Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-11-15
2001-11-13
Jastrzab, Jeffrey R. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C128S902000
Reexamination Certificate
active
06317625
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to system for use within an Implantable Medical Device for measuring physiological signals using analog-to-digital processing; and, more specifically, relates to a system for increasing the dynamic frequency range of signals that can be measured within a patient's body.
BACKGROUND INFORMATION
Many Implantable Medical Devices (IMDs) measure physiological parameters of a patient's body. These signals may be used for diagnosis, and to enable a physician to render appropriate treatment. Examples of physiologic parameters measured by an IMD may include body temperature and pressure, tissue impedance, and tissue oxygen levels. As another example, it is often desirable to measure a voltage existing between two points in the body. This measurement is typically obtained between two electrodes.
Many types of IMDs obtain voltage measurements for use in the diagnosis and treatment of medical conditions. For example, pacemakers, defibrillators, cardioverters, and hemodynamic monitors often measure electrocardiograms (EGMs), which are voltage signals measured within a patient's cardiovascular system and used in the diagnosis and treatment of heart conditions. These signals may be transferred immediately to external devices for use in diagnosis, or may be temporarily stored in the IMD and transferred for later external use. These signals may also be used by the IMD to adjust treatment.
One problem with obtaining accurate voltage measurements within a patient's body involves baseline wander. Baseline wander involves large amplitude, low-frequency, non-physiological signals that can saturate a measurement system, resulting in the loss of patient signal information. There are several sources of baseline wander that often affects internal devices. Patient movement, for example, may disturb the electrical connection of an electrode, causing a low frequency signal to be superimposed on the physiologic signal. Another source of baseline wander in pacing devices, cardioverters, and defibrillators, involves the delivery of electrical stimulus to tissue in the region of the electrode. This delivery of electrical energy creates an electrical field that interferes with the physiological signal being measured.
FIG. 2
shows how baseline wander can affect the measurement of a physiological signal such as an EGM. In
FIG. 2
, an initial portion
21
of curve
20
has a relatively large rate of change as will occur upon delivery of electrical stimulus to tissue surrounding an electrode. 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, wherein the combined signals include the baseline wander imposed on the physiological signal being measured. 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.
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, conventional compensation techniques are generally inadequate for the high slew rate of the combined signal caused during the initial period of the bias current signal as discussed above. Additionally, when conventional techniques are used to compensate for the offset, the waveform morphology is changed. This change makes patient diagnosis and monitoring more difficult. For example, by using a conventional filter having a cut-off frequency selected to remove the offset resulting from baseline wander, waveform characteristics used to diagnose ischemia are filtered from the measured signal.
FIG. 3
is a block diagram illustrative of conventional digital signal measuring system of the type that may be used to measure a physiological signal. Signal measuring system
10
includes a preamplifier
31
, a high pass filter (HPF)
33
, an analog-to-digital converter (ADC)
35
and a second HPF
37
. As will be appreciated by those skilled in the art, signal measuring system
10
includes an anti-aliasing filter (not shown) configured to filter out frequency components of the input 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 surface electrocardiograms. 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 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 physiological 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 signal measuring system. In addition, a sixteen-bit ADC utilizes more power than a twelve or eight-bit ADC. This is undesirable in the context of an Implantable Medical Device (IMD) wherein power conservation is a primary design consideration.
Another solution to a related problem of measuring an external electrocardiograph (ECG) signal 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 disclosed 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, energy-efficient, physiological signal measuring system for use in an IMD having a relatively large dynamic range and high resolution. The system should minimize the changes in the morphology of the physiological signal being measured so that the ability to provide accurate patient diagnoses is not compromised.
SUMMARY
In accordance with the present invention, a signal measuring system for an IMD is provided for measuring physiologic signals having a relatively large effective dynamic range. This system is adaptable for use in measuring electrocardiograms. 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 that is used to convert the sensed signal to digital format.
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 that includes the baseline wander imposed on the physiological signal. The HPF 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 output signal. The ADC then oversamples the output signal of the HPF
Lu Steven N.
Olson Dana J.
Ptak Tara N.
Stadler Robert W.
Van Ess David W.
Jastrzab Jeffrey R.
McMahon Beth L.
Medtronic Inc.
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