Method and apparatus for controlling infusion volume

A flow sensor detects drops of infusion fluid that enter the drip chamber of an infusion system and generates a drop-detection signal indicating each drop detected. A setting controller receives the drop-detection signals from the flow sensor and counts the drops detected and then determines the fluid-flow rate of infusion fluid into the drip chamber. The setting controller then sets the infusion pump so that the fluid-flow rate of the infusion pump substantially matches the fluid-flow rate of the drip chamber. A motion sensor detects drip-chamber movement and generates a motion-detection signal indicating the motion state of the drip chamber. A maintaining controller is coupled to receive the motion-detection signal from the motion sensor. The maintaining controller also stores a signal that indicates a stationary drip chamber. The maintaining controller compares the motion-detection signal to the stored signal to determine if drip chamber movement has occurred, and if drip-chamber movement has occurred it maintains the fluid-flow rate of the infusion pump as set immediately prior to the detection of drip-chamber movement.

BACKGROUND OF THE INVENTION 
The invention relates generally to infusion systems, and more particularly, 
to an apparatus and method for controlling the volume flow rate of 
infusion fluid through an infusion pump. 
A typical infusion system for use in the medical field includes an infusion 
fluid container that supplies infusion fluid to a drip chamber. The drip 
chamber is typically made of a transparent resin. The drip chamber, in 
turn, supplies the fluid to an infusion tube which passes through an 
infusion pump. As disclosed in Japanese patent publication No. 
Hei-4-51963, an apparatus and method for controlling infusion volume 
typically involves setting the rate at which fluid flows through the 
infusion pump to match the rate at which fluid flows into the drip 
chamber. A photo coupler flow sensor is used to detect drops of fluid 
entering the drip chamber. The number of drops of fluid dripping into the 
drip chamber is counted over a specified period of time by a computer, 
typically contained in the infusion pump. The volume flow rate of fluid 
into the drip chamber is then calculated by the computer. Using this 
calculated volume flow rate, the infusion-pump motor is adjusted so that 
the desired volume flow rate of fluid through the infusion pump matches 
the measured flow rate into the drip chamber. 
Under a steady-state environment, i.e., one in which the drip chamber 
remains motionless, the method and apparatus of controlling volume flow 
rate as just described provides smooth and continuous infusion. However, 
in some environments in which infusion systems operate, it is generally 
impracticable to expect a drip-chamber to remain motionless. A sway or 
vibration of the drip chamber may occur at a bedside or in a clinical 
environment due to the movement of a patient. Uncontrollable environmental 
conditions, such as wind, may also cause the drip chamber to move. 
When the drip chamber experiences such movement, existing sensors may not 
successfully accommodate for the effect such movement has on the operation 
of the drip chamber. This is because the accuracy of the flow rate is 
largely dependent on the accuracy of the drip-chamber drop count. If the 
drip chamber is caused to sway or vibrate, the flow sensor may not detect 
all the drops and accordingly the count is inaccurate. If movement occurs 
and the drip-chamber drop count is inaccurate, the infusion volume flow 
rate of the infusion pump does not accurately reflect the flow rate in the 
drip chamber. When such movement occurs, existing apparatus have the 
disadvantageous feature of stopping the infusion pump and ceasing 
infusion. 
Hence, those skilled in the art have recognized a need for an apparatus and 
a method to ensure continuous and accurate infusion even where 
uncontrollable environmental conditions cause the infusion system, 
specifically the drip chamber, to move. The invention fulfills these needs 
and other. 
SUMMARY OF THE INVENTION 
Briefly, and in general terms, the invention is directed to a method and 
apparatus for controlling infusion volume through an infusion pump. In one 
aspect, the invention comprises a method of controlling the fluid-flow 
rate of infusion fluid through an infusion pump that is receiving infusion 
fluid from a drip chamber. The method includes the step of measuring the 
flow rate of infusion fluid into the drip chamber and the step of 
monitoring the drip chamber for movement. If drip-chamber movement is not 
detected, the method further includes the step of setting the fluid-flow 
rate of the infusion pump to substantially match the fluid-flow rate of 
the drip chamber. If, however, drip-chamber movement is detected, the 
method then further includes the step of omitting the setting step and 
instead maintaining the fluid-flow rate of the infusion pump as set 
immediately prior to the detection of drip-chamber movement. 
In another aspect, the invention comprises an apparatus for controlling the 
flow rate of infusion fluid through an infusion pump. The infusion pump 
receives infusion fluid from a drip chamber that has a fluid-flow rate; 
the fluid-flow rate of the infusion pump is set to substantially match the 
fluid-flow rate of the drip chamber. The apparatus includes a motion 
sensor that detects drip-chamber movement and generates a motion-detection 
signal that indicates the motion state of the drip-chamber. Also included 
is a controller that is coupled to receive the motion-detection signal 
from the motion sensor. If drip-chamber movement is detected, the 
controller maintains the fluid-flow rate of the infusion pump as set 
immediately prior to the detection of drip-chamber movement. 
In yet another aspect, the invention comprises an apparatus for controlling 
the flow rate of infusion fluid through an infusion pump that receives 
infusion fluid from a drip chamber. The apparatus includes a flow sensor 
that detects drops of infusion fluid that enter the drip chamber and 
generates a drop-detection signal that indicates each drop detected. Also 
included is a setting controller coupled to receive the drop-detection 
signals from the flow sensor. The setting controller counts the detected 
drops and determines the fluid-flow rate of infusion fluid into the drip 
chamber. The setting controller then sets the fluid-flow rate of the 
infusion pump to substantially match the fluid-flow rate of the drip 
chamber. The apparatus further includes a motion sensor that detects 
drip-chamber movement and generates a motion-detection signal that 
indicates the motion state of the drip-chamber. A maintaining controller 
is coupled to receive the motion-detection signal from the motion sensor. 
The maintaining controller stores a signal that indicates a stationary 
drip chamber. The maintaining controller compares the motion-detection 
signal to the stored signal to determine if drip chamber movement has 
occurred. If drip-chamber movement has occurred the maintaining controller 
then maintains the fluid-flow rate of the infusion pump as set immediately 
prior to the detection of drip-chamber movement. 
These and other aspects and advantages of the present invention will become 
apparent from the following more detailed description, when taken in 
conjunction with the accompanying drawings which illustrate, by way of 
example, the preferred embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Turning now to the drawings with more particularity, in FIG. 1 there is 
shown an infusion system that is equipped with an infusion liquid 
container 2. A drip chamber 3 communicates with, and receives fluid from 
the infusion liquid container 2. An infusion tube 4, in turn, communicates 
with the drip chamber 3 and receives fluid therefrom. The infusion tube 4 
passes through an infusion pump 5 which controls the rate at which fluid 
flows through the infusion tube 4. The infusion liquid container 2 and 
infusion pump 5 are supported by an infusion stand 1. A drip monitoring 
device 6 is attached to the drip chamber 3. A controller (not shown) 
controls the operation of the infusion pump and is preferably incorporated 
into the infusion pump 5. The drip monitoring device 6 and controller 
interface through a cable 7. 
The infusion pump 5 as shown in FIG. 2 is provided with a plurality of 
horizontally movable finger members 8 disposed in a multi-layered fashion. 
A plurality of eccentric cams 10, one for each finger member 8, are 
pivotally stacked on a rotatable drive shaft 9. The eccentric cams 10 
rotate together with the drive shaft 9. The finger members 8 are 
constructed so as to carry out peristaltic movements in line with the 
rotation of the drive shaft 9 and eccentric cams 10. This motion causes 
the finger members 8 to press against the infusion tube 4. Pressure plate 
11 limits the movement of the infusion tube 4 and, together with the 
finger members 8, acts to compress the infusion tube 4 thereby 
transferring the infusion fluid in the tube 4 downward. The upper end of 
the drive shaft 9 is connected to a motor 13 through a transmission 
mechanism 12. The transmission mechanism includes a worm gear 12a; engaged 
with the worm gear 12a is a gear 12b. An encoder 14 is attached to the 
drive shaft 9 while a rotation position detector 15 is aligned with the 
encoder 14. 
The drip monitoring device 6 as shown in FIG. 3 has a flow sensor 16 which 
detects fluid drops 25 entering the drip chamber 3. The flow sensor 16 is 
a photo coupler consisting of a light-emitting element 16a and 
light-receiving element 16b which are disposed in a casing 17. The casing 
17 is attached to the drip chamber 3 so that the drip chamber 3 is 
positioned between the light-emitting element 16a and light-receiving 
element 16b. When a drop of fluid 25 passes through the drip chamber 3 it 
passes between the light-emitting element 16a and light-receiving element 
16b. When this occurs the intensity of light received by the 
light-receiving element 16b is reduced and a drop is thereby detected. The 
flow sensor 16 generates an electrical signal based on the light intensity 
received by the light-receiving element 16b. Typically, a reduction in 
light intensity results in a reduction in the voltage of the electrical 
signal. The flow sensor 16 is electrically coupled to the controller 22 
which is typically a microcomputer and is usually incorporated into the 
infusion pump. 
A motion sensor 18 for detecting swaying or vibration of the drip chamber 3 
is disposed in the drip monitoring device 6. Because the drip monitor 
device 6 is attached to the drip chamber 3, any movement of the drip 
chamber 3 results in a corresponding movement of the motion sensor 18. The 
motion sensor 18 includes a vibration detector 20. In an alternate 
configuration the motion sensor 18 also includes a vibration generator 21 
and a metallic shim 19 positioned between the vibration detector 20 and 
the vibration generator 21. The vibration detector 20 and a vibration 
generator 21 are preferably piezoelectric ceramic elements. The motion 
sensor 18 is electrically coupled to the controller 22. 
In operation, the voltage level of the electrical signal generated by the 
flow sensor 16 is monitored by a setting controller 23, contained in the 
controller 22. Based on the fluctuation in the voltage level of the 
electrical signal caused by fluid drops passing between the light emitting 
element 16a and the light receiving element 16b, the setting controller 23 
counts the number of drops entering the drip chamber for a set period of 
time. The setting controller 23 then calculates the volume flow rate of 
fluid into the drip chamber 3 and, based on this calculation, sets the 
number of revolutions of the motor 13 which, in turn, sets the infusion 
volume flow rate of the infusion pump 5. 
Movement of the drip chamber causes a corresponding movement or vibration 
of the vibration detector 20. The frequency of vibration of the vibration 
detector 20 due to drip-chamber movement is generally low, as for example 
10 Hz. The movement of the vibration detector 20, in turn, produces an 
electrical signal whose voltage level fluctuates with the frequency of 
vibration of the vibration detector 20. In the alternate configuration of 
the motion sensor 18, the vibration generator 21 is used to verify the 
operation of the vibration detector 20. The vibration generator 21 is 
forced to vibrate at a known frequency, typically 60 Hz, through the 
application of an AC voltage signal. The vibration of the vibration 
generator 21 is transmitted to the vibration detector 20 through the shim 
19. If operating properly, the vibration detector 20 vibrates at the same 
frequency of the vibration generator 21 and, again, produces a 
corresponding voltage. 
A maintaining controller 24, also contained in the controller 22, monitors 
the voltage level received from the motion sensor 18 and compares it to 
the voltage level associated with a stationary drip chamber. The 
maintaining controller is programmed to distinguish voltages indicating 
drip-chamber movement, i.e., voltages associated with frequencies around 
10 Hz, from voltages indicating a stationary drip chamber, i.e., the 
voltage associated with a frequency of around 0 Hz for a motion sensor 18 
consisting only of a vibration detector 20 and the voltage associated with 
a frequency of around 60 Hz for a motion sensor 18 which includes the 
vibration generator 21 and the shim 19. 
If a voltage indicating drip-chamber movement is received by the 
maintaining controller 24, the maintaining controller 24 overrides the 
setting controller 23 and the number of revolutions of the motor 13 
remains as set immediately prior to the detection of drip-chamber 
movement. Otherwise the setting controller 23 sets the number of 
revolutions of the motor 13 so that the fluid-flow rate through the 
infusion pump 5 matches the fluid-flow rate through the drip chamber 3. 
While the setting controller 23 and the maintaining controller 24 are 
described as separate components of the controller 22, their functions may 
actually be performed by a single component, as for example a programmed 
microprocessor. 
In summary, the controller performs a series of steps as shown in FIG. 4 
that determines the volume flow rate of the infusion pump. In step S1, 
operation of the infusion pump is commenced. In step S2, the controller 
determines whether the signal it receives from the motion sensor indicates 
that drip-chamber movement has occurred. If drip-chamber movement has not 
occurred, in step S3 the controller determines, based on the signals 
received from the flow sensor, whether drops (D) are being detected by the 
flow sensor. 
If drops are being detected, in step S4 the controller counts the number of 
drops (Nd) for a fixed period of time, e.g., 100 msec. In step S5, the 
controller calculates the volume (Q) of the infusion fluid that has 
entered the drip chamber in the fixed period of time based on the number 
of drops counted in step S4 and a known volume of one drop (Qo). The total 
volume is calculated using the expression Q=Nd.times.Qo. In step S6, the 
controller, having calculated the infusion volume in the drip chamber, 
calculates the number of revolutions (RN) of the motor necessary to set 
the infusion volume of the infusion pump to match that of the drip 
chamber. In step S7, the controller sends a control signal to the 
infusion-pump motor to set the number of revolutions of the motor to match 
that calculated in step S6. Thereafter, the process is returned to step 
S2. 
In step S3, when no drops are detected for a fixed period of time, an 
infusion malfunction is issued. In step S8, the controller sets the number 
of revolutions of the motor in step S7 to zero, thereby causing the motor 
to a stop. 
In step S2, when drip-chamber movement is detected and infusion has just 
commenced, the controller sets the number of revolutions of the motor in 
step S7 to a predetermined value as set in step S9. The predetermined 
value is typically programmed into the controller. When drip-chamber 
movement is sensed in step S2 and infusion has previously commenced the 
operation of the controller advances to step S7 and thereby bypasses a 
change in the motor revolutions (RN). Accordingly, the motor continues to 
run at the number of revolutions that had been obtained immediately 
beforehand. In this situation, where the flow sensor cannot detect all 
drops entering the drip chamber and the controller cannot accurately count 
the number of drops, the infusion volume of the infusion pump is 
maintained at the infusion rate obtained immediately prior to drip-chamber 
movement. As a result, it is possible to secure accurate and continuous 
infusion regardless of the environmental conditions under which the 
infusion system is operating. 
While several particular forms of the invention have been illustrated and 
described, it will be apparent that various modifications can be made 
without departing from the spirit and scope of the invention. Accordingly, 
it is not intended that the invention be limited, except by the appended 
claims.