Syringe pump abstract
A syringe pump is disclosed in which the pressure in the syringe
is monitored. The syringe pump includes a force detector which detects
the force on the plunger of the syringe. The force on the plunger
of the syringe is converted into a force by means of an algorithm
which is independent of the cross-sectional area of the syringe.
The algorithm calculates the pressure by subtracting a predetermined
frictional force from the measurement force and multiplying the
result by a calibration pressure divided by the difference between
a calibration force and the predetermined frictional force.
Syringe pump claims
1. A method of determining the pressure in a syringe used in a
syringe pump comprising the steps of:
measuring the force on the plunger of the syringe;
subtracting a predetermined frictional force (Ff) in the syringe
from the measured force to produce a scaled force;
multiplying the scaled force by a correction factor dependent on
a predetermined calibration pressure (Pc), the predetermined frictional
force (Ff) and a predetermined calibration force (Fc).
2. The method of claim 1 wherein Ff, Fc and Pc are empirically
3. The method of claim 1 wherein the correction factor is obtained
by dividing Pc by the difference between Fc and Ff.
4. The method of claim 2 wherein Fc is determined by measuring
the force on the plunger of the syringe when a pressure of Pc exists
in the syringe.
5. The method of claim 2 wherein Pc is determined by measuring
the pressure in the syringe when a force of Fc is applied to the
plunger of the syringe.
6. The method of claim 2 wherein Ff is the frictional force between
the wall of the syringe and the stopper of the syringe when the
pressure inside the syringe is the same as ambient pressure.
7. The method of claim 1 further comprising the step of comparing
the pressure with a threshold pressure determined to determine whether
an occlusion has occurred in the syringe pump.
8. A syringe pump for pumping fluid from a syringe having a plunger,
a stopper and a wall, the syringe pump comprising:
means for detecting the force on the plunger;
means for converting the force into a calculated pressure using
means for storing predetermined pressure values;
means for comparing the calculated pressure with a predetermined
means for informing a user if the calculated pressure exceeds the
predetermined value with which the calculated pressure was compared;
wherein the algorithm calculates the calculated pressure by subtracting
a frictional force from the detected force, multiplying result by
a calibration pressure divided by the difference between a calibration
force and the frictional force.
Syringe pump description
1. Field of the Invention
The invention relates generally to the detection of occlusions
in the infusion line of a syringe pump. In particular, the invention
relates to a programmable syringe pump which utilizes a novel algorithm
to determine whether an occlusion exists in the infusion line.
2. Background of the Invention
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 force between
the pusher of the syringe pump and the syringe plunger to increase
due to increased pressure in the syringe. In the prior art, an occlusion
in the infusion line has been detected by a pre-loaded spring which
collapses when the force between the pusher of the syringe pump
and the plunger increases beyond the predetermined force in the
spring. This in turn triggers a switch which alerts the user or
shuts off the syringe pump.
More sophisticated syringe pumps monitor the force between the
pusher and the plunger by means of a force transducer. The amount
of force increase in the syringe corresponding to an occlusion requiring
remedial action varies from one syringe to another. For this reason,
the force is translated into a pressure. This translation takes
into account the frictional force in the syringe and the cross-sectional
area of the syringe by use of the formula: ##EQU1## where P=the
liquid pressure in the syringe
F=the pushing force measured by the transducer
Ff=the frictional force in the syringe
A=the cross sectional area of the syringe
The frictional force in the syringe and the cross sectional area
of the syringe are assumed to be constant. However, in reality (1)
the frictional force in the syringe is not constant and varies with
pressure; and (2) The cross sectional area of the syringe may also
vary with pressure. The prior art force to pressure conversion is
there not very accurate.
SUMMARY OF THE INVENTION
The invention is a syringe pump having a transducer to detect the
force exerted on the plunger of the syringe. The detected force
is used to calculate the pressure in the syringe by means of an
algorithm which compensates for the frictional force in the syringe
and scales the detected force by an empirically derived scaling
factor. The scaling factor is determined by measuring the force
on the plunger when the pressure in the syringe is at a predetermined
value. Because the scaling factor is obtained from force and pressure
measurements, there is no need to consider the cross sectional area
of the syringe and the actual (and variable) frictional force in
The calculated pressure may then be compared with a predetermined
occlusion pressure. Remedial action may be taken if the calculated
pressure exceeds the predetermined occlusion pressure.
The scaling factor varies depending on the type of syringe used.
The invention thus permits a variety of types of syringes to be
used in the syringe pump by storing syringe type-dependent parameters
in a memory and calculating the pressure in the syringe. The invention
also results in far more accurate pressure readings than were available
using the prior art, because cross sectional area and the actual
frictional force are not used in the algorithm.