Coherent signal rejection in ECG
A method for monitoring an electrocardiogram (ECG) signal of a subject, includes digitally sampling an average signal from at least a first ECG electrode, determining an average interference frequency, and digitally sampling and buffering a raw ECG signal from at least a second ECG electrode. The method further includes: filtering the raw ECG signal to generate a residual signal; calculating, based on the residual signal, a first amplitude and a first phase shift of a primary interference signal at the average interference frequency and a second amplitude and a second phase shift of one or more harmonic interference signals at respective multiples of the average interference frequency; and digitally subtracting the primary interference signal and the one or more harmonic interference signals from the raw ECG signal so as to generate and output a clean ECG signal.
1. A method for monitoring an electrocardiogram (ECG) signal of a subject, comprising:
digitally sampling an average signal from ECG electrodes attached to the subject, the average signal comprising a Wilson Central Terminal (WCT) signal;
processing the WCT signal with a band-pass filter to establish a band-pass filter signal, the band-pass filter signal having a line interference component;
processing the band-pass filter signal with a frequency meter and determining a frequency of the line interference component;
determining an average interference frequency of the WCT signal based on the frequency of the line interference component;
digitally sampling and buffering a raw ECG signal from another ECG electrode attached to the subject;
calculating a moving average of the raw ECG signal;
calculating a residual signal using the moving average of the raw ECG signal wherein the residual signal comprises frequency components at and above the average interference frequency;
calculating, based on the residual signal, a first amplitude and a first phase shift of a primary interference signal at the average interference frequency and a second amplitude and a second phase shift of one or more harmonic interference signals at respective multiples of the average interference frequency; and
digitally subtracting the primary interference signal and the one or more harmonic interference signals from the raw ECG signal so as to generate and output a clean ECG signal.
2. The method according to claim 1 , and comprising using an artifact processor for imposing a limit on a rate of change in the first amplitude over time prior to subtracting the primary interference signal from the raw ECG signal.
3. The method according to claim 2 , wherein the limit on the rate of change in the first amplitude over time is a first limit and further comprising imposing a second limit on a rate of change in the second amplitude over time prior to subtracting the one or more harmonic interference signals from the raw ECG signal.
4. The method according to claim 1 , wherein filtering the raw ECG signal comprises applying to the raw ECG signal a moving average filter, and wherein generating the residual signal comprises subtracting an output of the moving average filter from the raw ECG signal.
5. The method according to claim 1 , wherein calculating the first amplitude and the first phase shift of the primary interference signal comprises finding a correlation between the residual signal and a periodic function having a frequency equal to the average interference frequency.
6. The method according to claim 1 , wherein calculating the second amplitude and the second phase shift of the one or more harmonic interference signals comprises finding a correlation between the residual signal and a periodic function having a frequency equal to a multiple of the average interference frequency.