Robust time-diversity filter and method for removing electromagnetic interference

Apparatus for filtering electrocardiogram (ECG) signals of a series of heart beats each having a cardiac event episode including an event epoch suspected of containing signals of interest and a quiet epoch suspected to be free of said signals of interest, the apparatus comprising:

• (a) detector for detecting analog ECG signals from a patient;
• (b) analog-to-digital converter connected for converting the analog ECG signals to digitized signals;
• and
• (c) computer connected for receiving and storing the digitized signals of the event episode, for determining the event epochs and the quiet epochs of the event episodes in the digitized signals of the series of the art beats, for analyzing power spectra of the digitized signals, each spectrum being based on the signals of at least one event episode, for estimating electromagnetic interference (EMI) to determine an EMI estimate for each of the event epochs based on the frequencies obtained in the spectrum corresponding to said each of the event epochs such that the estimator by means of the spectra can adapt to a shift in frequency in the EMI for EMI that varies in frequency with time;
• and for subtracting the EMI estimates from the digitized signals of the corresponding event epoch such that the resulting signals for the event epochs have reduced EMI, said estimating for each event epoch comprises filtering the signals of a quiet epoch of the at least one event episode with a high-pass filter to supress non-EMI signals in the signals of the quiet epoch, combining a sine wave and a cosine wave of an EMI frequency determined from the spectral analysis to approximate data of the signals of the quiet epoch after non-EMI-signal-suppression such that a block of data of the signals of the quiet epoch is compared to a block of data of sine and cosine waves to generate a first EMI estimate, and scaling up the first EMI estimate to compensate for attenuation of EMI in the signals of the quiet epoch by the high-pass filter to arrive at the EMI estimate for the subtraction.

A method for filtering signals of a series of generally periodic events each having an event episode including an event epoch and a quiet epoch, the method comprising:

• (a) detecting signals of the event epochs and quiet epochs of the generally periodic events;
• (b) analyzing power spectra, each spectrum being based on signals of at least one of the event episodes;
• (c) estimating electromagnetic interference (EMI) of the event epochs, said estimating from each event including matching signals of at least one quiet epoch of the at least one of the event episodes to basis functions of one or more frequencies identified in a spectrum corresponding to said at least one of the event episodes, such that the estimating step can adapt to a shift in frequency, phase, or amplitude in the EMI for EMI that varies with time;
• and
• (d) subtracting the corresponding estimated EMI from the signals of the event epoch such that the resulting signals are signals of the event epochs with reduced EMI.

A program storage medium readably by a computer, tangibly embodying a program of instruction including code means executable by the computer to digitally filter signals of a series of events each having an event episode including an event epoch and a quiet epoch, the program storage medium comprising:

• a memory device having code means including,
• (a) code means for detecting the event epochs and the quiet epochs for the series prevents;
• (b) code means for analyzing power spectra of digitized signals, each spectrum being based on at least one event episodes of the events;
• (c) code means for estimating electromagnetic interference (EMI) of the event epoch, for each event epoch said code means for estimating matches signals of a quiet epoch of the at least one event episode of a corresponding spectrum to a combination of a sine wave and a cosine wave at an EMI frequency determined from the spectrum to fit data of the signals of the quiet epoch to arrive at a first estimate, said estimating code means including a high-pass-filter for suppressing non-EMI signals from the signals of the quiet epoch and a means to scale up the first estimate obtained by said matching to compensate for attenuation of EMI the signals of the quiet epoch by the high-pass-filter to arrive at the EMI estimate, such that said code means for estimating can adapt to a shift in frequency in the EMI for EMI that varies in frequency with time;
• and
• (d) code means for subtracting the corresponding estimated EMI from the signals of the event epochs such that the resulting signals are signals of the event epochs with reduced EMI.