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Quantification of the Safety Distance Between ICDs and Phones Equipped With MagnetsFree Access

Research Letter

J Am Coll Cardiol EP, 7 (8) 1066–1068
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Introduction

Modern implantable cardioverter-defibrillators (ICDs) are equipped with switches that can be triggered by an externally applied magnetic field to deliberately activate certain functions or to suspend therapy. The European standard EN 45502-2 for active implantable medical devices requires a full functionality up to 10 G. Ryf et al (1) observed in vitro a mode switch at a field strength of 5 G in several devices. They report magnet interferences between ICDs and neodymium magnets for office use exhibiting 5 G at distances up to 10.5 cm (1). Recent phones are now equipped with strong magnets to facilitate the alignment of the phone with accessories and accelerate the speed of wireless charging.

Greenberg et al (2) recently presented the first case of an inhibition of an ICD (Medtronic, Minneapolis, Minnesota), while applying the Apple Inc iPhone 12 Pro Max on a patient’s chest. The application of this iPhone near the ICD device implanted on the left chest triggered a suspension of the ICD high-voltage shock therapies. An ex vivo analysis by Patterson et al (3) supported these findings. Held et al (4) repeated the experiment with 4 different smartphones without triggering the investigated cardiac implantable electronic devices. They concluded that the alignment of the smartphones relative to the devices must be extremely precise and that a 3-dimensional mapping of the magnetic fields should be performed to characterize the risk of activating ICD magnet modes.

To address this need, we performed a quantitative magnetic field mapping near the surface of the iPhone 12 Pro Max at several distances to the back side of the phone to identify the distances where the field strength reaches values of interest of 5 and 10 G (Figure 1A). As a comparison, the magnetic field map of a clinical ring magnet is shown in Figure 1B. The mapping has been performed with an in-house magnetic field mapper (Figure 1C), which consists of an arrangement of 4 × 4 = 16 3-axis Hall sensors (AK09970N, Asahi Kasei Microdevices Corporation, Tokyo, Japan). The sensors exhibit a resolution of 11 mG for continuous field measurements. The mapper has been characterized in a calibrated Helmholtz coil. This allows us to reach a measurement uncertainty of ±5%. We have not performed any experiments on patients. This work did not require any ethical approval. We observed that even though the magnetic field of the phone is approximately 16 times weaker than the medical ring magnet for all distances, the 10G threshold is exceeded for distances smaller than 23.65 mm above the surface of the phone over an area of 48 × 48 mm. Increasing the distance to the phone to 30.80 mm reduced the magnetic field below 5 G.

Figure 1
Figure 1

Magnetic Field Maps of an iPhone 12 Pro Max Compared to Maps of a Clinical Ring Magnet

Experimental mapping of the magnetic field distribution (A) near the surface of the back side of the iPhone 12 Pro Max and (B) near the surface of a clinical ring magnet used to disable ICD therapy (B). The map was measured over a 48 × 48 mm area at 3 different heights along the z-axis. (C) Each magnetic field map has been obtained with a magnetic field mapper, which consists of an arrangement of 4 × 4 = 16 magnetic field sensors.

In conclusion, this quantitative assessment of the field strength and distribution might help physicians define recommendations on the positions of the phone to be avoided by the patient, for instance, in the pocket of a shirt. Our findings show that with the safety distance of 150 mm provided by Apple for the iPhone 12 Pro Max (5), the patient is on the safe side, and smaller distances might be tolerated. Compared to multiple single-point measurements, the custom-designed magnetic field mapper will allow us to quickly map the field strength of many samples of the iPhone 12 Pro Max to obtain a statistical distribution of the entire magnetic field map for this phone model. Subsequently, further phone models and wearables equipped with magnets will be characterized with the mapper.

References

  • 1. Ryf S., Wolber T., Duru F., Luechinger R. "Interference of neodymium magnets with cardiac pacemakers and implantable cardioverter-defibrillators: an in vitro study". Technol Health Care 2008;16:1: 13-18.

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  • 2. Greenberg J.C., Altawil M.R., Singh G. "Lifesaving therapy inhibition by phones containing magnets [letter]". Heart Rhythm 2021;18:1040-1041.

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  • 3. Patterson Z, Straw S, Drozd M, et al. "New phones, old problem? Interference with cardiovascular implantable electronic devices by phones containing magnets [letter]". Heart Rhythm. 2021;18:1041.

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  • 4. Held EP, Shehata M, Swerdlow CD, Sandhu R. "Interference of smartphones containing magnets and cardiac implantable electronic devices—is this common? [letter]". Heart Rhythm. 2021;18:1042-1043.

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  • 5. "About the magnets inside iPhone 12, iPhone 12 mini, iPhone 12 Pro, iPhone 12 Pro Max, and MagSafe accessories". Available at: https://support.apple.com/en-us/HT211900. Accessed April 15, 2021.

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Footnotes

The authors have reported that they have no relationships relevant to the contents of this paper to disclose.

The authors attest they are in compliance with human studies committees and animal welfare regulations of the authors’ institutions and Food and Drug Administration guidelines, including patient consent where appropriate. For more information, visit the Author Center.