Originally Published MDDI August 2004

EMC 



The microprocessor revolution and the proliferation of electronics that followed introduced the medical industry to electromagnetic interference.

William D. Kimmel and Daryl D. Gerke

We were pleased to be invited to present our perspective on electromagnetic interference (EMI) and electromagnetic compatibility (EMC) in medical electronics for MD&DI's 25th anniversary issue. We emphasize that this is our perspective. We have tried to maintain historical accuracy, but we're not infallible and, of course, we only see a part of the picture.

Our relationship with MD&DI extends back to 1994, when we wrote the magazine's first series of medical EMI articles, the impetus being a flurry of EMI activity surrounding medical devices. But the EMI story goes back even farther.

In the Beginning

Electromagnetic interference has long been known to the military. Military personnel were early users of electronic equipment operating in close proximity to radio transceivers. They had to deal with both susceptibility and emissions to and from nearby electronic equipment. 

As electronics proliferated to other disciplines, EMI was walking right alongside. The trouble was that EMI often didn't show up as a problem until the density of interference sources and recipients reached a point where a collision was probable.

The electrocardiograph (ECG) was among the earliest useful medical electronic instruments, having been coupled to an amplifier about 75 years ago. (Earlier, nonelectronic versions date all the way back to 1900—that must have been some design!) The ECG had always been operated in a hospital or a clinic by skilled persons. When interference did occur, there was always someone to recognize the problem and take corrective action. And rarely was the interference life threatening.

The major growth in the electronics industry awaited the invention of the transistor, soon followed by the integrated circuit, a quantum leap over the vacuum tube. By the late 1950s, transistor radios appeared on the consumer market. The earliest beneficiary of transistor technology in the medical world was the pacemaker, which was first implanted in a human in 1960 and was in common use by 1970.

Unlike the ECG, the pacemaker had to operate in the real world, exposed to interference from nearby radio sources operated by police, taxis, etc., and in industrial settings where interference sources such as radio-frequency (RF) heaters were employed. Clearly, interference to the pacemaker could have serious consequences.

Fortunately, the pacemaker is relatively easy to immunize against radio-frequency interference (RFI). You have a battery-powered device in a metal shield and only the pacer lead (or leads) must be protected with an RF filter. This was fairly easy to do in the early units that worked much like a metronome. They pulsed at a regular interval day and night. There were no sensitive input devices to protect, so you just had to keep RFI from entering the lead and jamming the internal timing. Modern models monitor heart activity, so the input leads are far more sensitive, increasing the vulnerability. More recently, digital cell phones have been shown to be troublesome interference sources.

In 1975, the emergence of the pacemaker prompted FDA (specifically, CDRH) to release the first pacemaker EMI requirements.¹ And, as electronics proliferated, FDA released the first general EMI requirements in 1979. But this was only the beginning.

The Past 25 Years

When the first issue of MD&DI was published in 1979, the world was in the midst of the microprocessor revolution. The home computer, led by the Apple II, exploded onto the scene. While the home computer got all the visibility, the microprocessor brought about an undercurrent of development in all electronics, including medical electronics. Many devices originated in someone's garage, funded by a group of physicians who had an idea to be developed.

Electronics had shrunk to the point where it was feasible to dedicate a computer to a specific function, and even to be portable, where needed. The neat thing about computers was that they could be used to perform complex computations—and very rapidly at that—opening the door to applications that were previously not possible. Furthermore, much of the development could be handled by software (or firmware), meaning that prototyping was relatively inexpensive. Once the basic hardware was working, programming patches could be quickly implemented.

So the mid-1970s saw lots of new devices being developed and marketed, and by the mid-1980s, the installed electronics customer base had reached substantial proportions.

But this exponential growth also brought EMI to the forefront. We now had electronics being operated in all sorts of environments, benign or hostile. Medical electronics were hit hard. Anything measuring sensitive physiological processes necessarily involved sensitive amplifiers, and these proved to be very vulnerable to RFI.

At about the same time, we were seeing an increase in radio transceivers and other radio sources. Vehicle-based radios were already commonplace, and handheld radios were invading the marketplace, being adopted by police, fire, factory maintenance, and security personnel. But they were still primarily in the hands of a select group who were somewhat controllable. Maintenance and security people, in particular, could be advised when and where to cease radio use.

The cell phone was not introduced until the late 1980s, but when it was, it essentially put RFI in everyone's pocket or purse. A cell phone in a breast pocket two inches away from a pacemaker lead or on top of an ICU monitoring console is a bad combination. 

And thus, we had a proliferation of sensitive electronic devices being used in uncontrolled environments and a proliferation of radio sources to provide the necessary interference. This set the stage for two prominent (and well publicized) cases: the infant apnea monitor and the motorized wheelchair.

The Apnea Monitor

The apnea monitor is used for monitoring the at-risk (notably premature) infant's breathing while sleeping. If the infant does not draw a breath within a prescribed time, an alarm sounds enable a parent to rush in to wake the infant, hopefully successfully.

The apnea monitor senses respiration by detecting the very slight difference in body resistance (in the chest cavity) between inhaling and exhaling. The difference is so slight that we wonder how anyone discovered it in the first place. The instrument must detect the difference, because the actual resistance is uncontrolled and may vary widely from patient to patient and from minute to minute as the infant moves. This raises the possibility that conditions will occur in which the instrument fails to detect a cessation of breathing, or does not detect a breath and sounds a false alarm.

Still, in most cases, the infant apnea monitor did its job faithfully. But apparent failures were reported, and, when attributed to EMI, these failures led to a steady stream of lawsuits. Lawyers were tripping over each other to get a piece of the action. Unfortunately, the plaintiff was usually a distraught parent left with a brain-damaged child, and the jury was usually generous, even when the evidence didn't back up the claims.

FDA found some monitors that were very sensitive to RFI and, further, found that certain RFI conditions could result in failure to alarm. That was the start of a vigorous effort to get the manufacturers to shore up the immunity of their apnea monitors.

We did investigative work on a number of these devices, both in the EMI test labs and in the residences where the reported failures occurred. In our investigations, interference was far more likely to trip a false alarm than it was to fail to detect an apnea. And we found a number of cases in which the RF levels were minimal, and not nearly enough to cause failure. These conditions led us to suspect that the parents just simply turned off the monitor so they could get a decent night's rest.

This brings up an interesting point, perhaps unique to patient-monitoring equipment. This equipment is characterized by tenuous signal levels and a lack of control of the environment (outside of the clinic). The variables are so numerous that it is difficult to develop a reliable detection system that will work in all cases. The bottom line is that your detection equipment may be far from bulletproof. There are going to be cases that the equipment can't cope with. But that's often the case in medical treatment. You never have 100% success. In some cases, the success ratio may be very low. Does that mean we shouldn't try? The alternative is to hold the equipment to a standard so high that none could comply. The result may deny physicians a valuable tool that could save lives.

So, we have to take the bad with the good. Unfortunately, that leaves lots of room for litigation.

Motorized Wheelchair

The development of motorized wheelchairs provided patients with new mobility. They could now more easily navigate down the street. This freedom, however, could bring them in close proximity to a radio transmitter, such as a police car or a taxi. The RFI would jam the electronic controls, causing the wheelchair to perform an unintended operation, such as executing an abrupt left turn, and dumping the patient onto the pavement in the process.

FDA investigations verified that radios could, in fact, seize control of motorized wheelchairs. Clearly, we needed a fix for this problem. FDA was able to remedy the problem by using ordinary grounding and shielding modifications to the electronics and cables. Because the wheelchairs operated at high signal levels and had no patient connections, there was no reason why the signal cables couldn't be shielded. The solution was simple, in contrast with patient-connected devices where the signals are very weak, and the cables can't be shielded adequately.

Lots of Improvement

The apnea monitor was an early indicator of what was to come. It had all the cards stacked against it. Subsequently, some or all of these conditions appeared in many other medical applications.

The good news is that EMI was beginning to get the attention it deserved, and we made rapid progress. The bad news is that because medical electronics covers such a wide range of needs, it is not a simple task to generate standards. You have to define a substantial number of categories and specify how each one is to be handled. This would include the critical nature of the mission (life supporting?), sensitivity of the instrument (physiological process?), and intended location of operation (clinical setting?).

We know that, considering the sensitivity of some of the equipment, it will never be feasible to categorically control EMI in all cases. Modern standards are considering the environment as well as the equipment, and noting that sensitive equipment needs to be used in an environment with minimal interference.

The Chain of Standards

Initial EMI problems revolved around RFI, but the standards cover power and electrostatic discharge (ESD) as well. It is interesting to look at the evolution of EMI standards over the years. A representative (but not complete) comparison of several important standards is shown in Table I.

FDA got actively involved in EMC in the late 1960s. The agency's first standard was the “Pacemaker Standard,” prepared by AAMI under an FDA contract and released in 1975. It was a fairly simple test, testing immunity for only a few frequencies: 50 Hz, 60 Hz, and 400 MHz fields. The 50–60-Hz requirement is easy to understand, but we wonder how the 400-MHz requirement came about. Our best guess is that the investigators determined that the antenna effects would be at maximum at about that frequency. Certainly the standard was not comprehensive, but it seemed to be adequate, at least for the time. The requirement was for a field of 200 V/m peak (100%-modulation square wave). This standard was specifically aimed at the pacemaker and was unsuitable for noninvasive devices.

This was followed by FDA's “Electromagnetic Compatibility Standard for Medical Devices,” MDS-201-0004, dated October 1, 1979.² This document was prepared by McDonnell Douglas, which basically tailored MIL-STD-461B to the hospital environment. This standard was obviously more general than the pacemaker standard. As is FDA's general practice, use of this document was voluntary. The final word lies with the individual FDA reviewer, even to this day.

To support the reviewer and the manufacturer, FDA publishes reviewer guides for various technologies. The apnea monitor, for example, was covered in the Respiratory Devices Guidance.

When FDA investigated reported failures of the apnea monitor, it identified failure modes that were not uncovered by MDS-201 testing (largely in the modulation requirements). This discovery led to modifications in the reviewer guidance. This revision practice continues on a regular basis. The last revision was published in 2002.³

At about the same time FDA was focusing on EMC, the International Electrotechnical Commission (IEC) was also working on medical EMC and released IEC 601 in 1993 (although draft versions had been around for some time). This IEC document referenced existing IEC standards, namely IEC 801-x, a series of EMC basic standards, and covered ESD, RFI, and power disturbances.

IEC 801-x was written for industrial control equipment, but that document was all we had to work with, so that is what we used. IEC 801-x subsequently was generalized to other equipment and was renamed IEC 1000-4-x and later as IEC 61000-4-x (it was becoming hard to keep up with the numbers). IEC 601 became as IEC 60601.⁴ The European Union has adopted the IEC documents essentially intact, so IEC 60601-1-2 is known as EN 60601-1-2.

We were making progress, but the first edition of IEC 60601 was a pretty brief document, and it raised more questions than it answered. Most notably, it did not give any guidance as to what constituted failure. In addition, the test levels were inadequate for some categories.

FDA, in response to the apnea monitor problem, continued to upgrade the reviewer guidance, leaning more toward the IEC standards and ultimately abandoning MDS 201-0004. A reviewer revision dated 1995 drew heavily from the IEC 801 series, but retained some requirements from MIL-STD-461 (by then in revision D). FDA had even added some of its own guidance (notably the quasi-static electric field test). 

Both FDA and IEC continued working on improvements, separately and in cooperation with each other. By the late 1990s, the two organizations had come close enough together that FDA essentially threw in its lot with IEC. The result is IEC 60601-1-2, 2nd ed., released in 2001.⁵

This document is a major improvement over the first edition (and other previous EMI documents). In addition to plugging a lot of holes left by the first edition, it makes a decided move to the systems aspect of medical EMC, which puts some of the onus for successful operation on the end-user. Even more importantly, it brings the United States and the European Union a big step closer to harmonized requirements. Poetically, for MD&DI's 25th anniversary, this document will become a mandatory requirement in both the United States and the European Union later this year.

Where Do We Go from Here?

Having looked over the last 25 years of EMC, what will happen in the next 25 years? We have made a major leap forward in recent years, and while we still get reports of interference, we don't see the huge numbers that we saw a decade or so ago. We do see problems popping up, but we also see responsible organizations addressing them. A few noteworthy groups are the University of Oklahoma, ANSI/IEEE, AAMI, and, of course, FDA in the United States and IEC internationally.

We will still have problems as new technologies emerge. We will still find problems popping up in the field. We will see improvements in existing standards. And when a problem does occur, we're certain that it will get immediate attention. So we won't see the changes occurring at the same pace in the future. Changes will be evolutionary, not revolutionary. Of course, revolutionary technologies bring unforeseen problems. We're sure there will be some surprises. We'll just have to be alert and work them out when they appear.

References

1.AAMI (FDA/HFK-76-38), “Pacemaker Standard,” (Rockville, MD: FDA, 1975).
2.MDS-201-0004, “Electromagnetic Compatibility Standard for Medical Devices,” developed by McDonnell Douglas under FDA contract, (Rockville, MD: FDA, 1979).
3.“Class II Special Controls Guidance Document: Apnea Monitors; Guidance for Industry and FDA,” (Rockville, MD: FDA, 2002). 
4.IEC 60601-1-2:1993, “Medical Electrical Equipment Part 1: General Requirements for Safety, 2, Collateral Standard: Electromagnetic Compatibility—Requirements and Tests,” (Geneva: International Organization for Standardization, 1993).
5.IEC 60601-1-2:2001, “Medical Electrical Equipment Part 1: General Requirements for Safety, 2, Collateral Standard: Electromagnetic Compatibility—Requirements and Tests,” (Geneva: International Organization for Standardization, 2001).   

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