Note: Descriptions are shown in the official language in which they were submitted.
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Pump System With Error Detection for Clinical Nutrition
This invention relates generally to a pump system for administering
liquids to a patient; for example medicinal or nutritional solutions. The
liquids
may be administered enterally or parenterally. The invention also relates to a
pump for use in the system and to methods of administering liquids to a
patient.
Systems for administering liquids to a patient are widely used in clinical
settings. All of these systems comprise a container for the liquid and a flow
set
for delivering the liquid to the patient. In general, the liquid is either
allowed to
drain through the flow set to the patient under the action of gravity or is
pumped
through the flow set. Systems using pressure sleeves on the container are also
used. Systems using a pump are referred to in this specification as "pump
systems".
The rate of flow of the liquid through the system is usually set to a desired
rate depending on the needs of the patient. In pump systems this may be
achieved
by controlling the pump rate. However, particularly when intended for
intravenous administration of liquids, it is important to ensure that there
will be
no back flow of liquid in the tubing, that is away from the patient. To
prevent
this, a one-way valve is typically installed in the flow set. Further, because
the
container is typically mounted on a stand it is necessary to ensure that free-
flow
of liquid due to the liquid head will not occur when the pump is at rest. For
this
purpose, the valve, in addition to being a one-way valve, also needs to
prevent
free flow. Therefore the valve has a certain threshold pressure which is
required
to open it to allow flow of liquid. The threshold pressure is also known as
the
"cracking point". Pump systems containing such a valve are described in PCT
Application WO 95/16480 and US. Patent 5,472,420. However, incorrect valves
are occasionally connected in the flow sets with serious consequences.
It is also important to ensure that the flow set, which is typically provided
as an integral disposable set, is correctly connected to the pump to avoid
pumping of liquid in a reverse direction, away from the patient. This is often
left
to the supervising staff and errors do occur. Also, flow sets occasionally
fail and
this is often not noticed until too late.
It is therefore an object of the invention to provide a pump system which
automatically detects errors which may impair the proper functioning of the
system.
Accordingly, in one aspect, this invention provides a pump for delivering
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a liquid from a container to a patient through a flow set, the pump including:
a sensing means for sensing a parameter indicative of the pressure in the
flow set; and
a controller having a computing and memory means for determining
deviation of the parameter from a standard, the deviation being indicative of
an
error in the flow set.
It has been surprisingly discovered that, by measuring the pressure in the
flow set and comparing it to a standard, errors in the system may be detected.
This enables supervising staff to be alerted. These errors may, for example,
be
incorrect assembly of the system or one of its components, inclusion in the
system of an incorrect valve, impairment in the integrity of the flow set, and
the
existence of air bubbles in the system.
In another aspect. this invention provides a pump system for administering
a liquid from a container to a patient, the system comprising:
flow set comprising a tubing set connectable at one end to the container
for delivery of liquid to the patient, and a one-way valve system coupled to
the
tubing set which permits liquid flow to the patient when a pressure
differential
over the valve exceeds a threshold pressure, and which prevents back flow;
a pump as defined above coupled to the tubing set.
Preferably, the controller causes the pump, during operation of the pump,
to enter into a test phase at selected intervals, the test phase comprising a
first test
sequence in which the pump propels a first amount of liquid in a first
direction
through the flow set, and a second test sequence in which the pump propels a
second amount of liquid through the flow set in a second direction, opposite
the
first. The sensing means senses the parameter during the first test sequence
and
the second test sequence.
The valve system preferably comprises a valve having a liquid flow path
sealed by a resilient membrane, the membrane being deformable in a desired
flow direction at or above a threshold pressure for opening perforations in
the
membrane to permit flow. Further, the valve preferably has a support
preventing
the membrane from deforming sufficiently in an opposite flow direction for
preventing back flow.
In further aspect, this invention provides a method for administering a
liquid from a container to patient using a pump system, the method comprising:
pumping liquid through a flow set from the container to the patient
through a one-way valve system which permits flow to the patient when the
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pressure differential over the valve system exceeds a threshold pressure, and
which prevents back flow; and
intermittently entering a test phase comprising
pumping a first test amount of liquid in a first direction and then pumping
a second test amount of liquid in an opposite direction,
sampling a parameter indicative of pressure within the flow set during
pumping of the first test amount of liquid and during pumping of second test
amount of liquid, and
comparing the sampled parameters to a standard and, upon determining a
difference of selected magnitude between the sampled parameters and the
standard, indicating the existence of an error in the pump system.
The error which is diagnosed by the system, may, for example be:
the impairment of liquid flow through the flow set as a result of an
occlusion, a rupture or a hole in the tubing, or disengagement of components
of
the flow set, etc.;
incorrect engagement of the pump with the flow set, for example
engagement in a reverse direction;
the use of incompatible components in the flow set, for example the use of
an improper valve having improper flow specifications; or
changes in the flow parameters of the valve during operation, for example
the existence of gas bubbles or gas pockets in the tubing; etc.
The present invention also provides a flow set for use in the system of the
Invention.
Embodiments of the invention are now described, by way of example
only, with reference to the drawings in which:
Figure 1 is a schematic illustration of a pump system;
Figure 2A is a longitudinal cross-sectional view of a valve for use in the
system of Figure 1 in a rest state;
Figure 2B is a longitudinal cross-sectional view of the valve of Figure 2A
in an operational state;
Figure 3A is a graph of the flow of liquid versus time during a test phase
of the pump system; and
Figure 3B is a graph of the pressure build-up versus time during a test
phase of the pump system.
A pump system 10 is illustrated in Fig. 1. The pump system 10 comprises
a pump 14 with a control unit 15, and a flow set 12. The pump 14 is preferably
a
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peristaltic pump but any type of pump which is able to pump liquid at
controlled
flow rates suitable for clinical applications may be used. The control unit 1
S
typically comprises a control panel 1 Sa which has a display 15b and a key pad
15c. The key pad 1 Sc may be used for manual control of the pump, data entry,
and the like. The control unit 15 also includes a microprocessor (not shown)
for
controlling and activating the pump and for data storage. A memory (not shown)
may be associated with, or be incorporated in, the microprocessor. If desired,
the
control unit 15 may also include an audio, visual or dual alarm signalling
means.
The flow set 12 is made up of a tubing set 16 which is connected to a
liquid container 18 at one end and a connector 20 at the other end. A drip
chamber 21 and a one-way valve 22 are coupled to the tubing set 16 between its
ends. In this embodiment, the drip chamber 21 is positioned beneath the liquid
container 18, upstream from the pump 14. The one-way valve 22 is positioned
downstream from the pump 14. As is conventional, the connector 20 may be
connected to a catheter, an enteral feeding tube, etc. When not in use, the
free
end of the connector 20 is covered by a cover 26.
The pump 14 is coupled to the tubing set 16 and is able to pump liquid in
either direction. Therefore, from the container 18 to the connector 20 (the
forward direction), and towards the container 18 (the reverse direction).
The pump 14 also includes a sensing means for sensing a parameter
indicative of the pressure in the flow set 12. The sensing means (not shown)
is
conveniently a tube diameter gauge which measures the diameter of the tubing
set 16. Then, using the known resiliency of the tubing set 16, the pressure in
the
tubing set 16 may be determined by the microprocessor. The tube diameter may
for example, be a strain gauge, an optical sensor, and the like.
Alternatively,
other known means of determining pressure in the tubing set 16 may be used.
For example, conventional pressure gauges may be connected into the tubing set
16. The pressure parameter is preferably repeatedly sampled at short time
intervals so that a curve of pressure change with time may be developed.
The one-way valve 22, shown in cross-section in Fig. 2, has a housing 30
formed of a first housing member 32 and a second housing member 34. The first
housing member 32 has a recess in it into which the second housing member 34
is accommodated in a sealed manner. The second housing member 34 also has a
recess in it so that a chamber 35 is defined between the first and second
housing
members 32, 34.
The first housing member 32 has an inlet tube 36 which is connected to
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the chamber 35 by means of an entry port 37. An annular rim 38 projects into
the chamber 35 from the floor of the recess of the first housing member 32,
about
entry port 37. The second housing member 34 has an outlet tube 39 which is
connected to the chamber 35 by means of an exit port 40. Both the inlet tube
36
and the outlet tube 39 are sized to be sealingly engaged by the tubing set 16.
which permits liquid to flow from the chamber.
The first housing member 32 has an annular shoulder 41 projecting from
the floor of its recess at the circumference of it recess. The annular
shoulder 41
and the annular rim 42 of the second housing member 34, when the second
housing member 34 is fitted in the recess of the first housing member 32, form
an
annular clamp.
A resilient membrane 44 is clamped between the annular shoulder 41 and
the annular rim 42 in the annular clamp. In the rest state of the valve 22
shown
in Fig 2A, the membrane rests on the annular rim 38 projecting from the first
housing member 32. The membrane 44 is made of a resilient flexible material,
typically sterilisable material such as silicon, rubber or any other suitable
material. The membrane 44 has a plurality of slits 46 (two shown in this cross-
sectional view) which, in the rest state shown in Fig, 2A, are closed and do
not
permit flow of liquid through it.
When liquid is propelled through the inlet tube 36, the membrane 44 is
stretched and deflected as shown in Fig. 2B. Once a selected threshold
pressure
differential is reached and the membrane 44 is sufficiently stretched, the
slits 46
widen and open to allow flow of liquid from the inlet tube 36, through the
chamber 35, to the outlet tube 39. The flow is represented by the arrows in
Fig.
2B. Typically, the membrane 44 is designed so that slits 46 will open only
when
the pressure differential over the membrane exceeds about 20 kPa. This
prevents
undesired free flow of the liquid from the container 18, which in a clinical
setting
is typically placed on a stand of a height of about 2 metres.
For flow in the reserve direction, the membrane 44 cannot deflect
suff ciently since it is held against the floor of the recess of the first
housing
member 32. Therefore the valve 22 also prevents back flaw of liquid.
In use, the control unit 1 S causes the pump 14 to operate in a duty cycle
which has an administration phase and a test phase. The test phase is entered
at
selected, intermittent intervals. Typically, test phase is entered immediately
after
the pump system 10 has been set up, prior to initiation of the first
administration
phase. Thereafter, the test phase is entered at selected intervals, which may
be
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randomly selected, between consecutive administration phases. Thus, the pump
14 may operate in a duty cycle of a first test phase, followed by an
administration
phase and then repeatedly through test phases and administration phases. In
general, the test phases are of much shorter duration than the administration
phases.
During the test phase, the integrity of the flow set 12 is checked. Also,
correct assembly of the system 10 and the presence of the correct components
of
the flow set 12, and particularly the valve 22, are checked. Further, the
existence
of air pockets or bubbles in the tubing set 16 may be detected.
A test phase sequence is shown graphically in Fig. 3. As illustrated in Fig.
3A, during a first step 50 of the test phase, the pump 14 propels a small
amount
of liquid, for example about 0.5 ml, in a reverse direction, and then, in a
second
step 52, propels another small amount of liquid, for example about 0.4 ml, in
a
forward direction. The pressure change, relative to atmospheric, in the
downstream portion of the flow set 12 (that is between the pump 14 and the
valve
22) is shown in Fig, 3B.
If the pump system 10 has no faults, the pressure change is given by the
solid line in Fig. 3B. In the first step 50, the pressure drops below
atmospheric.
In the second step 52, the pressure increases above atmospheric. The pressure
is
expected, in the second step 52, to increase to the cracking (threshold)
pressure
of the valve 22. As mentioned above, this is typically about 20 kPa. This
pressure is maintained while the pump 14 is operating. When the pump 14 is
then
stopped, the pressure slowly declines to the zero level. This pressure curve,
the
no-fault curve, forms a standard which is stored in the microprocessor.
There may be several operative faults in the pump system 10. One
possible fault is reverse assembly of the valve 22 in the flow set 12. Another
possible fault is the reverse engagement of the pump 14 with the flow set 12
(in
which case the pump 14 in a "forward" operational state in fact propels liquid
in
a reverse direction). Both of these faults will result in a pressure curve
which is
essentially a mirror image of the standard pressure curve. This faulty
pressure
curve is shown in Fig. 3B by the dashed line marked I. Another possible fault
is
leakage in the flow set 12 or the existence of air pockets or air bubbles
(e.g. as
foam) in the flow set 12. In this case, the pressure changes will be more
moderate pressure than that of the standard curve; this is shown in Fig. 3B by
the
dashed-dotted lines marked II. A further possible fault state occurs when a
valve
22 with an incorrect cracking pressure is used. In this case, the pressure
curve
T
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during the first step 50 will be essentially the same as the standard curve.
However, during the second step 52, the pressure will reach higher or lower
maximum value than the standard curve; this is represented in Fig. 3B by
dotted
lines III' and III", respectively. Yet another possible fault is where the
valve 22
S is omitted entirely. In this case there will be substantially no pressure
change and
the pressure curve will essentially follow the abscissa (marked IV in Fig.
3B).
It will be appreciated that, in the test phase, the pump 14 need not first
propel liquid in the reverse direction and then in the forward direction. In
particular, this sequence may be reversed such that, during the test phase,
the
pump 14 first propels liquid in the forward direction and then in the reverse
direction. This is merely a matter of appropriately setting the control unit
15.
However, in this case, the standard pressure curve should be appropriate for
an
inverted test sequence.
During the test phase, the pressure curve which is determined is compared
to the standard pressure curve stored in the memory in the control unit 15. In
the
event that the determined pressure curve deviates from the standard curve, the
microprocessor indicates the presence of an error. It will be appreciated that
the
microprocessor may permit small deviations from the standard curve prior to
indicating the presence of an error.
Upon the microprocessor indicating the presence of an error, the control
unit 15 may, depending upon the error detected, initiate an alarm signal and
prevent the pump 14 from entering into an administration phase. This may not
be necessary if the error is the existence of air bubbles or air pockets. In
this
case, the control unit may halt the pump 14 for a short period of time,
typically
about 30 seconds, to allow possible air pockets to rise up in the tubing set
16
towards the container 18. Then the control unit 15 causes the pump 15 to enter
into another test phase. If this fault is not detected again, the pump 14 will
then
be induced to enter into an administration state.
It will be appreciated that numerous modifications may be made to the
preferred embodiments without departing from the scope of the invention as set
out in the claims. For example, it is not essential for a drip chamber 21 to
be
connected in the flow set 12. Similarly, it is not essential that the flow set
12 use
a one way valve 22 as described above. Other valve types and arrangements may
be used; for example a combination of a one way valve and a valve which opens
upon a threshold pressure being reached.
