
Once the leads are positioned and tested, they’re connected to the pulse generator, a small metal device about the size of a matchbox, which is placed in a pocket created just under the skin below the collarbone. The whole process generally takes one to three hours. Most people stay in the hospital overnight so the care team can check and adjust the device settings before discharge.
Recovery After Implantation
Recovery is relatively quick. You can eat normally right away, and most people return to their usual routine within a few weeks. The main restriction is avoiding lifting, straining, or stretching the arm on the side of the implant for the first four to six weeks. This protects the leads while they settle into position and scar tissue anchors them in place. The incision site needs to stay clean and dry until your care team clears you to shower.
After that initial healing window, most daily activities are fine. Your care team will schedule follow-up visits to check the device’s battery, lead function, and pacing settings. Many modern CRT devices also transmit data remotely, allowing your doctor to monitor how the device is performing between office visits.
Benefits of CRT
For patients who respond well, CRT can meaningfully improve quality of life. The heart pumps more efficiently, which eases symptoms like breathlessness and fatigue and can increase exercise tolerance. Over time, the heart may actually remodel, shrinking back toward a more normal size as the coordinated contractions reduce the mechanical stress on the muscle.
The survival data is encouraging too. A large study of over 50,000 implantations found that outcomes have improved steadily over time, with patients implanted in more recent years showing significantly lower mortality than those treated earlier, likely reflecting better patient selection and lead placement techniques. Women appear to benefit particularly strongly. A meta-analysis of more than 33,000 patients found that women had roughly 33% lower rates of death and heart failure hospitalization after CRT compared to men.
Why CRT Doesn’t Work for Everyone
One of the most important things to know about CRT is that it doesn’t help everyone. Roughly 30 to 50% of patients are classified as non-responders, meaning they don’t see meaningful improvement in heart function or symptoms. That’s a wide range, and the exact figure depends on how “response” is defined, whether by symptom relief, improved pumping function, or heart remodeling on imaging.
The reasons for non-response are varied. Before the implant, factors like the type and width of the electrical delay, the underlying cause of heart failure, the presence of scar tissue in the heart muscle, and how advanced the disease is all influence the odds of success. During the procedure, where exactly the left ventricular lead ends up matters enormously. Placing it away from scar tissue and in the region of latest electrical activation gives the best results. After implantation, maintaining a high percentage of biventricular pacing (ideally above 98% of heartbeats) is critical, and problems like atrial fibrillation can reduce that percentage.
Risks and Complications
CRT implantation is considered safe, but like any procedure involving the heart, it carries risks. The most common complication is lead-related problems, including lead dislodgement or failure to capture the heart’s electrical signal properly. Across large real-world databases, lead complications occur in roughly 3.5 to 6.8% of implantations. Infection at the device pocket or along the leads occurs in about 1 to 1.5% of cases. Pneumothorax, a small puncture of the lung lining during lead insertion, happens in about 0.6 to 1.4% of procedures. Procedural death is rare, at around 0.5%.
For CRT-D devices specifically, there are additional concerns beyond those shared with CRT-P. Generator-related problems are more common due to the added complexity, and inappropriate shocks, where the defibrillator fires when it shouldn’t, remain a recognized issue that can affect both physical comfort and mental health.