Cardiovascular · Topic 22 of 48

Pacemakers, ICDs & Temporary Pacing

Cardiovascular · cardiac devices

Pacemakers, ICDs & Temporary Pacing

Spikes without capture, pauses from oversensing — and a magnet makes a pacemaker pace asynchronously but only silences an ICD's shocks.

Key numbers

Magnet on a pacemaker: asynchronous pacingmagnet rate 85–100/min

Electrical storm: 3 or more VT episodes/24 h≥ 3 VT episodes in 24 h

ICD for LVEF ≤ 35% despite therapyLVEF ≤ 35%

01 · Look forFailure to capture: spike with no QRS

Failure to capture = pacing spikes not followed by a P wave or QRS; causes include lead dislodgement or disconnection, fibrosis, MI, electrolyte imbalance and drugs. Check connections, raise output and correct the cause.

02 · The ruleMagnet on a pacemaker: asynchronous pacing

A magnet over a pacemaker switches it to asynchronous pacing (AOO, VOO or DOO) at a fixed manufacturer rate of about 85–100/min, ignoring sensed beats; removing it restores the programmed mode.

03 · The ruleMagnet on ICD: shocks stop, pacing unchanged

A magnet over most ICDs suspends tachycardia detection and shocks without changing bradycardia pacing, so a pacing-dependent ICD patient needs reprogramming to asynchronous pacing before electrocautery.

What it is

Core idea

Pacing code: chamber paced, sensed, responseThe pacemaker code lists chamber paced, chamber sensed and response to a sensed beat (I = inhibit, T = trigger, D = both), then R for rate modulation — eg, VVIR.

Presentation

What you'll see

Oversensing: pacing withheld, causing pauses must knowOversensing: the device reads large T waves, skeletal myopotentials or electromagnetic interference as cardiac activity and inhibits pacing, causing pauses or bradycardia; a magnet (asynchronous mode) or a less sensitive setting restores pacing.

Undersensing: spikes fire despite native beatsUndersensing: the device misses intrinsic beats and paces inappropriately, so spikes fall on or soon after native complexes; correct by making the device more sensitive (lower the mV value).

Failure to pace: no spikes when neededFailure to pace (no output) — absent spikes during bradycardia — comes from oversensing, battery or lead failure, or a loose connection: check connections, maximize output, switch to asynchronous mode and prepare transcutaneous pacing.

Diagnosis & monitoring

Work-up

Electrical storm: 3 or more VT episodes/24 hElectrical storm = 3 or more distinct VT episodes in 24 hours or incessant VT: correct potassium and magnesium, sedate, give IV amiodarone or a nonselective beta-blocker (propranolol), and involve electrophysiology for ablation.

ICD for LVEF ≤ 35% despite therapyImplant an ICD after VF or unstable VT without a reversible cause, or for LVEF ≤ 35% with NYHA II–III on optimal therapy (if ischemic, ≥ 40 days post-MI and ≥ 90 days post-revascularization).

Do right now

In this order

First moves

Unstable bradycardia: transvenous pacing if drugs fail

→

Transcutaneous pacing: confirm capture with a pulse

→

Failure to capture: spike with no QRS

Why each step

Unstable bradycardia: transvenous pacing if drugs failFor unstable bradycardia unresponsive to atropine, bridge with transcutaneous pacing or an epinephrine or dopamine infusion and place a temporary transvenous pacer; transcutaneous pacing is unreliable for prolonged pacing.

Transcutaneous pacing: confirm capture with a pulseTranscutaneous pacing: anterior–posterior pads, rate about 70–80/min, raise current until every spike has a wide QRS with a distinct ST–T, then confirm a matching femoral pulse; give analgesia and sedation.

Failure to capture: spike with no QRSFailure to capture = pacing spikes not followed by a P wave or QRS; causes include lead dislodgement or disconnection, fibrosis, MI, electrolyte imbalance and drugs. Check connections, raise output and correct the cause.

Management

Orders

Inappropriate ICD shocks: magnet, pads, interrogateFor unnecessary or inappropriate shocks (supraventricular rhythm, T-wave oversensing, lead noise) in a stable patient, place a magnet to suspend therapy, keep external defibrillator pads on and monitor, and have the device interrogated.

Before surgery: plan device reprogrammingFor surgery above the umbilicus, reprogram pacing-dependent patients to asynchronous pacing and suspend ICD therapies (programming or magnet), prefer bipolar cautery or short bursts, keep pads on, and interrogate afterward.

Red flags & don'ts

Avoid harm

Pacemaker-mediated tachycardia: magnet or adenosine ends it complicationPacemaker-mediated (endless-loop) tachycardia in dual-chamber devices: a retrograde P wave is sensed and triggers another ventricular paced beat in a repeating cycle at the upper rate; a magnet or adenosine terminates it.

Pacemaker syndrome: restore AV synchrony complicationPacemaker syndrome: ventricular pacing without AV synchrony causes light-headedness, neck pulsations or dyspnea; treat by restoring AV synchrony, most often with dual-chamber (DDD) pacing.

Undersensing or oversensing?

Commonly confused

What goes wrong	Device misses the patient's own beats
On the ECG	Spikes fire on or right after native complexes
Clinical effect	Inappropriate, competing pacing
Sensitivity fix	More sensitive: lower the mV value
Bedside rescue	Reprogram sensing

More detail

Reference

Show 6 reference facts

Most patients capture at 40–80 mA transcutaneously; thresholds are higher with emphysema, pericardial effusion or positive-pressure ventilation. In unconscious patients start at maximum output, then set 5–10 mA above threshold.

Transcutaneous pacing is relatively contraindicated in severe hypothermia (bradycardia may be physiologic) and is not routinely recommended for asystolic arrest.

ICDs terminate VT/VF with antitachycardia pacing or shocks but do not prevent them; antiarrhythmic drugs are often added to reduce episodes and shocks. Generators typically last 5–7 years.

Other platforms: leadless VVI(R) pacemakers sit entirely in the right ventricle; the subcutaneous ICD has no intravascular lead; a wearable defibrillator vest covers short-term risk.

Electromagnetic interference comes from electrocautery and MRI (unless an MRI-conditional system); phones should not rest over the device, and walking through metal detectors is safe without lingering.

For temporary pacing wires, find the capture threshold and set output at about double it; if failure to pace, raise output to maximum (about 20 mA atrial, 25 mA ventricular).

Pearl

Magnet on a pacemaker: it paces at a fixed rate and ignores the heart. Magnet on an ICD: the shocks stop, the pacing does not change.

Sources · Merck Manual Professional — Cardiac Pacemakers · Merck Manual Professional — Implantable Cardioverter-Defibrillators (ICDs) · Using transcutaneous cardiac pacing to best advantage: how to ensure successful capture and avoid complications — PMC6376978 · Perioperative management of patients with cardiac implantable electronic devices. Korean J Anesthesiol 2024 — PMC11150116 · Management of hemodynamically stable wide QRS complex tachycardia in patients with ICDs. Front Cardiovasc Med 2022 · 2025 AHA Guidelines for CPR and ECC — Part 9: Adult Advanced Life Support (bradycardia) · AHA 2025 ACLS algorithms poster set (JN-2788, 12/25): Adult Bradycardia With a Pulse · LITFL — Temporary Pacemaker Troubleshooting · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).