Syringe pump abstract
A syringe pump is disclosed having a transducer for detecting the
force on the syringe pusher. The detected force is translated into
a pressure which is continuously displayed so that the user can
anticipate the occurrence of an occlusion. The syringe pump also
has a device for pre-selecting a range of acceptable pressures.
Syringe pump claims
What is claimed is:
1. A syringe pump for pumping fluid from a syringe, the pump comprising:
means for detecting the pressure in the syringe substantially continuously;
means for displaying the pressure in the syringe substantially
continuously such that the pressure in the syringe can be monitored
2. The syringe pump of claim 1 wherein the means for detecting
the pressure comprises:
means for measuring the force on the plunger of the syringe; and
means for converting the measured force to a pressure.
3. The syringe pump of claim 1 further comprising:
means for selecting an acceptable pressure range in the syringe;
means for indicating the selected acceptable pressure range.
4. The syringe pump of claim 3 wherein the means for displaying
comprise a digital segmented display.
5. The syringe pump of claim 4 wherein the digital segmented display
comprises means for displaying the pressure in analog form.
6. The syringe pump of claim 3 wherein the means for displaying
comprise a pointer for indicating the pressure in the syringe.
7. The syringe pump of claim 3 wherein the means for indicating
the selected acceptable pressure range comprise three regions, a
first region for low pressures, a second region for medium pressures
and a third region for high pressures.
Syringe pump description
FIELD OF THE INVENTION
The invention relates generally to the field of syringe pumps.
In particular, the invention relates to a syringe pump having a
continuous pressure monitoring and display device.
A syringe pump is a device for pumping fluid from a syringe into
a patient. The syringe is placed in the pump and connected to the
patient via an infusion line. During the course of infusing medication
into a patient, it is possible for an occlusion to arise in the
infusion line. Such a condition, if undetected may cause injury
to the patient.
An occlusion in the infusion line will cause the pressure in the
syringe to increase. This in turn will cause the force between the
pusher of the syringe pump and the syringe plunger to increase.
Syringe pumps exist in the prior art in which the force between
the pusher of the syringe pump and the syringe plunger or the pressure
in the syringe are monitored. In prior art pumps, when the force
between the pusher and the plunger or the pressure in the syringe
increased above a predetermined threshold, an alarm was generated.
This alarm was essentially a binary alarm. That is to say, it was
either "on" or "off." Therefore, the user of
the syringe pump would not know whether the pressure in the syringe
was building up to an unacceptable level. The user would only know
when the alarm limit was reached. Thus, remedial action could only
be taken once the user was aware of the occlusion. The binary nature
of the alarm therefore prevented preemptive action from being taken
to remove the occlusion prior to the alarm limit being reached.
SUMMARY OF THE INVENTION
The invention is a syringe pump in which the pushing force on the
syringe plunger is continuously measured and displayed so that the
user can monitor the development of occlusions in the infusion line.
The invention is made up of a syringe pump having a housing on which
a syringe is mounted. The syringe plunger is pushed by means of
a motor driven pusher. A transducer is provided to measure the force
on the pusher. The force on the pusher is translated into a pressure
and the pressure is displayed on a pressure display.
The syringe pump may also include means for selecting ranges of
occlusion pressures so that the user may monitor the pressure inside
the syringe and insure that it remains within the pre-selected range.
The invention also displays the actual pressure in the syringe.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a syringe pump embodying the invention;
FIG. 2 is a perspective view of the drive mechanism of the syringe
FIG. 3 is a cross sectional view of the pusher mechanism of the
FIG. 4 is a perspective view of the pusher disc and force transducer;
FIG. 5 is a block diagram of the main components of the invention;
FIG. 6a-f are schematic diagrams of the electronics of the invention;
FIG. 7 is a diagram showing the segments of the pressure display.
DETAILED DESCRIPTION OF THE DRAWINGS
A syringe pump 8 embodying the invention is shown in FIG. 1. Housing
10 supports syringe 12 pusher 14 and syringe clamp 16. Syringe
clamp 16 holds syringe 12 in place on housing 10. Plunger 18 of
syringe 12 is pushed by pusher 14 which is driven by an electric
motor via a lead screw (see FIG. 2).
Pusher 14 is provided with antisiphon catch 20 which engages flange
18a of plunger 18 thus preventing plunger 18 from moving independently
of pusher 14. Pusher 14 is also provided with pressure plate 22
for pushing directly against flange 18a thereby pumping fluid from
FIG. 2 shows the chassis and mechanical components of pump 8. Chassis
226 carries motor 230 and lead screw 222. Motor 230 drives lead
screw 222 via gear assembly 232. Pusher 14 is driven by the interaction
of pusher block 228 with lead screw 222. Pusher block contains half
nuts 322 324 which interact with lead screw 222 (see FIG. 3).
FIG. 5 is a block diagram showing the main electronic components
of the invention. Transducers are provided to detect various parameters
of the syringe pump which are displayed on panel 24. The transducers
are: force transducer 36 antisiphon catch detector 38 disengage
detector 40 and syringe clamp detector 42. The outputs of these
transducers 60 62 64 and 66 respectively are fed into central
processing unit 44 via various signal processing modules. Schematic
diagrams of the various electronics modules are shown in FIGS. 8a.
The values and types of the components are indicated on the schematic
Central processing unit 44 comprises microprocessor 46 (FIG. 6a)
with random access memory 53 (FIG. 8a), watchdog 48 (FIG. 6b), EPROM
50 (FIG. 6a) and EEPROM 52 (FIG. 6c). Watchdog 48 monitors microprocessor
46 to ensure its proper operation. EEPROM 52 contains data concerning
the parameters of the syringes used in the pump. EPROM 50 contains
a software program which controls the operation of the syringe pump.
The output of force transducer 36 is conditioned by signal conditioning
circuit 54 (FIG. 6d), which converts the output of force transducer
36 into a form suitable for input into analog to digital converter
56 (FIG. 6e). Analog to digital converter 56 digitizes the analog
output and produces serial output 58 which is in turn fed into input
port 60 of microprocessor 46.
FIGS. 4 shows force transducer 36 in greater detail. Force transducer
36 is made up of four strain gauges in a wheatstone bridge configuration.
The bridge has an impedance of 350 ohms or 1 Kohm with a tolerance
of +/-15%. The range of force measurements is 0 to 150N. The bridge
sensitivity is 1.7 mV/V to 2.4 mv/V under a load of 150N at 20 degrees
centigrade. The bridge is powered intermittently under the control
of microprocessor 46 (line CDANA in FIGS. 6a and 6d) in order to
As seen in FIGS. 3 and 4 strain gauges 112 are glued onto beam
114. When force is applied to pressure plate 22 beam 114 flexes,
causing strain gauges 112 to distort and produce output 60.
Output 60 of force transducer 36 is fed into conditioning module
54 (FIG. 6d) and thereafter into analog to digital converter 56
which converts the conditioned output of force detector 36 into
serial output 58. Serial output 58 is then fed into input 60 of
Resident in EPROM 50 is a software program for microprocessor 46
which calculates the pressure inside syringe 12 continuously as
the force on the plunger 18 is measured by force transducer 36.
Certain parameters which are used by the program to calculate the
pressure in the syringe and stored in EPROM 52. Since syringe pump
8 is programmable to accommodate various types of syringe, a set
of parameters for each type of syringe, is stored in EPROM 52.
The parameters stored in EEPROM 52 include:
Ff=average frictional force between the syringe plunger and the
syringe barrel at null (atmospheric) pressure.
PC=the pressure in the syringe when a calibration force is applied
to the plunger. The calibration force is typically 5 kgF and leads
to a value of Pc of around 0.7 bar, a usual pressure threshold.
Fc=the force with which the plunger is loaded to obtain a pressure
of Pc in the syringe.
The program in EPROM 50 is used by microprocessor 46 to calculate
the pressure in the syringe. Microprocessor 46 then compares the
calculated pressure with a pressure value or values stored in EEPROM
52 for that syringe. If the calculated pressure exceeds the stored
pressure, an occlusion alarm is generated by microprocessor 46.
The algorithm for calculating the pressure in the syringe is: ##EQU1##
where F is the force measured by force transducer 36 and Fc, Ff
and Pc are the parameters defined above.
The main advantages of this formula over the traditional formula
described in the BACKGROUND section above are (1) it is not highly
dependent on the frictional force in the syringe which is known
to vary with pressure and (2) that the cross-sectional area of the
syringe need not be determined. Rather, the pressure in the syringe
is calculated using parameters which are easy to determine empirically.
Thus it can be seen that the error in the pressure measurement
using the present invention is substantially reduced in comparison
to that of the prior art.
Pressure display 27 is supplied with data from microprocessor 46.
Microprocessor 46 calculates the pressure in syringe 12 and outputs
the calculated pressure to display 27 via a serial data bus 29 (SI
in FIGS. 6a and 6f). Display 27 (See FIG. 7) is a liquid crystal
display made up of three segments 27a, 27b and 27c. Segments 27a,
27b and 27c correspond to low, medium and high pressure ranges which
may be pre-selected by the user using switch 35 on panel 24 (FIG.
1). Switch 35 is linked to microprocessor 46 which communicates
the selection of segments 27a, 27b or 27c via serial bus 29. Display
27 also includes pointer 28 which indicates the measured pressure
to the user.
Pointer 28 is made up of segments 27a-j each of which corresponds
to a measured pressure. LCD driver (IC6 in FIG. 8f) actuates segments
27a, 27b or 27c depending on the pressure range selected by the
user. The LCD driver also actuates the segment in 27a-j which corresponds
to the pressure data provided by microprocessor 46. Thus, the user
is provided with a digital display providing information as to (1)
whether the pressure in the syringe is approaching a pre-set occlusion
pressure and (2) what the actual pressure in the syringe is. The
number of segments results in an analog form despite the fact that
it is implemented by means of a digital segmented display. This
allows the user to take remedial action well before an occlusion
becomes a problem. It also permits more accurate use of the syringe