Convective warming of intravenously-administered fluids

IV fluid is warmed while being intravenously-administered to a patient through an IV tube by positioning a portion of the tube at a warming location and flowing a heated gas past the warming location to heat the tube, which thereby heats the IV fluids flowing in the tube.

BACKGROUND OF THE INVENTION 
The invention is in the field of intravenous administration of fluids to a 
patient in a clinical setting, and particularly concerns the warming of 
such fluids while they are being intravenously-administered to a patient. 
Fluids which are administered intravenously to a patient consist typically 
of blood-based fluids and non-blood fluids, all referred to as "IV 
fluids." As is known, blood-based fluids are held in cool storage at 
approximately 4.degree. C. until they are used. Non-blood fluids are 
usually stored at room-temperature. 
Intraveneously-administered fluids are a major cause of conductive heat 
loss in patients and can contribute to patient hypothermia. As is known, 
hypothermia poses a significant peril in an emergency, and during or after 
surgery. When fluid must be intraveneously-administered to patients in 
such circumstances, the threat of hypothermia is compounded. 
In the art, it is known to warm fluids prior to administering them 
intravenously. Further, mechanisms are available for heating fluids during 
intravenous administration. In one such mechanism, the IV tube used to 
deliver the fluid is immersed in a liquid, such as a water bath, whose 
temperature is elevated by an electrical hot plate heater. In another 
apparatus, a plastic cassette in series with the IV tube is placed against 
a hot plate to warm the fluid as it runs through the cassette. 
All of these methods of fluid warming have limitations. The water bath 
apparatus is bulky, poses the danger of electrical shock and is 
inconvenient to use because care must be given to storage and transport of 
the heating liquid in which the tubing is immersed. The drawbacks of the 
cassette heater are manifold. The cassette is expensive and it adds 
resistance to the flow of fluid. The heating element must directly contact 
the cassette. Because of the high thermal resistance from the element to 
the cassette, the rate of heating may be insufficient to adequately heat 
IV fluid flowing at the usual clinical rates. Adjustment of the rate of 
heating or flow poses the danger of over-heating the fluid, which can 
damage the components of blood-based compositions. 
Therefore, there is an evident requirement for a means and method by which 
intravenously-administered fluid can be warmed quickly, efficiently, 
safely, and without the bulkiness and inconvenience of the prior art of 
warming devices. Preferably, addition of a fluid heating capacity to the 
normal set of surgical or emergency equipment would not result in a need 
for extra equipment to heat the fluid. 
SUMMARY OF THE INVENTION 
The inventor has observed that hypothermia can be effectively combatted by 
use of convective warming technology as exemplified in U.S. Pat. No. 
4,572,188, of which the applicant is a co-inventor. In convective body 
temperature control technology, a heated gas, such as air, is provided 
from a source to a thermal blanket or air flow cover which the heated gas 
inflates and erects. Apertures in the blanket deliver the inflating gas to 
the patient in the form of a warm bath which uniformly and efficiently 
elevates the patient's body temperature. This invention is based upon the 
applicant's critical observation that the flow of heated gas can quickly 
and efficiently elevate the temperature of an intravenously-introduced 
fluid while the fluid is being administered to a patient. 
A significant objective of this invention is therefore to provide an 
apparatus and method to heat intravenously-administered fluids by an 
apparatus which transfers thermal energy from a flowing, heated gas to the 
fluid during intravenous application. 
Advantageously, an apparatus which warms intraveneously-administered fluids 
by a flow of heated gas eliminates the need for containment of a liquid 
heating agent and a means to heat the liquid. A gas flow is simple and 
inexpensive to direct and deliver. 
A further advantage is the provision of heated IV fluid without an increase 
in the usual set of surgical or emergency equipment. 
The invention adequately and efficiently heats IV fluid at the IV flow 
rates encountered in clinical practice without danger to the fluid 
components. 
Other objects and advantages of this invention will become clear when its 
detailed description is read with reference to the below-described 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 illustrate an inflatable, self-erecting thermal blanket that 
is described in detail and claimed in U.S. Pat. No. 4,572,188, which is 
incorporated herein by reference in its entirety. The thermal blanket of 
FIGS. 1 and 2 utilizes convection of heated air for temperature management 
of a patient in a clinical setting. The patient 10 lies on a surface 12. 
The patient's body temperature is controlled by an inflatable thermal 
blanket 13, connected by a flexible conduit (hose) 15 to a source of 
heated air 16. The heated air source 16 heats air and forces a flow of 
heated air through the conduit 15. The conduit 15 is connected at 18 to 
the inflatable blanket 14. 
As FIGS. 1 and 2 illustrate, the blanket is constructed of a plurality of 
parallel inflatable chambers, one of which is indicated by reference 
numeral 19. Each of the chambers 19 has one or more openings to at least 
one adjacent neighboring chamber so that the heated air forced into the 
blanket 14 through the conduit 15 flows into all of the chambers. The 
chambers are made of a flexible material and are inflated when the heated 
air flows into them. Preferably, the chambers are carried on a relatively 
flat, non-inflatable undersheet 20. A plurality of apertures are provided 
which extend through the undersheet 20 into the tubes. One of these 
apertures is indicated by reference numeral 22. 
When the heated air is forced into the blanket, it inflates the chambers, 
erecting the blanket into the rounded structure illustrated in FIGS. 1 and 
2. The inflating heated air flows throughout the chambers and is expelled 
into the interior of the erected structure through the apertures 22. The 
expelled air, still heated, bathes and warms the patient 10. 
The thermal blanket of FIGS. 1 and 2 is used most frequently in 
intra-operative, post-operative, or emergency applications. In these 
situations, patient maintenance frequently requires, in addition to 
warming, the intravenous administration of fluids, such as blood, blood 
products, or other IV fluids. 
An apparatus for the intravenous adminstration of fluid includes a fluid 
source consisting of a stand 25 which holds an elevated container 26 of 
fluid 28. Normally, the container 26 would be an elevated plastic bag if 
the fluid 28 were blood. If the fluid 28 were a non-blood composition, the 
container would be either a plastic bag or a bottle. The container 26 is 
elevated over the patient and held upside down on the stand 25 and its 
stopper 29 is connected to a valved fluid path including a plastic IV tube 
31. The intravenuous connection (not shown) of the tube 31 to the patient 
is conventional. In operation, the fluid 28 flows out of the container 26 
through the tube 31 at a rate which is set by valving 33 in the fluid 
path. 
In order to obviate the cooling effects that cold or room temperature fluid 
will have on the patient 10, it is useful to warm the fluid 28 as it is 
being administered. Of course, for the surgical or emergency context 
illustrated in FIG. 1, it is probable that a host of instrumentation will 
be connected to the patient 10. Therefore, the addition of the prior art 
plumbing and heating elements to warm the fluid 28 only adds to the 
clutter and confusion of instrumentation used with the patient 10. 
FIGS. 3, 4A, and 4B illustrate a first embodiment for utilizing the flow of 
heated air generated by the heater/blower 16 to warm 
intravenously-administered fluid. In FIG. 3, an inflatable self-erecting 
thermal blanket 50 of the kind illustrated in FIGS. 1 and 2 is connected 
at 52 to a conduit 54 which delivers heated air flowing in the direction 
56 from a heater/blower assembly (not shown). The heated air inflates the 
blanket 50 in the manner described above, and the inflating air flows out 
of the under side of the blanket to warm a patient (not shown). An IV 
fluid 61 is delivered from a container 60 through a tube 62 for 
conventional intravenous administration to a patient. The fluid flowing in 
the tube 62 is warmed by an apparatus 66 which is positioned in the center 
chamber 65 of the thermal blanket 50. The apparatus 66 positions the tube 
adjacent to the conduit connection 52 at a location in which the inflating 
heated air flows across and past the tube portion 67. The apparatus 66 is 
wholly within the center chamber 65 and is warmed by the flow of heated 
gas which inflates the blanket 50. Warming the tube warms the fluid 
upstream of the patient; therefore, the fluid is warm when administered to 
the patient. 
FIGS. 4A and 4B illustrate the apparatus 66 which positions the tube 
portion 67 for warming by the flow of heated air entering the blanket 50 
in the conduit 54. As shown in these figures the tube portion 67 is held 
in a serpentine array by a series of roughly coaxial rings 69. Each of the 
rings 69 has an outer edge with a set of notches or indentations 71. The 
rings are aligned, and the tube is snapped into the indentations where it 
is held to form the serpentine array of tube portion 67. Of course, the 
serpentine array maximizes the total tube surface area which is exposed to 
the flow of heated air, which maximizes the amount of heat conducted 
through the tube to the circulating IV fluid. 
FIG. 3 shows a test apparatus by which the inventor has measured the 
heating effectiveness of the illustrated arrangement. According to FIG. 3, 
a liquid was conducted through the tube 62 into the serpentine array 67 at 
74, and was returned at 75 through the tube section 77. The fluid was 
collected in a beaker 79 through an outflow valve 81. The flow rate of the 
liquid through the tube path 62, 66, 77, 81 was set by valving 85. A 
thermometer 88 was conventionally connected to thermisters 85a in the 
container 60, 85b in the center chamber 65 adjacent to the tube portion 
66, and 85c in the outflow valve 81. The thermometer was conventional, an 
example being the HH-51 apparatus available from Omega. The thermocouples 
85a, 85b, and 85c were conventional K-type thermocouple sensors. 
The heating efficiency of the setup of FIG. 3 was tested using chilled ice 
water in the container 60, which was placed at the top of an IV stand 87, 
with the termperature probe 85a lowered into the ice water. The valving 85 
consisted of inseries, a drip chamber, a 15 drop/minute venoset back 
check, and a cair clamp IV set all having a combined length of 48 inches. 
This valving connected the container 60 with the tube 62 and provided the 
means by which the flow rate of the water was regulated. The IV tube 62 
consisted of 168" of 3/16" o.d. .times.1/8" i.d. vinyl tube. The tube 
section 67 was looped into a serpentine array and held on 2" diameter 
plastic rings, with the loops placed inside the center chamber 65 of the 
thermal blanket 50. The ambient temperature inside the center chamber 65 
was measured by the probe 85b; the outflow temperature of the water was 
measured at the outflow valve 81 by the probe 85c with the valve 81 placed 
12" from the thermal blanket. The results of the test are illustrated in 
Table I. These results indicate that the first embodiment exhibits highly 
effective operation in heating a chilled liquid during intravenous 
administration of the liquid under typical flow rate conditions 
encountered in everyday clinical environments. 
A second embodiment for using a flow of heated gas to elevate the 
temperature of an intravenously-administered fluid is illustrated in FIGS. 
5 and 6. In FIG. 5, the means for flowing the heated gas (air) is 
identical with that illustrated in FIG. 3. The means include the thermal 
blanket 50, the air conduit 54 connected to the center chamber 65 of the 
blanket 55, and a heater/blower (not shown). In FIG. 5, the tube portion 
90 which is heated by the flow of heated air within the blanket 50 is 
wedged in indentations formed between the center chamber 65 and two other 
blanket chambers 98 and 99, each abutting one respective side of the 
center chamber 65. An elongated partial loop of the tube 62 is held in 
this manner between the tubes 98, 65, and 99 of the blanket. The test 
setup for measuring the heating efficiency of the second embodiment 
illustrated in FIG. 5 was the same as that illustrated in FIG. 3. However, 
the test conditions were varied by using room-temperature water in the 
container 60. In the test of the FIG. 5 embodiment, the length of the tube 
portion, measured from point 101 where the tube first contacts the blanket 
50 between the chambers 65 and 98 and the point 102 where the tube exits 
from between the chambers 65 and 99 was varied. Various diameters of the 
tubing were used for the tube 62. The temperature gain was then measured 
between the container 60 and the outflow valve 81 for various combinations 
of lengths and diameters of the tube 62. A cotton hospital blanket (not 
shown) was placed over the blanket 50 to simulate actual usage of the 
blanket, but the blanket did not cover the entering or exiting portions of 
the tube 62. The results of the tests performed for the second embodiment 
of FIG. 5 are illustrated in Table II. These results indicate that the 
invention operates very effectively to warm an IV fluid being administered 
to a patient. 
TABLE I 
______________________________________ 
INFLOW OUTFLOW 
BLANKET H.sub.2 O H.sub.2 O 
FLOW 
TEST TEMP. TEMP. TEMP. RATE 
# .degree.F. .degree.F. 
.degree.F. 
liters/hr 
______________________________________ 
1 116.8- 43.0 85.2- 2.58 
120.0 85.8 
2 116.0- 42.6 95.2- 1.56 
121.2 99.8 
3 114.8- 43.4 104.8- .96 
121.6 105.2 
______________________________________ 
TABLE II 
__________________________________________________________________________ 
INFLOW 
OUTFLOW IV TUBE 
BLANKET 
H.sub.2 O 
H.sub.2 O 
FLOW IV TUBE 
OUTER-INNER 
TEST 
TEMP. TEMP. TEMP. RATE LENGTH 
DIAMETER 
# .degree.F. 
.degree.F. 
.degree.F. 
liters/hr 
inches 
inches 
__________________________________________________________________________ 
1 121.4- 73.4 83.6- 1.8 80 5/32 7/64 
125.6 84.2 
2 122.6- 75.2 88.8- 1.08 80 5/32 7/64 
125.0 90.4 
3 122.2- 73.6 91.4- .9 80 5/32 7/64 
125.4 91.8 
4 119.8- 75.4 85.4- 3.1 140 5/32 7/64 
121.4 88.2 
5 119.6- 76.0 96.2- 1.0 140 5/32 7/64 
121.4 98.4 
6 116.6- 76.6 81.4- 5.6 86 1/4 13/64 
118.0 82.0 
7 116.6- 76.8 87.4- 2.1 86 1/4 13/64 
118.2 88.2 
__________________________________________________________________________ 
Those skilled in the art will appreciate the ease and efficiency with which 
intravenously-administered fluid is heated according to the invention. Use 
of a flow of heated air results in efficient and very uniform transfer of 
heat through the IV tubing to the fluid. This conclusion is supported for 
both of the illustrated embodiments by the results presented in Tables I 
and II. In both cases, the fluid flow rates were those normally 
encountered; in both cases, the temperature of fluid flowing in a typical 
IV set-up was increased significantly; in neither case was the fluid 
heated above a level which would damage blood components. 
Moreover, in a setting where a flowing, heated gas is used to warm a 
patient, little, or no, additional equipment is required to heat IV fluid 
with the flowing gas. Although these embodiments show the gas being 
transported past the IV apparatus by means of a thermal blanket, it should 
be manifest that other means may be employed to divert and deliver a 
portion of the gas stream from the heater/blower to the IV tube. 
While I have described several preferred embodiments of my invention for 
convectively warming intravenously-administered fluids, it should be 
understood that modifications and adapations thereof will occur to persons 
skilled in the art. Therefore, the protection afforded my invention should 
only be limited in accordance with the scope of the following claims.