Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-01-15
2001-06-19
Jastrzab, Jeffrey R. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C128S901000
Reexamination Certificate
active
06249696
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 electrocardiograph (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
.
Large amplitude, low-frequency, non-physiological signals, commonly referred to as baseline wander, can saturate an ECG measurement system, resulting in the loss of patient ECG signal information. There are several sources of baseline wander including; DC bias currents, patient movement and changing patient impedance, which 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.
A similar problem is 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 slew rate 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.
Conventional techniques can be used to compensate for the “offset” caused by the baseline wander in order to keep the combined input signal from saturating the system. However, the inventors of the present invention have observed conventional compensation techniques are inadequate for the high slew rate of the combined signal caused during the initial period of the bias current signal.
FIG. 3
is a block diagram illustrative of conventional digital ECG signal measuring system
10
(FIG.
1
). ECG signal measuring system
10
includes a preamplifier
31
, a high pass filter (BPF)
33
, an analog-to-digital converter (ADC)
35
and a second HPF
37
. As will be appreciated by those skilled in the art, ECG signal measuring system
10
includes an anti-aliasing filter (not shown) configured to filter out frequency components of the input ECG signal above one-half of the sample rate of ADC
35
.
In this example, the passband of HPF
33
is set at about 0.03 Hz, while the passband of HPF
37
is set at about 0.02 Hz. This gives a passband with a lower edge of 0.05 Hz. This performance is consistent with industry standards for diagnostic quality ECG systems (AAMI EC-11). Unfortunately, the baseline wander signal has frequency components above 0.05 Hz. Thus, in this example, HPF
33
passes the baseline wander signal along with the ECG input signal to cause the saturation problem described above.
One conventional solution to this problem is to increase the dynamic range of the system. Current industry standards require a dynamic range of at least 10 mV (i.e. ranging from ±5 mV). Diagnostic and interpretive algorithms require resolution of 5.0 &mgr;V. This range is adequate for patients ECG signals that do not include baseline wander. Sources of baseline wander discussed above dictate that the dynamic range would have to be increased to greater than 150 mV. However, to increase the dynamic range and maintain a given resolution would require an increase in the number of bits of the analog-to-digital conversion. For example, a twelve-bit ADC can be used for 20 mV dynamic range and 5 &mgr;V resolution. However, a sixteen-bit ADC may be required for 160 mV dynamic range and the same 5 &mgr;V resolution. The cost of a sixteen-bit ADC is significantly higher than a twelve-bit ADC, which undesirably increases the cost of the ECG signal measuring system. Another solution to this problem is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 09/013,570, entitled “Digital Sliding Pole Fast Restore For An Electrocardiograph Display,” Stice, et al. Although the digital sliding pole invention represents a substantial improvement over the prior art, further improvement is, of course, generally desirable. Thus, there is a need for a low cost ECG measuring system having a relatively large dynamic range and high resolution.
SUMMARY
In accordance with the present invention, an ECG signal measuring system having a relatively large effective dynamic range and high resolution is provided. In one aspect of the present invention, low frequency compression/enhancement techniques are combined with dither techniques to effectively increase the dynamic range while maintaining resolution. This aspect of the present invention is achieved without increasing the number of bits of the ADC.
In one embodiment, the system includes a HPF, an ADC, a decimation filter (DF), and a compensation filter (CF). The HPF receives an input signal (i.e., baseline wander combined with ECG input signal) and attenuates the low frequency components of the input signal. Unlike conventional systems, the HPF serves to attenuate the bias current signal so that the sampled signal remains within the dynamic range of the system. In one embodiment, the HPF attenuates frequency components that are within the frequency bandwidth of the desired ECG output signal. The ADC then oversamples the output signal of the HPF. The DF receives the output samples of the ADC and generates output samples at rate that is at least twice the maximum frequency of the desired ECG output signal. The CF then amplifies the low frequency end of the DF output samples. The gain and cutoff frequency of the CF are, ideally, set to exactly offset the HPF's attenuation of those low frequency components of the input signal below the cutoff frequency of the HPF and above the minimum frequency of the desired ECG output signal. Although it would appear that the resolution of these low frequency components has been degraded, dither techniques are used, in effect, to exchange sample rate for resolution. In one embodiment, system noise (noise inherent in the system due to imperfections in the components, thermal noise, etc.) is used as the dither. As a result of the compression/enhancement and dither techniques, the ECG output signal remains within the dynamic range of the system with the desired resolution, which allows the system to display an accurate ECG significantly faster than conventional systems.
REFERENCES:
patent: 3569852 (1971-03-01), Berkovits
patent: 3868567 (1975-02-01), Ekstrom
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: 4494551 (1985-01-01), Little, III et al.
patent:
Olson Dana J.
Van Ess David W.
Christensen O'Connor Johnson & Kindness PLLC
Jastrzab Jeffrey R.
Medtronic Physio-Control Manufacturing Corp.
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