| Popis: |
The present clinical study examined anatomical and physiological factors that influence an interference voltage at the input of an implanted pacemaker or cardioverter defibrillator (ICD) in a low-frequency electromagnetic field (EMF). "Worst case" conditions were sought that could interfere with proper function of the implanted device and thus endanger the patient with the implant. This investigation was carried out with 15 subjects in a defined 60 Hz electromagnetic field. The interference voltage was recorded using a bipolar as well as a unipolar electrode configuration; the electrodes were fitted to the chest wall and in an oesophageal catheter positioned immediately adjacent to the right ventricle. The factors examined were: body height, body circumference (in particular chest circumference), body mass index (BMI), respiration, and extent of stomach and cardiac filling. The main findings were that anatomical and physiological factors had a significant effect on the interference voltage, especially with the unipolar system and the 60 Hz electric field. Body circumference and height were the most important anatomical factors. For each 10 cm reduction in thoracic circumference, there was an increase of about 10 percent in interference voltage. With body height, there was an increase in interference voltage of about 11-12% for each 10 cm increase in height. Respiration had the most marked physiological effect, since there was a difference in the interference voltage of over 60 percent between maximal inspiration and maximal expiration. In conclusion, we can state that a combination of the following factors could be regarded as "worst-case": small body circumference, above-average body height, and maximum inspiration. Taken together, these factors can lead to a difference in the interference voltage of over 100% with a unipolar system in an electrical field. Measurements in a 60 Hz magnetic field showed that this field does contribute to the interference voltage, but this component is only influenced to a limited extent by physiological and anatomical factors. Regardless of anatomical characteristics, our findings demonstrate that with a unipolar system in an electric field, the interference voltage increases as the pacemaker housing is positioned more cranially. It is also clear that the unipolar system is significantly inferior to the bipolar system with regard to its sensitivity to disturbance by a low-frequency EMF. In a magnetic or electric field, a bipolar system only picks up about 5-10% of the noise induced in a unipolar system. The extent of filling of the heart and stomach had no significant effect on the interference voltage in our study. The consequence of our results is that the worst-case-factors outlined above must be taken into account and documented in future studies for getting comparable results. The findings from this study getting realized in a provocation trial with pacemaker and ICD patients. |
