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Syringe Pump Patent
 

Method and apparatus for determining pressure and detecting occlusions in a syringe pump

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

We claim:

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 derived.

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 an algorithm;

means for storing predetermined pressure values;

means for comparing the calculated pressure with a predetermined pressure value;

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

BACKGROUND

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 syringe.

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.


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