Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
2001-12-07
2004-02-03
Lefkowitz, Edward (Department: 3746)
Measuring and testing
Fluid pressure gauge
Diaphragm
Reexamination Certificate
active
06684710
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for measuring the pressure of blood.
More particularly, the present invention relates to a device for measuring the pressure of blood, which is used in an extra corporeal blood treatment device in which the blood is taken from a patient in order to be treated then reintroduced into the body of the patient (especially for the purpose of carrying out dialysis) by means of an extracorporeal blood circuit comprising pipes and including at least one section for measuring the pressure of blood flowing in a pipe.
BACKGROUND OF THE INVENTION
A known type of pressure measurement section comprises, in a substantially rigid wall, a hole which is sealed by a closure element, the internal face of which is in contact with the blood and the external face of which is in contact with the ambient air, it being possible to elastically deform or displace the closure element overall along a deformation or displacement axis which is substantially orthogonal to its general plane, under the effect of the blood pressure; a portion of the external face of the closure element, in its rest state, is in direct or indirect contact with a load sensor which can measure the force applied axially to the internal face of the closure element by the pressure of the blood, in order to calculate therefrom the value of this pressure.
Generally, this type of extracorporeal blood treatment device comprises a circuit part which is made from a casing, or cassette, of the disposable type, incorporating pipes which are connected to the extracorporeal blood circuit.
The pressure measurement section may be an attached module which is mounted in an associated housing of the casing.
The casing is mounted on a support apparatus which comprises, for example, sensors, display means, pumping means, a control interface, an electronic control unit, etc.
In this type of extracorporeal blood treatment device, the blood pressure must be measured without contact between the measurement member and the blood.
Several systems for carrying out this pressure measurement are known.
In a first pressure measurement system, which is shown in
FIG. 1
, a pressure measurement section
10
in a pipe
12
comprises a measurement chamber
14
in which a membrane
16
, or diaphragm, separates the blood flowing in the pipe
12
from the air contained in a compartment
18
.
The membrane
16
can be deformed along a deformation axis A—A which is orthogonal to its general plane, so that it is displaced axially according to the pressure of the blood in the pipe
12
.
The extreme deformation positions of the membrane
16
are shown by dotted lines.
The air compartment
18
is sealed shut when the pressure measurement section
10
is mounted on a support apparatus
20
.
The support apparatus
20
comprises a sensor
22
which directly measures the pressure in the air compartment
18
.
When the blood pressure changes, the membrane
16
is axially displaced to an equilibrium position in which the pressure on both sides of the membrane
16
is equal.
The pressure measured by the sensor
22
in the air compartment
18
is therefore equal to the pressure of the blood in the pipe
12
.
By virtue of a suitable geometry, in particular by virtue of a suitable volume for the compartment
18
and a suitable surface-area for the membrane
16
, this first pressure measurement system makes it possible to measure, on the one hand, so-called “positive” blood pressures, that is to say, blood pressures which are greater than a reference pressure, in this case atmospheric pressure, and, on the other hand, so-called “negative” blood pressures, that is to say blood pressures which are less than the reference pressure.
This measurement system operates correctly provided that there are no leaks in the air compartment
18
, otherwise the membrane
16
is displaced right up to its end stop and it no longer carries out the function of transmitting pressure.
The seal of the air compartment
18
during mounting of the pressure measurement section
10
on the support apparatus
20
, is a weak point of the measurement system.
In particular, the seal may be impaired during use of the measurement system.
In a second pressure measurement system, which is shown in
FIG. 2
, the pressure measurement section
10
forms a compartment
24
containing the blood and one wall
26
of which comprises a hole
28
which is sealed by a flexible membrane
30
.
When the pressure measurement section
10
is mounted on the support apparatus
20
, the external face of the central part of the flexible membrane
30
is in contact with a load transmitter
32
which is inserted between the membrane
30
and a load sensor
34
.
The load sensor
34
makes it possible to measure the forces applied to the internal face of the membrane
30
because of the effect of the blood pressure in the compartment
24
, where the blood pressure is greater than the ambient air pressure.
The blood pressure is determined by the equation:
P
=
F
-
F
0
S
a
(
1
)
In this equation, F is the force measured by the load sensor
34
, F
0
is the force measured in the rest state, that is to say, in the absence of a pressure gradient between the two sides (internal and external faces) of the membrane
30
, and S
a
is the active area or active surface area of the membrane
30
.
The active surface area S
a
of the membrane
30
is equivalent to an area intermediate between the total area of the internal face of the membrane
30
in contact with the blood and the area of contact between the membrane
30
and the load transmitter
32
.
This measurement system allows positive pressures to be measured but it does not allow negative pressures to be measured.
This is because, for negative pressures, the membrane
30
tends to come away from the load transmitter
32
. The load sensor
34
can then no longer measure the forces which are applied to the membrane
30
.
This system has therefore been adapted to measure negative pressure.
In order that the load sensor
34
can continue to measure the forces which are applied to the membrane
30
, when the blood pressure is negative, the membrane
30
is secured in axial displacement to the load transmitter
32
.
Thus, according to one improved embodiment of the second pressure measurement system, which is shown in
FIG. 3
, the membrane has a metal disc
36
on its external face and the load transmitter
32
has a magnet
38
at its axial end facing the membrane
30
.
The magnetic attraction exerted by the magnet
38
on the metal disc
36
makes it possible to secure the membrane
30
in axial displacement to the load transmitter
32
.
When the pressure is positive, the membrane
30
exerts a force which pushes axially against the load transmitter
32
.
When the pressure is negative, the membrane
30
exerts a force which axially pulls the load transmitter
32
.
This device for securing the membrane
30
to the load transmitter
32
is expensive since it requires a special membrane
30
fitted with a metal disc
36
and a special load transmitter
32
fitted with a magnet
38
.
The metal disc
36
must have a large area in order to allow effective magnetic coupling.
In addition, the membrane
30
is subject to a significant jolt when the metal disc
36
“sticks” to the magnet
38
of the load transmitter
32
, which may impair its mechanical characteristics.
Moreover, it is noted that the known measurement systems require an attached membrane
30
, which is made from a material different to that of the pressure measurement section
10
.
The two pressure measurement systems generally use flexible membranes
30
made of silicone.
An attached membrane
30
is relatively complex to mount since the membranes
30
must completely seal the hole
28
of the associated wall
26
, which involves high manufacturing and assembly costs for the pressure measurement system.
SUMMARY OF THE INVENTION
The purpose of the invention is to remedy these drawbacks and to provide a pressure measurement system which is simpler than the existing systems.
For this
Chevallet Jacques
Court Thierry
Mercier Guy
Allen Andre
Hospal International Marketing Management
Lefkowitz Edward
Young & Thompson
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