 
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Journal of the New Zealand Medical Association,
19-June-2009, Vol 122 No 1297
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Interference with the operation of medical devices
resulting from the use of radio frequency identification
technology
Bryan Houliston, David Parry, Craig S Webster, Alan F
Merry
Recent years have seen a slow but steady increase in the use
of RFID systems in hospitals. The two major components of RFID
systems—readers and tags—are comparable in function to barcode
scanners and labels. An RFID reader transmits an RF signal, which is picked up
by any nearby tags. Each tag responds with a signal of its own, which encodes
data such as a unique identification number.
The reader picks up each tag’s response signal,
decodes the data, and passes it on to a suitable information system for
processing.1 This wireless communication gives
RFID technology a number of advantages over barcodes, in particular the ability
to read multiple tags simultaneously without requiring a direct
line-of-sight.
These advantages have seen RFID used in hospitals for
applications such as patient and staff identification; ‘smart
cabinets’ for secure storage of drugs and supplies; real-time tracking of
beds, wheelchairs, and other equipment; and checking for retained surgical
items.2
Many types of RFID technology are available to suit the
requirements of different applications. Systems that operate within a very small
area, such as checking for retained surgical items, typically use handheld,
battery-operated, low-power readers, and passive tags, which are powered by the
energy in the reader’s RF signal.
Systems that operate over a large area, such as tracking
equipment, are more likely to use mains-powered, wall- or ceiling-mounted,
high-power readers and/or active tags, which are powered by on-board
batteries.
A widely acknowledged risk of deploying RFID technology in
the hospital environment is electromagnetic interference (EMI) affecting the
operation of other electronic medical devices. Research on other technologies
that produce EMI, such as mobile phones and wireless computer networks, have
shown that they can cause electronic medical devices to function in unexpected
ways, or even to fail.3–5 However, there
has been little published research testing the effect of EMI produced by RFID
technology specifically. What has been published has tested RFID systems in
non-hospital environments, such as Irnich’s experiments with pacemakers
and electronic security systems in
shops.6
Two recent articles,7,8
testing common types of RFID technology and a wide range of electronic medical
devices in realistic hospital settings, are welcome additions to the literature.
Both research teams adopted a similar approach: An RFID reader was placed around
2 m away from various electronic medical devices, and then moved closer or
further away to determine the maximum range at which EMI affected each device.
Despite this common method, the two teams reported quite
disparate findings. Christe et al7 report that,
in 1600 tests of passive RFID technology operating in the ultra-high frequency
(UHF) range, they found no interference with any medical device at any distance
tested (from 30 cm up to 1.8 m).
Van der Togt et al8 report
that, in 246 tests of passive and active RFID technology operating in the UHF
range and the low frequency (LF) range, they found 68 instances of interference
with the medical devices. This interference ranged from minor effects (such as
unexpected noises coming from computer monitors) to potentially hazardous
failures (such as infusion pumps and ventilators stopping), and occurred at
distances from 1 cm to 6 m away from the device.
Given the other published research, the results of Christe
et al7 seem surprising. The complete lack of
interference found by these authors raises the possibility that some of the
results were false negatives, perhaps due to a flaw in the design or execution
of the experiments.
Both research teams were using high-power RFID readers,
transmitting at their maximum output power of 4 W. Van der Togt et
al8 note this as a limitation of their
research, and note its impact on their results: “the number of EMI
incidents increased with higher output power of transmitting RFID systems”
(pg 2889).
In many developed countries, telecommunications regulators
limit the maximum power output of RF transmitters to 4 W in the UHF frequency
band. Under New Zealand regulations,9 4 W is
permitted only for RF transmitters that employ specific measures to reduce EMI,
such as frequency-hopping. Otherwise the maximum power level permitted is only 1
W.
Our group is currently using low-power, handheld UHF RFID
technology as part of a research project at Auckland City Hospital (ACH) and the
University of Auckland’s Advanced Clinical Skills Centre (ACSC) on
improving patient safety during anaesthesia.10
RFID readers are placed at various locations around theatre, including close to
infusion pumps, ventilators, and other electronic medical devices.
It was therefore considered prudent to replicate one of the
serious device failures reported by van der Togt et
al,8 in order to evaluate the EMI risk posed by
low-power RFID readers and whether further study was warranted. This paper
presents the results.
MethodOf the medical devices tested by van der Togt et
al,8 the Graseby 3500 infusion pump appeared to
be one of the most sensitive to EMI, experiencing a serious failure at a
distance of up to 1 m. Therefore we decided to test a Graseby 3500 infusion pump
in these experiments. The pump tested had been in regular use, without any
apparent malfunction.
The research project under way at ACH uses the Tracient
Padl-R passive UHF RFID reader. This reader has been designed specifically to
produce very low levels of EMI. Note that this paper uses the term
‘power’ to refer to the power output by an RFID reader. When
referring to the ‘power’ experienced by a medical device at some
distance from the reader, the term ‘field strength’ is used,
expressed in mcW/cm2. As shown in figure 1, the
Tracient’s field strength at 10 cm peaks at
2 mcW/cm2.
 It was expected that the Tracient’s low field
strength would not be sufficient to interfere with the infusion pump. Bearing in
mind the results of Christe et al,7 it was
desirable to create interference in at least one test, to provide some
confidence that negative results (i.e. no interference) were not false
negatives. Thus additional experiments were planned in which the infusion pump
would be exposed to stronger RF fields, created by:
  
The final test plan called for up to 15 tests. The
first test replicated those of Christe et al7
and van der Togt et al.8 One Tracient reader
was placed 1 m away from the pump, in line with the results from van der Togt et
al.8 The reader was then moved closer or
further away to find the maximum range at which interference occurred.
If no interference resulted, then additional tests were
performed, covering combinations of the following three variables:
Each test was performed twice to
determine reproducibility.
The tests were performed over two days at the ACSC, in
an RF-controlled area. Readings taken before testing showed a background field
strength of 0.01 mcW/cm.2 Tests were conducted
on a plastic surface approximately 1 m above the floor and at least 1 m
from any conductive surface. In each test the infusion pump was observed for a
minimum of three seconds, to determine whether it experienced
interference.
ResultsTen tests were conducted. The results are summarised in
Table 1. As expected, initial tests using a single Tracient reader did not
produce any interference with the infusion pump. The same was true for tests
with four Tracient readers. Interference occurred in two tests: using the
SkyeTek reader with a tag on the pump, and using all five readers
simultaneously.
Table 1. Test results
In both cases the pump failed completely. It stopped
working, sounded an alarm, and displayed the message ‘FAULT CODE
10’. The pump could not be reset, and had to be switched off and back on.
Each failure occurred on only one execution of the test. In other executions of
the same test, the pump functioned normally. However, once the pump had failed
the first time, the same failure occurred three times between tests, when no
RFID readers were active.
DiscussionEMI from high strength RF fields did appear to interfere
with a Graseby 3500 infusion pump, producing a failure similar to that described
by van der Togt et al.8 This provides some
confidence that the lack of failures in the other tests were not all false
negative results. That the failure could not be reproduced reliably is probably
due to the marked fluctuation in the strength of RF fields produced by the RFID
readers used, as illustrated in Figures 1–4. Interference is most likely
caused only near peak field strength.
‘FAULT CODE 10’ in the Graseby 3500 indicates a
failure in the motor driving the pump. The fault is normally caused by
mechanical failure in, or interruption of the power supply to, the motor. It is
not clear how EMI could cause such a failure. It seems likely that the
interference is not affecting the motor itself, but the pump’s control
circuitry.
After initial failure, the infusion pump failed subsequently
at times when no RFID readers were active. In the controlled test environment it
seems unlikely that such failures were caused by EMI from other sources. A more
likely explanation is that the process for resetting the infusion pump after the
initial failure (i.e. switching it off and then back on) may not have cleared
the condition that led to the failure, leaving the pump able to fail again
spontaneously. This highlights another possible risk of RFID interference with
medical devices, namely the uncertain reliability of a device once it has failed
and been reset in the usual way.
It is therefore important that the definitive procedure for
resetting each medical device is determined and disseminated to the relevant
staff, and we consider this a priority for device manufacturers and theatre
technicians.
This research has been subject to the limitations common in
EMI testing. The results reflect the properties of the Tracient and SkyeTek
readers, notably the variability in RF field strength. The operation of the
infusion pump may have been affected by the lack of a recent electrical service
check, but it had shown no faults in regular use prior to this research.
The design and execution of the tests may have been improved
given better documentation for the infusion pump, particularly on reset
procedures. However, switching the pump off and then on again is the method
normally thought sufficient to reset such a device. Available time and resources
meant that only selected RFID technology and medical devices have been tested to
date.
It is, of course, not possible to extrapolate the results
for this one device to all the other electronic medical devices common in
theatre , such as other infusion pumps, pacemakers, ventilators, fluid warmers,
anaesthetic monitors, and diathermy units. Further testing is planned, but in
the meantime our data do provide some degree of reassurance in relation to the
safety of low power RFID readers.
The conclusion of van der Togt et
al,8 that the number of EMI incidents increases
with higher output power of transmitting RFID systems, is essentially confirmed
by our study but can now be refined. All RFID readers in these tests were
operating at the same maximum output power, around 0.5 W.
The SkyeTek’s broadband antenna allowed it to produce
2.2 W maximum output power, but this still did not interfere with the infusion
pump. However, attaching an RFID tag to the pump, increasing the RF field
strength experienced by the device but not the reader’s output power,
caused the pump to fail.
Thus the chance of EMI incidents appears to increase with
higher RF field strengths experienced by medical devices, determined by higher
output power of the RFID system, shorter distance between the RFID reader and
the medical device, and the presence of an RFID tag.
The low-power Tracient readers produced no failures, even
when multiple readers were in direct contact with the infusion pump. These
readers appear to be safer for use in theatre, and presumably in the wider
hospital environment, in the configuration used by our group.
Competing interests: Craig S Webster
and Alan F Merry have financial interests in Safer Sleep LLC, a company that
provides products to improve patient safety during anaesthesia.
Author information: Bryan Houliston,
Doctoral Candidate; David Parry, Senior Lecturer, AURA Laboratory, School of
Computing and Mathematical Sciences, Auckland University of Technology,
Auckland; Craig S Webster, Research Fellow; Alan F Merry, Professor of
Anaesthesiology, Department of Anaesthesiology, School of Medicine, University
of Auckland, Auckland and Specialist Anaesthetist, Auckland City Hospital,
Auckland, New Zealand..
Acknowledgements: Thanks to Tracient
Technologies, Christchurch for providing RFID readers, and to the University of
Auckland’s Advanced Clinical Skills Centre at Mercy Hospital, Auckland for
providing the testing facility and medical devices.
Correspondence: Bryan Houliston, AURA
Laboratory, School of Computing and Mathematical Sciences, Auckland University
of Technology, P O Box 92006, Auckland 1010, New Zealand. Fax: +64 9 9219944;
email: bryan.houliston@aut.ac.nz
References:
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| | Current
issue | Search journal |
Archived issues | Classifieds
| Hotline (free ads) Subscribe | Contribute | Advertise | Contact Us | Copyright | Other Journals |
|