ICD and Cardiac Resynchronisation Therapy
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Direct answer
An implantable cardioverter-defibrillator (ICD) detects and terminates life-threatening ventricular arrhythmias, whereas cardiac resynchronisation therapy (CRT) paces both ventricles to restore coordinated contraction in heart failure. Primary prevention ICD is indicated for an ejection fraction of 35 percent or less despite at least three months of optimal medical therapy, and CRT is considered when the QRS is wide with left bundle branch block morphology. Secondary prevention ICD suits survivors of sustained ventricular arrhythmia.
What you must remember
- Primary prevention ICD: LVEF of 35 percent or less, NYHA class II or III, on at least three months of optimal therapy, with expected survival over one year.
- Secondary prevention ICD: survivors of sustained ventricular tachycardia, ventricular fibrillation or sudden cardiac arrest not due to a reversible cause.
- Waiting periods: ICD is deferred for about 40 days after MI and about 90 days after revascularisation, because EF often recovers.
- Strongest CRT indication: LVEF of 35 percent or less in sinus rhythm with LBBB morphology and a QRS of 150 milliseconds or more.
- Weaker CRT indication: LBBB with a QRS of 120 to 149 milliseconds, or non-LBBB widening, which benefits less consistently.
- Narrow QRS: CRT is not indicated when the QRS is under 120 milliseconds, because benefit is unlikely.
- Device choice: CRT-D adds defibrillation to resynchronisation, while CRT-P provides pacing alone.
- Complications: lead dislodgement, infection and inappropriate shocks are the main device problems to recognise.
Deciding between ICD and CRT in heart failure
A 61-year-old man with ischaemic cardiomyopathy has an EF of 30 percent five months after an MI, despite optimal medical therapy. His 12-lead ECG shows LBBB with a QRS of 160 milliseconds, and he remains in sinus rhythm with NYHA class II symptoms.
- Confirm the EF after therapy: Echocardiography after at least three months of medical therapy shows whether the EF remains 35 percent or less.
- Check the timing: An MI five months ago with revascularisation completed is past the waiting periods, so implantation can be considered.
- Read the QRS and morphology: LBBB with a QRS of 150 milliseconds or more gives the strongest case for CRT, which can be combined with a defibrillator.
- Exclude reversible triggers: Ischaemia, electrolyte disturbance and drug effects are corrected before deciding that an implant is needed.
- Choose the device: CRT-D is selected when both prevention and resynchronisation are indicated, while CRT-P suits patients who need pacing without defibrillation.
- Plan follow-up: Device checks track arrhythmia, shocks, lead function and battery, and the heart failure team reviews clinical response over time.
Clinical response, with fewer symptoms and reverse LV remodelling, is the real measure of CRT benefit. A patient whose symptoms and LV volumes do not improve is called a non-responder, and reviewing the lead position and the programming is the usual next step.
Where the exam tries to catch you
ICD and CRT questions usually hinge on timing, EF thresholds and QRS morphology.
- Trap: Implanting a primary prevention ICD in the first weeks after MI for a low EF. Correct: EF often improves early, so implantation waits about 40 days after MI.
- Trap: Offering CRT to any patient with a wide QRS. Correct: Morphology matters, and LBBB with a QRS of 150 milliseconds or more predicts the greatest benefit.
- Trap: Implanting an ICD for ventricular tachycardia caused by hypokalaemia. Correct: Reversible causes are corrected first, because an ICD treats the arrhythmia but not its cause.
- Trap: Using CRT in heart failure with a narrow QRS. Correct: A QRS under 120 milliseconds is not an indication for resynchronisation.
Frequently asked questions
What EF threshold defines primary prevention ICD eligibility?
Primary prevention ICD is generally indicated for a left ventricular ejection fraction of 35 percent or less after at least three months of optimal medical therapy. Eligibility also requires NYHA class II or III symptoms and an expected survival of more than one year.
Which QRS duration and morphology favour cardiac resynchronisation therapy?
Cardiac resynchronisation therapy is favoured in heart failure with LBBB morphology and a QRS of 150 milliseconds or more, with an EF of 35 percent or less. Narrow QRS complexes under 120 milliseconds have little expected benefit from resynchronisation.
Is an ICD indicated for survivors of cardiac arrest?
Survivors of sudden cardiac arrest from ventricular fibrillation or sustained ventricular tachycardia are candidates for a secondary prevention ICD, unless the arrhythmia had a reversible cause. The device is placed to prevent recurrence of the same life-threatening arrhythmia.
Why is an ICD delayed after myocardial infarction?
An ICD is delayed for about 40 days after myocardial infarction and about 90 days after revascularisation, because the ejection fraction often recovers during that time. Implanting too early could mean fitting a device that may not be needed.