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CRT and subcutaneous ICD (where they differ)

Variants of the pacemaker/ICD, read against that page. CRT-D and S-ICD shock; CRT-P does not. A CRT-P is a pacemaker and a magnet will not switch it off.

AdultCardiac & circulatory supportGuideline

Also called: CRT, CRT-P, CRT-D, biventricular pacemaker, biV, cardiac resynchronisation therapy, subcutaneous ICD, S-ICD

What it is

These are variants of the devices on the pacemaker/ICD page, grouped here only where they change what you do.

A CRT (cardiac resynchronisation therapy, or biventricular) device paces both ventricles to improve the pumping of a failing heart. It comes two ways: CRT-P paces only; CRT-D also has a defibrillator. The UK guidance notes the two functions of a CRT-D can be adjusted or deactivated independently from each other.

A subcutaneous ICD (S-ICD) is an ICD with no lead in the heart: the generator sits in the left lateral chest wall and a single lead runs under the skin alongside the sternum. It can shock but gives only brief post-shock pacing - it cannot provide long-term bradycardia pacing. Written for adults, as on the pacemaker/ICD page.

How to recognise it

What it gets confused with

Read the pacemaker/ICD page first - the magnet rule, pad placement, the shockable-arrest drill and the after-death rules are identical. The only reason this entry exists is the differences below: a CRT-P is a pacemaker (a magnet speeds it, does not switch it off), and an S-ICD sits laterally and cannot pace for bradycardia.

Open that entry →

What goes wrong

CRT-P mistaken for an ICD (or vice versa)

Someone reaches for a magnet to 'turn off the pacing' in a CRT-P, or assumes a CRT-D cannot shock.

Red flag
A magnet over a CRT-P will not deactivate its pacing function but will cause the pacemaker to fire at a faster than usual rate. On a CRT-D a magnet suspends the shock only, without interrupting its biventricular pacemaker function. Confirm P versus D from the card before acting.

S-ICD shocks, and cannot rescue bradycardia

An S-ICD delivering shocks behaves like any ICD. But if the problem is a bradyarrhythmia or asystole, the S-ICD offers only transient post-shock pacing.

Red flag
Do not expect an S-ICD to pace a bradycardic patient. Use transcutaneous pacing as you would in a patient with no device, with pads placed clear of the laterally-sited generator.

Worsening heart failure after loss of CRT

A CRT patient whose resynchronisation has been lost (lead problem, AF with loss of biventricular capture, or deactivation) may deteriorate into heart failure over days.

Red flag
Cessation of biventricular pacing is likely to be followed by worsening of symptoms of heart failure. Treat the heart failure and get the device checked; this is a subacute decline, not usually a crash.

The first ten minutes

  1. Work the pacemaker/ICD page. Everything there applies: arrest drill, magnet use for inappropriate/intolerable shocks, pad placement, interrogation after ROSC.
  2. Read the card for P versus D. It decides whether the device can shock and therefore what a magnet achieves.
  3. For an S-ICD, place defibrillation and pacing pads clear of the left lateral generator - anterior-posterior usually achieves this - and do not rely on it for bradycardia pacing.
  4. Call cardiology / device physiology with the card details, exactly as for any pacemaker or ICD.

Do not

Do not put a magnet on a CRT-P expecting it to stop.
A CRT-P is a pacemaker. A magnet makes it pace fast and asynchronously; it does not deactivate it. Only a CRT-D or S-ICD has a shock function for a magnet to suspend.
Do not expect a subcutaneous ICD to pace a slow heart.
The S-ICD delivers shocks and only brief post-shock pacing. A bradycardic or asystolic patient with an S-ICD needs transcutaneous pacing, not reliance on the device.
Do not place a pad over a left-lateral S-ICD generator.
As with any device, a pad over the generator risks damaging it and reduces the energy reaching the heart. Keep the same clearance from the generator as for any device (see the defibrillation panel); an anterior-posterior position usually helps, but check it - a left-lateral generator can sit close to the posterior pad.

Who owns it

Cardiology and cardiac physiology, as for any CIED. Tell them whether the card says CRT-P, CRT-D or S-ICD, because that determines both the shock question and the pacing question.

What a magnet does to each device

This is the single most-confused fact in the whole tool, and it is dangerous because the same magnet does opposite things depending on the device. The rule beneath the table: a magnet suspends shocks on anything that can shock (ICD, CRT-D, S-ICD) and does nothing to stop pacing on anything that only paces - on a pacemaker it forces fast, asynchronous pacing. Always identify the device from its card before reaching for a magnet. Two caveats: the magnet response is not universal - a leadless pacemaker (Micra/Aveir) has no magnet sensor and the response can be programmed off, so no magnet response does not prove the device is inactive; and these rules apply to a child with one of these devices too (paediatric pad size, energy and specialist advice are not covered here).

ICD (including CRT-D, S-ICD)
A magnet suspends shock therapy only while it is held in place - the guidance: Placing a strong magnet on the skin over the ICD generator will suspend tachyarrhythmia detection (and thereby suspend shock delivery). It does not stop pacing. Tape it on and check it stays: removal or displacement of the magnet will immediately restore the ability of the ICD to deliver a shock. One manufacturer (Biotronik) restores shocks after only 8 hours, so with that or an unknown device lift and replace the magnet periodically. Before applying the magnet for a shock storm, put external defibrillator pads on and get a rhythm strip - the magnet removes the safety net, so if the shocks are appropriate (VT/VF) you must be ready to defibrillate externally at once.
Pacemaker (and CRT-P)
A magnet does NOT switch it off. It will not deactivate it but will cause the pacemaker to fire at a faster than usual rate and stops it being inhibited by the patient's own beats (asynchronous pacing). The guidance adds that magnets should not be used in a misguided attempt to deactivate a pacemaker. The one time a pacemaker magnet helps: if the pacemaker is being wrongly inhibited by oversensing (surgical diathermy, myopotentials, lead noise) and the patient is bradycardic or pausing, holding a magnet on forces asynchronous pacing and restores a reliable rate.
On a device that both paces and shocks
The magnet suspends the shock function only; pacing continues. The appendix states plainly: Magnet application does NOT affect the programmed pacemaker function of the device. So a magnet makes a pacing-dependent ICD patient no less paced - it only stops shocks.
Vagus nerve stimulator (VNS)
Opposite logic to a cardiac magnet. Holding the magnet over the generator deactivates stimulation (continuous application of the magnet to the generator will deactivate the device); swiping it delivers an extra burst to abort a seizure. In an emergency without the patient's VNS magnet, any magnet with a strength over 50 gauss can be used to deactivate the device.
Loop recorder
Nothing to suspend - it delivers no therapy. A magnet is irrelevant to it.
After death with an active ICD
A magnet is only a temporary hold, not deactivation: Temporary deactivation using a magnet is not adequate before autopsy, device removal or cremation. A physiologist must deactivate the ICD, and if the patient has died with the magnet suspending shocks it must stay taped in place until they do. Note a brand that auto-reverts (e.g. Biotronik at 8 h) - after that the magnet protects no one, so tell the mortuary and device service.

Defibrillation and pacing with an implanted device

You still defibrillate an arresting patient who has a device - you just keep the pads away from the generator. Standard ALS applies; the device changes pad position, not the algorithm. These pad rules apply to a child in arrest too; paediatric pad size and energy are not covered here.

Keep pads off the generator
The guidance: external electrodes should not be placed over or close to implanted electronic devices. A pad over the box risks damaging the device and the myocardium where the leads sit, and wastes energy.
How far, and which way
A distance of at least 10-15 cm between the edge of the device and the edge of the defibrillator electrode is recommended. Placing the pads approximately perpendicular to the device and its leads reduces current entering the device; an anterior-posterior position is a clean way to achieve this with a pectoral or a left-lateral (S-ICD) generator.
Do not withhold the shock
In a shockable arrest, if the ICD has not fired or its shocks have not worked, external defibrillation should be attempted in the usual way. The device is a reason to move the pad, never a reason to delay defibrillation.
External (transcutaneous) pacing
Can be used over an implanted device. If the implanted device is still emitting pacing spikes, set the external pacer faster than the programmed rate and/or out of demand mode, or its output may be inhibited by the implanted device's spikes.
Loop recorders and chest neurostimulators
Avoid placing a pad directly over a loop recorder. Similar clearance precautions apply to a neurostimulator implanted in the chest wall - the guidance advises Similar precautions are advised in people with neurostimulators implanted in the chest.

Who to call

The team and the criteria are national. The phone number is local, and this tool does not know it. Find it once, now, and write it on the department's own card — that is the part no website can do for you.

Say the device name first
Open with laryngectomy, LVAD, shunt — the single word changes who comes and what they bring. Do not bury it in the history.
Say what you have already established
Whether the device is patent, whether it has been removed or disturbed, and what you have already tried. The specialist is deciding whether to walk or run.
Ask who owns it out of hours
Device ownership and the on-call route often differ from the implanting specialty, and it is frequently a regional centre rather than your own hospital.
The patient's own team is a resource, not an afterthought
Named nurse specialists, home-care companies and device helplines answer questions nobody else can, and the patient usually has the number.

What the patient will be carrying

The device identification card distinguishes CRT-P, CRT-D and S-ICD and names the manufacturer - carry it to every attendance, as for any implanted cardiac device.

Scope of this page

This page covers only the points where a CRT or subcutaneous ICD changes ED action compared with the main pacemaker/ICD page: the CRT-P-is-a-pacemaker trap, the S-ICD's lateral position and inability to pace for bradycardia, and loss of resynchronisation. For the arrest drill, magnet use, pad placement and after-death rules, use the pacemaker/ICD page. Reprogramming and device procedures remain with cardiology. Adults only.

Sources