Surgery – Diagnostic testing – Measuring electrical impedance or conductance of body portion
Reexamination Certificate
1999-12-14
2002-04-09
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Measuring electrical impedance or conductance of body portion
C600S536000, C600S506000, C607S009000, C607S027000, C607S032000
Reexamination Certificate
active
06370424
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to methods and apparatus for generating an impedance cardiogram and, more particularly, to methods and apparatus for generating an impedance cardiogram for patients having cardiac stimulators which include bioimpedance sensors, as well as methods and apparatus for calibrating cardiac stimulators which include bioimpedance sensors.
2. Background of the Related Art
For a variety of reasons, a person's heart may not function properly and, thus, endanger the person's well-being. Medical devices have been developed to assist physicians in the diagnosis and treatment of cardiac dysfunction. In regard to the diagnosis of cardiac dysfunction, it has been found that the volume of blood that a person's heart is able to pump, commonly referred to as “cardiac output,” is one of the most important cardiovascular parameters. The cardiac output reflects the supply of oxygen and nutrients to the body. Measurements of cardiac output provide information for quantifying the extent of cardiac dysfunction and for indicating the optimal course of treatment.
Both invasive and non-invasive instruments are available for measuring a person's cardiac output. The invasive techniques for measuring cardiac output require complex instrumentation, which must be operated by skilled personnel, and involve the penetration of the skin by a catheter. Due to the various disadvantages of invasive techniques, non-invasive techniques are generally preferred in the majority of cases.
Although a variety of non-invasive techniques exist, one of the more popular techniques is referred to as “impedance cardiography.” Impedance cardiography is a method of measuring the electrical impedance of the body to determine cardiac output. In impedance cardiography, electrodes are typically connected at two locations on the body. A device, referred to as a cardiorespiratory monitor, generates an electric current that flows through the body from one electrode to the other. A second pair of electrodes, which are positioned between the first pair of electrodes, sense the potential developed by the electrical current as it flows through the body and delivers this sensed potential to the monitor. Based on the sensed potential and the injected current, the monitor calculates the impedance of the body.
In general, the impedance of the portion of the body between the electrodes varies inversely with the amount of blood flowing through the vessels in that region. Such impedance is often referred to as “bioimpedance” because it is the impedance of a set of biological tissues. In particular, if the first pair of electrodes are placed such that the current flows through the thorax, i.e., the cavity in which the heart and lungs lie, then the changes in the measured impedance result from changes in the amount of blood pumped by the heart.
The instantaneous amount of blood in the vessels is directly related to the performance of the heart. When blood is pumped out of the heart, the vessels in the thorax become momentarily filled with blood, and the impedance in the thorax rapidly decreases. After the ventricular contraction is complete, the impedance increases to its former level. Analysis of bioimpedance can therefore provide information related to cardiac output. Specifically, to obtain the cardiac output, the stroke volume, which is the amount of blood being ejected during each cardiac cycle, is first computed. The stroke volume may be calculated in a number of different ways, but, generally, it relates to the derivative of the impedance signal. Once the stroke volume has been determined, the cardiac output is computed by multiplying the stroke volume by the heart rate.
However, as anyone familiar with the bioelectrical characteristics of the human body is well aware, a variety of different factors can influence a bioimpedance measurement. For instance, one of the problems encountered in using thoracic impedance to derive the stroke volume is that the thoracic impedance is influenced by the effects of respiration. Similarly, if the patient is moving, during a stress test for instance, the movement also interferes with the thoracic impedance measurement and, thus, the subsequent calculation of stroke volume and cardiac output. Furthermore, when this technique is applied to patients suffering from severe cardiac dysfunction, the measured thoracic impedance may vary markedly from one cycle to another, thus making a qualitative determination of cardiac output difficult to obtain. In view of various problems such as these, a variety of different techniques have been developed for better correlating the measured thoracic impedance to cardiac output by eliminating the influences of these various problematic factors. As a result, impedance cardiography has improved vastly over the past several years and has become an important technique in the detection and treatment of cardiac dysfunction.
Bioimpedance signals are not only useful in the generation of impedance cardiographs using non-invasive monitors as discussed above. For instance, once a person has been diagnosed as having cardiac dysfunction, a physician may determine that a cardiac stimulator may be used to treat the condition. A cardiac stimulator is a medical device that delivers electrical stimulation to a patient's heart. The cardiac stimulator generally includes a pulse generator for creating electrical stimulation pulses and a conductive lead for delivering these electrical stimulation pulses to the designated portion of the heart.
To understand how impedance measurement may be used to enhance the operation of a cardiac stimulator, it is beneficial to understand how cardiac stimulators have evolved. Early pacemakers did not monitor the condition of the heart. Rather, early pacemakers simply provided stimulation pulses at a fixed rate and, thus, kept the heart beating at that fixed rate. However, it was found that pacemakers of this type used an inordinate amount of energy because the stimulation pulses were not always needed. The human heart includes a sinus node located above the atria. The sinus node provides the electrical stimulation that causes a heart to contract. Even the sinus node of a heart in need of a pacemaker often provides such stimulation. Accordingly, if a heart, even for a short period, is able to beat on its own, providing an electrical stimulation pulse using a pacemaker wastes the pacemaker's energy.
To conserve power, pacemakers were subsequently designed to monitor the heart and to provide stimulation pulses only when necessary. These pacemakers were referred to as “demand” pacemakers because they provided stimulation only when the heart demanded stimulation. If a demand pacemaker detected a natural heartbeat within a prescribed period of time, typically referred to as the “escape interval”, the pacemaker provided no stimulation pulse. Because monitoring uses much less power than generating stimulation pulses, the demand pacemakers took a large step toward conserving the limited energy contained in the pacemaker's battery.
Clearly, the evolution of the pacemaker did not cease with the advent of monitoring capability. Indeed, the complexity of pacemakers has continued to increase in order to address the physiological needs of patients as well as the efficiency, longevity, and reliability of the pacemaker. For instance, even the early demand pacemakers provided stimulation pulses, when needed, at a fixed rate, such as 70 pulses per minute. To provide a more physiological response, pacemakers having a programmably selectable rate were developed. So long as the heart was beating above this programmably selected rate, the pacemaker did not provide any stimulation pulses. However, if the heart rate fell below this programmably selected rate, the pacemaker sensed the condition and provided stimulation pulses as appropriate.
To provide even further physiological accuracy, pacemakers have now been developed that automatically change the rate at whic
Intermedics Inc.
Nasser Robert L.
Natnithithadha Navin
Schwegman Lundberg Woessner & Kluth P.A.
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