A CRT-D (cardiac resynchronisation therapy defibrillator) is a device that combines two functions in one: a biventricular pacemaker and an implantable defibrillator.
The biventricular pacing component (the CRT part) sends precisely timed electrical signals to both the left and right ventricles, helping them contract in better synchrony with each other. In people whose ventricles are out of step — a condition that shows up on an ECG as a wide QRS complex — this can significantly improve the efficiency of the heart’s pumping action. The defibrillator component works exactly like a standard ICD: it monitors the heart rhythm continuously and delivers a shock if a life-threatening arrhythmia is detected.
A CRT-D typically has three leads rather than the one or two used in a standard ICD. The extra lead is positioned via the coronary sinus, a vein on the back of the heart, to pace the left ventricle directly. This three-lead system makes the implant procedure more complex than a standard ICD implant.