Pacemaker sensitivity to 50 Hz noise voltages.

AIMS

European standards specify that pacemakers (PM) should be resistant to electromagnetic interference (EMI) up to an upper borderline voltage as a function of frequency. Electromagnetic interference fields should either remain below this upper borderline voltage level or be identified and isolated from the general population. Physicians caring for PM patients need to be aware of potential problems relating to EMI. For example, sensitivity should be programmed to avoid sensing EMI below the recommended borderline voltage level. The susceptibility of a pacemaker (PM) to 50 Hz noise is an important parameter of EMI and depends on the programmed sensitivity [sensitivity setting (SS)]. We studied SS and 50 Hz noise thresholds in a large population and determined the borderline SS, defined as the SS below which 50 Hz noise was sensed. Our results should be taken into consideration in programming the SS to protect patients from the adverse effects of EMI.

METHODS AND RESULTS

Measurements were performed on 189 PMs explanted after death. All PMs studied were implanted in 1998 or later. Sensitivity setting and sensing configuration (unipolar or bipolar) were left as programmed during lifetime. The ventricular SS and 50 Hz noise thresholds were measured according to the European pacemaker standard. The signal-to-noise ratios (S/N) were derived from the heart test and noise test signal thresholds. The S/N for pulsed 50 Hz noise of five manufacturers tested ranged from 0.435 to 0.59. The S/N for 50 Hz continuous noise for four manufactures other than Medtronic was higher, ranging from 0.458 to 0.623. No PM showed a ratio of 1 or better. Medtronic PMs reacted differently to 50 Hz continuous noise than the other brands. In 24 Medtronic PMs, the continuous noise threshold was evaluated with two heart test signal amplitudes: either 10 mV or threshold level. In tests at threshold amplitudes, voltages between 0.1 and 0.85 mV elicited interference proving that Medtronic PM reacted extremely sensitively to noise. At 10 mV heart test amplitude, the noise threshold was inversely proportional to the SS, i.e. higher SS resulted in lower noise thresholds. Noise immunity increases with increasing heart test signal amplitude.

CONCLUSION

All tested PMs reacted to pulsed 50 Hz waves as if they were heart signals and were inhibited. Continuous noise above noise threshold evoked asynchronous pacing at noise rate. All PMs had an S/N ratio <1, indicating that the heart signals were amplified less than noise. The European Standard requires that unipolarly sensing PMs tolerate noise up to 2 mV. However, an SS of 2 mV does not guarantee a noise tolerance of 2 mV. In order to fulfil this requirement, SS in the majority of PMs must be programmed >2 mV. In Medtronic PMs, the continuous noise threshold is paradoxical as it is higher with decreasing SS. As a good compromise in Medtronic PMs, SS should be ∼3 mV to guarantee sufficient protection from pulsed and continuous noise, assuming ventricular heart signals of 10 mV or more.