Surgery – Diagnostic testing – Measuring electrical impedance or conductance of body portion
Reexamination Certificate
2000-01-20
2002-10-29
Kamm, William E. (Department: 3762)
Surgery
Diagnostic testing
Measuring electrical impedance or conductance of body portion
Reexamination Certificate
active
06473640
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an implantable device and method for long term detection and monitoring of congestive heart failure. More particularly, this invention relates to a device and method of assessing pulmonary congestive and/or systemic venous congestion.
As is known, congestive heart failure (CHF) in a patient is caused, in part, by a build-up of fluid in the lungs and body of a patient. Typically, the of build-up of fluid in the. lungs and body (i.e. edema) is a sign of failing heart circulation, for example, as described in U.S. Pat. No. 5,876,353. Accordingly, attempts have been made for detecting the occurrence of edema in the lungs, for example, using impedance monitoring. In some cases, proposals have been made to take external or internal measurements of impedance as an index of lung water content (edema). However, suitable results have not been obtained to permit long term monitoring of pulmonary and systemic venous congestion.
By way of example, U.S. Pat. Nos. 5,876,353 and 5,957,861 describe the use of an impedance monitor for discerning edema in a patient through the use of respiratory rate and direct current (DC) impedance. In particular, an impedance and respiratory monitoring circuit is added to a pacemaker system in order to obtain measurements of transthoracic impedance and respiratory rate. These measurements are taken over a long term to obtain a long term average signal so that differences can be used to algorithmically process the variance over time for use to assess the amount of tissue edema over the long term changing condition. However, such techniques fail to distinguish between left heart failure causing lung (pulmonary) edema and right heart failure causing systemic venous congestion. In clinical cases, right and left heart failure may occur in concert or independent of each other. DC impedance in particular is more reflective of systemic venous congestion and may be insensitive for detection of pulmonary edema. Furthermore, respiratory rate changes are a relatively late consequence of CHF and may not occur far enough in advance of dangerous clinical consequences to permit intervention. In addition, respiratory rate changes are not specific to CHF. They occur with exacerbations of various ling diseases such as emphysema, asthma and the like, as well as with anxiety, fever, ascent to high altitudes, and other common conditions.
U.S. Pat. No. 5,282,840 describes a multiple frequency impedance measurement system for monitoring a condition of a patient's body tissue in order to obtain an indication of the condition of the tissue. As described, the measurement system employs a pair of electrodes which are located in contact with the tissue to be monitored. During use, electrode signals are to be applied to the two electrodes at different frequencies with the impedance between the electrodes being measured at the different frequencies. Any changes which occur in the measured impedances over time are then used to indicate changes in tissue condition, such as those induced by ischemia, drug therapies, allograft rejection, lead fractures or insulation degradation. When used with a cardiac pacemaker, the impedance values may be compared over a period of time such as hours, days, weeks or even months and may be employed to provide an increase in minimum or base pacing rate in an attempt to counteract a detected ischemia. The measurement system may also be employed to measure respiration for control of the pacing rate. In this case, one electrode is located within the right ventricle in spaced relation to a can electrode of a pacemaker with a substantial amount of lung tissue located within the sensing field of the electrodes. Changes in the impedance may then be used to calculate a respiration rate.
U.S. Pat. No. 4,676,252 describes a double indicator pulmonary edema measurement system for measuring in vivo extra vascular lung water (pulmonary edema). As described, the detection of pulmonary edema is provided by detecting the response of the pulmonary vascular network to indicator dilution of thermal and conductivity modifiers as a function of not only detected thermal and conductivity value but additional body parameters including blood characteristics and a temperature modifier of conductivity. From these, a value for extra vascular heat capacity is determined from which a quantified lung water measurement is obtained.
U.S. Pat. No. 5,003,980 describes a method and apparatus for measuring lung density by Compton Backscattering.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a relatively simple technique for long term detection and monitoring of right and left sided congestive heart failure.
It is another object of the invention to assess congestive heart failure by measuring bioimpedance with an implant device.
It is another object of the invention to utilize a cardiac pacemaker, defibrillator and/or cardioverter/defibrillator with circuitry for monitoring of congestive heart failure in a patient.
Briefly, the invention provides an implantable device and method for long term monitoring of congestive heart failure. In particular, the invention provides a method of measuring bioimpedance to assess congestive heart failure and particularly lung capacitance as an index of pulmonary congestion.
The invention recognizes that the different bio-electric properties of blood and lung tissue permit separate assessments of systemic venous congestion and pulmonary congestion. That is, the lung has a high resistance to current flow as compared to venous blood with the structure of the lung being similar to a nearly dry sponge. As is known, the lung is a honeycomb of air spaces (i.e. dielectric) surrounded by blood filled capillaries and associated pulmonary arterial and venous vessels (i.e. conductors). Thus, the lung may be modeled as an array of resistors and capacitors which can be simplified to a parallel resistive-capacitive circuit. As the lung becomes congested with edema fluid, its resistance decreases and its capacitance changes as well.
Electrical impedance is a vector quantity. Vector quantities have a scalar magnitude and a direction given as an angle (&PHgr;) to the horizontal. Using trigonometry, a vector can be resolved into its horizontal and vertical components. In the case of electrical impedance, the scalar magnitude of impedance (Z) is given by the Voltage across a circuit divided by the Current through the circuit. The angle (&PHgr;) is the phase angle between the voltage and current. The horizontal or “real” component of impedance is the resistance (R) and the vertical “complex” component of impedance is the reactance (X). Reactance may be due to inductance or capacitance, but in the case of thoracic impedance, reactance is purely capacitive (X
c
). Capacitive reactance is given by the formula X
c
=1/(2&pgr;fC), where f is the frequency of an alternating sine wave signal, and C is the capacitance.
By measuring the capacitive component of lung impedance, an index can be obtained which is independent of the systemic venous resistane That is, there would be separate indexes of systemic venous congestion and of pulmonary congestion
Development of the implantable device is based, in part, on a realization that the blood in the right ventricle and venous system provides the lowest resistance path between an electrode in the heart and an electrode implanted in an upper chest of the patient, usually not far from the subclavian vein. The resistance of this blood path is expected to be much lower than an alternate parallel current path through aerated lung tissue. Thus, the right ventricle and systemic venous system of the upper body are likely to dominate in a resistance measurement where the low resistance of the venous system is in a parallel circuit with the high resistance lung.
With direct current, i.e. a constant unvarying signal with a constant amplitude, the capacitive reactance is infinite, so at DC and very low frequencies, almost all of the current flow is through the blood of the
Carella Byrne et al
Hand Francis C.
Jastrzab Jeffrey R.
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