The archetypal wireless medical device is the telemetry monitor for measuring electrocardiographs. First introduced in the 1970s, cardiac telemetry systems were pretty straight forward. Analog signals were transmitted with each telemetry transmitter/receiver using its own dedicated channel. Medical device vendors placed ceiling mounted antennas connected with coaxial cable back to central radio frequency (RF) transmitter/receivers in a wiring closet. There were no other wireless medical devices. Nor were there any wireless LANs – or even wired local area networks, for that matter.

A lot has changed in almost 30 years – I mean besides feeling older.

The nirvana that was the 1970s came to an abrupt end on February 27, 1998 at 2:17 pm, when, “WFAA-TV channel 8 television began broadcasting on digital TV channel 9 and continued until 10:35 p.m., shutting down transmission a few times to allow a tower crew to work on the antenna.” This and subsequent tests of digital television broadcasts by the Dallas broadcaster, knocked Baylor University Medical Center’s (BUMC) telemetry off the air. Fallout from this intentional (and completely legal) interference resulted in the creation of the new WMTS (what FCC called Wireless Medical Telemetry Service) frequencies for use by telemetry monitors. Between that fateful day in 1998 and 2006, BUMC has spent $6.6 million shifting frequency and upgrading the telemetry systems at their hospitals. (You can read about BUMC’s ordeal reprinted from the AAMI publication Biomedical Instrumentation and Technology Journal story on this FDA web page.) So the new WMTS solved all our wireless medical device problems, right? Although some may differ, the bottom line to the foregoing question is a definite “no.”

About the only thing WMTS has going for it is that the designated spectrum is “protected.” A “protected” frequency is one where you can make someone generating intentional interference cease and desist – once you’ve successfully identified the offending source of interference, made the appropriate legal requests, and perhaps responded to rebuttals. As you might guess, this whole process can take weeks or months, which is a problem is your wireless medical devices are unusable in the interim. The incidence of intentional interference where a hospital has greater rights than the interfering party is almost nonexistent. By far, the greatest source of RF interference in WTMS (or almost any other band) is unintentional, resulting from bad brushes in a hair dryer motor, faulty fluorescent light ballasts, noisy paper shredder motors, and a myriad of other sources.

There are numerous flaws with WMTS.

  • First the frequency bands are not contiguous (608-614 MHz, 1,395-1,400 MHz, and 1,429-1,432 MHz) which adds to the cost and complexity of developing and deploying products using WMTS.
  • The bandwidth available in WMTS is just 13 MHz – barely enough to deploy a few hundred channelized patient monitors in a large hospital. This was sufficient for current requirements in 1999, when telemetry transceivers were the only wireless medical device in use, but not in today’s hospitals.
  • The WMTS band specifies frequency only; there are no provisions to ensure reliable coexistence and maximum utilization of the bandwidth between vendors. Consequently, it has taken years for most coexistence problems to be worked out between vendors. One can argue whether the solutions reached to date actually maximize WMTS spectrum.
  • The 608-614 MHz portion of WMTS is susceptible to co-channel interference from near by digital television broadcasters. This adjacent channel interference is perfectly legal and hospitals have no recourse but to narrow their use of that portion of WMTS (that represents almost 40% of all WMTS bandwidth).

About the same time FCC designated WMTS, the IEEE ratified the first standards for wireless networking. These new standards included 802.11 (also known as 802.11FH or frequency hopping), 802.11a and 802.11b. Some medical device vendors, looking to develop next generation wireless medical devices at the time, evaluated both WMTS and the new IEEE 802.11x standards. One vendor that chose to go with 802.11 was Protocol Systems (acquired by Welch Allyn). GE launched Apex Pro using WMTS, while using 802.11 for their patient monitors. Spacelabs, Datascope and others also ran their telemetry on WMTS, putting 802.11 in their wireless medical devices. The exception here is Philips, who operates all of their patient monitoring devices – telemetry, patient monitors, defibrillators – on WMTS. (An exception might be their EKG carts which I believe use 802.11x, while all other Philips factories have built 802.11x into their medical devices.)

Vendors with existing wireless telemetry products (notably GE and Philips) rushed revisions of existing products that supported the new 608-614 MHz WMTS frequency. Referred to as “re crystaled” these upgrades and new products simply shifted the old frequencies used by BUMC by incorporating the appropriate RF components. Late 1999 and 2000 were big revenue growth years for the patient monitoring market, as many hospitals looked to upgrade or replace telemetry systems in response to unexpected interference from digital TV stations. Upgrades were less expensive than outright replacement with non-WMTS technologies, and most hospitals went this route. This significant turnover in telemetry systems was leveraged most effectively by Philips Medical Systems, and the resulting sales cemented their position as the number one patient monitoring vendor in the U.S. – a distinction they’ve maintained since.

During the late 1990s new wireless medical devices came to market, notably “smart” infusion pumps. The past few years have seen the advent of wireless point of care testing devices from Abbott and Johnson & Johnson. Vendors like iSirona and Capsule Tech have launched wireless modules to connect legacy medical devices. Expect all point of care testing devices that are carried to the point of care to eventually go wireless. With the recent news that CMS plans to end reimbursement for ventilator acquired pneumonia in 2009, we can expect to see ventilators go wireless too.

The fact is, that 802.11x has become the defacto standard for wireless medical devices. There are two basic reasons 802.11x has come out ahead of WMTS. First, 802.11 has proven to be safe and effective after years of experience. Patient monitors using 802.11 have been shown to be more reliable than channelized telemetry (regardless of frequency used) and virtually as reliable as wired Ethernet. The second reason for 802.11x’s ascendance is that the technology is much less expensive to develop and build in to medical devices – not to mention being less expensive for customers too.

In fairness, I should mention another advantage of WMTS: placing some medical devices on a separate wireless infrastructure does eliminate a single point of failure. If WMTS fails and you lose telemetry (and possibly other patient monitoring stuff), all your wireless medical devices on 802.11x will probably still be operational. Planning enterprise architecture to minimize single points of failure is a good thing, although you don’t have to use WMTS to accomplish this objective.

There are two big advantages to vendors using WMTS. First, by running on a separate infrastructure service and support is greatly simplified. In most installations, the medical device vendor has WMTS all to themselves and doesn’t have to worry about pesky variables introduced by third parties (including the customer fiddling with things). Finally, requiring a dedicated infrastructure for your solution increases customer’s changing costs (even if it doesn’t necessarily raise the acquisition cost too). This factor comes into play when the buyer (inevitably) gets into a tiff with the vendor and wants to replace them, not only must the customer replace the medical devices they also need to replace the wireless infrastructure. Given that 802.11x is a shared infrastructure this is more a matter of when a buyer may make certain investments into their wireless LAN, rather than having to make infrastructure investments that are dedicated to a specific medical device or vendor.

That’s not to say that there aren’t costs involved in deploying medical devices on 802.11x wireless networks – there are, and they’re getting ready to go higher.

To bring us up to the present state of the art, you must read Medical-Grade, Mission-Critical Wireless Networks, by Steve Baker and Dave Hoglund, in the March/April issue of the IEEE Engineering in Medicine and Biology Magazine. You can buy an electronic copy of this peer reviewed paper for $35. I’m usually pretty critical of journals that sell the published results of research funded by my own tax dollars, but that is not the case here. Baker and Hoglund wrote their paper based on years of industry experience developing and deploying wireless medical devices – no tax dollars were harmed in the making of this journal article – and the content is well worth the cost.