US 5,511,554 AGranted

Real-time artifact removal from waveforms using a dynamic filter having a fixed delay

AssigneeHewlett-Packard Company

InventorsEric D. Helfenbein, Richard D. Pering, James M. Lindauer, Don Goodnature

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Abstract

During sampling, a finite impulse response (FIR) filter separates a composite signal into two components, non-cardiac physiological phenomena and cardiac artifacts. The length of the filter window dynamically varies to produce an output signal in which the cardiac frequency and any associated harmonics are suppressed with a fixed delay. The FIR filter is applied to the patient signal and lags the patient signal by at least one interval between triggering events. When the present interval between triggering events passes by the filter window, the window length is gradually adjusted to have the same length as the next period such that the window length is centered between two adjacent triggering events. The difference between the present interval and the next interval is evenly distributed or subtracted across the filter window between centerings.

Claims19

Dependent claims

A patient monitor, receiving a cyclical cardiac signal and a composite signal containing cardiac artifacts from a patient, comprising:

a sampler, detecting the composite signal at a fixed rate, generating composite samples from the composite signal;
a cardiac event detector, detecting the cyclical cardiac signal, and generating a marked stable reference point for each cardiac cycle;
a cardiac interval detector, detecting the marked stable reference points and generating a cardiac output signal having a variable cardiac frequency and associated harmonics, wherein the variable cardiac frequency corresponds to a cardiac interval between adjacent marked stable reference points;
memory, detecting and storing the composite samples, having a left and a right composite sample pointer;
a first digital filter having a fixed delay, connected to the memory, having an adaptive window with a midpoint and two edges defining a length, wherein one edge corresponds to the left composite sample pointer and the other edge corresponds to the right composite sample pointer, wherein the midpoint corresponds to the composite sample centered between two adjacent reference points, for removing events which occur at the variable cardiac frequency and the associated harmonics and producing a filtered composite signal;
and
memory controlling means, connected to the memory and detecting the cardiac output signal, for updating the left and right sample pointers such that the length of the adaptive window is equal to the cardiac interval when the midpoint is between two of the marked stable reference points;
wherein the filtered composite signal is produced by removing events which occur at the variable cardiac frequency and the associated harmonies of the cardiac frequency from the composite signal such that the cardiac artifacts are reduced.

5 more elements

A filtering method for reducing cardiac artifacts in a composite signal by receiving from a patient a first, second, and third cardiac event and a composite signal containing cardiac artifacts, the filtering method comprising the steps of:

measuring a first and a second cardiac period, wherein the first cardiac period is a duration between the first and second cardiac events, wherein the second cardiac period is a duration between the second and third cardiac events;
sampling the composite signal at a sampling rate so as to produce discrete composite values;
defining a first left and a first right window length such that a summation of the first left and first right window lengths is equal to the first cardiac period;
defining a second left and a second right window length such that a summation of the second left and second right window lengths is equal to the second cardiac period;
storing the discrete composite values in a series of memory addresses;
defining a left and a right window correction factor;
pointing a midpointer to the address containing the discrete composite value corresponding to the midpoint of the first cardiac period;
pointing a left end pointer to an address in the memory addresses containing the discrete composite value corresponding to the first left window length behind the midpointer and a right end pointer to the address containing the discrete composite value corresponding to the first right window length ahead of the midpointer;
and
producing a filtered composite signal using a finite impulse response filter having a dynamic window length, the step of producing comprising:
averaging the discrete composite values between the left and right end pointers, inclusively;
shifting the midpointer to the next discrete composite value such that there is a fixed delay;
updating the left end pointer such that the left window length is changed at multiples of the left window correction factor when the multiples correspond to an integral number of discrete composite values and the right end pointer such that the right window length is changed at multiples of the right window correction factor when the multiples correspond to an integral number of discrete composite values;
and
repeating, the steps of averaging, and updating until the midpointer points at the address containing the discrete composite value corresponding to the midpoint of the next cardiac interval.

12 more elements

Dependencies are inferred from the claim text.