Chapter 4 - Electromagnetic Compatibility And Medical Devices: Susceptibility

SUSCEPTIBILITY

It is really surprising that regulatory agencies around the world took so long to take steps
that would protect patients and health-care providers from EMI-induced medical-device
failures. Although the military and aviation industries had been developing hardware
immune to EMI for many years, it was only in 1994 that the FDA started taking action by
warning the medical device industry about their concerns regarding EMI-induced failures,
as well as by making specific recommendations for immunity levels for critical devices. In
1998, European agencies advanced this process by making it mandatory for medical devices
to comply with a fairly comprehensive EMC standard to be marketable in Europe. Today,
most nations which require medical devices to comply with EMC requirements make use
of standards based on the EN-60601-1-2 immunity requirements shown in Table 4.6, which
cover electrostatic discharge (ESD), radio-frequency interference (RFI), and a variety of
power disturbances. IEC-60601-1-2 itself cites extensively the test methods and immunity
levels of the basic IEC-61000-4 series of standards.

At the time of this writing, EN-60601-1-2, pass/fail criteria are ultimately defined by
the manufacturer. This is because the current immunity criteria for medical products are
defined in this standard as “equipment and/or system continues to perform its intended
function as specified by the manufacturer or fails without creating a safety hazard.”A safety
hazard is then defined as a “potentially detrimental effect on the patient, other persons,
animals, or the surroundings arising directly from equipment.” As a result, the manufacturer

TABLE 4.6 EN-60601-1-2 Immunity Requirements

may chose to classify a failure mode that does not pose a risk to the patient (or other surrounding
targets) as a pass. For example, the manufacturer may pass a device that fails to
start operating or stops operating when exposed to EMI threats as long as these modes do
not pose a threat to the patient. A common classification for device performance during
testing is as follows:

Criteria level A: normal performance within equipment specifications
Criteria level B: degradation or loss of function or performance which is self-
recoverable when the interfering signal is removed
Criteria level C: degradation or loss of function or performance that requires system
reset or operator intervention when the interfering signal is removed

In the United States, the FDA is adopting many of the IEC-60601-1-2 requirements but
imposes restrictions on the manufacturer’s ability to adopt pass/fail criteria. The FDA prescribes
that a passing result corresponds to maintaining clinical utility. Some of the same
concerns are also being adopted for revised versions of IEC-60601-1-2 and drafts show
that the failure criteria will change in focus from the safety hazard to specified performance
compliance. Because of the fast-changing nature of this field, we present this section
as a primer on the issue of immunity to EMI, but strongly advise you to keep updated on
the latest versions of applicable standards.

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