Surgery – Diagnostic testing – Measuring electrical impedance or conductance of body portion
Reexamination Certificate
1999-10-08
2001-06-26
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Measuring electrical impedance or conductance of body portion
C607S008000, C324S607000
Reexamination Certificate
active
06253103
ABSTRACT:
FIELD OF INVENTION
The present invention pertains generally to the field of electrical impedance estimation, including methods and apparatus for estimating electrode, transthoracic and transmyocardial impedance in conjunction with electrical cardiac therapy.
BACKGROUND
Defibrillation is the process of applying a strong electrical “countershock” to a patient's heart in an effort to convert an excessively fast heart rhythm—i.e., in order to slow and correct the heart rhythms and thereby allow the heart to pump more blood. A defibrillator is a device which applies such “countershocks.” For example, “external” defibrillators produce and store energy for delivery to the heart through a pair of externally attached electrodes, wherein the actual delivered energy is a function of both the stored energy in the defibrillator device and the “transthoracic impedance” of the patient. Thus, for a given stored energy level, the transthoracic impedance determines the amount of current which flows to the myocardium of the patient. In particular, this current flow must be adequate to depolarize a critical mass of myocardium to achieve defibrillation.
The transthoracic impedance consists of two parts: the impedance due to the electrode-electrolyte interface at the connection site to the patient, hereafter referred to as “electrode impedance,” and the impedance due to patient tissue. Impedance itself is comprised of two parts: resistance, which is constant, and reactance, which is frequency dependent. Historically, transthoracic and electrode impedance have been estimated using a method first proposed by Geddes in 1975. With this method, an “apparent” impedance is calculated as a ratio of peak voltage to peak current obtained when defibrillation energy is applied across a patient's chest, or, for purposes of testing patient electrodes, to face-to-face electrodes. As a variation of this method, Geddes showed that in dogs the ratio of peak voltage to peak current obtained from a high frequency (approximately 30 kHz) sinusoidal current approximates the ratio obtained from application of defibrillation energy.
Kerber later “validated” this approximation in 19 human subjects using a 31 kHz square wave voltage and a Hewlett Packard 78660 defibrillator, obtaining a mean impedance of 78.1±19.4 &OHgr;. In a paper describing these tests, Kerber used the description of “actual” impedance. However, this “actual” impedance, as calculated by Geddes and Kerber, assumes this impedance is a pure resistance, whereas the true impedance is not purely resistive, since it varies with frequency. Nevertheless, this method of impedance estimation has been used consistently for the past two decades to estimate transthoracic and electrode impedance. For example, Hewlett Packard has incorporated the high frequency ratio measurement in its defibrillators in the operation of a patient contact indicator, as described in U.S. Pat. No. 4,840,177.
Thus, it has been assumed since 1975 that the “apparent” impedance accurately estimates the true resistance. However, a simple experiment can be used to determine the accuracy of the apparent impedance ratio calculation, based on connecting the positive and negative outputs of a recently calibrated Hewlett Packard 43110 defibrillator directly to various resistors, each rated for 25 W, placed in series with a 50.6 &mgr;F capacitor, rated for 5 kV. For each resistor value, the apparent impedance, as estimated according to the Geddes/Kerber methodology, was measured from the mean of three measurements as:
True Resistance (&OHgr;)
“Apparent” Impedance (&OHgr;)
50.4
105
502
90
999
105
As can be observed from these figures, regardless of the true resistance value taken over a large resistive range, the measured apparent impedance is approximately constant. In other words, the apparent impedance method is predisposed to estimating resistance values of approximately 100 &OHgr;, even if the true resistance is an order of magnitude higher. In fact, Geddes himself has acknowledged the shortcomings of his measurement, stating, in a recent writing, that “in defibrillation studies many authors use the term impedance [Geddes' italics] to mean the ratio of peak voltage to peak current for the defibrillating current pulse, regardless of waveform. Although this practice violates the strictest definition of impedance, the ratio so obtained informs about the quality of the conducting pathway.”
It is believed, however, that the Geddes/Kerber “apparent” impedance ratio may not, in reality, be providing sufficient information about the “quality of the conducting pathway,” especially given the notable flaws in the underlying assumptions of this methodology pointed out by the above experiment. A significant need therefore exists for determining true electrode and transthoracic impedance. In particular, the true impedance estimate should consider both the resistance and reactance components of impedance, as opposed to a “lumped” resistance approach of Geddes/Kerber.
In an international patent application PCT/US94/08134, published in international publication no. WO 95/05215, entitled “Electrotherapy Method and Apparatus,” the effect of impedance on the decay of a biphasic truncated exponential defibrillation waveform is measured during the actual external defibrillation discharge. In particular, this “impedance effect” is measured as the time interval required for a either predetermined charge, or the defibrillator capacitor voltage, to be delivered to a patient. The duration of each waveform phase is determined in real time, based on either measurement. However, this measured time interval is equally dependent on the respective electrode, transthoracic and transmyocardial resistance and capacitance. Thus, without separating the effect of each parameter on the time interval and then measuring the decay interval for a significant portion of the duration of the waveform, the measured time interval can not accurately predict the waveform decay.
In contrast, by measuring the electrode, transthoracic and transmyocardial resistance and capacitance separately after a single defibrillator discharge, it is believed that the decay of future waveforms can be accurately predicted, since the resistance and capacitance should not change significantly after further discharges.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for accurately estimating the actual electrode and/or transthoracic impedance, taking fully into account both the resistance and reactance components of impedance.
In a preferred embodiment, the actual impedance across a pair of face-to-face electrodes, or across electrodes in contact with a patient's chest cavity, respectively, is modeled as a resistor in series with a capacitor, wherein the “reactance” component of the impedance equals 2&pgr;*frequency/capacitance. A reference square wave voltage is applied to the electrodes in series with a selected load resistor and a response voltage across the electrodes is measured. Both the reference voltage and the response voltage are then used to estimate a transfer function between them. Equating this transfer function to a resistor-capacitor circuit model results in estimation of the actual resistance and “capacitance” components of the true impedance.
An advantageous application of the present invention is that these true impedance measurements can be made within an external defibrillator connected to an electrode pair, e.g., for purposes of adjusting the stored energy level required to deliver a desired charge to a patient. In such preferred embodiments, an impedance measurement module is preferably isolated from any defibrillation energy or pacing current; e.g., by employing a high voltage relay. For example, if such a device is in “defibrillation mode”, impedance is preferably measured every n heartbeats, where n is selected in accordance with a desired defibrillation algorithm and where the impedance measurement is halted if a defibrillation request is made by the operator. If such a
Garrison Scott B.
Jaworski Francis J.
Kimberly--Clark Worldwide, Inc.
Moriss Bateman OBryant & Compagni PC
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