Implanted Cardiac Devices: Pacemakers, ICDs and Wearable Defibrillators

In depth

The one-page summary above is what the PDF prints. These sections go further.

Remote monitoring and device data

• Remote monitoring is standard care: the home unit sends stored data (rhythms, therapies, leads, battery), often daily. It is not an emergency service.
• Device-detected AF (atrial high-rate episodes) is confirmed on stored electrograms and reported for a stroke-risk and anticoagulation decision.
• CRT works only with near-constant biventricular pacing (target above 95%); PVCs or AF that cut the percentage are reported.

Pacemaker code, guideline sources and exam points

Unnecessary right ventricular pacing can worsen heart function; programming aims to keep it low (ESC 2021 cites about 20% as an observational threshold).

More bedside detail: pacemaker and ICD problems

Causes of inappropriate shocks: sinus tachycardia or SVT above the detection rate, AF or flutter, T-wave oversensing (the main S-ICD cause) and noise from a broken lead.

Signs a patient should report at home: dizziness, fainting or near-fainting, shortness of breath, chest pain, return of the old symptoms, or abdominal pulsing or twitching near the device.

More on procedures, MRI, the vest and follow-up

• Before surgery: know the device type and whether the patient is pacing dependent.
• Prefer bipolar cautery; otherwise short bursts, cut mode and the return pad placed so current does not cross the device.
• Monitor ECG and pulse oximetry throughout, with a defibrillator and external pacing at hand and trained staff.
• Abandoned or epicardial leads and older non-conditional systems need specialist review; risks include lead heating, threshold change and power-on reset.
• After the elective replacement indicator, about 90 days of normal function remain.
• Abnormal lead impedance, noise and more than 2 shocks are high-priority alerts.
• Other early problems after implant: pneumothorax, lead dislodgement, infection.

How the systems are built

A transvenous pacemaker or ICD has a generator in a pocket under the skin or muscle of the upper chest and leads passed through a vein into the right atrium, the right ventricle, or both. CRT adds a third lead that reaches the outside of the left ventricle through the coronary sinus. Modern generators use low-energy circuits and steroid-eluting leads, which helps them last longer. Merck gives 5 to 7 years for ICD generators; MedlinePlus tells patients 6 to 15 years for a pacemaker battery and 4 to 8 years for an ICD. Replacement is a minor operation that swaps the generator and usually keeps the leads.

Leadless pacemakers combine the generator and electrode in a small capsule placed through the femoral vein into the right ventricle and held by tines or a screw. Current devices pace the ventricle only. ESC 2021 notes the lack of randomized comparisons with transvenous single-chamber systems and the difficulty of retrieving a capsule at the end of its life.

The S-ICD places the generator on the side of the chest wall and tunnels a single lead under the skin beside the sternum, so nothing sits inside a vein or the heart. Its shock is larger than a transvenous ICD shock (80 J). Before implant the surface ECG is screened to check that the device will tell the R wave from the T wave; about 8 to 15% of people fail this screen. Lead complications and systemic infection are less common than with transvenous systems, which is why it suits young patients, people with poor venous access, and people at high infection risk such as those on dialysis. It is a poor choice when bradycardia pacing, CRT or antitachycardia pacing for monomorphic VT is likely to be needed.

Modes, timing and pacemaker-specific problems

Merck notes that VVI and DDD give similar survival, but physiologic pacing that keeps the atrium in the loop (AAI, DDD, VDD) may lower the risk of AF and heart failure and slightly improves quality of life. ESC 2021 makes DDD(R) the first choice in sinus node disease and prefers dual-chamber pacing in AV block when the patient is in sinus rhythm, with VVIR reasonable in frail elderly patients or paroxysmal block.

Mode switching protects dual-chamber patients in AF: the device stops tracking the fast atrial rate and paces the ventricle at a programmed rate instead. Pacemaker-mediated tachycardia is a loop in a normal dual-chamber system: a premature or paced ventricular beat conducts back to the atrium, the device senses that atrial signal and paces the ventricle again, and the cycle repeats at a fast rate. The device team reprograms the device to prevent it. Cross-talk is the ventricular channel sensing the atrial pacing pulse and withholding a ventricular beat.

Pacemaker syndrome comes from ventricular pacing without AV synchrony: atria contract against closed valves, causing light-headedness, neck pulsation and dyspnea. The fix is a mode that restores AV timing, most often DDD.

In twiddler syndrome the patient repeatedly rotates the generator in a loose pocket. The leads wind around the generator and their tips pull out of the heart, so capture is lost, and the leads may stimulate the phrenic nerve (diaphragm pacing with abdominal pulsation) or the brachial plexus (rhythmic arm movements). The chest x-ray shows the twisted leads. Older and obese patients with loose tissue are at higher risk; suturing the generator to the fascia helps prevent it. A pacing-dependent patient may need temporary pacing until the leads are repositioned.

Lead impedance is trended at every check, and a sudden change matters more than any single value. Classic teaching: a sudden large rise suggests a conductor fracture or loose connection, while a sudden fall suggests an insulation breach. A fracture also produces bursts of very short, irregular, nonphysiologic signals that an ICD can count as VF, giving shocks while the patient sits in sinus rhythm.

ICD and WCD decisions in more depth

Secondary prevention: after VF or hemodynamically significant VT that was not caused by a reversible or transient problem, the ICD is the preferred treatment (Merck). EF does not decide it. The class III list in Merck's table explains when not to implant: VT or VF from a correctable cause, incessant VT or VF, arrhythmias that ablation can cure, and no reasonable expectation of at least a year of survival with acceptable function.

Primary prevention in ischemic cardiomyopathy waits at least 40 days after MI and 90 days after revascularization, because EF often recovers. The thresholds in Merck's table are EF 35% or less with NYHA class II or III, or EF 30% or less with class I. Nonischemic dilated cardiomyopathy with EF 35% or less and class II or III symptoms qualifies after optimal medical therapy, which the ESC frames as about 3 months. Merck's class IIa list adds cardiac sarcoidosis with EF above 35% plus unexplained syncope, scar on MRI or PET, inducible VT or VF, or a pacing indication.

The wearable defibrillator covers the waiting periods. In the VEST trial (2,302 post-MI patients with EF 35% or less), the primary endpoint of sudden or VT death was not significantly reduced (1.6% vs 2.4%), but all-cause death was lower (3.1% vs 4.9%). ESC 2022 gives the WCD class IIa for adults with a secondary-prevention indication who temporarily cannot have an ICD and class IIb in selected patients early after MI. Shocks are 75 to 150 J, up to five per episode. Observational and registry studies report wear times of 20 to 23.5 hours a day (the VEST median was 18); inappropriate shocks run about 2% in pooled data.

Electrical storm (ESC 2022): ventricular arrhythmia three or more times within 24 hours, separated by at least 5 minutes, each needing an intervention to stop it. Sympathetic drive feeds it, so sedation and a nonselective beta-blocker such as propranolol, usually with amiodarone, come first, along with correcting ischemia, electrolytes and heart failure. Deep sedation and intubation are considered for intractable storm, then autonomic modulation or catheter ablation.

Before any implant, and again as illness advances, the team and patient discuss what the ICD will do near the end of life, including whether and when to turn shocks off.

Procedures, magnets and MRI in more depth

Magnet rates differ by maker (Fernando 2026): 100/min for Abbott and Boston Scientific, 90/min for Biotronik, 85/min for Medtronic and 96/min for Microport, with lower rates when the battery reaches its replacement point. Biotronik's magnet response times out after 8 hours unless the magnet is lifted and replaced. A transvenous ICD does not pace asynchronously with a magnet; it only stops tachy therapy. The S-ICD tone lasts about 60 seconds; if no tone is heard, reprogramming is preferred. The Micra does not respond to a magnet, while the AVEIR switches to asynchronous pacing.

Societies divide surgical risk at the umbilicus, and EHRA/HRS also uses 15 cm from the generator. Bipolar instruments carry essentially no interference risk. With monopolar cautery, lower power, cut rather than coagulation mode, short bursts and a return pad on the opposite lower limb keep current away from the device. The reviewers recommend disabling ICD tachytherapies whenever electrosurgery is used. Afterward the device is interrogated, especially after emergency surgery without a recent check, interference close to the device, a suspected permanent change, or any delivered therapy.

For MRI, Maass and colleagues describe programming a dependent patient to VOO or DOO at 80 to 90/min with high output (RV-only in a biventricular device), and a non-dependent patient with a resting rate above 50/min to VVI or DDI at 30 to 40/min. Rate response, magnet response and ICD therapies are turned off. For non-conditional systems, 1.5 T is preferred. Power-on reset can change the mode and make the device prone to oversensing; a device near battery depletion should not be scanned without a plan.

Implant, anticoagulation and the first weeks

Pocket hematoma occurs in about 2 to 5% of implants, usually within the first week. It matters because it raises the chance of device infection several-fold and can force reoperation and a pause in anticoagulation. In BRUISE CONTROL (patients with an annual clot risk of 5% or more), continuing warfarin gave clinically significant hematoma in 3.5% versus 16.0% with heparin bridging. DOACs are handled by individual risk; dual antiplatelet therapy is a strong bleeding risk factor.

Arm restrictions protect the new leads while scar tissue anchors them. The most common late problems are infection (fever, chills, pocket pain or redness, which can appear months after surgery), lead dislodgement or fracture, and device recalls.

Living with a device: teaching in plain words

• Most household appliances are safe when they work properly.
• Hold the phone to the ear on the other side and keep it out of the shirt pocket over the device.
• Airport body scanners appear safe; show the card if the alarm sounds.
• Tell every doctor and dentist about the device before a procedure.
• Shocks: one shock and you feel fine, call the clinic; several shocks or you feel ill, call 911.
• Remote monitoring works only if the home unit is plugged in and paired; many patients never activate it.