Negligible Electromagnetic Interaction Between ... - Springer Link

P1: gcp Journal of Medical Systems [joms]

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Style file version June 5th, 2002

C 2002) Journal of Medical Systems, Vol. 26, No. 4, August 2002 (°

Negligible Electromagnetic Interaction Between Medical Electronic Equipment and 2.4 GHz Band Wireless LAN Eisuke Hanada,1,6 Yasushi Hoshino,2 Hiroshi Oyama,3 Yoshiaki Watanabe,4 and Yoshiaki Nose5

Wireless LANs using radio waves have recently gained popularity for installation in hospitals. Because electromagnetic waves transmitted from mobile telephones have been shown to cause interference with medical electronic equipment, prudence would seem necessary when introducing radio wave communication devices into hospitals. Therefore, we tested the effect of wireless LAN communication on medical electronic equipment and the effect of electronic equipment on wireless LAN communication. We observed nine pieces of electronic equipment in the operating mode while transmitting radio waves from a wireless LAN. Even when the access point was put very close to the medical electronic equipment surface and data was transmitted, no malfunction of the equipment was observed. The medical electronic equipment caused little change in the effectiveness of the communication device, although radio waves emitted from electric knives and a remote patient monitor reduced the reception rate to about 60%. The communication speed of the wireless LAN was temporarily reduced only when a microwave oven was located close to and facing the access point. Because output in Japan is limited to a maximum of 10 mW, wireless LAN following the IEEE802.11b standard should be able to be installed safely in Japanese hospitals. However, wireless LAN access points should not be installed near microwave ovens. KEY WORDS: wireless LAN; radio waves; electronic medical equipment; electromagnetic effect.

1 Department

of Medical Informatics, School of Medicine, Shimane Medical University Hospital, Izumo 693-8501, Japan. 2 Architectural Engineering Division, Nippon Sheet Glass Environment Amenity Co., Ltd., Tokyo 1050014, Japan. 3 Department of Medical Informatics, Kyoto University Hospital, Kyoto 606-8507, Japan. 4 Department of Information Science, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan. 5 Department of Medical Information Science, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, Japan. 6 To whom correspondence should be addressed; e-mail: [email protected]. 301 C 2002 Plenum Publishing Corporation 0148-5598/02/0800-0301/0 °


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Hanada, Hoshino, Oyama, Watanabe, and Nose

INTRODUCTION Wireless LANs, which connect terminals by antennas using radio waves as shown in Fig. 1, have recently gained popularity for installation in hospitals.(1,2) Because electromagnetic waves transmitted from mobile telephones have been shown to cause interference with medical electronic equipment, prudence would seem necessary when introducing radio wave communication devices into hospitals.(3,4) No reports concerning whether or not the use of wireless LANs cause electromagnetic interference with medical electronic equipment are to be found in the current literature. We, therefore, investigated the interaction between common wireless LAN equipment following the IEEE802.11b standard(5,6) and electronic equipment in hospitals. We also examined the effect of various types of electronic equipment commonly used in hospitals on wireless LAN communication. METHODS A commercially available piece of wireless LAN communication equipment, WR211API (Allied Telesis Co.), was used as the access point for communication with a card type terminal side antenna, WR211PCM (Allied Telesis Co.). The investigation was done in an anechoic chamber 10 m each in height, width, and depth. The Effect of Wireless LAN Communication on Medical Electronic Equipment For nine pieces of medical electronic equipment in the operating mode, observation was done while transmitting radio waves from the wireless LAN, as shown in Table I(a) and (b). First, a 2.45 GHz sine wave generated by the radio wave generator was transmitted through a dipole antenna. The output was 32 mW, about three times the

Fig. 1. Wireless LAN construction.


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Electromagnetic Interaction Between Medical Electronic Equipment and Wireless LAN

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Table I. Equipment Used in This Study Equipment

Type

(a) Subject equipment: Nontransmitting Ventilator Servo ventilator I Infusion pump TE-172 Syringe pump TE-3311N Bedside patient monitor V24 M1205A Ultrasound cleaner

VT-205

(b) Subject equipment: Transmission capable Remote patient monitor ZB-860P (transmitter), WEP-3214 (receiver/ display) Electric knife 7500ABC Microwave oven A Microwave oven B

Supplier

Casing

Siemens Terumo Terumo Philips Medical Systems Sharp Manufacturing System

Aluminum Partial aluminum Plastic Plastic

Nihon Kohden

Acrylics butadiene styrene (metal coated inside)

Conmed Sharp Koizumi

Aluminum molding Aluminum with gap Aluminum with gap

Aluminum

maximum output of the terminal. Each piece of such irradiated medical electronic equipment was put at 1 m height on styrene foam in the operation mode. To test for malfunctions, the antenna was brought by degrees increasingly closer from the far wall of the chamber until it almost touched the equipment. This was done from each direction: right, left, front, back, and from above the equipment. Next, using the system shown in Fig. 2, the access point was put very close to the medical electronic equipment surface, and data communication was carried out. Observation was done as to whether or not malfunction of the medical electronic equipment occurred. The electric-field intensity induced by radio waves emitted from the wireless LAN equipment was also measured. The Effect of Electronic Equipment on Wireless LAN Communication The electronic equipment used for this observation was one remote patient monitor, one electric knife system that has three modes, and two kinds of microwave oven, as shown in Table I(b). The equipment shown in Table I(a) was not used in this study since it does not transmit any electromagnetic waves. First, the electric-field intensity induced by electromagnetic waves emitted from the wireless LAN equipment was measured by focusing on the frequency band utilized by the wireless LAN. Next, with the electric knife located and operating as

Fig. 2. Wireless LAN system used in this study.


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Fig. 3. Equipment location used for the observation of an electric knife system.

shown in Fig. 3 and the remote patient monitor and the microwave ovens as shown in Fig. 4, the effect on wireless LAN communication was observed. The microwave ovens were tested in two ways. In one test the distance (x in Fig. 4) was 1 m (point “a”), and in the other the distance was 50 cm (point “b”). The distance in the test of the remote patient monitor was 2 m. Each test was done with the access point facing the oven in four positions: front, back, left side, and right side. Details of the observation

Fig. 4. Equipment location used for the observation of a remote patient monitor and microwave ovens.


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are as follows. First, a continuous file transfer was made from a personal computer at the access point side to a personal computer on the terminal side for about 1 min under conditions that automatically controlled the communication speed. During the transmission, the reception condition of the radio wave and changes in communication speed were confirmed using software attached to the wireless LAN driver software installed in the terminal side personal computer. The computer software has a function that displays the “reception condition” as a percentage using the S/N ratio and the electric-field intensity of radio waves emitted periodically from the access point. It also has a function that displays the data transfer speed every second. When the communication speed changed, the lowest speed was recorded. When communication speed did not change, the lowest value for “reception condition” displayed on the terminal was recorded. RESULTS The Effect of Wireless LAN Communication on Medical Electronic Equipment None of the nine pieces of medical electronic equipment malfunctioned. The “reception condition” rate was lowered to 46% when medical electronic equipment with metal casing was in a straight line between the access point and the terminal. When the same equipment was put in other positions, the reception rate and communication speed continued at their maximum potential without interference. The Effect of Electronic Equipment on Wireless LAN Communication The electric-field intensities at the frequencies emitted by the tested electronic equipment are shown in Table II. Although the electromagnetic waves emitted by the remote patient monitor and electric knives were comparatively strong, the field intensity at frequencies more than 10% from the frequency at which the strongest electric field intensity was observed (hereafter, called the “center frequency”) was very small. The frequency of the electromagnetic waves emitted from the microwave ovens was almost the same as the IEEE802.11b standard, as shown in Table II. In some directions from the microwave oven, strong electric-field intensity was observed at frequencies even 20% from the center frequency. Table II. Electromagnetic Wave Emitting Equipment and Frequencies Equipment Remote patient monitor (transmitter) Electric knife

Microwave oven A Microwave oven B

Function

Monopolar Bipolar Argon beam coagulator

Frequency of emitted waves

Maximum electric intensity

Equivalent channel number

441.3 MHz

86.8 dBµV/m

—

500 KHz 461.8 KHz 620 KHz

97.4 dBµV/m 104.5 dBµV/m 91.1 dBµV/m

— — —

121.3 dBµV/m 117.2 dBµV/m

11ch. 13ch.

2.465 GHz 2.472 GHz


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Operation of electric medical equipment, including the electric knives and the remote patient monitor, only reduced the reception rate to about 60%. Communication speed was not reduced. On the other hand, the electric field induced by the microwave ovens located at point “a” lowered reception on the order of 20%. At point “b,” not only was reception reduced but the communication speed sometimes was reduced from 11 to 5.5 Mbps. However, the degree of reduction greatly differed depending on which side of the microwave oven faced the access point. In order to clarify the effect of the microwave ovens on the radio wave, we examined the number of packets transmitted and received by the access point. When doing a bidirectional file transfer for about 3 min, almost 6000 packets are normally transmitted and received. The rate of packet retransmission was 0.05% when microwave oven B was placed at point “b” with its door facing the access point. DISCUSSION In this investigation, medical electronic equipment was not affected by radio waves emitted from wireless LAN equipment. This is possibly because the medical electronic equipment used in this study was the newest available and has been improved with features resistant to electromagnetic waves. For example, some parts, including the sensor, are now covered with metals, such as aluminum. Our results show the possibility of safe use of wireless LANs in hospitals that use the latest electronic medical equipment when the output follows international standards. However, radio wave irradiation testing of all medical electronic equipment to be used in hospitals should be done.(1) Testing at strong radio wave outputs was not done in this study, and only a small number of pieces of medical electronic equipment were tested. In Europe and the United States strong radio wave output from wireless LAN equipment is permitted: 100 times higher than in Japan where a ministry ordinance restricts output to 10 mW or less.(5,7) For this reason, irradiation testing is indispensable. The communication speed of the wireless LAN was temporarily lowered by electromagnetic waves emitted from a microwave oven. In some equipment that follows the IEEE802.11b standard, the terminal or access point can periodically monitor the field intensity of radio waves and the S/N ratio. When these values become lower than the fixed ones, the communication speed is automatically lowered. Some equipment has a function by which it changes the data coding method to ensure communication. Wireless LAN communication is thought to be interfered with in two patterns: one in which noise and radio waves in the same space overlap and another in which the communication equipment is directly interfered with by noise. In this observation, the electromagnetic waves emitted by the microwave oven interfered with reception at the access point antenna. However, the communication speed was affected only when the microwave oven was close to the access point. Most of the noise superimposed on the radio waves seemed to be overcome when the wireless LAN was digitized and had spectrum modulation, or by resending to correct errors. Wireless LAN access points should not be installed close to microwave ovens. A hospital information system should be able to offer a data communication environment that can access patient records quickly. In the past when a doctor wanted to


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refer to a patient record at the bedside, communication terminals had to be connected by communication cables with sockets at the bedside. Now, with the access point of a wireless LAN installed in the ceiling, there is no need to install a communication cable. Relocation of equipment can be minimized when room usage changes. Wireless LAN equipment following the IEEE802.11b standard has begun to spread rapidly for the following reasons. The communication speed is now equal that of cable communication. The usable number of frequencies has expanded, as has the number of units that can concurrently communicate. The cost of the equipment has dropped to under $400. Installation of the access point is cheaper and easier than installing information cable plugs. PDA (Personal Digital Assistant) and notebooktype computers are much improved, especially in terms of processing speed, size, and weight. It is unnecessary to change settings or restart when using wireless LAN. Lastly, continuous data communication is possible even when a user is moving around. The radio wave frequency band used by the IEEE802.11b standard is 2.4000– 2.4835 GHz. This band is called the ISM band (Industrial, Scientific and Medical band) and has a wide range of applications. For radio data communication, a de facto standard called Bluetooth(8,9) uses this ISM band. Bluetooth data communication equipment for short-range communication between household electrical appliances is inexpensively marketed. If the introduction of wireless LAN is haphazard, the possibility exists for interference with communication between Bluetooth equipment and IEEE802.11b equipment. Infrared rays are also used for wireless LAN. The newest infrared ray LAN equipment can communicate at 100 Mbps, which surpasses the transmission rate of wireless LANs using radio waves. However, objects between the access point and the terminal block the communication when infrared rays are used. Infrared ray LANs are not suitable for environments in which people and things frequently move, such as patient rooms. Interference with communication rarely occurs with wireless LAN equipment using radio waves by such diffraction and reflection. In the near future, wireless LAN communication equipment with higher communication speeds that follow the IEEE802.11a standard will become commercially available. The frequency band used in IEEE802.11a is different from that of IEEE802.11b, which was investigated in this study. A standard called IEEE802.1x has been proposed for strengthened security.(10) Further study to confirm the safety of equipment using these new standards will be necessary.

ACKNOWLEDGMENTS The authors deeply thank the following companies, which supported them by supplying medical equipment and allowing them to use their anechoic chamber: Allied Telesis Co., Ltd.; Fukuda Denshi Co., Ltd.; Terumo Co., Ltd.; Sharp Manufacturing System Co., Ltd.; Nihon Kohden Co., Ltd.; Philips Medical Systems Co., Ltd.; Kobayashi Pharmaceutical Co., Ltd.; and Kishiya Co., Ltd. This study was supported by grants-in-aid from the Japan Society for the Promotion of Science (No. 12771451) and the “Research for the Future Program” of the Japan Society for the Promotion of Science.


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REFERENCES 1. Tan, K. S., and Hinberg, I., Effects of a wireless local area network (LAN) system, a telemetry system, and electrosurgucal devices on a medical devices in a hospital environment. Biomed. Instrum. Technol. 34:115–118, 2000. 2. Nelson, L., Step-by-step guide to selecting mobile wireless devices. Nu. Manage. 30(11):12–13, 1999. 3. Silberberg, J., Performance degradation of electronic medical devices due to electromagnetic interference. Compliance Eng. Fall, 1993. 4. Anonymous, Radiofrequency interference with medical devices. IEEE Eng. Med. Biomed. Mag. 17(3):111–114, 1998. 5. IEEE-SA Standards Board, IEEE Std 802.11b-1999 (Supplement to IEEE Standard for Information Technology—Telecommunications and Information Exchange Between Systems—LAN/MAN Specific Requirements—Specific Requirement—Part 11: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band), The Institute of Electrical and Electronics Engineers, New York, 1999. 6. ARIB STD-T66 Second Generation Low Power Data Communication System/Wireless LAN System, Association of Radio Industries and Business, Tokyo, 1999. 7. Ministry of Public Management, Home Affairs, Posts and Telecommunications (Japan), Ordinance for Regulating Radio Equipment, Article 49-20, 2001. 8. Specification of the Bluetooth System, Bluetooth, 1999. 9. ARIB STD-33A Low Power Data Communication System/Wireless LAN system, Association of Radio Industries and Business, Tokyo, 1999. 10. Release Notes for Cisco Aironet Workgroup Bridges, Cisco Systems, San Jose, 2001.