THERE is growing awareness regarding potential problems that may be induced with two-way radios and cellular telephones if used in critical-care and telemetry units and in nurseries,* but electromagnetic interference has not been reported as a source of error in capnometry. We report three incidents in which hand-held radio transceivers, in use outside the operating room, interfered with the output of a capnographic monitor, resulting in false readings that could have compromised patient management. Given the proliferation of radiofrequency devices in and around the operating room, these incidents may warn of a problem more serious than a design error by a single manufacturer. In other reported incidents that could have had serious clinical implications, cellular telephones caused an infant incubator to increase the heat to maximum setting, a ventilator to change its delivered gas volume, and a dialysis machine to sound an "air-in-blood" alarm and stop its blood pump. .
Case Reports
Case 1
A 4-kg, 4-month-old infant (postconceptual age 38 weeks) was anesthetized for an exploratory laparotomy. A 3.5-mm ID nasal endotracheal tube was in place and the lungs were being mechanically ventilated via a pediatric circle system. A CO2monitor (Marquette Advantage, Milwaukee, WI) indicated the end-inspired CO2(PICO2) was 0 mmHg and the end-expired (PETCO2) was 25 mmHg. During surgery, the CO2monitor suddenly indicated that the PICO2was 24 mmHg and that the PETCO2was 60 mmHg. The peak and end-expiratory pressures had not changed. Oxygen saturation, blood pressure, heart rate, and temperature also remained normal. We recalibrated the CO2monitor manually, and it indicated inspired and expired end-tidal CO2levels of 0 and 24 mmHg, respectively. Later, the hospital patient transport personnel used a two-way radio transceiver outside the operating room, and the CO2monitor again indicated increases of 20 and 25 mmHg in inspired and expired CO2, respectively.
Case 2
A 65-yr-old, morbidly obese woman underwent surgery to remove a parathyroid adenoma. After a somewhat difficult intubation, and for the first 30 min of surgery, she had low blood pressures (80/55 mmHg), despite intravenous infusions, intermittent phenylephrine administration, and light anesthesia (0.4–0.6% isoflurane and Oxygen2and N2O in a 1:1 ratio). The surgeon requested that the patient not receive muscle relaxants. Suddenly, the patient started "bucking," and the lungs became difficult to ventilate. We wondered if the endotracheal tube had become dislodged. At this moment, the CO2monitor (Marquette Advantage) suddenly displayed a continuous, high PETCO2pattern that filled the entire CO2display area and a "no-breathing" warning appeared, despite cycling of the mechanical ventilator and, eventually, manual ventilation. The abnormal readout persisted. While this was happening, a patient transportation crew was using a "walkie-talkie" in front of the operating room door, which was open. The patient's obesity made auscultating the lungs difficult. We therefore performed a direct laryngoscopy, to make sure the endotracheal tube was still in place. The capnography pattern spontaneously returned to normal after several minutes, and the remainder of the operation was uneventful.
Case 3
A 57-yr-old man was scheduled for colorectal surgery. Ten minutes after induction of anesthesia (which was uneventful), he had a blood pressure of 145/85 mmHg, a heart rate of 98 beats/min, and an esophageal temperature of 36.5 degrees Celsius. Suddenly, the PETCO2, as measured by a Marquette Advantage monitor, appeared to increase from 32 to 75 mmHg. The capnography tracing showed a rebreathing pattern and an PI sub CO2of 25 mmHg, but all other clinical measurements were normal. Arterial blood gas analysis was normal: the PaCO2was 28 mmHg. By the time the blood-gas results were available (10 min), the PETCOsub 2 reading had decreased spontaneously to 23 mmHg. The entire event coincided with the testing of a two-way radio transceiver by a clinical engineer in an adjacent operating room, which shares a walk-through utility room with our operating room.
Discussion
Normally, inspired gas in an anesthesia circle system contains no CO sub 2. A real increase in inspired CO2in a circle system may indicate rebreathing (the failure of a one-way valve), administration of CO2, and/or CO2production greater than the CO2absorbent's capacity. Numerous other technical factors can also falsely alter the CO sub 2 readout, but the absolute increases are usually small.
The Marquette-Advantage monitor contains two systems that measure CO2, a time-shared mass spectrometer, and an infrared analyzer that provides a breath-to-breath readout. Abel and Eisenkraft reported that, if the mass spectrometer in the Marquette Advantage monitor is not properly configured, desflurane or sevoflurane can produce small, spurious increases in the end-tidal CO2reading. Also, the PET sub CO2may be falsely increased as a result of a high respiratory rate in a mass spectrometry system that uses long sampling catheters.**
Raemer and Calalang analyzed different technical factors that can induce errors in CO2measurements in uncompensated infrared analyzers; inaccuracies ranged from 0.5 to 6.5 mmHg. However, other factors, such as a faulty algorithm for end-tidal CO2detection, poor calibration technique, or water contamination, can all lead to gross inaccuracies in CO2monitoring. Radio-frequency waves have not been reported as a source of error in CO2readings. Medical personnel should be aware of this increased radio-frequency "pollution." Cellular telephones may interfere with medical electrical equipment, and restriction of use in hospitals has been suggested. .
To speed operating room turnaround times and to facilitate patient transport, our institution recently purchased 4-watt, UHF FM two-way hand-held radio transceivers (Kenwood TK-320, Long Beach, CA; and Motorola Visar, Schaumburg, IL) which both operate in the 450–480 MHz frequency band. We were surprised to discover that, if used in or near the operating room, these transceivers can cause inaccuracies in end-tidal CO sub 2 readings as measured by the infrared analyzer of the Marquette Advantage monitor.
After the events described above (cases 1–3), we found we could reproduce the problem by transmitting with different transceivers at varying distances from the CO2monitor (Figure 1). We were able to reproduce these changes from any place in our largest operating rooms (8–10 meters). The lead-shielded and grounded door of the operating room blocked radio-frequency interference when it was closed, but when it was open, we were able to induce the interference from outside the operating room, 10 meters from the monitor.
These radio-frequency transmissions typically interrupted and distorted the end-tidal CO2waveform and then abruptly increased the PICO2and PETCO2readings to between 20 and 25 mmHg and between 65 and 75 mmHg, respectively. Occasionally, the monitor continuously displayed a PETCO2tracing and a high CO2level, and a "no-breathing" warning appeared, such as in our second patient.
If the Advantage monitor is in a stand-by mode (as it is when not connected to the patient), a 5- to 10-s radio transmission even from outside the operating room (approximately 8–9 meters, with an unobstructed view of the monitor) can activate the monitor in the same way as the presence of CO2in the breath. A shorter radio transmission will not do this (a minimum of three breaths is necessary to activate the Marquette Advantage monitor), but it will alter the calibration of the end-tidal CO sub 2 monitor. If the anesthesiologist were to connect the patient's endotracheal tube to the monitoring system at that moment, the monitor would become activated, but the CO2calibration would be altered and would produce a CO2pattern consistent with hypercapnia and rebreathing (high end-inspired and end-expired concentrations). Fortunately, the end-tidal CO2reading recalibrates several minutes after the transceiver is turned off or removed from the vicinity. This self correction is the result of the autocalibration of the infrared analyzer signal to the more accurate mass spectrometer signal. A typical example of how electromagnetic interference from two-way radio transceivers can affect the capnography as measured by the Marquette Advantage monitor is presented in F1-27.
We also tested a 2-watt hand-held radio transceiver (Johnson, Waseka, MN; operating frequency band 450–480 MHz) and found that, in contrast to the 4-watt transceivers, it had minimal impact on the Marquette Advantage CO2monitor, and then only when placed several centimeters from the monitor screen. We also tested a different monitor (Datex, Instrumentarium Corp., Helsinki, Finland) that appeared to be properly shielded against radio-frequency signals; not even use of a 4-watt radio transceiver in the immediate vicinity influenced its function.
In conclusion, radio interference from 4-watt hand-held radio transceivers can cause spuriously high PETCO2readings on the Marquette Advantage monitor, shifting PICO2reading from zero to between 20 and 25 mmHg. If these high CO2values are taken seriously, the patient may be mismanaged.
The authors wish to thank Ray Borazanian, The Department of Scientific Publications, Cleveland Clinic Foundation, for suggestions and help in preparing this manuscript.
*Anonymous: Exploring Medical Technology with Hewlett Packard; Managing EMC: Issues and challenges, PROBE. Andover, Hewlett-Packard Company Medical Products, 1995, pp 1–3.
**Eisenkraft JB: Monitoring gas/vapors. ASA Refresher Courses 1994;125:1–7.
