Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-02-08
2001-08-28
Kamm, William E. (Department: 3262)
Surgery
Diagnostic testing
Cardiovascular
C607S005000
Reexamination Certificate
active
06280391
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to ECG measuring systems and, more particularly, to ECG measuring systems using analog-to-digital processing.
BACKGROUND INFORMATION
When providing emergency cardiac patient care, it is essential to generate the patient's electro-cardio graph (ECG) quickly and accurately for proper diagnosis and successful treatment. A typical ECG signal measuring system
10
is shown in FIG.
1
. In this example, ECG signal measuring system
10
is part of diagnostic-quality monitor/defibrillator
12
. To measure the ECG signal of a patient
14
, ECG signal measuring system
10
is coupled to patient
14
through electrodes
15
and
16
and a signal acquisition and digitizing circuit
17
. Signal acquisition and digitizing circuit
17
is a standard circuit configured to receive the analog ECG signals from the electrodes and convert this signal into a digital signal. ECG signal measuring system also includes a baseline wander filter (BWF)
18
.
Large amplitude, low-frequency, non-physiological signals, commonly referred to as baseline wander, are generally sensed along with the patient's ECG. There are several sources of baseline wander including; DC bias currents, patient movement and changing patient impedance. Several of the sources of baseline wander are described further below.
ECG signal measuring systems used for emergency medical applications typically use a DC bias current to detect disconnected electrode leads. This current interacts with the patient's impedance to cause a relatively high amplitude but low frequency signal that is superimposed on the relatively low voltage ECG signal when electrodes
15
and
16
are initially applied to patient
14
. For convenience, this signal is referred to herein as the bias current signal. This bias current signal is illustrated in
FIG. 2
by a curve
20
. As can be seen in
FIG. 2
, an initial portion
21
of curve
20
has a relatively large rate of change. The bias current signal eventually begins to stabilize, as indicated by a portion
23
of curve
20
. The bias current signal results in a significant rate of change of the combined input signal (i.e., the baseline wander combined with the patient ECG signal) during the initial period. This rate of change of the combined input signal is referred to herein as the slew rate. When the bias current signal eventually starts to stabilize, the slew rate of the combined input signal is reduced.
Baseline wander can also be caused by movement of patient
14
or electrodes
15
or
16
that disturbs the electrical connection of electrodes
15
and
16
to patient
14
. This movement can result in a significant change in the impedance presented to ECG signal measuring system
10
. This change in impedance can result in a change in the bias current signal, which results in a change in the level of the combined input signal.
Baseline wander can also be caused by interaction of the bias current with changing patient impedance caused by the electrodes forming a better electrical connection to the patient over time.
Generally, the baseline wander is estimated and the estimate is subtracted from the combined signal before displaying the output ECG. One conventional system for estimating baseline wander is illustrated in FIG.
3
.
FIG. 3
is a functional block diagram illustrative of conventional BWF
18
(
FIG. 1
) for use in a digital ECG measuring system. BWF
18
includes an infinite impulse response (IIR) baseline wander estimator (BWE)
30
, an adder
32
and a delay circuit
34
. BWE
30
is connected to receive input ECG samples, denoted ECG
i
(n), and output an average of the samples that represents the estimated baseline wander samples, denoted BW(n). Delay circuit
34
also receives ECG
i
(n), and delays each sample by the delay of BWE
30
. In this way, each BW(n) sample is synchronized with the corresponding ECG
i
(n) sample. Adder
32
receives each estimated baseline wander BW(n) sample and subtracts it from the corresponding input ECG sample ECG
i
(n), thereby generating output ECG samples, ECG
o
(n).
One problem with using a BWE based on an IIR filter is that, in some conventional systems, the IIR filter normally provides non-linear phase response. To linearize the phase response in these conventional systems, the data must be filtered, then the output samples must be reversed in order, and filtered through the IIR filter again. This “forward-backward” operation adds delay to the process, and requires that the data be processed in batches. Thus, each filtered “batch” must then be appended to the previous filtered “batch”, which, undesirably, tends to cause discontinuities in the output ECG. Other conventional systems that use IIR filters to remove baseline wander employ a slew-rate limiter to clip portions of the input ECG signal. The slew-rate limiter adds complexity and cost to the system. Generally, these conventional systems use IIR filters to avoid the relatively high computation load of conventional finite impulse response (FIR) systems. Thus, there is a need for a BWE that is computationally efficient, has linear phase response, and does not require an additional circuitry such as a slew-rate limiter.
SUMMARY
In accordance with the present invention, an efficient, low-cost BWE with linear phase response for an ECG signal measuring system is provided. In one aspect of the present invention, the BWE includes two cascaded box car FIR filters to estimate the baseline wander. The cascaded box car filters form, in effect, a triangular FIR filter (i.e., the co-efficients form a triangular profile), which generates a weighted sliding window average of the input samples to serve as the estimated baseline wander. The estimated baseline wander samples generated by the BWE are then subtracted from the corresponding input ECG samples. This aspect of the present invention significantly reduces the computational burden of the FIR filter implementation because the boxcar filters can be designed to avoid multiplication. In one embodiment, the samples are added and the resulting sum is divided by the number of coefficients (e.g., N). By choosing the number of coefficients as a power of two (i.e., N=2
n
), binary division can be performed by shifting the bits of the resulting sum by n places to the right. Accordingly, the present invention avoids the relatively large computational load of conventional FIR filter-based systems. In addition, this aspect of the present invention allows the BWE to operate on the input samples continuously as they are received, thereby avoiding the discontinuities caused by batch processing required by some conventional IIR filter-based systems. In addition, the input ECG need not be clipped as in other conventional IIR filter-based systems.
In another aspect of the present invention, the BWE is implemented in software. Each boxcar filter uses an accumulator data structure that generates the sum of the previous N samples. When adding the next sample to the sum, it subtracts the Nth previous sample. For example, each sample may be stored in a circular buffer and added to a value in a sum buffer. Before storing the current sample into the circular buffer, the existing sample is subtracted from the value in the sum buffer. The output sample would then be equal to the value in the sum buffer, appropriately shifted. Alternatively, the shifting may be performed after being processed by the second box filter.
REFERENCES:
patent: 3569852 (1971-03-01), Berkovits
patent: 4147162 (1979-04-01), Gatzke
patent: 4153049 (1979-05-01), Gatzke et al.
patent: 4194511 (1980-03-01), Feldman
patent: 4479922 (1984-10-01), Haynes et al.
patent: 4492235 (1985-01-01), Sitrick
patent: 4494551 (1985-01-01), Little, III et al.
patent: 5042026 (1991-08-01), Koike et al.
patent: 5297557 (1994-03-01), Reichl
patent: 5318036 (1994-06-01), Arand et al.
patent: 5357969 (1994-10-01), Herleikson
patent: 5402795 (1995-04-01), Reichi
patent: 5433208 (1995-07-01), Lundstrom et al.
patent: 5532951 (1996-07-01), Ohlsson et al.
patent:
Olson Dana J.
Seguine Dennis R.
Christensen O'Connor Johnson & Kindness PLLC
Kamm William E.
Physio-Control Manufacturing Corporation
LandOfFree
Method and apparatus for removing baseline wander from an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for removing baseline wander from an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for removing baseline wander from an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2498523