Telemetry Systems

In the purest sense, telemetry is a technology that allows data acquired in one location to be monitored and displayed in another. The word telemetry is formed by combining two Greek words: tele meaning remote and metron meaning measure. Its use is not unique to the medical community. For decades, applications of telemetry ranged from agriculture (soil moisture sensors) to rocketry (monitoring flight parameters) and beyond by remotely measuring radiation in areas inaccessible or hostile to humans. Early telemetry systems used wires, such as dedicated leased telephone lines, or infrared (a point-to-point beam). Today, most telemetry, including medical telemetry, is wireless, using some type of radio frequency (RF) transmitter.Most of today's medical telemetry systems allow clinicians to monitor continuously selected vital signs of patients on intermediate care ("step-down") units as they move around the facility. More dedicated systems are used as part of a cardiac rehabilitation program employing software programs that generate an exercise protocol, then monitor and trend the patient's progress. This allows a much greater exercise range than wired monitor systems, reduces the possibility of lead entanglement as the patient exercises, and prevents artifact pick up by long patient cables. The most common vital sign—and one monitored by all systems—is the electrocardiogram or ECG. Some systems also monitor other physiologic parameters such as the saturation level of oxygen in hemoglobin (pulse oximetry or SpO2), respiration, temperature, noninvasive blood pressure (NIBP), as well as additional derived parameters.Most telemetry systems consist of a battery-powered transmitter carried by the patient—usually in a pocket in the gown, clipped to the gown, or carried in a pouch attached to a harness—and a receiver located at a central monitoring location, typically at the nurse's station of the step-down unit. However, some large hospital systems have established a central facility at one hospital to monitor all patient monitors, both wired and wireless. This remote location, sometimes referred to as a "war room" or "centralized monitoring room," can be miles from the patient, employs CCD cameras and their wide-area network for visual monitoring, and communicates any alarms back to the nursing staff. For engineering and technical reasons, some systems even employ limited two-way communication. That is, they are able to both transmit patient vital signs and receive signals sent from the base or central station unit. Bilateral communication facilitates the ability to employ "frequency hopping spread spectrum" (FHSS) to minimize interference and "direct sequence spread spectrum" (DSSS) transmissions to facilitate data integrity.Every medical telemetry system consists of one or more of the following modules: a patient (yes, in this case the patient is part of the system), preamplifier/modulator electronics and transmitter electronics located in a small case, antenna/antenna network, receiver electronics, display, alarms, and recorder(s). Each module will be discussed generically in this article since designs vary widely by manufacturer.The patient provides the raw input for the preamplifier and related electronics. The ECG signal is taken as a differential signal between a pair of electrodes applied to the patient's body. Small electrodes are placed on the traditional ECG locations on the body. Three different readings may be obtained from three leads. Many systems use just one reading—either lead I, lead II, or lead III— but more sophisticated systems allow multiple leads to be sent to the central station. Additional leads equate to additional readings, up to the typical 10 leads for a diagnostic "12-lead" ECG, although most telemetry systems only employ no more than five leads. The electrodes are connected to lead wires (not to be confused with the term "lead" when referring to standard differential readings) that are in turn connected to a single yoke or patient cable where they run as a single cable from the connector to the preamplifier/modulator electronics contained inside the "telemetry transmitter."The preamplifier takes the differential milli-voltages obtained from the electrodes and increases their potential to obtain usable signals for the modulator electronics. In doing so, the preamplifier also reduces noise and cancels out some interference picked up by the electrodes and lead wires. The modulator electronics attaches the captured signals to, and in some way modulates, a radio frequency signal that is then transmitted through the air. Some modulators simply use the signal to slightly alter the frequency of the transmitted signal (this technique is known in the communications industry as frequency modulation or FM) while others employ digital technology that ranges from the very basic to the very elegant, such as bidirectional, multilead digital packet transmission, and frequency-hopping spread-spectrum transmissions. Some telemetry systems incorporate a button to alert staff when the patient feels something unusual is occurring. This alert is perceived at the central station as an event and may trigger one or more system responses, such as a patient-triggered alarm or hardcopy recording. High-end systems capable of the simultaneous monitoring of multiple ECG leads and other patient parameters usually employ digital electronics and are often referred to simply as "digital" telemetry systems.The transmitter portion is a low-wattage RF transmitter subassembly usually provided by another vendor (specializing in RF devices) and purchased as a single printed circuit board (PCB) or module for attachment to a primary PCB. Other designs incorporate the transmitter components on the main PCB, but the industry is increasingly going to an RF module that is attached to the primary PCB. From a design standpoint, it is fairly straightforward and standardized. The transmitter itself may operate only on a single frequency or be adjusted in the field to operate on one of a limited number of frequencies. In the United States, modern telemetry systems use the Wireless Medical Telemetry Service (WMTS) frequencies of 608–614, 1395–1400, and 1427–1432 MHz, but older systems may still operate on unused television channels (174–216 and 470–668 MHz) or the private land mobile radio band (450–470 MHz) as secondary users. Some digital telemetry systems even employ the facility's wireless local area network (WLAN) operating under the Institute of Electrical and Electronic Engineers (IEEE) 802.11 family of standards. A few telemetry transmitter designs also include small monitors that display the captured heart signals directly on the case of the transmitter module.The antenna system is literally a system within a system. That is, it consists of multiple antennas, strategically placed throughout the facility, wired together in parallel to form a subsystem that receives the signal transmitted by the patient-worn telemetry transmitter. The individual antennas come in two generic types, active and passive. Active antennas contain an integral preamplifier to boost received signal strength while passive antennas depend solely on signal strength to achieve acceptable signal output to the receiver. Due to the low power transmitted, the transmitter-to-antenna range may only be 100 feet or less; therefore, antennas must be strategically placed (ordinarily just inches above or below drop ceilings) in corridors, therapy and recreational areas, and other areas where monitored ambulatory patients may travel or congregate. Some nontherapeutic areas needing antenna placement include the cafeteria, gift shop, and hospital chapel, as well as corridors between those areas and the step-down unit(s). Even systems using the IEEE 802.11 family must plan transceiver placement throughout the WLAN to avoid signal dropouts.The receiver is the companion of the transmitter. The receiver "accepts" the transmitter's signal and converts the modulated data into usable patient ECG signals. The frequency and modulation method must be compatible with the transmitter. Otherwise, the receiver will not provide a usable ECG. Depending on the system design, some receivers can be tuned to match different transmitter frequencies while others use a fixed frequency. Some receivers will be analog and others will be digital. Similarly, the method of demodulation must match the method of modulation and varies from system to system. In all cases, the characteristics of the transmitter and receiver must match; otherwise, data cannot be passed from one to the other. Sometimes, disparate systems are used coincidently in different areas (medical step-down and cardiac rehab, for example) of the hospital. Although both systems may have been designed and installed by the same vendor, and carry the same manufacturer's name, the underlying designs can be so different, especially when their age is several years apart, that transmitters from one will not work correctly or at all with the other system, and vice versa.In facilities that do not employ remote central monitoring, the central station monitor display is the most visible item at the step-down unit's nursing station. This is where the primary human interface takes place and resembles a large television or computer screen. The central station monitor will display up to 16 ECG traces. Additionally, depending on the complexity of the monitoring system, other information ranging from just the patient name on bare-bones systems to nurses-electable multilead ECG displays, SpO2, patient temperature, respiration, NIBP, and even (user-selectable) trending information for the past eight, 24, or more hours, depending on the manufacturer's design, on demand. Some of these complex high-end systems even track the patient throughout the facility using signal triangulation or GPS coordinate information. Typically, there is a personal computer or small server behind this display, multiplexing the signals from the individual receivers into a single video feed for each monitor. The user interfacing consists of a keyboard and mouse or a touchscreen. The monitor display may be monochrome (green or amber on a black background) or multicolored. Multicolored displays are used on higher-end systems and may display current and past alarm information. Virtually the same bells and whistles available on today's wired physiological monitoring systems, such as editing and trending capabilities, are available on telemetry systems if the facility wishes to invest in a high-end digital system.The alarm function—separate from the display—accounts for a major portion of the information provided at the central station console. Alarms can range from the simple (triggering of both audible and visual indications, and storing the types, number, and direction of arrhythmia events) to the complex (ST-segment analysis and alarming when variations beyond selected limits occur). Some systems signal an alarm condition by changing the trace or screen color for that patient and display or flash a message stating the reason for the alarm and add an audible indicator for major alarms. Other systems will indicate an alarm condition and then allow a staff member to respond with the severity of the alarm or even an indication that the situation is normal for this patient. The system then "learns" what constitutes an alarm for this patient and thereafter acts accordingly. Some telemetry systems even employ a hierarchical form of alarm management. When two or more alarm conditions occur simultaneously or near simultaneously, the more serious condition triggers the primary alarm. The other conditions are noted, tracked, and documented. Should the more serious condition be resolved while the other(s) still exist, the alarm is reactivated. Other than some type of audible and visual indicator, the number and types of alarm conditions and indicators vary so widely from system to system that there is no across the board standard.Recorders work in conjunction with the alarms to record the event triggering the alarm. Normally all systems have at least a strip-chart recorder to provide immediate hardcopy documentation of alarm events. This is typically a thermal printer using temperature sensitive, chemically treated paper. The downside of this form of documentation is the relatively short storage time before the image begins to deteriorate. The workaround for this is to photocopy the image to standard paper and store that in the patient record. Better systems typically employ two or more printers. At least one is the traditional strip-chart recorder, while at least one other is an administrative-type laser printer. The laser printer allows the system to print out multilead ECGs and historical information on standard size paper suitable for inclusion in the medical record. High-end systems usually contain two thermal printers, in case one runs out of paper or is busy with another alarming patient, plus the laser printer. Many times, the system contains a built-in time delay, which allows it to print a recording of the patient parameter(s) for some period, typically 30 seconds, before the alarm triggered. This allows the physician to see heart activity (multiple premature ventricular contractions (PVCs), for example) leading up to the alarm event (the arrhythmia that triggered the alarm).Telemetry systems should be managed as a single item, but with component-unique tracking of both scheduled and remedial services. Likewise, a detailed maintenance history should show all services performed, modifications and additions to the system, along with a complete diagram (a blueprint overlay is recommended) showing the exact locations of antennas and line amplifiers in the system.All of the system's components, with the exception of the telemetry transmitter and receiver, are good candidates for in-house maintenance—especially if the telemetry system is similar to or shares components with one or more physiological monitoring system(s) in the facility. Most of the components will be identical or bear a family resemblance to each other. With proper acquisition planning, additional major components (e.g., monitor, printer), as well as modules (e.g., I/O interface boards, power supplies) should be purchased as maintenance "float" items and can be used with both the telemetry and other monitoring systems in the facility. Having these float items immediately available will substantially reduce downtime and overall system maintenance cost.The two items this author recommends be maintained by either the original equipment manufacturer (OEM) or a third-party specialty firm are the telemetry transmitter and receiver. Maintenance of these items requires specialized communications industry expertise, test equipment, and service aids rarely found in a biomed shop. Additionally, the use (utilization rate) of these items is typically so low that investment in them for just the telemetry system is neither warranted nor justified. Out-of-house or one-time contract maintenance is cost-effective for the times when this specialized maintenance is required. A first-call service contract should be considered, especially if RF-component coverage is included.In the United States, the Federal Communications Commission (FCC) has jurisdiction over all aspects of the telemetry transmitters. Old legacy telemetry systems operated as a secondary user on locally unused television channels 7 to 46 (174–216 MHz and 470–668 MHz) and the private land mobile radio portion of the 450–470 MHz band. As several large medical centers learned the hard way, being a secondary user became both costly and dangerous once the FCC began assigning the unused frequencies for digital television service. To address industry concern and to ensure that patient care would not be adversely impacted, the FCC established the WMTS under 47 CFR Part 95H in 2000. The service allocated certain radio frequencies (608–614 MHz, 1395–1400 MHz, and 1427–1432 MHz) to WMTS as a primary user. Unlike traditional radio stations, the language used in Part 95H meant that an individual license is not required to operate the transmitter; it is "licensed by rule." However, Part 95 H also contained a number of restrictions. For example, it restricted WMTS frequency use to authorized healthcare providers (i.e., properly licensed to provide healthcare services). It also called for the designation of a frequency coordinator (the American Society for Healthcare Engineering accepted this role) and the subsequent registration of WMTS devices. Since the transmitter output power must, by law, be less than 1.5 watts and most WMTS transmitters operate within 20 cms of the wearer's body, they are required to meet the RF exposure limits of 47 CFR 2.1093.The most obvious risk management challenge is legacy systems operating on unused television channels and private land mobile radio frequencies. Although it has been 12 years since the establishment of the WMTS, it is possible that, due to economic constraints and questionable management decisions, telemetry systems are still operating outside of the protection afforded by the WMTS.Another is a patient wandering outside of the radio reception area, resulting in a decrease in, then the total loss of, the signal and a lack of monitoring. Just prior to being totally out of range, the ECG signal can be distorted, resulting in an incorrect interpretation and possibly a false arrhythmia alarm.A similar situation occurs when there is a loss of signal strength due to poor antenna placement, loss of an antenna preamplifier, outside interference caused by some other RF-producing device either operating on the same frequency or generating harmonics, cross talk, or heterodyning. Cross talk is especially insidious because it can cause the receiver to capture the signal from a transmitter with which it is not associated. Heterodyning occurs when one radio frequency modulates another slightly different frequency to produce beat frequencies or sidebands. The receiver may perceive these signals as a problematic ECG. Heterodyning commonly occurs when two or more unused active transmitters are stored in close proximity, such as in a drawer at the nurse's station.Manufacturers employ one or more fault tolerant and redundant system designs to mitigate these risks. Examples include WMTS systems employing spread-spectrum and frequency hopping transmitters, bidirectional digital systems that verify each digital packet, and unique transmitter codes programmed into the transmitted signal. Institutions and users can mitigate these issues by careful system selection, controlling RF-producing devices on the grounds of the facility, and always removing the batteries from unused transmitters.The most common problem biomeds encounter with telemetry systems is the dropping of a transmitter. While not appearing to be a biomed issue at first, a transmitter can end up in the biomed's bench for repair—with a piece of white tape stuck to it saying, "doesn't work," without mentioning it was dropped. Upon disassembly, the biomed may find a damaged printed circuit board (PCB) or even droplets inside the case. A damaged PCB can be replaced, but alignment is a challenge since it is rare to find RF alignment equipment in a biomed shop. In such a case, either turning to the OEM or a third-party repair service is recommended. Dealing with droplets is trickier: Are they from pure water, did the patient shower while wearing the transmitter, or was it dropped into a toilet? Often there is no clear answer unless there is a tell-tale smell or visible residue—another good reason to use either OEM or third-party repair.Other common problems with telemetry systems mirror those encountered in conventional bedside monitors, for example, poor electrode-to-skin contact and broken electrode wires. The same resolutions apply to these for telemetry patients as for wired patients. Additionally, loss of transmitter signal will always be a problem since patients are ambulatory and do not always follow prescribed pathways or stay in monitored areas.Except for the RF components (transmitter and receiver), troubleshooting of most of the telemetry system requires the same tools and common test equipment used for wired physiological monitoring systems. Maintenance of telemetry systems requires an experienced biomed, backed up by good service literature (including block diagrams, schematics, circuit descriptions, and installation layout). Model-specific training is not required, but is recommended.Telemetry system technology is considered mature. The only future improvements foreseen are further reductions in size and weight.