Telemetry

For so many people working in hospitals, the word telemetry evokes an image of patients walking around with transmitters that seamlessly communicate patient cardiac activity to a central monitoring system. While that representation is mostly correct, it certainly is not that simple.

Vital signs monitors, such as the one pictured above, can be set up as telemetry receivers and should be run on emergency power.

Take a few minutes to look at the frequency table on page 31 and you will realize that there is a lot more besides telemetry activity going on in the wireless world. These other wireless technologies create challenges for biomeds trying to keep telemetry systems running smoothly. Due to all of the different sources of wireless signal transmissions, the FCC established the Wireless Medical Telemetry Service (WMTS) report and order.

What is WMTS?
Wireless medical telemetry, the remote monitoring of a patient’s health through radiofrequency (RF) technology, allows patients greater mobility and increased comfort. But challenges are involved. In 1998, a Texas digital television station located near a hospital started testing its broadcasting ability. The nearby hospital’s wireless telemetry system stopped working. An investigation revealed that the monitoring system was halted due to interference generated by the digital television transmission. This incident prompted the FCC and American Hospital Association to create WMTS bands. The FCC’s WMTS report and order sets aside the frequencies of 608 to 614 MHz, 1395 to 1400 MHz, and 1427 to 1432 MHz for primary or coprimary use by eligible wireless medical telemetry users. This action prevents interference from other in-band RF sources.

The FCC has designated the American Society for Healthcare Engineering (ASHE) as the frequency coordinator for the WMTS bands. As part of the WMTS order, the FCC has mandated that all transmitters operating in this band be registered to ensure interference-free operation.

They Keep Going and Going?
Cardiac telemetry has been around for more than 40 years and the basics are still mostly the same. The majority of transmitters only send a single parameter, usually ECG, and have a button that the patient pushes when they feel pain to trigger the strip chart recorder. Some of the newer transmitters also have the ability to transmit pulse oximetry and noninvasive blood pressure data. More parameters usually equates to shorter transmitter battery life. Batteries are still one of the major causes for trouble calls with telemetry systems. Also, the cost of batteries is often overlooked when planning new installations or establishing how much patients should be billed for monitoring.

Signal Strength
Telemetry transmitters work on an assigned frequency; most modern ones can be “tuned” to another frequency to avoid local interference problems or to replace lost frequencies. Very few transmitters still require the replacement of a crystal to change frequencies. This has reduced trouble calls due to the crystal frequency drifting over time. Most newer transmitters can be retuned in the biomed shop.

Patient electrode wires are made up of multi-stranded wires with a female connection that attaches to the transmitter. The other end of the lead wire can have various types of patient electrode connectors. In many systems, one of the electrode wires serves as the transmitting antenna. When you are getting signal strength problems on a signal transmitter, check the lead wires. High resistance, one or more broken wire strands, or intermittent contact is probably present. If a lead wire is questionable, destroy it. Don’t just put it back in the drawer or drop it in the wastebasket.

Antenna Systems
Since the output power of biomedical transmitters is low, a good antenna system is essential in a hospital. Antenna systems can be either active or passive. Passive systems are used mostly in small spaces with few walls to block

signals. Active systems can cover a multitude of floors as they have amplifiers as part of the system. If you receive calls about multiple transmitters dropping their signals, check your amplifier’s connection. If you have an active antenna system, the amplifiers must be on the emergency power system or you will lose much of the system’s capacity when an emergency situation forces you to run on emergency power. While the interior designers may frown on antennas sticking out of the ceilings every 10 m or so, putting them above the ceiling can cause signal degrading. Remember, you are dealing with low power so don’t hide antennas.

Receivers
The receivers are at the central station, often as a separate box or mounted under the desk or in the ceiling over the desk. Again, it is important to run them on emergency power. The waveforms are showed on displays, cathode ray tubes on the older designs, LCD, plasma, or LEDs on the new designs. Setting alarm limits and entering patient names and room numbers is done either by keyboard or touch screen. Some systems feature a built-in arrhythmia computer to capture and review the ECG information. There have been many papers and presentations on how useful or useless these arrhythmia reports are from telemetry systems. But if you have a good signal, good electrode prep and placement, and limited movement artifacts, such systems are very valuable.

Limitations and Tips
Telemetry systems do have limitations that you need to be concerned about. First and foremost is that locating a patient can be a problem since they can be anywhere that the antenna system covers. Some years back, we had a problem here in Boston where the transmitter signals from one hospital were picked up by the antenna system at a hospital on the next block. Outside interference is another problem. While the new frequency bands should help, it still will occur.

The following are some suggestions for avoiding problems with telemetry systems:

• When planning a telemetry installation, check with ASHE to see what frequencies are in use within 5 miles of your installation site. Don’t rely on the vendor to do it.
• Register the frequencies you purchase quickly. Again, the vendor may offer, but do it yourself.
• Consider the cost of batteries and lead wires as part of operating expenses.
• Help develop a system that will keep track of the transmitters when they are not in use. Telemetry transmitters get lost. These units can be thrown out accidentally with the laundry, left in a different department after a patient has some type of test or treatment, transferred with a patient moved to a new location or hospital, or even taken home by patients who may believe they have to keep them on.
• Finally, get a good floor plan of the area(s) in which a telemetry system will be installed, so you can plan the system installation. Locate the electrical risers, the air shafts, and any transformers, as they all could have an impact on the system.

David Harrington, PhD, director of staff development and training at Technology in Medicine (TiM), Holliston, Mass, is a member of 24x7’s editorial advisory board.

Thomas Carney, Jr, is a TiM account manager at MetroWest Medical Center in Framingham, Mass.