Magnetic interference from a tramway in electrocardiogram signals

Abstract: ECG is a method used in diagnostics and monitoring of the heart. The ECG measures the electrical activity between two or more predetermined positions on the body surface. These ECG signals might be disturbed by surrounding magnetic _elds, which might induce an interference into the signals. A tram passing by an ECG equipment has the possibility to induce interference into the ECG signals, due to changing currents on the tram line. In this study experimental and real life measurements have been made to analyse whether or not a tram track passing by Sk_ane University Hospital, Lund, would interfere destructively with ECG signals Experiments were made to simulate an accelerating/decelerating tram, contact loss between pantograph and overhead caternary and AC ripple of the DC current powering the tram network. Experimental simulations were made representing the magnetic _eld of these interfering sources at distances 10 meters and 25 meters, respectively. The real life measurements were made in Goteborg at Slottsparken, where the equipment was placed at an approximate distance of 10meters and 25meters from the tram track, respectively. All measurements were made with both a monitor and an oscilloscope since the monitor has a filter that removes some of the signals which were to be investigated. Measurement showed that AC Ripple of the DC current interfere with ECG signals. In the case of ripple the interference is larger measured with an oscilloscope than the monitor. The ripple due to a 6 pulse recti_cation showed quite large interference. For a 12 pulse recti_cation the interference was reduced considerably. At a distance of 10 meters the ripple interference was large, though the interference was greatly reduced at a distance of 25 meters. In Göteborg a 6 pulse recti_cation of the current is used and quite large interference could be seen correlate with passing trams at a distance of 10 meters, though not all passing trams showed any disturbance in the ECG signals. For measurements at 25 meters no noticeable interference could be seen correlate with passing trams. Acceleration and deceleration showed no interference for any measurements. Contact loss between pantograph and overhead caternary showed only an interference in the experimental simulations without _lter. No cases of contact loss could be seen when measuring in Goteborg and therefore no measurements for this in a real life setup could be recorded. A passing tram will induce interfering disturbances into ECG signals. Whether if the interference will be of a problem in interpretation of ECG signals is a question that experienced physicians has to answer