Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-10-29
2003-09-30
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S485000
Reexamination Certificate
active
06626840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a noninvasive method and system for detecting various vascular conditions. More particularly, the present invention relates to a noninvasive method and system which utilizes an occlusive arm cuff as a plethysmograph to obtain arterial and endothelial data on a patient and to analyze the data for diagnosing and predicting various vascular conditions.
2. Description of the Related Art
An occlusive arm cuff capable of being filled with compressible air is routinely used to determine the arterial blood pressure. It has been proposed to also use the occlusive arm cuff as a plethysmograph, i.e., to measure the arterial volume, since it possesses compliance due to the fact that it is filled with compressible air. In theory, if the cuff compliance is known, the arterial pulsations, that are found in the cuff pressure, can be converted into the arterial volume pulse which can be used to measure arterial volume to render the occlusive arm cuff as a plethysmograph.
Unfortunately, cuff compliance is not a constant value. It is dependent on how tightly the arm cuff is wrapped on the subject's arm and the current level of cuff pressure. This has prevented the use of the occlusive arm cuff as a plethysmograph.
The benefit of simultaneously measuring the arterial volume and pressure by means of the occlusive arm cuff is the ability to find arterial mechanics and wall properties. Such arterial information could then be obtained on patients as part of a routine physical exam. Furthermore, arterial wall properties can provide an early indication of the presence of a vascular disease process.
U.S. patent application Ser. No. 09/205,042, filed on Dec. 3, 1998 by Drzewiecki et al., now U.S. Pat. No. 6,309,359 and having a common assignee with the present application describes a method of calibrating the occlusive arm cuff to measure arterial volume to allow the arm cuff to be used as a plethysmograph. With reference to
FIG. 1
, there is shown a block diagram of the prior art occlusive arm cuff plethysmograph designated generally by reference numeral
100
. The occlusive arm cuff plethysmograph includes a pump
102
, a needle valve
104
, a flow meter
106
, a blood pressure cuff
108
, and a pressure transducer
110
. Pump
102
provides a constant known volume of gas per pump stroke over the relevant pressure range. The output signal from the transducer
110
is connected to an electronic amplifier
111
, which in turn is connected to an analog-to-digital (A/D) converter and signal processing circuit
112
. The A/D converter
112
outputs a first component
118
and a second component
120
. The first component
118
is caused by compressed air in the arm cuff
108
by the pump
102
. The second component
120
is caused by the patient's arterial pulse. A motor speed control circuit
113
controls the frequency of the pump
102
. A power supply
114
provides power to motor speed control circuit
113
.
One typically begins the process by ascertaining the stroke volume of pump
102
. This can be done by measuring the volume of a fluid (e.g., a gas, such as air) pumped by pump
102
over a period of time (e.g., ten seconds) divided by the number of strokes during that period of time. The fluid used to ascertain the stroke volume of pump
102
is typically the same fluid as that used to inflate the arm cuff
108
during use of the arm cuff
108
. This process can take place simultaneously with data acquisition and is monitored throughout the data acquisition procedure by using the flow meter
106
and monitoring the signal received at the pressure transducer
110
.
In short, the method entails applying a known volume change to the arm cuff using a periodic pump and subsequently requiring a skilled operator to calculate the cuff compliance. Since the pump frequency and the arterial pulse frequency differ, electronic filtering is used to separate each pulse from the cuff pressure. This method, thus provides continuous measurement of cuff compliance, thereby solving the cuff compliance problem. The method is referred to as occlusive arm cuff plethysmography.
In short, the occlusive arm cuff, acting as a plethysmograph, measures the arterial volume pulse as cuff pressure is decreased. In this manner the brachial artery compliance can be measured over the full range of cuff pressures. This allows the examination of all three states of the artery: collapse, buckling and distension.
To find the arterial compliance, one divides the arterial volume pulse by the arterial pulse pressure according to,
C
(
P
t
)
=
Δ
V
(
P
systolic
-
P
diastolic
)
W
cuff
P
systolic
and P
diastolic
are the systolic and diastolic pressure obtained from the Korotkoff method. P
t
is the arterial transmural pressure found from the difference between mean arterial pressure and mean cuff pressure,
P
t
={overscore (P
&agr;
)}−{overscore (P
cuff
)}
W
cuff
is a constant that represents the cuff width. More precisely, it represents the actual volume of artery that is subjected to a uniform transmural pressure, P
t
.
The lumen area can be found by integrating the compliance curve, from negative infinity to P
t
,
A
(
P
t
)=∫
C
(
P
t
)
dP
t
where negative infinity is approximated by the lowest transmural pressure measured or when the arterial lumen is completely collapsed. Since complete collapse corresponds with a lumen area equal to zero, the initial constant of this integration is zero as well. This analysis permits the determination of lumen area from noninvasive compliance measurements.
Other parameters can also be determined, such as the volume of a segment of an artery by integrating the arterial compliance, and blood flow through the artery by multiplying the derivative of the arterial volume with respect to time with the arterial compliance.
In one study, the occlusive arm cuff plethysmography was employed to derive brachial artery pressure (arterial pressure) versus lumen area curves (P-A curves) for several subjects. Drzewiecki et al., “Noninvasive Measurement of the Human Brachial Artery Pressure—Area Relation In Collapse and Hypertension,” Annals of Biomed. Eng., vol. 26, pages 965-974, 1998. A P-A curve for a normotensive and hypertensive subject are shown by FIG.
2
. It is apparent from this data that the P-A curves are quite varied from subject to subject. First, the vessel is most compliant near zero pressure (i.e., when the blood pressure in the artery equals cuff pressure). Second, the brachial artery adapts to high blood pressure by increasing its lumen size. Hence, a single observation of the P-A curve in a given subject may be insufficient for diagnostic information. Accordingly, multiple observations of the same patient or perhaps the use of an intervention may be required.
The cross-section of normal, hypertensive and arteriosclerotic blood vessel changes are shown by FIG.
3
. It is apparent that the vascular smooth muscle wall
2
of the hypertensive vessel becomes thicker and the lumen
4
becomes larger or remains the same. The lumen
4
of an arteriosclerotic vessel simply becomes narrower due to plaque
6
lining the wall
2
. An important application of the occlusive arm cuff plethysmograph is to aid in the noninvasive detection of these important vessel changes, i.e., to measure smooth muscle or endothelial function. It is not certain how these conditions manifest themselves in the P-A curve or in a brachial artery pressure versus compliance curve (P-C curve) which represent a patient's resting arterial mechanical function.
Due to the presence of arterial wall smooth muscle, the subject's P-A and P-C curves may vary with muscle activity. Thus, the vascular smooth muscle is a part of the vascular function that must be examined. Prior research suggests that impaired muscle function, often termed as endothelial function, is predictive of atherosclerosis.
Hence, there is a need for additional research directed at measurin
Drzewiecki Gary M.
Drzewiecki Robert B.
Carmen, Esq. Michael E.
Dilworth & Barrese LLP
Mallari Patricia
Nasser Robert L.
Rutgers The State University of New Jersey
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