Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1998-01-26
2001-02-06
Evanisko, George R. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C128S901000
Reexamination Certificate
active
06185450
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a method and apparatus for monitoring an electrocardiograph waveform, and more particularly to a method and apparatus for returning an electrocardiograph trace to the middle of a display such as a chart recorder strip.
BACKGROUND OF THE INVENTION
One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique of restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using a cardiac defibrillator.
To determine if defibrillation is required, defibrillators usually rely on an interpretation of an electrocardiograph (ECG) signal that is displayed on an ECG monitor or plotted on a strip of paper by a chart recorder. In such systems, the ECG signal is displayed as a waveform normally containing the P and T waves, as well as the QRS peaks associated with ventricular contraction. These waveforms are interpreted to determine the presence of ventricular fibrillation, ventricular tachycardia, asystole (the absence of contractions of the heart), or other abnormal heartbeat patterns.
One of the problems related to ECG monitoring is the typical situation where the ECG electrodes and certain tissues have become charged due to currents drawn through them during a pace pulse or defibrillation pulse. Once charged, the electrodes discharge over a time interval that may last up to several hundred milliseconds, depending upon the chemistry of the electrodes. As they discharge, the voltage across them is not DC, but rather an approximately exponentially decaying waveform, that is substantially larger than the normal range of ECG signals. Such voltages drive the amplifiers of the ECG monitoring circuit into saturation. Thus, a common problem is how to return as quickly as possible to normal ECG monitoring following the application of a defibrillation or pacing pulse which charges the electrodes and drives the amplifiers into saturation.
One prior art method for dealing with this problem is illustrated in U.S. Pat. No. 5,609,611 to Bolz et al., which discloses a pacemaker system with a porous electrode and residual charge or after-potential reduction. In addition, U.S. Pat. No. 4,811,738 to Economides et al. discloses a cardiac pacemaker with fast stored charge reduction. Both of these patents deal with the problem of trying to measure the electrical “response signal” of the heart (such as an ECG signal) during the period following a stimulation pulse. As stated in the patents, during each stimulation pulse, electrical charge is stored on the electrodes and in the polarization of the stimulated tissue that can interfere with normal heart measurements. Both disclosures describe techniques of providing short charges of opposite polarity following a stimulation pulse, so as to counteract the stored charge and quickly restore normal heart measurements. While these methods address the stored charge issue, they have disadvantages in that they require extra energy and time to implement the countercharges, and require additional countercharge circuitry.
Another prior art method is shown in U.S. Pat. No. 5,447,518 to Pless, which discloses a method and apparatus for phase-related cardiac defibrillation. As shown in FIG. 1 of Pless, the output from a first amplifier is coupled to a DC baseline restoring circuit including a resistor, a second amplifier, and a capacitor. The baseline restoring circuit has a variable time constant controlled by a microprocessor through a switch. When the switch is closed, current from the first amplifier is shunted through the resistor, thus providing a faster time constant. In addition, the second amplifier has a variable DC set point, which is also under the control of the microprocessor. In general, the sensing circuit generates a defibrillation output signal in response to an ECG signal of a certain level. After a defibrillation output signal is generated, the microprocessor sets the DC baseline restoring circuit to the rapid time constant by closing the switch. After a few milliseconds, the output of the amplifier is back to the same potential as it was just prior to the application of the defibrillation output signal. The microprocessor then opens the switch to return the baseline restoring circuit to the slower time constant. While this circuit addresses some of the problems raised regarding monitoring an ECG waveform following the application of a defibrillation pulse, it also has certain disadvantages. For example, in some systems, the method of instantly switching the baseline restoring circuit between rapid and slower time constants in the presence of decaying offsets tends to produce erroneous QRS detect marks (related to the QRS waveforms that indicate ventricular contraction) that could be interpreted as indicating a properly functioning heart, when in fact the heart is experiencing ventricular fibrillation. This could lead an operator to refrain from applying a defibrillation pulse when one is needed.
Accordingly, a method and apparatus are needed for returning to normal ECG monitoring as quickly and seamlessly as possible following delivery of defibrillation therapy. The method and apparatus should not require the output of additional energy or the presence of a significant amount of additional circuitry. In addition, the method and apparatus should return to normal monitoring without producing erroneous QRS detect marks. As explained in the following, the present invention provides a method and apparatus that meet these criteria and solve other problems in the prior art.
SUMMARY OF THE INVENTION
In accordance with the present invention, a digitally controlled circuit using a sliding pole is provided to restore an electrocardiograph (ECG) trace to the middle of a display. The ECG display includes a monitoring circuit with an amplifier and a switch that is controlled by a microprocessor. The switch is switchable between at least two states, the first state causing the monitoring circuit to have a first frequency response curve with a first pole, and the second state causing the monitoring circuit to have a second frequency response curve with a second pole. Thus, a selection of the frequency response of the monitoring circuit is controlled by the control signal for the switch, which is produced by the microprocessor.
In accordance with one aspect of the invention, the control signal for the switch is a pulse waveform with a variable duty cycle. By adjusting the duty cycle of the pulse waveform, the frequency response of the monitoring circuit can be adjusted. In one actual embodiment, the duty cycle of the pulse waveform is adjusted in incremental steps, so as to avoid the production of erroneous QRS detect marks that can otherwise occur. Also, the operating frequency of the pulse waveform is preferably above the upper frequency of the ECG bandwidth so that adjustments to the pulse waveform duty cycle do not produce erroneous QRS detect marks.
In accordance with another aspect of the invention, the amplifier of the monitoring circuit is brought out of saturation according to an adjustment routine. The adjustment routine consists of initially setting the duty cycle of the pulse waveform control signal to a minimum so as to quickly bring the amplifier out of saturation. The duty cycle is then increased in incremental steps until a maximum duty cycle is achieved. The incremental steps may be predetermined or they may be customized according to feedback from the amplifier.
In accordance with still another aspect of the invention, when the adjustment routine for the duty cycle of the pulse waveform is to be customized according to feedback from the amplifier, a low pass filter is added so as to form an envelope filter. The envelope filter temporarily removes the ECG signals so that the exponential decay of the waveform can be observed. If the output of the envelope filter is less than
LaBrash Stephen P.
Seguine Dennis R.
Stice John R.
Christensen O'Connor Johnson & Kindness PLLC
Evanisko George R.
Physio-Control Manufacturing Corporation
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