Theory and Applications of Bio Telemetry

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Journal of Medical Systems [joms] PP354-365711 March 5, 2002 17:6 Style file version Nov. 19th, 1999

Journal of Medical Systems, Vol. 26, No. 2, April 2002 ( C© 2002)

Theory and Applications of Biotelemetry

Nihal Fatma Guler1,2 and Elif Derya Ubeyli1

In this study, biotelemetry and its evolution is explained in detail. Bioelectric and phys-iological variables could be measured by biotelemetry systems. The development of abiotelemetry system and its principal operation are presented. The components of abiotelemetry system are explained. Biomedical data has been telemetered through ev-ery medium between two sites by using a variety of modulated energy forms. Designingof the link between transmitter and receiver is described. Important factors in design-ing a backpack or implanted telemeter are explained. The main features of implantedbiotelemetry units are determined. Single-channel and multichannel biotelemetry sys-tems are defined. The types of telemetry, and a comparison thereof, are given. Thepower sources of biotelemetry systems and features of different power sources are ex-plained. A survey of biotelemetry applications on humans and animals is presentedand advantages of using biotelemetry systems are determined.

KEY WORDS: biotelemetry; remote monitoring; data gathering; implantable device; modulation;telemetry types; single channel and multichannel biotelemetry.

INTRODUCTION

Biotelemetry is defined as transmitting biological or physiological data from aremote location to a location that has the capability to interpret the data and affect de-cision making. Biomedical telemetry is a special field of biomedical instrumentationthat often permits transmission of biological information from an inaccessible loca-tion to a remote monitoring site. When direct observation is impossible, biotelemetrycan be used to obtain a wide spectrum of environmental, physiological, and behav-ioral data.(1) Biotelemetry includes the capability for monitoring humans and animalswith minimum restraint and for providing a reproduction of the transmitted data. Ifmeasurements and monitoring techniques are applied to restrained humans and an-imals, stress of immobilization causes alterations of measured variables. Accordingto this concept, the advantage of biotelemetry is the measurement of physiological

1Department of Electronic and Computer Education, Faculty of Technical Education, Gazi University,06500 Teknikokullar, Ankara, Turkey.

2To whom correspondence should be addressed; e-mail: [email protected].

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0148-5598/02/0400-0159/0 C© 2002 Plenum Publishing Corporation

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variables in conscious, unrestrained humans and animals. The method of bioteleme-try is offering wireless, restraint-free, simultaneous, long-term data gathering.(2−4)

It is obvious that any quantity that could be measured was adaptable tobiotelemetry. Measurements that have been done in biotelemetry can be determinedin two categories:

1. Bioelectrical variables, such as ECG, EMG, and EEG.2. Physiological variables that require transducers, such as blood pressure, gas-

trointestinal pressure, blood flow, and temperature. By using suitable trans-ducers, telemetry can be employed for the measurement of a wide variety ofphysiological variables.(5,6)

Biomedical telemetry like many other things began as a laboratory curiositybut has evolved into a useful, reliable tool for data gathering. In 1903, Einthoventransmitted electrocardiograms from a hospital to his laboratory. Immersion elec-trodes were connected to a remote galvanometer directly by telephone lines. In thisinstance, telephone lines were merely used as conductors for current produced bybiopotentials.(5) The use of wires in the transmission of biodata suited his purpose,but a major advantage modern telemetry has is the elimination of wires. In 1921,Winters transmitted heart sounds over a marine radio link. External transmitters ofvarious signals evolved as electronic methods evolved to produce small transmitters.Later, several groups inserted small coils and electrodes into the skulls of animalsso alternating currents could be induced for a primitive form of telestimulation.(6)

The transmission of signals from a subject was a technique that evolved slowly. In1950s transistors were invented and then signals were transmitted from the body. En-doradiosonde was one of the earliest biotelemetry units developed by Mackay andJacobson.(5) Since the invention of integrated circuit technology in 1958, contribu-tions of microelectronics to biomedicine and health care have been enormous. Manyadvanced diagnostic, therapeutic, and rehabilitative devices and systems would nothave been possible without these contributions. The circuits tested by Markevitchshowed that signals could be transmitted through the tissues of the body from quitesmall coils placed within the body.(7) Miniature and micropower are two conceptsof modern biotelemetry design and construction. Improvements in these areas haveclosely paralleled the evolution of semiconductor and microcircuit technologies. Re-liable, stable integrated sensors and biotelemeters on microcircuit designs and im-plementations are studied.(2−7)

Biotelemetry is an important method for monitoring physiological variables byproviding a wireless link between the subject and the data collection equipment.Biomedical data has been telemetered through every medium between two sites,including air, space, water, and biologic tissue, by using a variety of modulated energyforms like electromagnetic waves, light, and ultrasound. Physiological measurementsare frequently telemetered from a subject. This can be done by a transmitter carriedon a belt or in a pocket. However, there are cases in which the transmitter is swallowedor surgically implanted in subject. The transmitting unit can be carried outside themonitored subject as a backpack unit or can be implanted within the subject’s bodyafter appropriate miniaturization and sealing against body fluid. An implantablebiotelemetry unit is a device usually designed to sense a physiological event and

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transmit this information, at least over few centimeters of tissue, to an externalreceiver.(8,9)

Although there had been examples of biotelemetry in the 1940s, they did notreceive much attention until the advent of the NASA space programs. Biotelemetryis a specialized telemetry that NASA developed for shuttle flights in which electronicbiomedical data taken from astronauts is transformed into radio waves and sent backto the ground. NASA has studied the design of advanced and reliable biomedicalsensors and biotelemetry devices.(5,10,11)

THE COMPONENTS OF A BIOTELEMETRY SYSTEM

Size, cost, circuit complexity, power requirements (and operational lifetime),transducers, nature of data to be transmitted, and performance dictate the designof a telemeter. First of all, a simple system is described to illustrate the basic princi-ples involved in telemetry. The stages in a typical biotelemetry system are shown inFig. 1.(6)

Transmitter and Receiver

The stages of a typical biotelemetry system can be divided into functional blocks,as shown in Fig. 2 for the transmitter and in Fig. 3 for the receiver.(5) Physiologicalsignals are obtained from the subject by means of appropriate transducers. Then,signal is passed through a stage of amplification and processing circuits that includegeneration of a subcarrier and modulation stage for transmission. The receiver con-sists of a tuner to select transmitting frequency, a demodulator to separate the signalfrom the carrier wave so as to display or record it.

The transmitter generates the carrier and modulates it. The receiver is capableof receiving the transmitted signal and demodulating it to recover the information.

Fig. 1. Block diagram of a biotelemetry system.

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Fig. 2. Block diagram of a biotelemetry transmitter.

Information to be transmitted is impressed upon the carrier by a process known asmodulation. Amplitude-modulated (AM) and frequency-modulated (FM) carriershave been used in biotelemetry. In an amplitude-modulated system, amplitude of thecarrier is caused to vary with the transmitted information. In a frequency-modulatedsystem, frequency of the carrier is caused to vary with the modulated signal.

In biotelemetry systems, the physiological signal is sometimes used to modu-late a low frequency carrier, called a subcarrier. Radio frequency (RF) carrier ofthe transmitter is then modulated by the subcarrier. If several physiological signalsare transmitted simultaneously, each signal is placed on a subcarrier of a differentfrequency and all of the subcarriers are combined to simultaneously modulate theRF carrier. This process of transmitting many channels of data on a single RF car-rier is called frequency multiplexing. Frequency multiplexing is more efficient andless expensive than employing a separate transmitter for each channel. At the re-ceiver, a multiplexed RF carrier is first demodulated to recover each of the separate

Fig. 3. Receiver – storage – display units.

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subcarriers and then demodulated to retrieve the original physiological signals.(6) Indescribing this type of system, a designation is given in which the method of modu-lating subcarriers is followed by the method of modulating RF carrier. For example,a system in which subcarriers are frequency-modulated and RF carrier is amplitude-modulated is designated as FM/AM. FM/FM designation means that both subcar-riers and RF carrier are frequency modulated. Although FM/AM has been used inbiotelemetry, FM/FM systems have been more popular because overall performanceexpected of FM radio link is better.(7) Also, FM radio frequency oscillators are easyto implement with a single transistor. FM/FM biotelemeters have been popular ina variety of restraint-free monitoring studies. Most of the other approaches use atechnique known as pulse modulation, in which the transmission carrier is generatedin a series of short pulses. If the amplitude of the pulses is used to represent the trans-mitted information, the method is called pulse amplitude modulation (PAM). If thewidth (duration) of each pulse is varied according to the information, pulse widthmodulation (PWM) or pulse duration modulation (PDM) system results. In pulseposition modulation (PPM), timing of a very narrow pulse is varied with respect toa reference pulse. Other designations are pulse code modulation, (PCM) and pulseinterval modulation (PIM). In pulse code modulation, information is representedby a sequence of coded pulses, which is accomplished by representing the signal indiscrete form in both time and amplitude. Pulse interval modulation uses the spacingbetween constant width (length) pulses to transmit the data.(12) In all these systems,the designations can be defined as PIM/FM, PWM/FM, and so on. As in amplitude-and frequency-modulation systems, multiplexing of several channels of physiolog-ical data can be accomplished in a pulse modulation system. However, instead offrequency, time multiplexing is used. In time multiplexing, each of the physiologicalsignals is sampled and used to control either amplitude, width, or position of onepulse, depending on the type of pulse modulation. If sampling rate is several timesthe highest frequency component of each data signal, no loss of information resultsfrom the sampling process. The signal transmitted at low power on the FM transmit-ter is collected by the receiver and tuned to the correct frequency. The subcarrier isremoved from RF carrier and then demodulated to reproduce a signal the amplitudeand frequency of which can be transformed back to the original data waveform’s.Later, this signal can be displayed or recorded on a chart and stored on tape for otheruse.(5,13)

Antenna Tips

The distance the transmitted signal can be received is called as the range ofthe system. The range of the system depends on power and frequency of the trans-mitter, relative locations of transmitting and receiving antennas, and sensitivity ofthe receiver. There are several important factors that telemetry users should be fa-miliar with when using antennas. Some of these factors are as follows: (1) Keepclear of the antenna when taking a bearing. (2) Do not stand within 1/2 wave-length of the antenna elements. (3) Protect the antenna elements to prevent themfrom getting bent out of shape. (4) Keep all metal objects from interfering with the

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antenna. In this way, confusion will be reduced and success rate in tracking will beincreased.(14)

Transmitting devices built for patients must have light weight and be compactto ensure adequate user comfort. This physical constraint on package volume meansthat any built-in antenna must be electrically small, with correspondingly low effi-ciency. Further problems appear as the telemeter is usually worn next to the skinat chest or abdominal level, so the transmitting antenna is at extremely close prox-imity to body tissue. In practice, the most important operational parameters for abody-surface–worn antenna used for biotelemetry are its radiation efficiency and theradiation pattern in the azimuthal plane. The analysis of electrically small antennasunder near-body proximity conditions has received little consideration. The use ofnumerical electromagnetic modelling methods such as finite difference time domain(FDTD) technique can provide a flexible, more efficient alternative. Application ofFDTD to the analysis of body-surface–mounted radiators for use in 418 MHz ra-dio biotelemetry systems have been explained. Numerical simulation of whole bodyproblems using suitable models enables a rapid determination of all critical param-eters affecting close coupled antenna performance.(8,15)

Implantable Device

In some occasions, it is desirable to implant the telemetry transmitter or receiversubcutaneously. The transmitter is swallowed or surgically implanted in the subject.These systems allow monitoring and collecting data from conscious, freely movingsubjects. Conscious subjects provide data free from the effects of anesthesia. It hasbeen clearly shown in the literature that anesthetic agents can change blood pressure,heart rate, peripheral vascular resistance, thermoregulation, gastrointestinal func-tion, and other body functions. Comparing with the tethered animal model animalsinstrumented with implanted telemetry are free of exit-site infections. Also, animalsinstrumented with implanted telemetry are free of infections that result from exteri-orized catheters and lead wires that are often required when using jacketed telemetry.In this situation, animals are free of stress and discomfort of carrying instrumentationin tight-fitting clothes. Once the telemetry device is implanted, data can be monitored24 h per day without human intervention or contact while the animal remains in itshome cage. Highest quality data can be collected by implanted units.(16) The functionof the instrumented implant and the external system components are described.(17)

However, there are some requirements for the usage of an implantable telemetry.Implantable units must have relatively small size and be lightweight. Internal powersource has to be used for a long time. Miniaturization and long-term use of implantelectronic systems for medical applications have resulted in growing necessity foran external powering system. Another requirement is encapsulation of the unit. Im-plantable parts of the system must be encapsulated in a biocompatible material. Theouter case and any wiring must be impervious to body fluids and moisture. For manyimplantable electronic instruments, packaging is important. Requirements of pack-aging materials, used in current techniques are as follows: (1) epoxy, silicone rubber,and other polymeric material; (2) metal flat-packs with resistive, electron beam, orlaser beam welding, or solder sealing; (3) pyrex and ceramic outer cases with epoxy

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or metal solder sealing. The unmet needs in packaging, as well as potentially usefulmaterials and techniques for packaging implantable electronic devices or systems foruse in chronic situations, are given. Problems of packaging solid state transducersinclude the determination of the volume and weight of the packaging material.(18)

The use of implantable units also restricts the distance of transmission ofthe signal. Body fluids and skin greatly attenuate the signal, and because of thisthe implanted unit must be small. Therefore, the unit has little power and the rangeof signal is quite restricted. This disadvantage has been overcome by picking up thesignal with a nearby antenna and retransmitting it. If applications involve monitoringover relatively short distances then retransmission is not necessary.(19)

Single Channel and Multichannel Biotelemetry Units

There are two types of biotelemetry units: single-channel and multichannel. Ingeneral, more than one channel of physiological information is studied. The simplestencoding can be used for telemetering a single channel of slow data. Pulse inter-val modulation or pulse width modulation can be used for a single slow variable.Telemetering a single channel of fast data requires quite different encoding becausethe carrier must be continuously varied. Either its amplitude or its frequency may bemodulated.(8) A multiple subject biotelemetry system is composed of an implantablesystem, which consists of a command receiver, a subject selection receiver, a con-ditioner and a transmitter, and external systems, such as a power command signaltransmitter and a subject selection signal transmitter, as shown in Fig. 4.(19) As indi-cated in Fig. 4, such a system is capable of accepting signals from a variety of sensors.

The function of the command receiver is to connect or disconnect the batteryto each implantable telemetry system on demand. Subject selection receiver is de-signed for receiving the subject selection signal from an external subject selectiontransmitter and then selecting a specific subject from the eight subjects and thenswitching the power source to the selected subject. The conditioner circuit consistsof an 8-to-1 multiplexer, a comparator, a signal generator, and an 8-state ring counter.In order to transmit the measured biological signals at low power, the transmitteris designed for pulse width modulation, having high noise immunity and frequencymodulation (FM). The receiver system is for receiving, demodulating, and demulti-plexing telemetry signals. This system enables one subject to be selected from eightsubjects, and the biological signals from seven implantable sensors to be obtainedsequentially using a synchronization gap.(3) The custom asynchronous digital mul-tiplexer IC can combine the output signals from many digital sensors onto a singlepair of wires. A novel system architecture assures system startup as well as detectionand recovery from inactive sensing elements. Computer simulation of the systemarchitecture provides a verification of the digital logic design and an evaluation ofthe system performance.(20)

A CMOS integrated circuit for a noninvasive biological signal telemetry systemspecified for use in medical and physiological studies of the influence of weight-lessness in space is presented. The system can monitor multichannel (4 channelsmaximum) biological signals from multiple subjects (4 subjects maximum) in realtime by using time multiplexing.(21−23)

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Fig. 4. Block diagram of a multiple-subjects telemetry system.

TYPES OF TELEMETRY

Biomedical data has been telemetered through every medium between two sites,including air, space, water, and biologic tissue by using a variety of modulated energyforms like electromagnetic waves, light, and ultrasound.

Radio Telemetry

In general, biotelemetry systems involve the use of radio transmission. A radiofrequency carrier is a high efficiency sinusoidal signal propagated in the form of

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electromagnetic waves when applied to an appropriate transmitting antenna. Radiotelemetry is an excellent tool for gathering data on the biology of animals and theirinteractions with the environment they inhabit.(24) The choice of operating frequencyhas always been the subject of considerable controversy amongst researchers. Manyselect the radio frequency at which they conduct their studies basing solely upon theavailability of equipment at hand or simply on tradition. Researchers are often notfully aware of the proximal impact of frequency choice on overall system performanceand its ultimate impact upon the quality of the data that the study generates.(25)

Infrared Telemetry

Infrared (IR) telemetry also has a very wide field of application. The IR radia-tion enables transmission of different physiological parameters from moving subjectslike patients in intensive care units, wards, newborn babies in incubators and animalsin biological and hospital laboratories. In a typical IR biotelemetry system, the pa-tient carries a battery-powered transmitter and one or more small arrays of infraredlight emitting diodes (IRLEDs) that send encoded data to remotely located photo-detector–based receivers.(26)

IR radiation in information transmission is used in two ways: Narrow beam ordirect radiation and diffuse IR radiation. Diffuse IR radiation fill up almost homo-geneously the room where the IR transmitter and receiver are located. The mobilityof the transmitter worn by the patient is complete within the room, without any re-straint. The coverage of the room with IR radiation is based on reflections from thewalls, ceiling, and floor. This is the reason for greater application of diffuse IR radia-tion in biotelemetry than narrow beam or direct IR radiation in biological parametermeasurements.

For analyzing an IR telemetry system and its feasibility in a room, it is impor-tant to know the voltage amplitude and photoamplifier output related to the noiselevel voltage in any location at the room. At least one reflection must occur on thepathway between transmitter and receiver to realize a diffuse IR radiation. Thereare two opposite types of reflection. One is called specular (mirror-like) reflectionand the other is reflection from matt surface. In some large rooms there are somelocations where IR irradiance is not sufficient. This can happen in some corners withdark background, close to the windows, or in very long rooms. A repeater can solvesuch problems. The repeater consists of a sensitive enough IR receiver and trans-mitter; the receiver is situated in a place where there is enough irradiation so thatpulses can be received. The efficiency coefficient is the most important parameterthat enables the estimation of how large the room can be. Pulse frequency or pulsetime interval modulation is used for biopotential or some other biological signaltransmission.(26,27)

Ultrasonic Telemetry

Acoustic energy is the best available technique for marine fishes to send a sig-nal over a distance. Ultrasonic ranges 30–100 kHz are above most animal auditoryranges and are transmitted with low energy loss through seawater.(28) There is a

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definite need to reveal the daily activities of animals that range out of sight of anunderwater observer and to obtain information on behavior beyond release andrecapture sites. Ultrasonic telemetry appears to be well suited for such studies asanimals can be followed from boats or by other arrangements for up to several kilo-meters distance.(29) Sound travels at a predictable speed through water. If a pulsedsignal is detected at a series of hydrophones at known positions, the position of thesound source can be calculated from the differences in time taken for the sound toreach each of the hydrophones successively more distant from the source. A numberof tracking systems based on this principle have been used to track sound-producinganimals as well as acoustically tagged individuals. Several transmitters may also betracked simultaneously using such techniques. Consideration of position-fixing er-rors associated with fixed-array acoustic tracking techniques has generally centeredupon the consequences of inaccuracies in the timing of the arrival of the ultrasonicpulse at each hydrophone.(30−35)

The simplest type of ultrasonic transmitter the pinger, which emits pulses of agiven ultrasonic frequency and repetition rate to transmitters with sensors for mon-itoring swimming speed, tail-beat frequency, water depth, and water temperature.Most pingers are augmented with a coded output that allows for recognition of in-dividual animals.(36) Most acoustically telemetered field data are interval-encoded.Data can also be encoded in the duration of the ping, but power constraints usuallymean that “off-time” is a better way to carry signal than “on-time,” which is minimizedfor signaling. Most standard, available acoustic receiving equipment is designed todecode data in this off-interval format. Receivers are usually set to ignore later pulsesin a blanking interval, typically 200–300 ms.(37)

COMPARISON OF TELEMETRY TYPES

Telemetry is an active process requiring energy output to send a signal over a dis-tance. Electromagnetic energy, particularly at radio frequencies, is rapidly absorbedas it passes through even a few centimeters of seawater. Thus, acoustic energy is thebest available technique for marine fishes. Acoustic energy is transmitted with lowenergy loss through seawater.(28)

The use of modulated infrared (IR) light as an alternative to RF carriers inbiotelemetry systems has many advantages: man-made electrical impulse noisecauses interference in radio telemetry links.(38) In IR telemetry there is much lessman-made and natural interference noise. The space occupied by the transmittingand receiving antennas is not required in IR telemetry. The transmitter worn by thepatient is more compact and does not require an inductive coil so that it can be re-alized in surface-mount technology in a very small size.(27) When IR biotelemetrysystems are used for monitoring and locating hospital patients, there are no band-width restrictions. Some reports suggest that RF transmission causes electromagneticinterference in medical devices such as cardiac pacemakers or infusion pumps.(39−44)

The possible effects of electromagnetic interference during wireless connectivityare searched and no evidence was found of electromagnetic interference of IRmodems with any of the medical devices. Furthermore, IR modems showed similar

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performance to a wired system even in an electrically noisy environment. As a result,IR wireless connectivity can be safely and effectively used in operating rooms.(45−48)

The disadvantages of IR biotelemetry are as the following: range of IRbiotelemetry is short and restricted to a single room. The short range is not par-ticularly troublesome in medical telemetry, since patients are usually confined tosmall rooms or wards. Diffuse IR biotelemetry system has difficulties with multisys-tem operation in the same room. Realization of a multisystem operation is not assimple as in radio frequency telemetry. There are two possibilities: frequency multi-plex and time division multiplex. Transmitters are difficult to separate by wavelengthdivision multiplex (frequency multiplex). It cannot be realized easily because of thebroad frequency response characteristics of photodiodes. In IR biotelemetry systempower consumption of transmitter is relatively high. Transmitter power consump-tion may be kept to a minimum by operating the infrared light emitting diodes in apulsed mode with a favorable duty cycle. The pulse duration is usually set to just afew microseconds, and pulse position modulation (PPM) is used to convey the data.It is only necessary to transmit one IR pulse for each physiological signal or datachannel.(26)

POWER SOURCES OF BIOTELEMETRY SYSTEMS

In many applications of short range biotelemetry, long operational life of theremote unit is a prime requirement. Many different types of biotelemetry systemsfeaturing low battery drain have been developed over the years, but adequate batterylife can still be a problem.(49)

Various environmental power sources have been investigated, such as light,atomic radiation, radio waves, and biomechanical and biochemical energy convert-ers. Biotelemetry systems that do not actively use their own internal power to transmitinformation have been described in the past and take two basic forms. RF-powereddevices could be considered one class since these derive their power from a base unittransmitter and use the power to excite an internal low power transmitter broadcast-ing telemetry data on another frequency. Another approach is to use the same RFfrequency for powering and transmission, but on a time-sharing basis. When prop-erly designed, this technique eliminates the crosstalk between the powering circuitand the signal circuit and only requires a single coil set for the implant. Two fac-tors in RF-powered telemetry need to be considered: (1) the RF interference of thesignal—either on the transducer or on the biological system—and (2) the biological,particularly the long term, effect of RF radiation on the subject.(50)

Another class might be considered passive biotelemetry systems. A passivetelemetry system has been developed, operating according to the principle of theimpedance transformation of two inductively coupled coils. Signal-dependent mod-ulation of the implanted coil’s load, which can be achieved with practically zeropower consumption, reduces the data transmission to the impedance measurementof the external receiver coil.(51,52)

Long operating life of implantable electronic circuits can be obtained by usinglow power transmission techniques. For example, pulse code modulation combined

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with remote switching systems to turn the circuit on only when monitoring isnecessary.(53) A single-transistor low power underdamped RF pulse position modula-tor, with remote switching, for implantable biotelemetry units has been presented.(54)

Inductive powering of implantable monitoring devices is a widely accepted so-lution for replacing implanted batteries. Inductive powering is based on the magneticcoupling between an internal coil and an external coil that is driven by an alternatingcurrent.(49,55) Both coils form a loosely coupled, coreless transformer. Parallel reso-nance of the internal coil with a capacitor is most often used for higher link efficiency.This method demands technically skilled operators to tune constantly the power linkparameters. This becomes particularly important when the external system must alsobe portable. Acceptable lifetimes for the external batteries can be obtained by a pre-cise optimization of the power efficiency of the inductive link. Inductive poweringsystem can be successfully developed for hospital use.(56,57)

APPLICATIONS OF BIOTELEMETRY

In the early days of human space flight, NASA used biotelemetry to providebiomedical data from orbiting astronauts to medical personnel. Biomedical datatransmitted to earth from space included astronaut’s heart rate, body temperature,ECG, and oxygen and carbon dioxide concentration. Telemetry was employed toestablish an understanding and to monitor health and well-being of the astronautswhile they were in orbit. So NASA has been involved in the development and ap-plication of biotelemetry since the Agency’s beginnings.(5,11,58) Because of the greatdistance from the earth, systems and procedures were developed to support med-ical operations in flight. All astronauts wore a biosensor harness, which providedfor transmitting critical physiological data back to the earth from the space craftand lunar surface. This real-time telemetry was also available to monitor astronautsin the event of illness in flight.(58) NASA had role in the development of spaceflight animal habitats and monitoring hardware in 1970s. In response to this de-velopment advanced biosensor and bioinstrumentation technologies were required.Miniaturized, specific-application biosensors, biotelemetry, and noninvasive moni-toring has become vital to the telemedicine industry. Sensors 2000! (S2K!) programat NASA has implemented a variety of advanced biosensor and bioinstrumentationtechnologies for space research and ground medical and surgical applications. Forthese applications, miniaturized implantable biosensor to measure blood pH, ionicsensors for Ca+, K+, and Na+, biophysical sensors for flow, pressure, and dimension,and miniature high resolution CO2 sensors, as well as advanced biotelemetry andinstrumentation and data systems, are developed.(10,59)

Advanced biosensors and biotelemetry systems are used in sensing a wide va-riety of phenomena in the body and transmitting this information to receivers nearthe body. These sensors can provide remote, continuous biomedical monitoring ofpatient data. Continuous ambulatory monitoring of the condition of the mother andfetus following in utero surgical repair of life-threatening congenital birth defects isrequired by the University of California, Fetal Treatment Center (FTC).(11) In 1993,Sensors 2000! established a relationship with FTC to adapt NASA’s implantable

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biotelemetry devices to their monitoring needs of human fetus and its uterineenvironment.(60) Sensors 2000! used NASA’s technology to design a system thatcould accurately measure intrauterine pressure changes, and the body temperatureand heart rate of the fetus.(10) A transmitting unit in a uterus monitors physiologi-cal parameters and tranmits its reading to external equipment. The fully developedtransmitting unit, resembling a large pill, would be small enough to be implantableby minimally invasive surgery.(60) A pressure/temperature pill transmitter is the firstof a family of implantable and/or ingestible pill transmitters that will measure avariety of physiological parameters. Other measurements will include pH, ions ofinterest (Ca+, Na+, K+), heart rate, ECG, EEG, EMG, blood gases (O2, CO2), andglucose. Testing had been done on a prototype pH pill transmitter similar in designto the pressure/temperature device. The pill shape and small size of the transmitter,its ultra-low power consumption, long life, and the powerful capabilities of its dataanalysis software make this system unique. The portability of the system makes iteasily adaptable to any hospital setting and ideal for use in a home-based monitor-ing environment. Applications of these pill transmitters are very common and gobeyond fetal surgery.(61) A complete biotelemetry system is designed for telemetryof various physiological signals such as ECG, EEG, etc. The main advantage of thisbiotelemetry system is that it provides an easy and practical way for long-term moni-toring of various physiological signals from a patient’s body while maintaining a lowimplementation cost.(4)

An intraoral plaque pH measuring system has been developed incorporatinga hydrogen-ion–selective field effect transistor. The telemeter is a temperature-compensated FM–FM system utilizing voltage feedback to the input sensingcircuit.(62) A telemetry pill is proposed to investigate bladder pressure under normallife circumstances. Extremely high compactness could be achieved using integratedand hybrid technology. A 3-mm wide thick-film substrate carries all electronics fromsensor to transmitter, including a dedicated control chip for minimal power consump-tion and is sealed by a micromachined package.(63) The use of telemetry to monitora swimmer’s rectal core temperature has been presented and measurements takenfrom swimmers have been shown.(64) Progress in the development of an implantabletelemetry system for assessing blood oxygen saturation and hematocrit is described.The key element of the system is an optical sensor, which employs optoelectronicsand on-chip signal processing electronics to measure light backscattered by blood.(65)

Long-term monitoring of central haemodynamics with implanted monitoring systemsmight be valuable in managing heart failure patients.(66) Emergency medical care hasbecome an important part of the overall health delivery system. In many areas am-bulances and emergency rescue teams are equipped with telemetry equipment toallow electrocardiogram’s and other physiological data to be transmitted to a nearbyhospital for interpretation.(5,67)

A compact, low power, implantable system for in vivo monitoring of oxygenand glucose concentrations is developed. The telemetry instrumentation systemconsists of two amperometric sensors: one oxygen and one glucose biosensor andtwo potentiostats for biasing the sensors, an instrumentation amplifier to subtractand amplify sensor output signals, and a signal transmitter subunit to convert andtransmit glucose-dependent signal from the sensors to a remote data acquisition

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system.(59,68) A system is designed to simultaneously acquire ECG and respirationdata and send them to a receiver over a telephone line. Respiration data is acquiredby measuring the transthoracic impedance between two electrodes.(69) The dataeach FM-modulate subcarriers which then FM-modulate a carrier. The transmit-ter is battery powered to assure patient safety. An electrohydraulic pulsatile bloodpump has been developed for implantation in the thoracic cavity. The system canbe used as a circulatory assist device or as an artificial heart with modifications.Remote biotelemetry systems provide power, remote monitoring, and control.(70)

A single-channel implantable microstimulator for functional neuromuscular stim-ulation is developed. This device can be inserted into paralyzed muscle groups byexpulsion from a hypodermic needle, thus reducing the risk and discomfort associ-ated with surgical placement. The device receives power and data from outside byRF telemetry.(9)

The walking (gait) analysis telemetry system (walking analyzer) consists ofminiature sensors/transmitters that are affixed directly over the leg muscle groupbeing studied. The muscle activity sensed by the electrodes is transmitted to a com-puter by biotelemetry process. This system is used to determine the degree andlocation of abnormal muscle activity and in prescribing treatment.(71) A system formeasuring force in both legs and crutches or cane, for the patient, during walking isdeveloped. A special sensor based on infrared radiation changes is realized for forcemeasurement in the crutches or cane. To extend patients’ free mobility, an infraredtelemetry system is applied.(72,73)

The most reliable operation of IR biotelemetry has been found in hospitals andin biological laboratories.(45) An IR diffuse telemetry system is realized for ECG andtemperature transmission. The basic patient unit has an IR receiver and transmitter,because biological data and signals have to be transmitted and commands and iden-tification to the basic patient unit have to be received.(27) A wireless biotelemetrysystem for the transfer of digital data through intact skin tissue has been devel-oped to provide a safe and noninvasive communication between implanted medicaldevices and outside of the body. The system utilizes two miniature infrared trans-mitter/receiver modules. Data are transmitted through intact skin and subcutaneoustissue on an infrared carrier signal. The system has been evaluated in human cadaversand during in vivo implantation of artificial hearts.(2) The infrared telemetry systemprovides a reliable and effective way of performing continuous real-time ambula-tory urodynamic monitoring in infants and young children. With the development ofmore powerful telemetric data transmission technologies, such a method could be ex-tended in the near future to a truly ambulatory urodynamic recording with real-timeon-line facilities, either at home or in the clinic, both for adults and for children.(74)

A through-water ultrasonic data telemetry system burst-mode frequency shiftkeying (FSK) is described. The system can be adapted for the transmission of datafrom various sensors, but it has been designed principally for monitoring the res-piratory rate, heart rate, temperature, and depth of a free-swimming diver over arange of up to 300 m. This application requires the transmission of low rate digitalsignals through water from a moving source to a receiver that is either stationary ormoving.(75)

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Different biotelemetric applications are done on a wide variety of animal speciessince 1950s. Information about wildlife biotelemetry activity with some historicalperspective are presented.(76) For many species, determination of habitat selection isbased on habitat-use data obtained through radio telemetry. The effects of habitat-patch size, level of telemetry signal inhibition, and selection pattern are observed.Monte Carlo simulations are used to assess the effect of habitat-dependent bias inradio telemetry studies on the assessment of habitat selection. The characteristicsof habitat mosaics selected by animals can be studied in this way.(77,78) Animals ofvarious species are used in biomedical researches, and some of the radio teleme-try systems employed consist of implantable transmitter and receiver. Implantabledevices used to monitor various physiological parameters in mice, hamsters, rats,rabbits, ferrets, dogs, cows, sheep, bears, and other species. Implant sites for pressuretelemetry can vary with the objectives of the study.

Implants that measure ECG have flexible leads extending from the housing,similar to those used in heart pacemakers. When measuring telemetered ECG, sens-ing leads are placed under the skin at locations similar to the surface electrodes’.For EEG measurements, the transmitter leads can be connected to screw electrodesor deep electrodes to monitor various sites within the brain. For EMG measure-ments, the transmitter leads can be connected to fine-braided stainless steel wire tobe threaded through or buried in the muscle.(79−82) Implants that measure tempera-ture generally have a sensor imbedded in the electronics module. When measuringcore temperature, the device is often placed in the peritoneal cavity. Core body tem-perature is a critical measure in studies of behavioral and physiological control ofmetabolism and body temperature regulation.(83−86) Activity measurements are de-rived by the receiver and obtained by sensing changes in signal strength that occur asthe animal moves about its cage. Various biotelemetry systems have been developedfor small animals to record alterations in their autonomic and behavioral activity. Themajor asset of telemetry method is the possibility of recording various parameters ata time in the unrestrained, conscious animals. A wireless telemetry system to assessheart rate, core temperature, and gross locomotor activity in freely moving rats whileperforming a behavioral task. The telemetry system consists of a small implantabletransmitter, a receiver connected to a computer with data acquisition controlled bya computer board and software package.(87−91)

Biotelemetry can be used for monitoring aquatic species in their natural envi-ronment. Sustained direct observation of aquatic species is often impossible. Thus,biotelemetry has become an increasingly important tool for studying the behaviorof fishes. A combined acoustic and radio transmitting tag employing a dynamic con-ductivity switch suitable for investigating the migratory behavior of diadromous fishis described. The unique feature of the transmitter is its ability to sense the elec-trical conductivity of the ambient water and therefore operate in the appropriatesignal mode. Under freshwater conditions the transmitter operates in radio mode, inseawater it operates in acoustic mode.(92)

Modern telemetry systems can gather many kinds of data, indicating whichanimals are near telemetry receivers and what they are doing. Sharks were amongthe first marine animals to carry telemetry systems because of their size and the

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need to understand their interactions with humans. The examples show the parallelprogression of shark biology and acoustic biotelemetry illustrating that telemetrysystems are tools for gathering data.(28) Movement rates of sharks are often estimatedfrom the distance traveled by an animal over a certain time period, where the shark’sposition is obtained from a telemetry device. This resultant speed is referred to asrate of movement or point-to-point swimming speed. Instantaneous swimming speedrequires more complicated and expensive transmitters to be externally attached.(29,93)

Electromyographic (EMG) telemetry involves implantation of transmitters infish that relay muscular activity to aerial or submerged antennas and receiver systems.Muscular activity rates in free-swimming fish are used to describe upstream migra-tions, spawning behavior, swimming performance and oxygen consumption, activityassociated with stressors such as temperature changes and metabolic rates and totest bioenergetic models.(94−96) Telemetered EMG signals indicate that muscle ac-tivity varied significantly for electrodes implanted at different longitudinal positionsalong the fish. As a result, electrode placement is an important influence affectingthe signals obtained from radio transmitters.(97) The monitoring of neural signals ofaquatic animals in the freely behaving condition is essential to understand the neu-ral mechanism of their behavior. Underwater radio telemetry system is developedto receive electroencephalographic (EEG) signals from the fish freely swimming infreshwater areas. The system uses simple and generally available instrumentationand is composed of a transmitter and a receiver. By using the system, EEG signalsare successfully received from the fish freely swimming in an outdoor pond.(98) Heartrate telemetry has been utilized as a tool for the assessment of metabolic rate in wildfish by a number of investigators. It is obvious that remote monitoring of heart rateis a good indicator of physiological activity.(99) Animal movement and behavior isremotely assessed within a wide range of environments characterized by water con-ductivity and depth. The optimal mode of transmission is dependent upon ambientconductivity and water depth and determined by the transmitter’s microprocessorand sensing devices.

CONCLUSION

Biotelemetry systems have been used for about forty years and have becomea useful tool for obtaining bioelectrical and physiological data from humans andanimal species, and for monitoring these variables as well. Biomedical telemetryis a special area of biomedical instrumentation that permits transmission of phys-iologic information from an inaccessible location to a remote monitoring site. Thegoal of biotelemetry include the capability for monitoring humans and animals withminimum restraint and to provide faithful reproduction of the transmitted data.Biotelemetry is a reliable tool for data gathering and with the invention of inte-grated circuit technology in 1958 contributions of microelectronics to biomedicineand health care have been enormous. Many advanced diagnostic, therapeutic, andrehabilitative devices and systems would not have been possible without these contri-butions. Miniature and micropower are two concepts of modern biotelemetry designand construction. Improvements in these areas have closely paralleled the evolution

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of semiconductor and microcircuit technologies. Size, cost, circuit complexity, powerrequirements (and operational lifetime), packaging, transducers, nature of data tobe transmitted, and performance are important in the design of a backpack or im-planted biotelemetry unit. It seems likely that future development will be in thefurther miniaturization and integration of biotelemeters and transducers, improvedpower sources, and improved packaging. Techniques and applications of biotelemet-ric methods continue to expand and seem to be limited only by the imagination ofthe investigators using new technologies as they evolve. Since the rapid growth intechnology different applications of biotelemetry could be used for data gatheringfrom a remote location.

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Theory and Applications of Bio Telemetry