Cardiac resynchronization therapy, or CRT, is a medical technology used to help some people with heart failure. In these patients, the lower chambers of the heart may not contract at the same time, so each beat pumps less blood than it should. A CRT device sends carefully timed electrical pulses to coordinate the ventricles and improve pumping efficiency.
This matters because better synchronization can reduce symptoms such as fatigue, shortness of breath, and poor exercise tolerance.
A CRT system usually includes a pulse generator placed under the skin and leads that carry signals to the heart. One lead often paces the right ventricle, while another reaches the left ventricle through a vein on the heart surface, allowing both sides to be paced in a coordinated pattern. The goal is to reduce the delay between ventricular contractions so the heart ejects blood more effectively.
CRT does not cure heart failure, but it can improve heart function and quality of life for selected patients.
Understanding Medical Technology: Cardiac Resynchronization Therapy
A healthy heartbeat depends on an electrical pathway. A small group of cells near the top of the right atrium starts a signal. The signal spreads across the atria, pauses briefly at the atrioventricular node, then travels down specialised fibres into the ventricles.
The brief pause gives the ventricles time to fill before they squeeze. In some heart failure cases, damage or delay in these pathways means one region starts moving while another is still relaxed. The wall between the ventricles can move in an inefficient direction, and some force is wasted by pulling against other parts of the heart.
CRT devices work with very small time differences, often measured in milliseconds. The device senses natural electrical activity and chooses when to send a pacing pulse. Doctors can adjust the delay between atrial activity and ventricular pacing.
They can adjust the timing between the two ventricles as well. These settings are not identical for every person. The left ventricular lead is usually guided through a blood vessel called the coronary sinus, then into a vein on the outside of the heart.
Its position matters because scar tissue or poor contact can make pacing less effective. During implantation, imaging and electrical measurements help the team place and test the leads.
Doctors use several kinds of evidence when deciding whether CRT may help. An electrocardiogram can show a prolonged QRS complex, which suggests that ventricular electrical activation is taking too long. An echocardiogram shows chamber size, valve movement, and how strongly different regions contract.
Stroke volume is the amount of blood ejected in one beat. Cardiac output depends on heart rate multiplied by stroke volume, so improving the amount pumped per beat can help even when the heart rate stays similar. Ejection fraction compares the blood ejected with the amount present after filling.
It is useful, but it does not describe every part of heart performance. A person with extensive scar tissue, certain rhythm problems, or poorly placed leads may respond less strongly.
Some devices provide pacing only, while others combine CRT with a defibrillator. A defibrillator can detect dangerous fast rhythms and deliver treatment when needed. After implantation, regular checks confirm battery level, lead function, and the percentage of beats receiving coordinated pacing.
Many devices can send routine information to a clinic remotely. Students learning this topic should pay close attention to the difference between electrical timing and mechanical pumping. An electrical pulse happens first, while muscle contraction follows.
This delay is why accurate timing, signal sensing, and careful measurement are central to the technology. Like any implanted device, CRT can involve risks such as infection, bleeding, lead movement, or the need for later battery replacement.
Key Facts
- CRT stands for cardiac resynchronization therapy.
- CRT uses timed electrical pulses to coordinate contraction of the right and left ventricles.
- Heart rate can be calculated as HR = 60/T, where T is the time in seconds between beats.
- Cardiac output is CO = HR x SV, where HR is heart rate and SV is stroke volume.
- Ejection fraction is EF = SV/EDV x 100%, where EDV is end-diastolic volume.
- A typical CRT system includes a pulse generator, a right ventricular lead, a left ventricular lead, and often a right atrial lead.
Vocabulary
- Cardiac resynchronization therapy
- A treatment that uses an implanted device to pace the ventricles so they contract in a more coordinated way.
- Ventricle
- One of the two lower chambers of the heart that pump blood out to the lungs or the body.
- Pulse generator
- The implanted battery-powered part of a CRT system that produces electrical pacing signals.
- Lead
- A thin insulated wire that carries electrical signals between the CRT device and the heart tissue.
- Ejection fraction
- The percentage of blood in a ventricle that is pumped out during one heartbeat.
Common Mistakes to Avoid
- Thinking CRT makes the heart beat stronger by squeezing it directly is wrong because the device sends electrical signals that guide timing, while the heart muscle still performs the contraction.
- Confusing CRT with a standard pacemaker is wrong because standard pacing may control heart rate, while CRT specifically coordinates the timing of both ventricles.
- Assuming every heart failure patient benefits from CRT is wrong because CRT is usually used for selected patients with electrical conduction delay and reduced pumping function.
- Ignoring ventricular timing is wrong because two ventricles contracting out of sync can lower stroke volume even if the heart rate is normal.
Practice Questions
- 1 A patient has 0.80 s between heartbeats. Use HR = 60/T to find the heart rate in beats per minute.
- 2 Before CRT, a patient has HR = 75 beats/min and SV = 55 mL/beat. After CRT, SV increases to 68 mL/beat while HR stays the same. Calculate the cardiac output before and after CRT in L/min.
- 3 Explain why pacing both ventricles at carefully timed intervals can improve pumping efficiency in a heart where the ventricles contract out of sync.