A medical device that relies on an electrical energy source or any power source other than what is directly generated by the human body or gravity to function, and is entirely or partially introduced inside the human body, is called an active implantable medical device (AIMD). Like any medical device, AIMDs are expected to maintain basic safety and essential performance without interfering with other equipment in their vicinity or the intended electromagnetic (EM) environment. The intent is to ensure the medical device will maintain the EM equilibrium condition that exists when equipment performs its designed functions without causing or suffering unacceptable degradation due to electromagnetic interference (EMI) to or from other equipment.

Usually, emission is not a problem for most AIMDs because they use less power. Also, because they are implanted inside the body, emissions are attenuated by the body tissue. AIMDs are subject to limited EM disturbance tests compared to medical devices external to the body and fall into the scope of IEC 60601. However, AIMDs are subjected to more severe levels of EM disturbance than the collateral 60601-1-2 standard because of their portability and exposure to dense EM environments, due to growing radio frequency (RF) and wireless devices. 

EMC Considerations
AIMDs are evaluated to the ISO 14708 or EN 45502 family of standards. The U.S. Food and Drug Administration (FDA) typically requires ISO 14708 and notified bodies require EN 45502. Like the IEC 60601-1 family, the ISO 14708 and EN 45502 family has a general standard all AIMDs are required to meet and particular standards that specific AIMDs are additionally required to meet. The manufacturer’s risk management process may also determine other requirements beyond the scope of the standards required as well. The U.S. is one of the ISO committee members (TC 150 SC 6) that played an active role in the ISO AIMD standard revision. U.S. participation in TC 150 SC 6 is administered by the Association for the Advancement of Medical Instrumentation (AAMI), who encourages their committee to harmonize their work with international standards as much as possible. AAMI and the American National Standards Institute (ANSI) usually revise their standards every five years to reflect technological advances.

Clause 27 of all parts of ISO 14708 deals with tests to determine the effects of EMI on an AIMD. The intent is to verify the implantable components of an AIMD shall not result in an unacceptable risk when it becomes susceptible to EM fields. Some risks caused by EM fields are malfunctioning, damage, heating, or a local increase of induced electrical current density within the patient. Some AIMDs like cardiovascular implants are highly sensitive devices, and it is difficult to make them immune to commonly encountered EM environments. In these cases, the test’s intent is not just to make sure the AIMD is immune to EMD, but to also have an insight on its susceptibility threshold and address it so the user—doctors are aware of the risks associated with using the AIMD near this type of interference. (Susceptibility is the system’s inability to perform without degradation in the presence of an EMD; immunity is the system’s ability to perform without degradation in the presence of an EMD.)

Patients are exposed to general expected EM environments when they leave the controlled clinical environment with an AIMD and its non-implantable peripheral device. The tests outlined in ISO and EN standards for AIMDs verify the AIMD maintains basic safety and continues to provide essential performance in the presence of the expected EM disturbance a device can be exposed to during normal use. 

Clause 27 of the ISO standard includes tests verifying protection from static magnetic fields, AC magnetic fields of 10 Hz to 30 MHz, and EM fields of 30 MHz – 450 MHz and 450 MHz – 3 GHz. The test levels for these evaluations are derived from International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidance for the general public. The newer versions of particular ISO 14708 standards emphasize risk management and risk analysis to be done by the manufacturer. Each function of the implantable that can affect performance should be tested in a scenario that is critical to patient outcome based on risk, placing a lot of responsibility on the manufacturer’s shoulders. The standard expects and requires disclosure, explanation, and justification from the manufacturer for any unintended behavioral responses during testing. These types of responses are expected to be temporary and to end at the cessation of testing. Test labs are expected to make accurate notes on the AIMD’s behavioral responses during testing. Permanent changes in performance due to these tests, outside of specification, are not allowed.

Wireless Considerations
AIMDs can have wireless technologies such as Bluetooth, Wi-Fi, wireless induction charging, and RFID chips. Ensuring the wireless safety of AIMDs is important to help verify the devices’ safety and performance. As with any wireless devices, AIMDs are subject to standards and requirements regarding coexistence, security, and functionality. The governing bodies for these considerations vary by geography, but in many cases, wireless considerations will bring additional regulatory bodies into play.

In the U.S., the FDA provides guidance regarding wireless medical devices. This guidance highlights and discusses RF wireless technology considerations that can affect the safe and effective use of medical devices. These considerations include the selection of wireless technology, quality of service, coexistence, security, and electromagnetic compatibility (EMC).

Selection of wireless technology should be based on:

• The ability to function properly in intended use environments where other RF wireless technologies will likely be located
• The ability to provide correct, secure, and timely medical data transmission to ensure its safe operation—especially for a device that performs life-sustaining activities

Wireless Quality of Service
A medical device’s wireless quality of service (QoS) is different and more important than a cellular telephone network’s QoS. The performance criteria acceptable for voice communications might not be sufficient for medical purposes.

Wireless Coexistence
A key factor affecting a wireless medical device’s performance is the limited amount of RF spectrum available, which can result in potential competition among wireless technologies for simultaneous access to the same spectrum. To address this issue, the FDA recommends manufacturers address their device’s environmental specifications and needs, including:

• Associated sources of EMD expected in the medical device’s environments
• Co-channel and adjacent channel interference from medical devices and other RF band users 

If the wireless medical device is expected to be used in proximity to other RF wireless in-band (i.e., the same or nearby RF frequency) sources, the FDA recommends addressing such risks through testing for coexistence of the device wireless system regarding the number and type of in-band sources expected to be near the device.

Security of Wireless Signals and Data
Security of RF wireless technology is a means to prevent unauthorized access to patient data or hospital networks and to ensure information and data received by a device are intended for that device. Authentication and wireless encryption play vital roles in an effective wireless security scheme.

EMC of the Wireless Technology 
The FDA recommends that EMC be an integral part of RF wireless medical devices’ development, design, testing, and performance. This should include consideration of applicable telecommunications standards and regulations and the potential for device RF emissions that might cause EMI with other equipment. In addition, RF wireless technology (by itself and in conjunction with the medical device) needs to meet applicable requirements from the Federal Communications Commission (FCC).

The EMC collateral standard contains an exemption from the EM immunity provisions in the “exclusion band” (passband) where the medical device’s RF wireless receiver or transmitter operates. Consequently, such standards do not adequately address whether the wireless communications will operate properly in the presence of in-band EMD (e.g., other RF emissions overlapping the frequency band utilized by the medical device wireless signals). Therefore, wireless communication should be actively transmitting while testing for susceptibility during all EMC immunity testing. 

In the EU, AIMDs are regulated under the Medical Device Directive and the Active Implantable Medical Device Directive. Outside the U.S. and EU, countries generally adopt international EMC standards or amended versions thereof.

The most common sources for wireless requirements are the IEC and CISPR. These regulations cover output power, effective radiated power, occupied bandwidth, power spectral density, spurious emissions, frequency stability, and specific absorption rate (SAR). These considerations ensure the AIMD’s safety and performance as it interacts with other devices and functions within the body.

Because AIMDs are complex devices that stay with a patient for an extended period, it is imperative to ensure their safety and performance regarding EMC and wireless coexistence. Knowing which standards and requirements apply to a product and considering them throughout the production process will help to complete the testing and other requirements and get a marketable product to the industry more quickly and effectively.