A magnet is placed over a pacemaker in three main situations: during surgery to protect against electromagnetic interference, during certain device malfunctions, and in end-of-life care to modify therapy. The magnet activates an internal sensor that switches the pacemaker into a fixed-rate pacing mode, temporarily overriding its normal programming. This is a clinical tool, not something done casually, and the response varies by device manufacturer.
What the Magnet Actually Does
Under normal conditions, a pacemaker listens to the heart’s electrical activity and only fires when it detects a missed or slow beat. This is called “demand” pacing. When a clinical magnet is held directly over the pulse generator (the metal case under the skin, typically below the collarbone), it triggers an internal sensor that switches the device into asynchronous mode. In asynchronous mode, the pacemaker delivers pacing pulses at a steady, fixed rate regardless of what the heart is doing on its own.
For dual-chamber pacemakers, the magnet triggers pacing in both chambers at the fixed rate. For single-chamber devices, it paces just the one chamber. The effect is immediate in most devices, though some Abbott (formerly St. Jude Medical) pacemakers take up to 5 seconds to switch because the device stores an internal recording first. The moment you remove the magnet, the pacemaker returns to its normal programmed mode.
During Surgery
The most common reason to place a magnet over a pacemaker is to protect it from electromagnetic interference during a surgical procedure. Electrosurgical tools (the instruments used to cut tissue and stop bleeding) generate strong electromagnetic fields that can confuse the pacemaker’s sensing circuits. If the device misreads that electrical noise as a heartbeat, it may stop pacing when the patient actually needs it. Switching to asynchronous mode with a magnet eliminates this risk because the pacemaker ignores all incoming signals and just paces at its fixed rate.
Not every surgery requires magnet application. It depends on where the procedure is happening, what instruments are being used, and how dependent the patient is on the pacemaker. If someone’s heart rarely needs pacing support, the risk from interference is lower. But for patients who rely on the device for every heartbeat, having the magnet ready or applied before the procedure starts is standard practice.
During Device Malfunctions
A magnet can serve as a quick diagnostic and treatment tool when a pacemaker appears to be misbehaving. One specific scenario is pacemaker-mediated tachycardia, a type of abnormally fast heart rate caused by a feedback loop within the device itself. The pacemaker senses electrical activity traveling backward through the heart and responds by pacing faster, which triggers more backward conduction, which triggers more pacing. Placing a magnet over the device breaks this loop by switching to asynchronous mode. If the tachycardia resolves with magnet application, that strongly suggests the pacemaker was causing it.
The magnet also helps identify battery or lead problems. If the pacemaker fails to produce pacing signals even in magnet mode, that points to a hardware failure, such as a depleted battery or a broken lead wire, rather than a software or sensing issue.
Checking Battery Status
Each pacemaker manufacturer programs its devices to pace at a specific fixed rate during magnet application, and that rate drops as the battery drains. By measuring the pacing rate with a magnet applied, clinicians get a quick read on battery life without needing the full programmer.
The magnet rates vary considerably by manufacturer:
- Abbott (formerly St. Jude Medical): 100 beats per minute with a fresh battery, dropping to 85 when replacement is recommended, and gradually declining to 60 at end of life
- Boston Scientific: 100 beats per minute with a fresh battery, 85 at replacement time
- Biotronik: 90 beats per minute with a fresh battery, 80 at replacement time
- Medtronic: 85 beats per minute with a fresh battery, 65 at replacement time
- MicroPort (formerly Sorin): 96 beats per minute with a fresh battery, 80 at replacement time
A significant drop from the expected fresh-battery rate tells the clinical team it’s time to schedule a generator replacement. This is especially useful in situations where a full device interrogation isn’t immediately available.
In End-of-Life and Palliative Care
For patients with implantable cardioverter-defibrillators (ICDs), which combine pacing with the ability to deliver shocks for dangerous heart rhythms, magnet application plays an important role in comfort care. One of the most distressing experiences for a dying patient is receiving repeated shocks from a device that detects rhythm changes that are part of the natural dying process. Placing a magnet over an ICD suspends its shock function for as long as the magnet stays in place.
The Heart Rhythm Society’s expert consensus recommends that clinicians consider providing a doughnut magnet, along with clear instructions, to patients diagnosed with a terminal illness. This gives families or hospice staff a way to stop shocks quickly if they occur, even when a device specialist isn’t available. The magnet only suspends shock therapy in ICDs. It does not turn off basic pacing, which requires reprogramming by someone with the device’s proprietary programmer.
Pacemakers vs. ICDs: Different Magnet Responses
This distinction matters because the two device types respond to magnets in fundamentally different ways. A magnet placed over a standard pacemaker switches it to fixed-rate pacing. A magnet placed over an ICD suspends its ability to detect and treat fast, dangerous rhythms (the shock and rapid-pacing therapies) but does not necessarily change how it handles basic pacing. Confusing the two responses could be dangerous, so knowing what type of device a patient has is essential before applying a magnet.
Research using Apple’s iPhone 12 Pro Max demonstrated that even consumer electronics with strong magnets can trigger these responses. Medtronic and Abbott ICDs showed inhibition of shock therapies, while Boston Scientific and other pacemakers switched to asynchronous pacing, simply from proximity to the phone. This underscores why patients are advised to keep phones and magnetic accessories away from the area over their device.
Risks of Asynchronous Pacing
Asynchronous pacing is generally well tolerated, but it’s not without risk. Because the pacemaker fires regardless of the heart’s own rhythm, its electrical pulses can land at the same moment the heart is naturally contracting. Most of the time this just feels uncomfortable. In extremely rare cases, a pacing pulse that lands during a specific vulnerable window of the heartbeat can trigger a dangerous heart rhythm. This is sometimes called the R-on-T phenomenon. The risk is low enough that magnet application remains a routine clinical tool, but it’s one reason the magnet is applied intentionally and monitored rather than left in place indefinitely without oversight.
Some patients also notice symptoms from the loss of coordinated timing between the upper and lower chambers. In normal demand mode, the pacemaker fine-tunes the delay between atrial and ventricular pacing. In magnet mode, that coordination is simplified, which can cause a sensation of pounding or mild lightheadedness in some people.
Where to Place It
The magnet goes directly over the pulse generator, the rectangular or oval metal case implanted under the skin. In most patients, this is in the upper chest just below the collarbone, typically on the left side. You can usually see or feel a slight bulge. The magnet must be centered over the device, close enough that the magnetic field reaches the internal sensor. A standard clinical “doughnut” or ring magnet is used because it provides a uniform field across the surface of the generator. Holding it flat against the skin over the device is the correct positioning. If the magnet is off-center or too far away, the device won’t respond.