Interference between Telemetry Transmitters Programmed to the Same Frequency
User Experience Network [Health Devices Jan-Feb 1999;28(1-2):82-3]
A patient was being monitored with an ambulatory telemetry transmitter when the display of the patient's rhythm at the central station monitor changed from normal sinus rhythm to ventricular tachycardia and then to ventricular fibrillation. A defibrillator was rushed to the patient's room, but the patient did not appear to be in distress. When a portable bedside monitor was connected to the patient, it displayed a normal sinus rhythm. The telemetry transmitter was disconnected from the patient and the batteries were removed, but the abnormal rhythm was still displayed on the central station monitor for that telemetry channel.
Further investigation revealed that another telemetry system from the same supplier had just been introduced in another care unit, two floors below the room where the incident occurred, and that a training class on the system was being conducted at the time of the incident. Apparently, a telemetry transmitter being used for the training class had been programmed to the same frequency as the transmitter involved in the incident. Consequently, the simulated ECG arrhythmias used with the training transmitter were overriding the signal transmitted from the patient. We are concerned that this type of problem can affect telemetry systems from other suppliers, and we think that telemetry users should be made aware of this problem.
We believe that the described problem is unusual and is not likely to be a common occurrence with any supplier's telemetry system. In fact, we are not aware of any other similar reports. The problem occurred because of an improbable combination of circumstances: 1) the programming of the training transmitter to a frequency already in use in the hospital (this was an error by the person conducting the training), 2) the proximity of the two transmitters, and 3) their simultaneous use. The second and third circumstances were sheer chance, but even having two different transmitters on the same frequency is rare.
Regardless, this incident illustrates why healthcare facilities need to be careful—as most are—about assigning transmitter frequencies. To avoid inadvertent duplication, facilities usually keep a log of all the telemetry transmitter frequencies being used. This log, which is often located in the biomedical engineering department, is checked before a transmitter is programmed to a different frequency or new telemetry equipment is introduced into the facility. In addition, programming a transmitter typically requires password access so that only qualified personnel can change the frequency. This prevents an everyday user (such as a nurse) from inadvertently reprogramming one of the transmitters to another frequency.
Nevertheless, users, including hospital staff, supplier representatives, and third-party servicers, must be vigilant when programming transmitters or introducing new telemetry equipment (e.g., for training or demonstrations) into a facility. If the facility is not keeping a log or otherwise tracking telemetry frequencies, it should start immediately. Moreover, users must be aware that using telemetry systems from different suppliers does not eliminate the possibility of interference. Several suppliers' telemetry systems share the same frequency bands (e.g., VHF, UHF), so it is possible that two transmitters from different suppliers can be programmed to the same frequency and can interfere with each other.
- ECG Monitors, Telemetric [13-988]
- Physiologic Monitoring Systems, Telemetric
Cause of Device-Related Incident
Device factor: Device interaction
External factor: Electromagnetic or radio-frequency
interference (EMI and RFI)
Support system failure: Error in hospital policy
Mechanism of Injury or Death