4.6 Implantable and Wearable Cardioverter Defibrillators
Key Takeaways
- Primary-prevention ICD thresholds: LVEF 35% or less with NYHA II-III on at least 3 months of guideline-directed therapy, at least 40 days after MI and at least 90 days after revascularization; LVEF 30% or less qualifies at NYHA I.
- The subcutaneous ICD defibrillates but provides no bradycardia pacing, no antitachycardia pacing and no resynchronization, so it is wrong for patients who need pacing or CRT.
- Electrical storm is 3 or more separate appropriate device therapies in 24 hours; priorities are deep sedation, IV beta blockade, amiodarone, correction of ischemia and electrolytes (potassium 4.0-4.5 mEq/L), then stellate ganglion block or ablation.
- A magnet over an ICD suspends shocks and ATP only and does NOT make pacing asynchronous, whereas a magnet over a pacemaker forces asynchronous VOO/DOO pacing at 85-100 ppm - so a pacing-dependent ICD patient going to surgery needs reprogramming, not just a magnet.
- The wearable cardioverter defibrillator delivers up to a 150 J biphasic shock but does not pace; the conscious patient aborts a shock by holding the two response buttons, and efficacy requires 22-23 hours per day of wear time.
Why ICD and Wearable Defibrillator Content Appears on the CMC
Test-plan items III.D.4 and III.D.5 cover implantable and wearable defibrillators. On the exam these items are not about implant technique; they are about eligibility thresholds and waiting periods, what a device will and will not do, shock triage, and patient teaching. A cardiac medicine nurse is the person who fields "my device went off three times tonight," who explains why a patient with an ejection fraction of 30% four days after a myocardial infarction is going home in a vest instead of with an implant, and who has the deactivation conversation at end of life.
Indications: The Numbers That Decide
An implantable cardioverter-defibrillator (ICD) terminates ventricular tachycardia (VT) and ventricular fibrillation (VF). Primary prevention means no prior sustained ventricular arrhythmia; secondary prevention means the patient has already had one.
| Setting | Threshold and timing | Notes |
|---|---|---|
| Secondary prevention | Survivor of cardiac arrest from VF or hemodynamically unstable VT, or sustained VT with structural heart disease, not due to a reversible cause | Class I. "Reversible cause" is the trap: VF within 48 hours of an acute MI, or from profound hypokalemia, acute ischemia corrected by revascularization, or a QT-prolonging drug, is treated by removing the cause — not by implanting a device |
| Ischemic cardiomyopathy, primary prevention | LVEF ≤ 35%, NYHA class II–III, on ≥ 3 months of guideline-directed medical therapy, and ≥ 40 days after MI | Class I |
| Ischemic cardiomyopathy, NYHA I | LVEF ≤ 30%, ≥ 40 days post-MI, on GDMT | Class I |
| After revascularization | Wait ≥ 90 days after CABG or PCI before reassessing EF for primary prevention | EF often improves with revascularization |
| Nonischemic dilated cardiomyopathy | LVEF ≤ 35%, NYHA II–III, after ≥ 3 months of optimized GDMT | Class I in the 2022 AHA/ACC/HFSA heart failure guideline |
| All candidates | Reasonable expectation of meaningful survival > 1 year | Applies to every indication |
The recurring CMC vignette gives an EF of 28% on hospital day 3 after an anterior STEMI and asks what the patient needs. The answer is not an ICD: the 40-day and 90-day windows exist because the DINAMIT and IRIS trials showed no mortality benefit to early implantation, and because ejection fraction frequently recovers on beta blocker, ACE inhibitor or ARNI, mineralocorticoid receptor antagonist and SGLT2 inhibitor therapy. The bridge is a wearable device.
Transvenous vs Subcutaneous ICD
The subcutaneous ICD (S-ICD) places the pulse generator at the left mid-axillary line with a parasternal subcutaneous electrode. Nothing enters the heart or the vasculature.
| Capability | Transvenous ICD | Subcutaneous ICD |
|---|---|---|
| Defibrillation for VF | Yes (typically 30–40 J) | Yes (80 J shock) |
| Bradycardia pacing | Yes | No (only post-shock transthoracic pacing for up to about 30 seconds) |
| Antitachycardia pacing (ATP) | Yes | No |
| Cardiac resynchronization | Yes (CRT-D) | No |
| Intravascular lead | Yes | No |
| Risk profile | Lead fracture, venous occlusion, systemic lead infection, tricuspid regurgitation | No intravascular infection risk; higher inappropriate-shock rate from T-wave oversensing; requires pre-implant surface ECG screening |
The S-ICD suits young patients with long device lifetimes ahead of them, patients with no vascular access or occluded veins, dialysis and other high-infection-risk patients, patients with a prior device infection, and those with no pacing or resynchronization indication. If the patient has bradycardia, needs ATP for recurrent monomorphic VT, or qualifies for CRT, the S-ICD is the wrong device.
Detection Zones and Tiered Therapy
Devices are programmed with rate-based detection zones, usually a slower VT zone (commonly around 170–200 bpm) and a VF zone (commonly above 200–220 bpm), plus a monitor-only zone that records without treating. Therapy escalates in tiers:
- Antitachycardia pacing (ATP) — a burst or ramp of pacing stimuli faster than the VT rate, delivered to break a reentrant circuit. It is painless and imperceptible, terminates a large majority of monomorphic VT, and is attempted first in the VT zone (and, when programmed, even during VF-zone charging).
- Low-energy synchronized cardioversion — a small shock delivered on the R wave for organized VT that survives ATP.
- High-energy defibrillation — an unsynchronized maximal shock, typically 30–40 J in a transvenous device, for VF or for VT that has degenerated.
Modern programming deliberately uses long detection durations and high rate cutoffs (MADIT-RIT, RAID) because delaying therapy lets nonsustained VT self-terminate, which reduces shocks and reduces mortality.
Electrical Storm
Electrical storm is defined as three or more separate, appropriate ventricular arrhythmia episodes requiring device therapy (ATP or shocks) within 24 hours, each separated by at least 5 minutes of restored rhythm; in a patient without a device, three or more sustained VT/VF episodes in 24 hours. It carries high short-term mortality and is a critical-care emergency, not a device-clinic problem.
Nursing and management priorities, in the order they matter:
- Sedation. Each shock is a catecholamine surge that begets the next arrhythmia. Deep sedation with propofol or dexmedetomidine, and in refractory cases intubation with general anesthesia, breaks that loop and is an early, not a last, step.
- Sympathetic blockade. Intravenous beta blockade is the pharmacologic cornerstone — esmolol infusion, or non-selective blockade (propranolol has outperformed metoprolol in randomized comparison when combined with amiodarone).
- Antiarrhythmic. Amiodarone 150 mg IV over 10 minutes, then 1 mg/min for 6 hours, then 0.5 mg/min. Lidocaine is an alternative in ischemic VT.
- Reverse the substrate. Rule out and treat acute ischemia (12-lead ECG, troponin, urgent angiography if indicated), decompensated heart failure, hypoxemia, acidosis, and drug proarrhythmia. Correct potassium to 4.0–4.5 mEq/L and magnesium to above 2.0 mg/dL.
- Recognize the exception. If the storm is polymorphic VT with a long QT (torsades), amiodarone is the wrong drug: give magnesium 2 g IV, stop QT-prolonging agents, correct potassium, and use overdrive pacing or isoproterenol to abolish the pauses.
- Escalation. Stellate ganglion block or thoracic epidural for sympathetic modulation, catheter ablation of the VT substrate, and mechanical circulatory support (intra-aortic balloon pump, Impella, VA-ECMO) as a bridge.
- Device reprogramming. Raising rate cutoffs, extending detection, and maximizing ATP reduce shock burden while the underlying problem is treated.
Inappropriate Shocks and the Magnet
An inappropriate shock is a shock delivered for a rhythm that is not ventricular. It occurs in roughly one in five ICD recipients over the device's life, is painful and traumatic, and is independently associated with worse outcomes.
| Cause | Recognition clue |
|---|---|
| Atrial fibrillation with rapid ventricular response | The most common cause; irregularly irregular narrow-complex tachycardia above the VT zone cutoff |
| Other SVT (sinus tachycardia, atrial flutter, AVNRT) | Rate exceeds the programmed cutoff; discriminators failed |
| Lead fracture or insulation failure | Nonphysiologic, high-frequency, saturated noise on the electrogram; sudden rise in lead impedance (fracture) or fall (insulation break); shocks often clustered and unrelated to symptoms |
| T-wave oversensing / double counting | Counted rate is exactly double the true rate; more common in the S-ICD |
| Electromagnetic interference | Shocks near arc welders, industrial motors, electrocautery, or a malfunctioning appliance |
Nursing response to a patient who has been shocked: assess responsiveness and rhythm first, obtain a 12-lead ECG and continuous telemetry, place the patient on a monitor, and check electrolytes. If the patient is conscious and alert and shocks are recurring, the rhythm is very unlikely to be VF and the shocks are likely inappropriate — apply a magnet over the generator and leave it in place, notify the electrophysiology service, and prepare for interrogation. Treat the underlying driver (rate control for AF with rapid ventricular response). If the patient is unresponsive and pulseless, the ICD is not a reason to withhold CPR or external defibrillation: perform standard resuscitation with external pads placed at least 8 cm from the generator, and rescuers may safely touch a patient during an internal shock.
Magnet Effects: The Comparison That Is Always Tested
| Device | Effect of a magnet over the generator | What it does NOT do |
|---|---|---|
| Permanent pacemaker | Converts to asynchronous fixed-rate pacing (VOO / DOO) at the manufacturer's magnet rate, typically 85–100 ppm; a markedly lower magnet rate signals battery elective replacement indicator | Does not disable anything else; a magnet cannot "turn off" a pacemaker |
| ICD (transvenous or subcutaneous) | Suspends tachyarrhythmia detection and all shock/ATP therapy for as long as the magnet stays in place | Does NOT switch pacing to asynchronous mode. The device's bradycardia pacing continues in its programmed mode and remains inhibitable by electromagnetic interference |
That distinction drives perioperative management. A pacemaker-dependent patient with an ICD going to surgery with monopolar electrocautery needs the device reprogrammed to an asynchronous mode plus therapy suspension — a magnet alone protects against inappropriate shocks but leaves pacing vulnerable to oversensing and inhibition. External defibrillation pads must be applied and the patient continuously monitored whenever therapy is suspended, and the device must be reactivated and interrogated afterward. Other perioperative measures: prefer bipolar cautery, use short intermittent bursts, position the dispersive return pad so the current path does not cross the generator, and keep the cautery tip more than 15 cm from the device.
MRI and Device Management
Most contemporary systems are MRI-conditional, meaning the generator and all leads are labeled conditional, there are no abandoned or epicardial leads, and the scan follows the manufacturer's field-strength and specific-absorption-rate conditions. The device is placed in MRI mode before the scan (asynchronous pacing for the dependent patient, therapies off for an ICD), the patient is monitored with ECG and pulse oximetry throughout by staff capable of resuscitation, and the device is reprogrammed and interrogated immediately afterward. Non-conditional systems are no longer an absolute bar: scanning at 1.5 T under a formal institutional protocol with interrogation before and after is accepted practice for a clinically necessary scan. Nursing role: confirm the device card and model, verify the order and the pre/post interrogation are scheduled, and ensure therapies are restored before the patient leaves the monitored area.
The Wearable Cardioverter Defibrillator
The wearable cardioverter defibrillator (WCD), in the United States the LifeVest, is a garment with four dry ECG sensing electrodes and three defibrillation electrodes connected to a belt-worn monitor. It fills the gap where sudden death risk is real but an implant is either premature or impossible.
Indications (bridge therapy):
- LVEF ≤ 35% within the 40-day post-MI or 90-day post-revascularization waiting period.
- Newly diagnosed nonischemic cardiomyopathy during the 3-month window while guideline-directed therapy is titrated and ejection fraction is reassessed.
- After ICD explantation for pocket or lead infection, while the infection is treated and reimplantation is planned.
- Listed for cardiac transplant, awaiting an implant date, or temporarily not a candidate for implantation.
Current guidance is a Class IIb recommendation; the VEST trial did not meet its primary arrhythmic-death endpoint, so the WCD is positioned as reasonable bridge therapy, not a mandate.
How it works and what patients must be taught:
- It is a defibrillator only. It does not pace — no bradycardia backup, no antitachycardia pacing, no resynchronization. A patient who becomes bradycardic or asystolic gets nothing from the vest.
- On detecting a treatable ventricular rhythm the device alarms in escalating stages (vibration, then audible alarm, then a spoken bystander warning). The conscious patient holds down the two response buttons to withhold the shock — this is the conscious-patient abort, and it is the single most important teaching point, because a conscious patient with a perfusing rhythm can prevent an unnecessary shock. Releasing the buttons, or never pressing them because the patient has lost consciousness, allows the sequence to proceed.
- Immediately before discharge the device extrudes conductive blue gel from the therapy electrodes, then delivers up to a 150 J biphasic shock; up to five shocks may be given in one episode. Bystanders may safely touch the patient.
- Wear time drives efficacy. The garment must be worn at least 22–23 hours per day — during sleep and all activity — and removed only for showering, preferably with another adult present. The device must never be worn in the shower or bath.
- Practical teaching: keep a charged spare battery and swap daily; wash the garment and change it per schedule (typically weekly); wear it against bare skin with no lotions, powders or undergarments beneath the electrode belt; the garment should be snug, not loose, or sensing artifact and false alarms result; respond to every alarm; notify the monitoring service and the clinic after any shock or any alarm; carry the device information; do not drive until cleared.
- Common reasons patients abandon the vest — itching, poor fit, sleep disturbance, and false alarms from a loose garment — are all addressable, and adherence counseling is a nursing intervention that measurably changes outcomes.
End-of-Life Device Deactivation
Patients with advanced heart failure, terminal illness, or a decision to move to comfort-focused care should be offered ICD shock deactivation. Shocks at the end of life are painful, distressing to families, and do not change the trajectory. Key points for the nurse:
- Deactivating an ICD's tachytherapy is legally and ethically withdrawal of an unwanted therapy, equivalent to stopping any other treatment. It is not physician-assisted death or euthanasia, and it does not require any special legal instrument beyond informed consent from the patient or surrogate.
- Deactivation of shock therapy is separable from pacing. Most patients keep pacing on for symptom control; turning off pacing in a dependent patient is a distinct decision requiring its own explicit discussion.
- The conversation should occur before crisis — ideally at hospice enrollment, at the time a do-not-resuscitate order is written, or when goals of care shift — because DNR status alone does not deactivate an ICD.
- Formal deactivation is done by programmer. A magnet secured over the generator suspends therapy immediately and is the appropriate interim measure when a programmer is not available, including in the home or hospice setting; it must remain physically taped in place, and staff and family must understand that removing it restores shocks.
- Document the order, the discussion and participants, the method used, and confirm therapies are off; notify hospice or the receiving facility.
A 58-year-old woman is hospital day 4 after an anterior STEMI treated with primary PCI. Echocardiography shows an LVEF of 27%. She has NYHA class II symptoms on carvedilol, sacubitril-valsartan, spironolactone and empagliflozin. She asks why she is not getting a defibrillator before discharge. What is the nurse's best explanation?
A patient with a transvenous ICD and a history of paroxysmal atrial fibrillation arrives in the emergency department awake, anxious and reporting four shocks in the past hour. Telemetry shows atrial fibrillation at 195 bpm. The patient is talking in full sentences with a blood pressure of 118/70 mm Hg. What is the priority nursing action?
A patient is discharged with a wearable cardioverter defibrillator after ICD explantation for a pocket infection. Which statement by the patient indicates that further teaching is needed?