Automated radiofrequency electromagnetic interference testing of apnea monitors using an open area test site

Medical devices that electronically sense physiological functions may be affected by external radiofrequency (RF) radiation from sources such as television and radio stations. When RF disrupts normal device function, it is referred to as electromagnetic interference (EMI). The Center for Devices and Radiological Health (CDRH) has been testing infant apnea monitors for susceptibility to EMI. Laboratory testing and user site surveys were conducted that confirmed susceptibility to EMI in infant apnea monitors. The field strength value above which device performance was compromised will be referred to as the device´s threshold. An automated procedure has now been developed which enables us to obtain threshold EMI field-strength versus frequency plots and identify EMI regions by frequency band In addition, precise measurements at an outdoor open area test site have found that the ≃300-cm (≃10-ft) patient leads, when fully extended, act as significant receivers of RF signals below 50 MHz. Thresholds, averaged over the 10–54 MHz frequency range, were 0.22 V/m with minimums reaching 0.05 V/m for four of the nine monitors tested when 100% amplitude modulation at 0.5 Hz was imposed on the RF carrier. In the FM band (88–108 MHz) the thresholds averaged 0.88 V/m, with one model reaching a minimum of 0.08 V/m. These results were similar to our earlier laboratory data which were taken with the patient and power supply leads not fully extended in the RF exposure Held, indicating that testing with full lead length exposures may not be necessary at higher frequencies.