Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1998-12-03
2001-10-30
O'Connor, Cary (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S485000, C600S492000, C600S495000, C600S499000, C600S504000
Reexamination Certificate
active
06309359
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to a non-invasive apparatus and method for measuring arterial compliance, arterial lumen area, the amount of blood flowing through the artery, and the phase lag between the pressure and blood flow waveforms.
Occlusive cuffs are commonly used to measure blood pressure using the auscultatory method. During this method, the cuff is placed on a patient's arm, inflated, and gradually deflated while the attending physician relies on the generation of Korotkoff sounds to determine systolic and diastolic pressure. It is known in the art, however, that occlusive cuffs can be used in other ways to obtain other valuable information.
Measuring Changes in the Volume of an Artery with an Occlusive Cuff
Windsor, “The Segmental Plethysmograph,” Angiology, Vol. 8, p. 87, 1957, discusses using an occlusive cuff to measure variations in the size of a limb (e.g. an arm). Windsor is incorporated herein by reference.
FIG. 1
illustrates Windsor's apparatus, known as a plethysmograph, which includes an occlusive cuff
10
, a bulb
12
for pumping air into Windsor's apparatus, a valve
14
for isolating the apparatus from bulb
12
, and a differential pressure chamber
16
comprising a diaphragm
18
. As the volume of a patient's arm varies (e.g. because of blood pumped through the arm by the patient's heart) this variation in volume &Dgr;V can be measured with Windsor's apparatus using the equation
Ad=K&Dgr;V
where A is the area of diaphragm
18
, d is the displacement of diaphragm
18
, and K is a proportionality constant.
K=V
0
/(
V
0
+V
1
)
where V
0
is the original volume of an inactive portion
20
of Windsor's system, and V
1
is the original volume of an active portion
22
of the system.
Measuring Systolic and Diastolic Pressure with an Occlusive Cuff
It is known in the art that one can use an occlusive cuff to measure a patient's systolic and diastolic pressure using the “oscillometric method,” e.g. as described by Drzewiecki, et al., “Theory of the Oscillometric and the Systolic and Diastolic Detection Ratios,” Annals of Biomedical Engineering, Vol. 22, pp. 88-96 (1994), incorporated herein by reference. During this method, an occlusive cuff is placed on a patient's arm, inflated, and slowly deflated while the cuff pressure is monitored.
FIG. 2A
illustrates cuff pressure vs. time during this method. A portion
30
of
FIG. 2A
illustrates pressure while the cuff is being inflated, and a portion
32
illustrates pressure while the cuff is being deflated. As can be seen, there is a set of small ridges and valleys in the waveform of FIG.
2
A. These ridges and valleys are caused by the expansion and contraction of the patient's brachial artery that occur when the patient's heart pumps blood through the artery.
FIG. 2B
shows the waveform of
FIG. 2A
after it has been band-pass filtered to isolate the portion of the signal between 0.5 and 5 Hz and amplified. This permits isolation and observation of the portion of cuff pressure oscillation caused by the artery expanding and contracting. As can be seen, the amplitude of the pulses gradually increases, reaches a maximum, and then decreases as the cuff deflates. The pulses of
FIG. 2B
are at their maximum amplitude when the cuff pressure equals the mean arterial pressure (“MAP”). One can calculate the systolic pressure as that pressure, above the MAP, at which the oscillation pulses have an amplitude As such that:
As/Am=
0.55
where Am is the maximum pulse amplitude (which, as mentioned above, occurs when the cuff is at the MAP). In other words, the cuff pressure (above the MAP) which produces pulse amplitudes that equal 55% of the pulse amplitude at the MAP equals the systolic pressure.
The diastolic pressure equals that cuff pressure (below the MAP) which produces pulses having an amplitude Ad such that:
Ad/Am=
0.85
In other words, the cuff pressure (below the MAP) that produces pulse amplitudes that equal 85% of the pulse amplitude at the MAP equals the diastolic pressure.
Using an Occlusive Cuff to Measure Artery Lumen Size
Cuff pressure exerts a radial force on the brachial artery directed towards the center of the lumen of the artery As the cuff pressure decreases, the magnitude of the force acting on the artery directed toward the center of the lumen of the artery decreases. Also, as the cuff pressure increases, the magnitude of the force acting on the artery directed toward the center of the lumen of the artery increases. The difference between blood pressure and cuff pressure is called the “transmural pressure”. The brachial artery exhibits compliance (i.e., elasticity) which differs with transmural pressure. As the cuff pressure decreases transmural pressure increases. Thus, the artery lumen size increases as the transmural pressure increases.
If the cuff pressure exceeds the blood pressure in the brachial artery, the artery lumen contracts. At sufficiently high pressure (e.g. more than 200 mm Hg), the brachial artery is pinched closed, and the lumen area is effectively zero.
FIG. 3
illustrates the relation between an artery lumen area and the transmural pressure.
FIG. 4
illustrates the artery compliance (e.g., elasticity) with respect to pressure. As can be seen, the artery is most compliant when the transmural pressure is zero (i.e., when the blood pressure in the artery equals cuff pressure). At very low transmural pressures and high transmural pressures, artery compliance drops.
Pilla, “Calibrated Cuff Plethysmography: Development and Application of a Device For Use in Evaluation of the Effect of Arterial Pressure-Volume Curve Alterations on Systemic Blood Pressure,” PhD. Dissertation, Rutgers University (May, 1995) discusses using an occlusive cuff to determine the pressure versus lumen area characteristics of a patient's brachial artery. Pilla is incorporated herein by reference.
FIG. 5
schematically illustrates Pilla's apparatus
50
. As shown in
FIG. 5
, apparatus
50
includes an occlusive cuff
52
to be placed around a patient's arm (not shown), a pump
54
for pumping air into cuff
52
, a pair of valves
56
,
58
, a pressure transducer
60
, and an electrical circuit
62
for amplifying the signal provided by transducer
60
.
As explained below, Pilla uses pump
54
and transducer
60
to approximate the change in cuff volume caused by a change in pressure within cuff
52
(i.e., cuff compliance). Pilla then uses this approximation of the cuff compliance and the pressure measured by transducer
60
to calculate the change in the patient's arm diameter (e.g., caused by expansion and contraction of the brachial artery) caused by the patient's pulse.
Pilla begins his process by passing water through pump
54
to determine the stroke volume of the pump. This information is used in subsequent calculations of cuff compliance.
Pilla then places valve
56
in a first position such that air from the atmosphere flows into an input conduit
54
a
of pump
54
, and air from an output conduit
54
b
of pump
54
flows into cuff
52
to thereby inflate cuff
52
. After cuff
52
is inflated to a pressure in excess of the patient's systolic pressure, valve
56
is adjusted so that pump
54
removes air from cuff
52
and pumps air back into cuff
52
(e.g. as indicated by arrow A). Because of the manner in which valves
56
and
58
are adjusted, pump
54
cooperates with cuff
52
to superimpose a sinusoidal pressure variation on the air in cuff
52
.
Transducer
60
measures the pressure in cuff
52
The pressure in cuff
52
varies in response to two things:
a) air being pumped in and out of cuff
52
by pump
54
; and
b) the expansion and contraction of the patient's arm caused by the patient's heart pumping blood through the arm. (This change in arm size is mostly due to expansion and contraction of the patient's brachial artery.)
Pilla's pump has a stroke frequency of 50 to 60 Hz. The patient's heart beats at a frequency be
Drzewiecki Gary M.
Pilla James J.
Whitt Michael D.
Carter Ryan
O'Connor Cary
Skjerven Morrill & MacPherson LLP
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