Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Implantable cardioverter-defibrillators and CRT
Cardiology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending
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Time-critical presentation
Call 999 after more than one ICD shock, any shock with collapse or ongoing chest pain, breathlessness, palpitations or presyncope, or suspected electrical storm. If a shockable cardiac arrest persists, deliver external defibrillation with pads more than 8 cm from the generator; a magnet can suspend inappropriate ICD shocks but does not disable pacing.
Purpose and principles
Select ICD or cardiac resynchronisation therapy for appropriate arrhythmic and heart-failure indications, and respond safely to shocks and device complications.
ICD and CRT solve different problems. An ICD terminates dangerous ventricular rhythms; CRT treats electromechanical dyssynchrony and can improve symptoms and outcomes in selected HFrEF. A combined CRT-D is chosen when both benefits are expected.
Device eligibility is not based on LVEF alone. Clinicians confirm optimised medical therapy, QRS morphology and duration, functional class, arrhythmic history, reversible causes, comorbidity, patient goals and a reasonable prospect of meaningful survival.
Long-term care includes programming, remote monitoring, heart-failure optimisation, psychological support and advance-care planning. Deactivating ICD shock therapy near end of life does not ordinarily turn off pacing.
Key points
An ICD detects ventricular tachyarrhythmia and can deliver anti-tachycardia pacing or a shock; it does not prevent the arrhythmia from starting.
Secondary prevention considers survivors of VT/VF arrest or haemodynamically important sustained VT after reversible causes are excluded.
Primary prevention balances LVEF, heart-failure status, substrate, life expectancy and competing non-arrhythmic mortality after optimised treatment.
Indications, selection and cautions
Appropriate ICD therapy
Stored electrograms show sustained VT or VF meeting programmed criteria, treated by anti-tachycardia pacing or shock.
Inappropriate shock
Rapid AF/SVT, over-sensing, lead fracture or electromagnetic noise can trigger therapy without ventricular tachyarrhythmia.
Electrical stormRed flag
Three or more episodes of sustained ventricular arrhythmia or appropriate ICD therapies within 24 hours is a critical syndrome requiring specialist intensive management.
CRT candidate phenotype
Persistent symptomatic HFrEF with LVEF 35% or less and a broad QRS, especially LBBB at 150 ms or more, after optimised therapy is the classic profile.
CRT non-response
Persistent symptoms may reflect inadequate biventricular pacing, AF, suboptimal lead position, scar, untreated ischaemia/valve disease or insufficient medical optimisation.
01Device interrogation with stored electrogramsFirst step
Why
Determine rhythm, therapy sequence, appropriateness, battery and lead integrity.
Interpretation and limitations
Do not label a shock appropriate from symptoms alone; stored ventricular and atrial signals adjudicate mechanism.
0212-lead ECG
Why
Assess rhythm, QRS duration/morphology, pacing capture and post-shock changes.
Interpretation and limitations
Measure QRS before CRT selection; after CRT, morphology and pacing markers help confirm biventricular activation.
03Echocardiogram
Why
Measure LVEF, chamber size, valve disease and response to CRT.
Interpretation and limitations
Eligibility commonly uses LVEF 35% or less; repeat after optimised treatment and after CRT to assess reverse remodelling.
04U&Es, magnesium and troponin when indicated
Why
Find triggers for VT/VF and assess consequences of repeated therapy.
Interpretation and limitations
Correct electrolytes; interpret troponin with symptoms, shocks and ischaemic context.
05Coronary and structural assessment
Why
Treatment approach
01secondarySecondary-prevention ICD assessmentFirst stepThe patient survives VT/VF arrest or sustained haemodynamically important VT.+
1Stabilise, document the rhythm and identify reversible causes such as acute ischaemia, electrolyte disturbance, drug toxicity or a transient acute illness.
3If no fully reversible cause explains the event and recurrent malignant ventricular arrhythmia risk remains, refer for specialist ICD evaluation.
4Discuss transvenous versus subcutaneous/extravascular systems, pacing need, shocks, complications, driving and future shock deactivation.
02hf-selectionPrimary prevention and CRT selectionHFrEF persists despite optimised evidence-based treatment.+
1
Complications, monitoring and follow-up
Use scheduled and remote interrogation for battery, lead impedance, sensing, capture, arrhythmia episodes and delivered therapies.
Track heart-failure symptoms, weight, renal function, blood pressure and adherence to evidence-based HFrEF therapy.
After CRT, measure biventricular pacing percentage and reassess ECG and LVEF; investigate capture below the desired near-continuous range.
Review every shock using stored electrograms and address psychological distress as well as the arrhythmic cause.
Inspect the pocket and act on fever, erosion, redness, swelling or discharge.
Revisit goals of care and shock-deactivation preferences during progressive illness; document that pacing can remain active when shock therapy is withdrawn.
Special situations
ICD treats, it does not prevent
Anti-tachycardia pacing and shocks terminate detected rhythms. Treating ischaemia, heart failure and arrhythmic substrate reduces recurrence and shock burden.
CRT needs capture
AF with rapid conducted beats can prevent effective biventricular pacing; rate control or AV-junction ablation may be required to deliver the intended therapy.
A magnet is temporary
Over an ICD it usually suspends tachycardia detection/therapy only while present. It is a bridge to expert correction, not definitive treatment.
External shocks remain valid
An ICD does not preclude standard ALS defibrillation; move pads away from the generator to reduce device damage and current shunting.
CRT-P versus CRT-D is a prognosis decision
Adding defibrillation makes sense only when arrhythmic death risk and meaningful survival justify shocks beyond the resynchronisation benefit.
Shock deactivation is not euthanasia
Stopping burdensome tachy-therapies allows the underlying disease to take its course and can be separated from ongoing bradycardia pacing.
Common pitfalls
01
Assuming every ICD shock is appropriate without stored-electrogram review.
02
Leaving a patient at home after repeated shocks or a shock with ongoing symptoms.
03
Applying external pads directly over the device generator.
04
Choosing CRT from LVEF alone without QRS duration, morphology, NYHA class and optimised treatment.
05
Assuming ICD shock deactivation also stops pacing.
Practice
Two practice questions
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Question 1 of 2
0 correct
CardiologyOriginal SBA
Repeated ICD shocks
A patient with an ICD receives two shocks within 10 minutes and remains breathless with palpitations. What is the correct action?
Sources and review status6 sources · checked 27 Aug 2026 · clinical review pending+
Sources
Sources and review status
National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem ·
CRT coordinates ventricular contraction using biventricular or conduction-system pacing; CRT-P paces, while CRT-D also defibrillates.
NICE TA314 uses LVEF 35% or less plus NYHA class, QRS duration and LBBB morphology to select ICD, CRT-P or CRT-D.
The strongest conventional CRT response is expected with symptomatic HFrEF, sinus rhythm, LBBB and QRS 150 ms or more despite optimised therapy.
One shock with complete recovery still requires prompt device-team contact; repeated shocks or ongoing symptoms are an emergency.
Inappropriate shocks can result from AF/SVT, lead noise or programming; interrogation of stored electrograms is essential.
Identify reversible ischaemia, scar, cardiomyopathy or valve disease.
Interpretation and limitations
Revascularisation or disease-specific treatment may be needed alongside, before or instead of device therapy.
06Chest radiograph and blood cultures when indicated
Why
Assess lead position, pneumothorax or suspected infection.
Interpretation and limitations
Positive cultures or pocket erosion can mandate complete system extraction; a normal radiograph does not exclude electrical lead failure.
Confirm LVEF, NYHA class, sinus rhythm/AF, QRS duration and LBBB morphology after treatment optimisation.
2Use the NICE TA314 matrix for LVEF 35% or less: narrow QRS generally favours ICD only when sudden-death risk is high; broader QRS and LBBB increasingly favour CRT-P or CRT-D according to NYHA class and defibrillator indication.
3For the classic LBBB QRS 150 ms or more phenotype, recommend CRT when symptoms and prognosis support it; choose CRT-D when defibrillation benefit is also expected and CRT-P when it is not.
4In AF, ensure a plan for high biventricular capture; ESC advises considering AV-junction ablation if conducted AF prevents more than 90-95% capture.
03shockAfter an ICD shockThe patient reports device therapy.+
1If more than one shock, collapse or ongoing chest pain, breathlessness, palpitations or presyncope occurs, call 999 and use monitored emergency care.
2If a single shock occurs and the patient feels completely well, contact the device service promptly the same day or as instructed and transmit a remote interrogation; do not drive until advised.
3Interrogate stored electrograms, check electrolytes/ischaemia and distinguish appropriate VT/VF therapy from inappropriate sensing or SVT.
4Treat the trigger, optimise programming and medicines, and consider VT ablation for recurrent appropriate therapy rather than accepting repeated shocks.
04active-shocksCardiac arrest or inappropriate repeated shocksThe ICD is repeatedly discharging or cardiac arrest persists.+
1AlternativeFor cardiac arrest, start ALS and give external shocks when indicated, placing pads more than 8 cm from the generator or using an alternative position.
2If shocks are clearly inappropriate and a perfusing rhythm persists, place a clinical magnet over the ICD to suspend tachy-therapies while monitoring continuously and obtaining immediate device help.
3Remember the magnet ordinarily does not disable bradycardia pacing; remove it if malignant ventricular arrhythmia develops and external defibrillation is not immediately ready.
4After stabilisation, interrogate and correct programming, lead or rhythm causes before discharge.