INAPPROPRIATE RATE CHANGE IN MINUTE VENTILATION RATE-RESPONSIVE PACEMAKERS DUE TO INTERFERENCE BY CARDIAC MONITORS

Observations of inappropriate rate increase in five patients with minu te ventilation rate responsive implanted pacemakers (Telectronics Meta ) are reported. Pacing rate in creases were observed immediately upon connection of the resting patients to two brands of widely used cardia c monitors, and one commonly used echocardiograph. in some circumstanc es, the rate increase remained until monitor disconnection; in others the rate increase was transient, lasting about 20 seconds. A hardware thoracic resistance variation simulator uas constructed and connected to one of the pacemakers to test sensitivity to rate modifying interfe rence from external sources. This demonstrated that the senstivity to interference is dependent upon the frequency; of the interfering signa l and is highest in the range 10-60 kHz, that peak currents as low as 10 mu A can cause maximum rate increase, and that the signals injected into patients by several cardiac monitors, for purposes of lead-off d etection or respiratory monitoring, fall into the frequency range at w hich the pacemaker is most susceptible to interference.