Heart devices: how do a pacemaker, an ICD and a CRT-D differ?

According to the EHRA patient website (myrhythmdevice.org), these devices consist of two parts. The generator is a small metal box that holds the battery and the electronic circuit. The lead is a thin, insulated wire that connects the generator to the heart.

Device	What it does	Leads	Usual reason
Single-chamber pacemaker	Sends signals to a slow heart	1	Slow rhythm, for example in atrial fibrillation
Dual-chamber pacemaker	Coordinates the atrium and the ventricle	2	AV block
ICD	Stops a dangerous fast rhythm	1–2	Has had a dangerous ventricular arrhythmia, or is at high risk of one
CRT-P	Coordinates the two ventricles	2–3	Heart failure
CRT-D	Coordinates and delivers shocks	3	Heart failure and a risk of dangerous arrhythmia

The atrium is the upper chamber of the heart, and the ventricle is the lower chamber. AV block means the signal fails to pass from the atrium to the ventricle. The table gives a general idea; the exact decision is made by a specialist.

According to EHRA, a pacemaker monitors the rhythm all the time and sends a signal only when the heart beats too slowly. It may be needed in sinus bradycardia (when the sinus node, which starts the rhythm, works slowly) and in AV block.

• Single-chamber: one lead. If the signal starts in the sinus node but is slow, the lead is placed in the right atrium. If the rhythm is slow and does not start in the sinus node, for example in atrial fibrillation, it is placed in the right ventricle.
• Dual-chamber: two leads, in the right atrium and the right ventricle. It is used in AV block and coordinates these two chambers.

If possible, the cause is dealt with first, for example a medicine that slows the rhythm is stopped; the doctor decides this. A pacemaker may be needed when this is impossible or not enough. According to the ACC summary of the 2021 ESC/EHRA guidelines, not everyone who is found to have a slow rhythm needs a pacemaker.

An ICD (implantable cardioverter-defibrillator) monitors the rhythm all the time. According to EHRA, it stops a life-threatening ventricular arrhythmia with fast pacing or a shock (a brief electrical impulse). These rhythms are ventricular tachycardia (the heart beats very fast) and ventricular fibrillation (the ventricles quiver instead of pumping blood). An ICD placed through a vein can also send signals to a slow heart, like an ordinary pacemaker.

EHRA distinguishes two situations:

• Secondary prevention: the person has a confirmed or suspected dangerous ventricular arrhythmia.
• Primary prevention: there has been no such arrhythmia, but the risk is high because of a diagnosed heart disease, sometimes because of a disease or genetic change found in the family.

The exact criteria depend on the type of disease, and a specialist assesses them.

In heart failure, the ventricles may stop contracting at the same time, and this may make symptoms worse. The cause of this may be left bundle branch block, that is, a blockage of one of the signal pathways inside the heart.

According to EHRA, in some severe cases where medicines are not enough, CRT (cardiac resynchronization therapy) may be needed. It coordinates the right and left ventricles, so that each beat becomes more effective.

A CRT-P (a “biventricular pacemaker”) only paces. A CRT-D, with three leads, combines resynchronization with defibrillation. The choice depends on the risk of dangerous arrhythmia, and a specialist decides it.

According to the ACC summary of the 2021 ESC/EHRA guidelines, CRT is recommended in heart failure with symptoms if there is sinus rhythm, LVEF ≤ 35 %, QRS ≥ 150 ms and left bundle branch block. LVEF shows how much blood the left ventricle pumps out in one beat, and QRS is a wave on the ECG.

No, even though the names are similar, these are different things:

• Leadless pacemaker: only a generator, placed in the right ventricle through the femoral vein. It is only for those who need single-chamber pacing of that chamber.
• S-ICD (subcutaneous ICD): its box sits near the armpit and its lead lies under the skin. It only delivers shocks, does not pace, and its incision is larger.
• AED: an automated defibrillator in public places. Even a person with no special training can use it.
• Wearable defibrillator vest: usually worn for a short period.

According to the ACC summary of the 2021 ESC/EHRA guidelines, the decision is made jointly: the evidence, the individual risks and benefits, your wishes and the goals of treatment are taken into account. According to the same summary, infection occurs more often with CRT or an ICD than with an ordinary pacemaker. It also occurs more often in device replacement and upgrade procedures.

Questions you can ask your doctor:

• Which device is being recommended for me, and why?
• Is there another way of treatment?
• What checks will be needed after it is implanted?