Surgery – Diagnostic testing – Cardiovascular
Patent
1998-10-08
2000-12-12
Layno, Carl H.
Surgery
Diagnostic testing
Cardiovascular
600509, 600513, A61B 504
Patent
active
061610380
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates generally to cardiac monitoring and specifically to the determination of heart rate (HR), heart stroke volume (SV), and cardiac output (CO) according to detection and complex analyses of thoracic bioimpedance and electrocardiograph (ECG) signals, which permit precise detection of the start of left ventricular ejection.
BACKGROUND ART
Heart rate is the number of times the heart beats per minute. Heart stroke volume is the volume of blood pumped during each heart stroke. Cardiac output is the volume of blood pumped in one minute and is generally considered to be the most significant gauge of cardiac fitness. Physicians must frequently rely upon such cardiac parameters to diagnose heart disease, to assess a patient's overall health, to determine the most appropriate method of treatment, and to quickly discover sudden lapses in cardiac performance.
The currently existing methods for measuring cardiac output and other cardiac parameters may be divided into two categories: invasive and noninvasive. The invasive methods require that a medical practitioner insert a measuring device into the patient's body, such as a catheter in the throat, and present numerous disadvantages to both patient and physician. The patient must often endure substantial pain and discomfort and the physician must perform a relatively complicated procedure and occasionally expose himself or herself to the risk of contact with infectious blood. The noninvasive methods currently in use represent a major advancement, but still have significant shortcomings. Most take measurements using ultrasound, phonocardiography, or electrical bioimpedance in order to calculate cardiac parameters.
The methods which employ bioimpedance measurement involve placing a plurality of electrodes on a patient's skin (predominantly in the thoracic region), generating a high frequency, low amplitude electric current from certain of the electrodes into the patient's body, measuring the changes in the electrical impedance of the patient's tissue over time, and correlating the changes in electrical impedance with cardiac parameters.
The manner of arranging the electrodes on the patient's body plays an important part in the relative accuracy of the ultimate cardiac parameter measurements. Due to various anatomical factors, electrodes must be placed over certain areas of the body to achieve optimum correlation between measured changes in bioimpedance and cardiac parameters. Many of the electrode configurations currently in use fail to adequately take into account the paths followed by the lines of electrical potential through the thorax and thus create a distortion in the cardiac measurement. Moreover, a few electrode arrangements require the use of band electrodes, e.g., influencing band electrodes A, B and measuring band electrodes C, D each having a width "n" (see FIG. 1). These band electrodes typically wrap around a patient like a belt and further limit access to the patient, an especially undesirable condition curing reanimation procedures. The movements associated with respiration also make band electrodes very inconvenient when placed on the neck and chest.
Perhaps the most significant problem with the presently existing bioimpedance methods in the imprecise mathematical derivation of cardiac parameters from bioimpedance measurements. The ventricular ejection time (VET) is a measurement of the time between the opening and closing of the aortic valves during the systole-diastole cycle of the heartbeat and it must be calculated as an intermediate step in determining cardiac stroke volume. The prior art does not teach a method for determining ventricular ejection time with sufficient accuracy. Furthermore, the prior art fails to account for the fact that VET is not a single event. In reality, there is actually a left VET and a right VET. It has been shown that the time-derivative impedance signal is actually proportional to the peak aortic blood flow ejected by the left ventricle. The measurements of left VET and right
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Beliaev Konstantin R.
Morozov Aleksandr A.
Schookin Sergei I.
Yong Wen H.
Zubenko Viatcheslav G.
Layno Carl H.
Rheo-Graphic Pte Ltd.
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