Accuracy of 50 Hz interference subtraction from an electrocardiogram

THE FILTERING of power-line interference from the electrocardiogram (ECG) is a very important procedure in any modem electrocardiograph. The requirements for this type of procedure are generally well known and have been very well formulated by Cramer et al. (eRASER et al. , 1987). Despite the high common mode rejection ratio of the input amplifiers of the electrocardiographs, recordings are often contaminated by mains frequency interferences, usually phase-shifted with respect to the mains voltage. This event is due to parasitic currents flowing through the patient's body and the connecting cables, and to the difference in the electrode impedances, converting the common mode interference voltages into unwanted differential signals (HUHTA and WEBSTER, 1973; TOWE, 1981). The use of conventional filters affects the signal components surrounding the mains frequency. A variety of adaptive filters for this purpose have been proposed (FtJRNO and TOMPKINS, 1983; AHLSTROM and TOMPK1NS, 1985; FERRARA and WIDROW, 1982), but they require an adaptation period after each interference or abrupt signal change and are not so effective in cases of extra-systoles and arrhythmias. Another problem is the disturbance of the adaptation process by the ECG signal itself, as can be seen in the work of Thakor and Zhu (THAKOR and ZHU, 1991) where, after each QRS complex, a burst of interference reappears.