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• QCan the radio ment,but RF interference is sometimes a problem. When interference does occur,such as might waves from a Audio rectification is the typical cause of interfer- occur when a passing police car activates its base station affect ence,with signals picked up by the wires that run onboard radio transmitter,the interference results medical equipment or throughout the vehicle.Additional shielding of from audio rectification.Wires between the body- monitors? the affected module,and filtering of the wires run- worn transducers and the monitoring electronics, ping to it,are the cures.As is the case with audio or between the wheelchair's control joystick and rectification interference in home-stereo systems, control electronics,pick up the RF signal.A AIt takes a strong the field strengths at ground level from base stations component in the control electronics then rectifies radio wave for are so weak that interference to onboard systems the RF,and the detected signal interferes with interference to occur is extremely unlikely. normal functioning. to medical equipment. I. MEDICAL ELECTRONICS Once detected,the interference can be dealt with The radio waves from by adding shielding or filtering or both,using a base station are Today,electronics commonly monitor a patient's techniques common to resolving home-stereo usually much too weak heartbeat,breathing,brain waves,temperature, interference.One participant described curing Pan by the time they reach blood pressure,etc.Body-worn sensors(transducers) interference in a susceptible apnea monitor by anybody's residence or convert the vital signs into electrical signals,which installing a very simple, inexpensive electronic a medical facility to are then processed by electronic circuits.When a component which acts as a filter. affect biomedical critical variable falls outside acceptable limits,an equipment. alarm sounds.Often,body-worn sensors connect to To prevent such interference from arising in the the monitoring electronics via leads several feet long. first place,the Food and Drug Administration has issued guidelines and testing protocols for biomed Ambulatory patients sometimes wear"wireless" monitors that transmit signals about their vital signs ical equipment.Standards-setting organizations without tying the patients to the end of an electronic have also issued standards for such equipment. • cable.And such medical electronics are not just found The International Electrotechnical Commission's in the hospital.Apnea monitors,for example,which Standard No.601-1-3 (1992)requires medical monitor breathing irregularities,are used in the home. devices to be immune from interference when subjected to fields of 3 volts/meter over a frequency Sometimes,RF fields can affect such devices.The range of 26 to 1000 MI-h.That frequency span primary effect is audio rectification that can includes the band in which base stations operate. prompt false alarms,which are certainly a nuisance, or a device's failure to sound an alarm when it The question of biomedical interference involving should—which can be life-threatening.One base stations is expected to arise,if at all,only Symposium participant described a paper which when a base station is mounted on the hospital reported 100 such incidents, including the lock-up roof.A participant described a case in which a of a ventilator(which assists breathing)when a 150-watt paging transmitter mounted on a hospital security guard keyed his walkie-talkie or a visitor roof caused a blood-pressure monitor's reading to began using a portable cellular telephone. jump 10 mm of mercury—a"minor but not Portable cellular telephones have also reportedly insignificant failure."The same participant related interfered with infant incubators and drug-infusion that the Society of Biomedical Equipment pumps.Because of the potential for such incidents, T (SBET),which is an ancillary group many hospitals have policies restricting the use of of the Association for the Advancement of portable radios and cellular phones near intensive- Medical Instrumentation,has issued a recommen- care facilities.However,cellular base stations have dation that hospitals ban the use of portable cellular not been implicated in any of these incidents. telephones in hospitals.The recommendation does not prohibit the installation or operation of Powered wheelchairs depend on electric motors cellular base stations.The SBET has also designed and electronic control systems. Interference to the a warning sticker for placement on equipment control systems can occur in the presence of proven to be susceptible to the nearby use of a strong RF fields,causing unintended chair move- security guard's walkie-talkie,but again,base ment.As is the case with interference to home stations do not appear to cause any such problems. • audio systems and telephones,base stations do not appear to pose any significant problems to powered wheel chairs or electronic medical devices. 30