Note: Descriptions are shown in the official language in which they were submitted.
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1 GRAVITY-INDEPBNDENT INFUSION SYSTEM
8 BACKGROUND OF THE INVENTION
4 1. Field of the Invention
6 This invention relates to infusion apparatus and more
6 particularly to apparatus for infusing, intg a living patient, a
7 desired amount of infusate substantially independently of
8 gravitational forces or temperature chanyes.
9 2. Prior Art Problem
Intravenous (IV) infusion of fluids is one of the most
widespread and lifesaving procedures in medicine. However, the
~2 use of present IV infusion systems is playued by one or more of
18 three limitations: first, standard IV sets that depend upon
4 gravity for powering fluid flow are critically dependent upon
elevation level of the infusate and are too tall for many
16 necessary sites such as ambulances and battlefields ; second, IV
17 sets that utilize pumps or drop-counting electronics tend to be
both expensive and operationally fragile; and third, IV sets
9 which employ a tubing clamp to control the flow from a
pressurized source of infusate are difficult to adjust and may
~1 not reliably maintain a desired rate of flow. And, superimposed
æ upon these limitations is the tendency of temperature changes to
28 upset flow settings.
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SUMMARY OF THE INVENTION
26 It is accordingly the principal object of this invention to
27 provide an infusion system, of the class described, which
28 provides reliably predictable flow of infusate independently of
~9 elevation of infusate and without damage to infusate. It is a
~0 further object of this invention to provide a flow of infus~te
~1 which is substantially independent of temperature. According to
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64421-369
~his inventlon, the de~ired objects are attained by an in~usion
syste~ utllizing a pressurized source of lnfusate of Xnown and
controllable pressure which drives a known flow through a fixed
selectable fluld resistance compr:L ing a capillary flow
restrlctor havlng a sufficient l~ngth and suitably large ratio
of length to internal diameter to provide reliably repeatable
fluid resistance with mlnimum damage ~o infusate. Flow through
the fluid resistance may be made substantially independent o~
temperature by causing the internal diameter of the ~low
restrictor to decrease as temperature increases.
The invention may be æummarized, accordln~ to a first
aspect, as an infusion system comprising a source of liquid
infusate contalned in an infusate bag and pre~suri7ed by means
external to said bag to a substantially constant known driving
pressure, of at least 200 Torr, said pressure drlving a flow
rate of said liquid infusate through a cutof~ valve in series
with a flow restrictor of seleatable, substantially constant,
known hydraulic reæistance, sald flow restrictor consisting o~
at least one small-bore flow passage having a leng~h and a
diameter, the ratio of said length to said diameter being at
least 10, and sald length belng at least 3 mlllimeters, and
said restrictor providing substantially all o~ the resl~tance
to flow of said liquid infusate when sald cutoff valve is open
and said infusate is flowlng, whereby said flow rake is ~hen
substantially constant.
Accordlng to a second aspeat, the invention provides
a flow restrlctor, ~or use in an infuslon system comprising a
source of liquid infusate pressurized to a substantially
constant known driving pressure, from 200 up to about 500 Torr,
said pressure clriving a ~low rate o~ said liguid infusate
through said restrictor, said res~rictor aonsiæting of at least
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64421 369
one small-bore flow passage having a length and a dlameter, the
ratio of said length to said diameter being at least 10, and
said length being at least 3 millimeters, sald restrictor
having hydraulic resistance adequate to provlde a pressure
differential of flowing llquid in:Eusate o~ the order of 200 to
500 Torr, and said restrictor further comprislng connection
means for connecting said restrictor into said lnPusion sy~tem,
said connection means being adequate to with~and drlving
pressure up to at least 500 Torr.
According to a third ispect, ~he inventlon provldes a
flow restrictor, for use in an infusion system comprlsing a
source of liquld infusate pressurized to a substantially
constant known drlvlng pressure driving a flow rate of liquld
infusate through said restrictor, compri6ing a small-bore ~low
passage, de~ined within a leng~h of capillary tubing and having
a length and a dlameter, the ratio of said length to sald
dlameter beln~ at least 10, and said length belng at leas~ 3
mlllimeters, said flow restrictor being further capable o~
indlcating pressuxe and flow of sald llquld in~usate and
further comprising: a tubular duct sealably surrounding sald
caplllary tubing and extending upstrea~ and downstream thereo~
holes in the wall of said duct at locations ju~t upstream and
downstream of ald capillary tubing; thin-wall elastomeric
tublng surrouncling said duct and sealed thereto at dlstanaes
upstream and downstream of sald holes to provlde sealed annuli
inflatable by pressurized lnfusate flowing outward through sald
holes; and a transparent cylindrical ~acket surrounding sad
elastomeric tubing and radially spaced therefrom at a small
distance such that said elastomerlc tublng touches sald ~acket
when said annu:li are infla~ed.
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6~21-369
Other ob~ect~ of thls lnventlon, as well as means for
attaining them, are set ~orth in the accompanyin~ Speci~lcation
and Drawings, wherein 5
BRIEF DESCRIPTION OF THE DRAWINGS
Flgure 1 is a schematlc block diagram of an infuslon
system according to this inventlon;
Figure 2 ls a longitudlnal cross-section of a simple
flow restrictor which may be used with th~s system;
Figure 3 is a longltudinal cro~s-section of a
temperature compen~atlng flow rest:rictor which controls a flow
rate which may be substantially independent o~ temperature.
Figure 4 is a longitudinal cro~s-section of a flow
restrlctor which incorporates means for indicat1ng infu~ion
pressure and flow.
DESCRIPTION OF THE I~VENTION
Reference iB made to Figure 1, which is a schematic
block dlagram of an infusion 6ystem accordlng to this
invention. The system comprlses a source of pressurized gas
which may typically be a s~andard small carbon dioxide
car~ridge 10 which can be opened by actuator 11; thi~
combination is frequently provided to
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1 inflate life vests and the like. Carbon dioxide gas at a pressure
2 of 800-1100 psi flows through tubing 12 to adjustable pressure
regulator 13 where the pressure is reduced to a driving pressure
4 in the range of 200-500 Torr (4-10 psi). Pressure gauge 15
verifies the pressure of this gas which is then carried by tubing
6 14 to an inflatable gas bladcler 16 which presses upon a septum
7 18 which in turn presses upon an infusate bag l9 which contains
8 the liquid to be infused - physiologic saline, blood plasma,
9 citrated whole blood, or the like. Gas bladder 16, septum 18 and
infusate bag 19 are snugly housed in rigid case 20, so that the
11 liquid pressure in infusate bag 19 equals the driving pressure in
12 tubing 14. The infusate liquid flows through flexible tubing 21
18 through a tubing clamp 22 which can be manipulated to allow or
~4 cut off flow, and then flows through a flow restrictor 23 which
1~ provides a high and constant resistance to flow. The liauid then
16 flows through flexible tube 24 and hypodermic needle 25 to the
~q patient, indicated by 26. To monitor the progress of the
18 infusion, and the amount of infusate remaining, septum 18 is
9 provided with pointer 27 and s~cale 28.
Since the gas driving pressure is considerably greater than
21 changes in hydrostatic pressure of the order of 0.5 psi, due to
22 changes of the order of one foot in the relative altitudes of the
23 infusate bag and the patient, flow rates in this infusion system
24 are substantially independent of gravity forces.
If flow restrictor 23 exhibits a constant known resistance to
26 flow of infusate, the rate of flow may be ascertained by noting
27 the setting of pressure regulator 13 or the indication of
~8 pressure gauge 15, in the context of that resistance. Therefore,
29 the regulator and pressure gauge may conveniently be calibrated
ao in terms of flow, rather than merely in terms of pressure.
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1 ¦ Reference is now made to Figure 2, which is a longitudinal
¦~cross-section of a simple flow restrictor, acc~dLng ~o this
B ¦ invention, which can e~hibit the re~uis~te flow characteristics
4 ¦ Flow restrictor 23 comprises a small-bore flow passage 30 defined
¦ by a length 31 of hypodermic needle tubing snugly held in a
¦ longer length 32 of elastomeric tubing. Flexible tubes 21 and 24
¦ are slipped over the ends of tubing 32. ~hese components are
8 ¦ joined together by tight wralppings 34, and the joint between
9 ¦ tubing 32 and flexible tubes 21 and 24 may be reinforced by some
10 ¦ applied adhesive 33. Typically, flow passage 30 may be one to
11 ¦ several centimeters in length, with a bore of the order of a few
12 ¦ tenths of a millimeter. It is to be understood that, in the
~ Figures, it is necessary to exaggerate dimensions, such as tubing
14 bore and thickness, in the interest of ease of comprehension.
15 ¦ In order that flow restrictor 23 may exhibit a constant
¦ and linear known resistance, it is required that the flow be
~7 ¦ laminar and that the length-diameter ratio of flow passage 30 be
18 ¦ sufficient. And, to protect the formed elements of blood used as
19 ¦ an infusate, it is necessary that fluid shear forces not be
~0 ¦ excessive. These requirements can be met if the length-diameter
~1 ¦ ratio of flow passage 30 is at least 10, if the flow Reynolds
æ ¦ number is well below 2000 and, if the infusate is blood, if shear
23 ¦ rate is less than 10 sec . The Reynolds Number criterion is
24 ¦ easily met for all structures of interest. The shear rate
2~ criterion is met if the length, in centimeters, is at least
0.0012 times the product of the driving pressure, in Torr, times
~7 the cube root of the flow rate, in milliliters per hour.
Experience shows that these criteria ma~ be met, for infusion
29 flow rates of interest, by flow passage diameters of a few tenths
~0 of a millimeter and lengths of three millimeters or more for
~1 aqueous infusates or a few centimeters for blood, with driving
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pressure of 500 Torr or less, the recommended pressure limit for
2 many infusate bags. In practice, flow passage length may be
8 varied to compensate for variation of internal diameter among
4 different lots of hypodermic needle tubing, a length reduction of
4N percent compensating for a diameter reduction o~ N percent.
6 Longer flow passage lengths facilitate such trimming.
q Typical infusates, being largely water, exhibit a fluid
8 viscosity which decreases as temperature increases, roughly at
9 the rate of 2% viscosity change per degree Celsius. While this
viscosity change, and the consequent change in flow rate, can be
compensated by suitable scaling or interpretation of the pressure
2 settings or measurements, it would be convenient to use a flow
~8 restrictor having a fluid resistance which varies with
14 temperature in a compensating manner.
1~ Reference is now made to Figure 3, which is a longitudinal
16 cross-section of a flow restrictor which can exhibit flow
17 resistance which varies with temperature in such a way that flow
is substantially independent of changes in temperature. Flow
9 passage 30`is defined by the inner diameter of a thick-walled
elastomeric tube ~0, the outer diameter of which fits snugly in a
al surrounding metallic tube 41. Flexible tubes 21 and 24 are
22 slipped over the ends of tube 40 and, again, the components are
23 joined together by tight wrappings 34, and the joint between
24 flexible tubes 21 and 24 and metallic tube 41 may be reinforced
2B with some applied adhesive 33.
26 Typical metals have thermal coefficients of linear expansion
27 of the order of 10 per degree C, while those of typical
~8 elastomers are about ten times larger. Therefore, as temperature
2g increases, elastomeric tube 40 tends to swell more than can be
accommodated by the expansion of surrounding metal tube 41. Since
~1 elastomers are not very compressible, the swelling is
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1 accommodated by a decrease in the inner diameter of elastomeric
2 tube 40, the decrease being greater for larger ratios of outer to
B inner diameter of tube 40. For most combinations of metal and
4 elastomer, a ratio of 4 to 10 can yield a fluid resistance which
compensates reasonably well for viscosity change over the
8 temperature range of lnterest; a ratio of 6, in a typical case7
7 could compensate within 1.5% Erom 18 to 42 degrees C.
8 Reference is now made tlo Figure 4, which is a longitudinal
9 cross-section of a flow restrictor which incorporates means for
indicating infusion pressure and flow. The fluid resistance
11 element is similar to that shown in Figure 2, comprising a
12 length 31 of capillary tubing defining the flow passage 30 and
18 snugly held by a length 32 of elastomeric tubing. This assembly
is snugly mounted within a long metallic tube or duct 50 which is
6 provided with holes 51 just outside the ends of tubing 32.
16 Surrounding duct 50 is a long thin-wall elastomeric tube 52 which
is joined to duct 50 with tight wrappings 34 to form a pair of
~8 inflatable annuli 55. This assembly is then placed in a
9 transparent cylindrical spaced-apart jacket 53 which is provided
with small vent holes 54 . Flexible tubes 21 and 2~ may be
2~ slipped over the ends of duct 50 and fastened thereto with tight
æ wrappings, as in the case of the restrictor assemblies shown in
23 Figures 2 and 3. And the joint between thin-wall tube 52 and
24 duct 50 may be reinforced by some applied adhesive 33.
2~ The inner surface of transparent jacket 53 is rough or
26 frosted, so that it appears white unless touched by the outer
27 surface of an inflatable annulus 55, in which case it takes on
28 the color of that surface. To exploit this fact, the upstream
~9 annulus 55, nearest flexible tube 21, is colored green on the
outside, and the downstream annulus, nearest flexible tube 24, is
~1 colored red. Therefore, if flow pressure is applied, it will
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1 inflate the upstream annulus 55, and the upstream part of jacket
2 53 will exhibit a green band, signifying flow. But if flow is
8 blocked downstream, as b~ kinking of flexible tube 24 or
4 occlusion of the needle, pressure will rise in the downstream
annulus 55 and the downstream part of jacket 53 will exhibit a
6 red band, signifying blockage.
7 Given the foregoing teaching, those skilled in the art to
8 which this invention pertains may readily devise further or
extended embodiments. For one example, the restrictor assembly
0 shown in Figure 2, comprising a metallic tube encased in an
11 elastomeric tube, may be replaced by a thick-walled small-bore
12 plastic cylinder. For another example, the gas-powered pressure
18 source shown and described with respect to Figure 1 may be
4 replaced by a spring-driven mechanism for squeezing infusate bag
1~ 19. Also, it is not necessary to provide a pressure gauge 15 if
lB the setting of regulator 13 is readable and reliable. In case of
a regulator malfunction which might apply excessive pressure, I
8 prefer to add a pressure-relief safety valve to tubing 14,
9 connected to a shrill whistle to warn the operator of such an
2~ event. And, while I prefer to make the inner surface of the
21 transparent jacket rough, and to color the outside of the
22 inflatable annuli, in the embodiment of Fig. 4, neither of these
~3 provisions is nècessary. various other features and advantages
~4 not specifically enumerated will occur to those versed in the
art! as likewise many variations of ~he embodiments which have
26 been illustrated, all of which may be achieved without departing
27 from the spirit and scope of the invention as defined by the
following claims:
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