Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2000-01-21
2002-06-11
Shaver, Kevin (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S529000
Reexamination Certificate
active
06402697
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus for determining cardiac output, the amount of blood the heart is pumping, as well as identifying pulmonary functions, without resorting to invasive techniques which introduce foreign objects and the like, into a body.
2. Description of the Related Art
Monitoring the cardiovascular system to determine myocardial performance is of paramount importance in patient care, regardless of whether the patient is located in the physician's office, emergency or operating room, intensive care unit, at an accident scene or in transit (e.g., in an ambulance). Although routine cardiac monitoring usually begins with a determination of the patient's heart/pulse rate and blood pressure, in the case of patients who are experiencing cardiac difficulties or distress, additional diagnostic details regarding the operation of the heart are needed. Such additional monitoring may quickly progress to include an electrocardiogram (EKG) and the measurement of hemodynamic variables such as cardiac output.
The term cardiac output is defined as the mean or average total blood flow in the circulatory system per unit time. Cardiac output is associated with the strength of the heart and is consequently an important parameter in assessing the condition of a patient's health. Knowledge of cardiac output level and trends have important diagnostic value in that they provide the clinician with information to help him/her assess how well the myocardium is functioning so as to provide the basis for the timely delivery and prescription of appropriate therapeutic modalities. Pulmonary function relates to the ability of the body to make use of oxygen and to eliminate wastes such as carbon dioxide. This parameter is strongly affected by physiological conditions such as deadspace and shunts, and the ability to quantify these conditions forms a major part of cardiopulmonary therapy.
Owing to the uncertainties of the geometry of blood vessels (e.g., diameter, compliance, etc.) and the dynamic nature of the heart itself, conventional flowmetering techniques such as flow resistance measurement or velocity (e.g., Doppler and ultrasonic) measurements have proven unreliable in estimating cardiac output. As a result, cardiac output is routinely measured invasively; that is, by surgically placing an instrument into the arteries near the heart.
The current state-of-the-art, and arguably the “gold standard” for cardiac output measurement, is considered to be either the Direct Fick or thermodilution technique using a flow-directed catheter (Swan-Ganz catheter). The catheter is physically threaded through a large vein (femoral, internal jugular, etc.) into and through the right atrium and right ventricle of the heart into the pulmonary artery located between the heart and the lungs. At that point, thermal dilution techniques may be used to quantify the blood flow. Unfortunately, because of the invasive nature of the technique, the potential risk to the patient of hemorrhage, dysrhythmia or cardiac arrest is relatively high. Consequently, the routine use of invasive techniques such as thermal dilution to measure cardiac output is presently limited to specific clinical situations where the benefits far outweigh the risks.
A significant number of patients (as many as two percent) do not survive the surgery associated with the catheter insertion procedure itself. Hence, this technique is limited to those situations where patients are extremely ill and the increased risk for increased morbidity and mortality is acceptable. Efficacy studies in recent medical literature report data that raises questions as to the risk-benefit ratio of the information provided by invasive cardiac output measurement and whether invasive cardiac output measurement is in the best interest of the patient. In addition to the intrinsic danger of the invasive procedure, the monetary cost of the procedure is relatively high, as it is in itself a surgical procedure and, as with almost all surgical procedures, its hands-on labor intensity by expensive medical personnel results in high costs. It is estimated that nearly $200 million is spent in the U.S. alone for invasive cardiac procedures, equipment and materials, and recent medical literature also questions the cost effectiveness of these invasive techniques.
Adding to the increasing concern that the cost-benefit ratios may not be in the patient's best interest is the fact that many patients may not be adequately monitored and consequently are being put at risk from lack of diagnostic information. Presently, no reliable, accepted, cost-effective, non-invasive techniques are available for continuous monitoring; thus, only the sickest and highest risk patients are candidates for continuous cardiac monitoring. This leaves a huge population that goes unmonitored, of which it is well known that significant numbers encounter cardiac distress of one kind or another during non-cardiac-related procedures. There are many clinical situations such as most routine surgery/anesthesia, outpatient care, emergency medicine, and home care where monitoring cardiac output is not routine, but if it were, would be of significant benefit to patient care. There are significant complications that require treatment, many of which may have been prevented had myocardial function monitoring been available and appropriate responses initiated. Current estimates of the costs of aftercare treatment for such cardiac complications exceed $22 billion in the U.S. alone.
Non-invasive and less invasive techniques are therefore highly desirable. Unfortunately, because of the variability and complexity of the physiology of the circulatory system and the pathology of disease, no currently used non-invasive or less-invasive methodologies are known to be capable of obtaining reliable cardiac output values. Although less-invasive methodologies such as impedance cardiography, Doppler-shift techniques, and non-invasive rebreathing and single-breath Fick techniques are or have been available commercially to measure cardiac output, in their current implementations, they all suffer from significant problems and/or disadvantages. In general, all of these techniques are extremely expensive, require a highly trained technical staff, and are limited to a few well-defined clinical situations. In addition, each technique has unique specific limitations.
More particularly, impedance cardiography requires the correct placement of electrodes on the neck and abdomen that are excited by a high frequency (e.g., 100 kHz) current and the subsequent monitoring of the resulting impedance changes between the electrodes. The impedance changes of the chest are used to determine the cardiac stroke volume resulting from the expansion and contraction of the cardiac volume. Cardiac output can be calculated by combining this volume with heart rate in an appropriate algorithm. The limitations of this technique include: the need/ability to correctly place the electrodes, accurate accounting for the volume changes resulting from the inhalation and exhalation of the lungs, and patient movement. Furthermore, the high impedance electrodes act as antennas that pick up considerable amounts of electromagnetic interference (EMI), thereby interfering with the measurements.
The Doppler-shift technique is based on the effect of the shift in frequency of sound from a stationary source that is reflected by a moving object. With this method, the average velocity of the blood flowing in an artery can be readily measured. However, to determine the volumetric flowrate, the cross-sectional area of the artery must be known. Obviously, soft tissue visualization techniques such as MRI are not practical at this time for general use, and ultrasound imaging generally tends not to be accurate enough, although it is used to provide a relative measure in some applications such as transesophageal-echocardiography. Costs are prohibitively high, and in this age of managed care cannot be consider
Calkins Jerry M.
Drzewiecki Tadeusz M.
Metasensors, Inc.
Natnithithadha Navin
Shaver Kevin
LandOfFree
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