Awareness is growing about the challenges of developing and maintaining safe and effective wireless medical devices. What with IEC80001 moving forward (due to be finalized next year) and the recent series of wireless medical device workshops, people in hospitals and among vendors are asking more of the hard questions about wireless. Amongst the turmoil, participants are jostling for position. This post looks at common problems with Wi-Fi, a report from U.K. alliance ERBI, and some alternatives to Wi-Fi.
Problems with Wireless
Those of us who are old enough, think back to the golden age of wireless medical devices — channelized analog telemetry. These systems were so basic and limited in scope (a couple dozen transmitters typically covering just a single 30 bed unit) that they had few problems and required little maintenance. Today, larger hospitals are pushing the envelope with a few hundred patient monitors and a thousand or more wireless infusion pumps. These wireless devices are using sophisticated client radio/access point (AP) communications protocols to maximize capacity, whether using Wi-Fi or WMTS. We’ve since left the golden age far in the past.
Radio frequency (RF) spectrum is a shared resource. There’s no getting around that fact, even with “dedicated” spectrum. The ether in which wireless signals move is like gases in the atmosphere or chemicals in water. There are no ways to practically segregate RF signals to specific areas, except for a Faraday cage. In a health care facility, some shielded rooms in Radiology qualify as Faraday cages, but little else. Much of the rest of a health care facility consists of objects and structures that seem to perversely confound and obstruct RF communications in ways like partially blocking and attenuating signals, creating multipath interference, and radiating both intentional and unintentional interference. Intentional interference is where two or more users of a portion of wireless spectrum get in each others way, disrupting or degrading the communications of one or both parties. When there are problems with two or more wireless devices using the same spectrum, this is intentional interference, often referred to as coexistence problems. Unintentional interference comes from electromechanical devices that accidentally spew RF signals as a consequence of some degradation or failure. Common sources of unintentional interference are florescent light balasts, blow dryers, paper shredders, elevator motors, or faulty microwaves. You can see a bunch of examples of RF interference on a spectrum analyzer (which everyone doing wireless medical devices should have, and know how to use) here.Read More
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 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 Instrumentaiton and Technology Journal story on this FDA web page.)Read More
According to a survey by CHIME, more hospitals are reducing restrictions on cell phones.
Twenty-three percent of the 185 survey respondents reported their organization has lifted all restrictions on mobile phone use, up 5.5% from a similar survey conducted by the Ann Arbor, Mich.-based organization in 2004. Only 11 respondents, or 6%, indicated that cell phone use is entirely
prohibited at their hospitals.
Sixty-nine percent of respondents reported mobile phone use is restricted only in certain areas, such as the emergency department or intensive care unit. And 39% indicated their organization has or will install technology to enhance cell phone signals.
Respondents, however, also reported that some problems have arisen as a result of increased use of mobile phones in their hospitals. For example, some say privacy and noise pollution concerns are compelling them to continue some mobile phone restrictions. Further, some respondents indicated their organization has specific bans on camera phones in patient areas.
As I noted on the Biomed Listserv this week, RF interference is a fact of life and cell phones are but one contributor. Regarding RF interference risk, cell phone’s will never be proven to be perfectly safe – but then neither will hair dryers, florescent light ballasts, microwaves ovens, and elevator motors. The key is risk management.
Sadly there’s no link to the actual report on CHIME’s web site. (You’d think they could have found a corporate sponsor for the study, and then published it in support of their advocacy for effective use of IT in health care and as a service to the industry – that is why CHIME exists, isn’t it?)Read More
Another caution: avoid MIMO routers. These Multi Input, Multi Output gadgets achieve excellent signal quality and range by hogging the wireless spectrum up to 219 yards away. If you live in the city or suburbs, your MIMO router will knock out your wireless-enabled
And if your neighbors also have MIMO, you’ll all lose your connections. MIMO also won’t work with those free Wi-Fi hotspots that are popping up in increasing numbers of cafes and libraries.
And this is the reply from Glenn Fleishman, blogger at Wi-Fi Net News:
“Less Is More” (Apr. 9, 2005) contains a glaring error regarding multiple antenna wireless networking. The reporter says that MIMO (multiple not “multi” input/output) gateways hog spectrum and knock out neighbors’ reception. This is entirely untrue. MIMO gateways for Wi-Fi, unlike previous range-extending Wi-Fi, are more sensitive receivers not more powerful transmitters.Read More
A while back there was quite a discussion on the Biomed Listserv about cell phone usage policy in hospitals. Policies ranged from outright bans to anything goes. Most hospitals had a policy of excluding cell phone use in ICUs or within one meter of a medical device. Cell phones have been found to cause malfunctions in medical devices (ventilators are the classic example). All medical devices are tested by the vendor for susceptibility to interference, and should be able to produce a cell phone intereference statement upon request. You may also request a summary of their electro magnetic interference testing to learn what frequencies were tested (i.e., did they test common cell phone and 802.11 frequencies), and susceptibility (how close a source of RF radiation can come to a device before causing interference problems).
There are numerous hospital and visitor users who benefit from good in-building cell phone coverage including, patients and their families, medical staff, hospital management, and vendors. Good in-building coverage enables low power communications for voice in addition to those fancy Treos and Blackberrys. However, cell phone coverage will never be perfect; there will always be dead spots, and in-building coverage is frequently even more problematic.
Interference problems with cell phones are more about the amount of power of the cell phone radiates than the frequency used by the phone. The greater the distance between a handset and a cell tower, the greater the transmit power that must be used by the cell phone. The signal blocking properties of a building (especially a hospital) require even more power. Cell phones frequently radiate at or close to maximum power when in hospitals and are more likely to cause interference. Risk of interference is reduced significantly if you can get cellular devices to operate at their minimum transmit power.
There are three potential solutions to this problem:
- Convince the carrier to put up a new cell tower close to your hospital. New cell towers cost around $250,000 and the carrier will expect you to pick up part of the capital cost and commit to a minimum number of subscribers. Oh, and it will take them a year or two to deploy the new tower.
- Install a microcell in your hospital. This is considerably less expensive (I’ve heard numbers like $30,000 for one), but some dead spots will persist unless you use an antenna system like Innerwireless. Deployment of this solution is measured in months rather than years.
- Use an adaptive repeater system like Spotwave.
Spotwave products serve as active (they are powered) repeaters that take the signal from an existing cell site and project it into your building. Each unit is made up of a donor unit and two indoor coverage units that can cover up to 50,000 square feet. Spotwave does a site survey and places their products around your building to ensure coverage. Cost comes to around 15 cents per square foot — by far the least expensive approach to in-building coverage. Spotwave has units for each individual carrier (although I heard that they’ve got a multi carrier product in the works), and is approved by all the US carriers. Deployment for a mid sized hospital takes a couple weeks, and best of all you don’t have to mess with the carrier!Read More