There’s a small but mighty tool that’s been part of cardiac device management for decades: the cardiac magnet. Flat, circular, and deceptively simple, it can change the behavior of a pacemaker or ICD in seconds—no programmer, no electrophysiologist, no reprogramming required.
In the perioperative setting, understanding when and why to apply a magnet can be the difference between a safe case and a dangerous one.
What Is a Cardiac Magnet?
A cardiac magnet is a simple donut-shaped permanent magnet that, when placed over a patient’s implanted device, triggers a predictable, device-specific response. It works by activating a reed switch inside the device—a small sensor designed to detect an external magnetic field.
The key word here is predictable. While the exact response varies by manufacturer and model, the effects are well-defined and have been established for years. That’s precisely what makes the magnet so useful in emergencies.
What Happens When You Put a Magnet on a Pacemaker?
For a pacemaker, applying a magnet causes the device to switch to an asynchronous pacing mode—most commonly DOO (for dual-chamber devices) or VOO (for single-chamber ventricular devices). In this mode, the pacemaker delivers pacing pulses at a fixed rate regardless of the patient’s underlying rhythm.
Why does this matter in the OR? Electrosurgical units (ESUs)—the most common source of electromagnetic interference (EMI) during surgery—can be sensed by a pacemaker as intrinsic cardiac activity. When that happens, the device may inappropriately inhibit pacing. For a pacemaker-dependent patient, this inhibition can cause life-threatening bradycardia or even asystole.
Applying a magnet eliminates this risk by removing the sensing function entirely. The device simply paces—no listening, no inhibition, no interference.
What Happens When You Put a Magnet on an ICD?
For an ICD, the magnet response is different—and it’s a distinction worth emphasizing.
Placing a magnet over an ICD suspends tachyarrhythmia detection and therapy. This means the device will not deliver shocks or anti-tachycardia pacing (ATP) while the magnet is in place.
Importantly, a magnet does not convert an ICD to asynchronous pacing mode. The bradycardia pacing function of an ICD continues to operate normally, but shock therapy is disabled.
This is clinically critical in the OR. ESUs can generate electrical artifact that an ICD may misinterpret as ventricular fibrillation—prompting an inappropriate shock. A single inappropriate shock during surgery can be destabilizing and dangerous. Keeping a magnet applied throughout a case where EMI is expected can prevent this from happening.
When Would You Need to Use One in the OR?
The magnet is an intraoperative backup tool—most useful in unplanned or emergent situations where there isn’t time to have a device formally reprogrammed before surgery. Common scenarios include:
- Unplanned emergency surgery in a patient with a CIED, where preoperative device interrogation and reprogramming isn’t possible
- Unexpected or prolonged EMI during a case from electrosurgery, particularly above the umbilicus
- Loss of access to a device programmer or electrophysiology support
- Inappropriate ICD therapies occurring or anticipated during the procedure
For elective surgeries, the preferred approach is formal preoperative device interrogation and reprogramming by a qualified device team. But when that’s not possible, the magnet is a reliable, immediately available alternative.
Important Caveats
The magnet is powerful—but it’s not a universal solution. A few key points to keep in mind:
- Manufacturer differences matter. Most pacemakers respond to magnet application with asynchronous pacing at a fixed rate (often 85 bpm), but the exact behavior—including the asynchronous rate—varies by manufacturer and model. Some devices also have the magnet response programmed off.
- ICD magnet response can be disabled. In some patients, the ICD magnet response has been intentionally turned off. Applying the magnet in this scenario won’t suspend shock therapy. This is why knowing the device type and its programmed settings preoperatively is so important.
- Positioning matters. The magnet must be held or taped directly over the device to maintain consistent contact. If it slips, the device will revert to its programmed behavior.
- Removal = reactivation. As soon as the magnet is removed, the device returns to its normal settings. For an ICD, this means shock therapy is immediately restored—which is exactly what you want once the surgery is complete and EMI is no longer a concern.
The Bigger Picture
The cardiac magnet is a clinical workhorse—simple in design, but nuanced in application. Using it safely requires understanding not just what it does, but why the device responds the way it does, and how that response fits into the broader context of perioperative CIED management.
That’s exactly the kind of decision support the Periop Pacemaker app is built to provide. Whether you’re deciding if a magnet is appropriate, identifying the device type, or navigating a case where formal reprogramming wasn’t possible, the app puts expert-backed guidance at your fingertips—right when you need it most.
The Periop Pacemaker app is available on the App Store and Google Play.