Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-02-08
2001-09-11
Schaetzle, Kennedy (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C600S547000, C607S028000
Reexamination Certificate
active
06287263
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a system for processing bursted amplitude modulated signals and in particular to method and apparatus for processing bursted amplitude modulated signals using an impedance sensor in biomedical applications.
BACKGROUND
The human body has electrical characteristics which can be measured for characterizing organ function and for the application of different therapies. For instance, the heart is a complex network of nerve and muscle tissue which operates in synchrony to pump blood throughout the body. Cardiac function may be monitored by sensing the electrical signals naturally conducted at certain places in the heart.
Sometimes it is convenient to apply signals to the body to determine the function of the organs of the body. For example, in ultrasound measurements a sound wave is transmitted into the body and the resulting reflections of the sound are used to image internal organs or a fetus.
Another way to apply signals is to use an implanted series of electrodes which apply a known current and measure the resulting voltage. The relationship between applied current and measured voltage is known as impedance. Thus, impedance is measured by injecting a known current using electrodes and monitoring the electrical voltage required to pass the known current between electrodes. The higher the magnitude of impedance, the higher the magnitude of voltage measured across the load for a known current magnitude.
If the electrodes are placed such that the impedance is measured across a right ventricular portion of the heart, then the impedance measured is a function of the stroke of the right ventricle. The stroke volume of the right ventricle provides a measure of the blood volume pumped by the heart into the lungs in one stroke.
The change in impedance is due to the conductive nature of blood and its changing volume in the left ventricle between contractions. The measured impedance will vary depending on the placement of the electrodes. For example, as shown in FIG.
1
A and
FIG. 1B
, if a current is conducted between the housing of an implantable device
12
and a tip electrode
13
on the end of a catheter
14
with the tip electrode
13
positioned in the apex of the right ventricle
15
, then the impedance observed between two electrodes,
16
and
17
, located within the right ventricle (and before the tip electrode
13
) will measure an increased impedance for a contracted ventricle (systole—
FIG. 1B
) as opposed to when the ventricle is not contracted (diastole—FIG.
1
A). This is because in diastole, the ventricle is holding more blood and has more conductive volume to transfer current. In systole, the ventricle is contracted and has less blood, leaving less volume for conduction.
Impedance-based measurements of cardiac parameters such as stroke volume are known in the art. U.S. Pat. No. 4,674,518, issued to Salo, discloses an impedance catheter having plural pairs of spaced surface electrodes driven by a corresponding plurality of electrical signals comprising high frequency carrier signals. The carrier signals are modulated by the tidal flow of blood in and out of the ventricle. Raw signals are demodulated, converted to digital, then processed to obtain an extrapolated impedance value. When this value is divided into the product of blood resistivity times the square of the distance between the pairs of spaced electrodes, the result is a measure of blood volume held within the ventricle. These calculations may be made using spaced sensors placed within a catheter, as in the Salo '518 patent, or they may be derived from signals originating in electrodes disposed in the heart, as described in U.S. Pat. No. 4,686,987, issued to Salo and Pederson. The device of the '987 patent senses changes in impedance to determine either ventricular volume or stroke volume (volume of blood expelled from the ventricle during a single beat) to produce a rate control signal that can be injected into the timing circuit of another device, such as a cardiac pacer or drug infusion pump. In this manner, the rate of operation of the slaved device may be controlled. An example of application of this impedance sensing circuitry to a demand-type cardiac pacer is disclosed in U.S. Pat. No. 4,773,401, issued to Citak, et al.
However, many existing measurement systems in biomedical applications provide a continuous excitation of the tissue, and therefore current excitations must be carefully applied to avoid a current which would be unsafe or to avoid quickly depleting the batteries in an implantable device.
Thus, there is a need in the art for a low power signal processing system. The signal processing system should be flexible to provide low power processing of signals in biomedical applications, such as in the measurement of signals related to cardiac performance in implantable devices. In biomedical applications, the signal processing system should operate without requiring unsafe excitation signals and excessive power drain.
SUMMARY
Those skilled in the art, upon reading and understanding the present specification, will appreciate that the present signal processing system satisfies the aforementioned needs in the art and several other needs not expressly mentioned herein. A low power processing system for processing bursted amplitude modulated signals using an impedance sensor is provided. The processing system performs impedance-related measurements across a load including injecting current pulses of constant amplitude across the load using at least a first electrode and a second electrode, the current pulses including bursts of a plurality of pulses at a pulse frequency at which the current pulses are repeated, the bursts transmitted at a burst frequency; detecting voltages across at least a third electrode and a fourth electrode; high pass filtering the voltages to produce filtered voltages; amplifying the filtered voltages to produce amplified voltage signals; bandpass filtering the amplified voltage signals with a bandpass filter with a center frequency equal to approximately the pulse frequency to generate first filtered signals; rectifying the first filtered signals to produce rectified signals; integrating the rectified signals to produce integrated signals; sampling-and-holding the integrated signals after each burst to capture an integrated pulse value for each burst, creating a plurality of discrete integrated pulse values; and bandpass filtering the plurality of discrete integrated pulse values using a filter including an upper cutoff frequency less than the burst frequency to produce the output related to the time-varying impedance of the load.
In one application, the first electrode is positioned near to an apex of a right ventricle, the second electrode is outside of the right ventricle, the third electrode and fourth electrode are located within the right ventricle and wherein the output is related to the time-varying impedance of the right ventricle during systole and diastole.
In one embodiment, the output signal is analog-to-digital converted. The system may be used for estimating stroke volume using the output and/or estimating hemodynamic maximum sensor rate using the output. The system may be used for controlling pacing as a function of the output. Various electrode configurations and pulse parameters may be used. Low power embodiments are provided.
In one embodiment, the signal processing system comprises an excitation source coupled to at least a first electrode and a second electrode, the excitation source producing current pulses of constant current flowing between the first electrode and the second electrode, the pulses sent in bursts at a burst frequency and having a pulse frequency at which the pulses are repeated; a first high pass filter filtering voltage signals received by at least a third electrode and a fourth electrode to produce filtered voltage signals; an amplifier amplifying the filtered voltage signals; a first bandpass filter coupled to the amplifier and having a center frequency of approximately
Cardiac Pacemakers Inc.
Schaetzle Kennedy
Schwegman Lundberg Woessner & Kluth P.A.
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