Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-04-27
2003-09-09
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S500000, C600S300000
Reexamination Certificate
active
06616613
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a physiological signal monitoring system and more particularly to a system which allows a user to determine various types of physiological information and which allows a user to electronically access this information over a communication network.
BACKGROUND OF THE INVENTION
Various types of instrumentation for monitoring physiological signals are currently available to consumers and health professionals. Specifically, consumers have access to thermometers, weight scales, blood pressure cuffs, blood glucose monitors, urine testing strips and other similar diagnostic technology. In the field of cardiovascular physiological testing, there is currently a wide variety of blood pressure testing equipment which has been developed to determine arterial blood pressure related parameters, namely systolic pressure (maximum blood pressure) and diastolic pressure (minimum blood pressure). It has also been recognized that other parameters such as mean (average) blood pressure during a heart cycle, pulse pressure (the difference between systolic and diastolic pressure) as well as pulse rate and pulse rhythm are also important in assessing patient health.
In an attempt to provide consumers and health professionals with non-invasive blood pressure measuring equipment for patient safety and convenience, photoplethysmograph (PPG) sensors have been utilized within blood pressure testing equipment. PPG sensors are well-known instruments which use light for determining and registering variations in a patient's blood volume. They can instantaneously track arterial blood volume changes during the cardiac cycle and are used within physiological signs monitoring devices.
One such device is disclosed in U.S. Pat. No. 6,047,203 to Sackner et al. which uses PPG sensors to monitor the physiological signs of the user to identify when adverse health conditions are present within the user and to provide the user with appropriate directions or signals. However, many devices such as this one are only used to determine whether physiological signals indicate the presence of an adverse condition for the user and are not directed to identifying and/or determining accurate estimates of blood pressure and other cardiovascular values for diagnostic purposes.
Since PPG sensors operate non-invasively, efforts have been made to utilize them to determine estimates of mean, systolic and diastolic blood pressure. These devices either estimate mean blood pressure from the mean value of the blood volume pulse, a measure of pulse wave velocity or changes in the volume pulse contour using formulae and calibrated constants. However, these devices have not achieved widespread use due to a lack of accuracy and difficulty of use.
Specifically, the difficulties with estimation of mean, systolic and diastolic blood pressure from the volume pulse contour can be attributed to variability in the amplitude of the volume pulse contour due to volume changes unrelated to blood pressure effects and the nonlinear relationship between volume changes in an arterial vessel and associated pressure changes.
Also, there are measurement and instrumentation difficulties associated with PPG sensors such as the presence of mechanical alterations in the sensor/skin interface (i.e. vibrations and differing pressure), ambient light effects, and changes in the blood volume due to alteration in body position. Without carefully correcting for changes in the blood volume pulse signal that are due to factors other than blood pressure and without using conversion techniques which recognize the nonlinear relationship between arterial vessel volume and pressure, these methods cannot accurately predict blood pressure characteristics using PPG readings alone.
It has long been recognized that blood volume pulse contours change with aging and blood pressure. These changes are largely related to a shift in the occurrence of the aortic reflected wave within the pulse contour. The reflected wave is a complex pulse signal generated by reflections of the pulse wave originating at the heart. The pulse wave travels from the heart along the aorta with branches to the head and the arms, continues along the aorta to the trunk and from there to the legs. At about the level of the kidneys, a significant reflection of the pulse wave originates. The reflected waves from the arms and the legs are rapidly damped, travelling with relatively low amplitude back to the trunk. It is well known that as detected in the upper extremity the reflective wave originating in the abdominal aorta has an onset later than the reflected wave from the upper limbs, has significantly greater amplitude, travels almost without attenuation to the ends of the upper extremity, and has a significant presence in the volume pulse contour obtained from a fingertip, ear or other points on the surface of the body above the aortic origin of the reflecting wave.
By accurately characterizing the timing, amplitude and shape of the abdominal aortic reflected wave, a significant amount of information about aortic compliance, aortic pulse wave velocity and the health of the internal organs can be obtained. As discussed in “Wave Reflection in the Systemic Circulation and its Implications in Ventricular Function”, Michael O'Rourke et al., Journal of Hypertension 1993, 11 pgs. 327-337, human aortic pulse wave velocity more than doubles between 17 and 70 years of age. This phenomenon is a manifestation of arterial stiffening and is attributable to the fatiguing effects of cyclic stress causing fracture of load-bearing elastic lamellae in the wall, and degeneration of arterial wall. When mean blood pressure is decreased (i.e. using vasoactive drugs), the reflected wave has been observed to occur later in the pulse wave, whereas when blood pressure is increased, the reflected wave occurs earlier and moves into the systolic part of the wave. Readily observed ascending aortic pressure wave contours associated with ageing and hypertension can be explained on the basis of early wave reflection. Also, several authorities have observed a strong association between poor aortic compliance (i.e. arterial stiffness) and coronary artery disease and hypertension. For example, it has been observed that decreased aortic compliance results in an increase in systolic and a decrease in diastolic aortic pressure, both of which are deleterious to the heart (“Aortic Compliance in Human Hypertension”, Zharorong Liu, et al., Hypertension Vol. 14, No. 2, August 1989 pgs. 129-136). Accordingly, the aortic reflected wave is a powerful source of information relating to a user's cardiovascular health and relative risk.
While there are several techniques for utilizing the timing of the aortic reflected wave to derive physiologically useful parameters, the analysis used by most of these techniques does not accurately identify the onset of the reflected wave in the volume pulse contour. The subtle changes in the volume pulse signal associated with aortic reflection effects that follow the systolic peak are difficult to visualize. It is often extremely difficult to identify these effects, even with the help of computing means, without time consuming pattern recognition techniques.
For example, U.S. Pat. No. 5,265,011 to O'Rourke discloses a method for determining the systolic and diastolic pressures based on the specific contours of pressure pulses measured in an upper body peripheral artery. The method identifies pressure pulse peaks relating to systolic and diastolic components of the pulse contour and takes first and third derivatives of the pressure pulses to determine relevant minimum and maximum points. Specifically, the onset of the systolic pressure wave is determined by locating a zero crossing from negative-to-positive on a first derivative curve and the shoulder of the reflected wave is identified by finding the second negative-to-positive zero crossing on the third derivative. However, it is difficult in practise to identify the reflected wave peak in this fashion as the slope
Bereskin & Parr
Mallari Patricia
Nasser Robert L.
Vitalsines International, Inc.
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