Fluid warmer comprising leakage current reduction circuit

Abstract

The present disclosure relates to fluid warmers such as intravenous fluid warmers for medical applications that exhibit improved leakage current suppression or cancellation during fault conditions of an intravenous fluid delivery system.

Description

[0001] FLUID WARMER COMPRISING LEAKAGE CURRENT REDUCTION CIRCUIT

[0002] The present disclosure relates to fluid warmers such as intravenous fluid warmers for medical applications. The fluid warmer comprises a resistive heating element mounted to a support structure and extending from a proximal electrical terminal to a distant electrical terminal and a fluid channel thermally coupled to the resistive heating element. The resistive heating element comprises a first resistive segment and a second resistive segment that both are arranged at least partly above respective sections of the fluid channel. The first resistive segment comprises a first part that is closest to the fluid inlet and the second resistive segment comprises a first part that is closest to the fluid outlet. The fluid warmer comprises a first drive voltage to be applied to the first resistive segment to dissipate power therein and a second drive voltage to be applied the second resistive segment to dissipate power therein. The fluid warmer further comprises a first voltage source configured to supply a first drive voltage to the first resistive segment to dissipate power therein and configured to supply a second drive voltage to the second resistive segment to dissipate power therein. The first and second drive voltages are configured to cancel voltage differences between the first part of the first resistive segment and the first part of the second resistive segment.

[0003] BACKGROUND OF THE INVENTION

[0004] Intravenous, intraosseous or infusion fluids such as blood are commonly used in hospitals. Infusion fluids are also used in the field, for example during patient transportation in disaster areas or war zones from an accident site to a hospital. The patient may be transported in a vehicle such as an ambulance or helicopter. The infusion fluid is used during most medical procedures and applications. Such infusion fluid is typically delivered from an IV fluid bag or container into a blood vessel of a patient. Blood is refrigerated during storage while other types of infusion fluids may be kept at ambient temperatures.

[0005] It would potentially be life threatening for the patient and may lead to hypothermia if the infusion fluid is infused into the patient’s body directly from storage at storage temperatures. Therefore, it is necessary to warm the infusion fluid to a temperature close to desired body temperature of the patient to avoid administration of underheated infusion fluid causes a drop in the patient’s body temperature. The presence of leakage currents flowing into a patient during operation of intravenous fluid warmer systems represents a hazard to patient safety. Such intravenous fluid warmer systems comprise an intravenous fluid warmer which is coupled to the patient’s body through an IV tube. The IV tube is connected between an outlet of the intravenous fluid warmer that supplies appropriately heated intravenous fluid to the patient’s body via a suitable port into a blood vessel such as a needle.

[0006] However, intravenous fluids are generally electrically conductive, as discussed below, and therefore prone to conducting leakage current into the patient’s body through the fluid connection made by the IV tube during operation of the system. The leakage current may for example be caused by certain parasitic impedances coupled to a fluid channel of the intravenous fluid warmer through various mechanical components and electrical circuitry of the intravenous fluid warmer for the reasons discussed below. Furthermore, the problem with leakage current further increases in certain fault states or situations of the intravenous fluid warmer system where uncontrolled and unintended leakage of intravenous fluid to the patient’s body is caused by a defective component of the system such as a defective connector to the IV bag.

[0007] Consequently, one object of the invention is to eliminate, or at least reduce, the level of leakage current flowing into the patient’s body under the above-discussed fault state.

[0008] SUMMARY OF INVENTION

[0009] A first aspect of the invention relates to a fluid warmer comprising a resistive heating element mounted to a support structure or integrally formed with the support structure; said resistive heating element extending from a proximal electrical terminal of the resistive heating element to a distant electrical terminal of the resistive heating element. The fluid warmer further comprising a fluid channel comprising a fluid inlet and a fluid outlet and thermally coupled to the resistive heating element, e.g. via an interposed heat transfer member such as a heat exchanger, wherein said resistive heating element comprises

[0010] - a first resistive segment extending from the proximal electrical terminal to a first intermediate node of the resistive heating element; and

[0011] - a second resistive segment extending from the first intermediate node to the distal electrical terminal; wherein

[0012] - the first resistive segment is arranged above at least a first section of the fluid channel extending from the fluid inlet to an intermediate position of the fluid channel. The first resistive segment further comprises a first part that is closest to the fluid inlet. The second resistive segment is arranged above at least a second section of the fluid channel extending from the intermediate position of the fluid channel to the fluid outlet. The second resistive segment comprises a first part that is closest to the fluid outlet; said fluid warmer further comprising

[0013] - a first voltage source configured to supply a first drive voltage to the first resistive segment to dissipate power therein and configured to supply a second drive voltage to the second resistive segment to dissipate power therein, wherein the first and second drive voltages are configured to cancel voltage differences between the first part of the first resistive segment and the first part of the second resistive segment.

[0014] This cancellation, or at least significant reduction such as a reduction by more than 90 %, of the voltage differences between the respective first parts of the first and second resistive segments is advantageous at least but not exclusively in certain fault states of intravenous fluid delivery systems which comprise the fluid warmer. In these fault states or fault situations the cancellation, or at least reduction, of the voltage difference between the respective first parts of the first and second resistive segments also minimizes a corresponding voltage difference between the fluid inlet and fluid outlet. This minimization of the voltage difference between the fluid inlet and fluid outlet, eliminates, or at least markedly reduces, undesired flow of leakage current through the patient’s body for example as described in additional detail below with reference to the appended drawings.

[0015] According one embodiment of the fluid warmer, the first part of the first resistive segment comprises the proximal electrical terminal and / or the first part of the second resistive segment comprises the distal electrical terminal. In alternative embodiments, the first part of the first resistive segment may comprise an intermediate section of the first resistive segment and / or the first part of the second resistive segment may comprise an intermediate section of the second resistive segment depending on the layouts of the first and second resistive segments. The skilled person will appreciate that the first part of the first resistive segment generally is situated close to the fluid inlet and / or the first part of the second resistive segment generally is situated close to the fluid outlet. This is due to a desire for small dimensions of the fluid warmer and simultaneously a desire to cover at least a major part of the fluid channel by the resistive heating element to maximize heat transfer to the infusion fluid. A distance between the fluid inlet and the first part of the first resistive segment is preferably less than 15 mm such as less than 10 mm or 5 mm. The corresponding distance between the first part of the second resistive segment and distal electrical terminal 26 is preferably less than 15 mm such as less than 10 mm or 5 mm.

[0016] According to one embodiment of the fluid warmer each of the first drive voltage and the second drive voltage comprises a switched voltage waveform such as pulse width modulated (PWM) or pulse density modulation (PDM) of the first and / or second drive voltages. The fluid warmer may comprise a processor, such as a processing circuit for example comprising software programmable microprocessor, that may be configured to control dissipation of thermal energy or power in the first and second resistive segments such that a set point temperature of the transfusion fluid at the fluid outlet is reached for example as described in additional detail below with reference to the appended drawings.

[0017] In one embodiment of the fluid warmer first voltage source is configured to generate each of the first drive voltage and second drive voltage based on AC line voltage which may have voltage between 100 V - 230 V.

[0018] The first voltage source may comprise a controllable voltage regulator configured to control the voltage waveform of each of the first drive voltage and the second drive voltage such as its amplitude, its frequency, its modulation scheme etc. The controllable voltage regulator may be energized by a battery or battery pack which may comprise one or more rechargeable battery cells and / or one or more supercapacitors. The battery or battery pack may be configured to supply a DC input voltage between 12 V and 36 V to the controllable voltage regulator. The amplitude of the first drive voltage and / or second drive voltage may be the same as the DC input voltage. In one embodiment the fluid warmer comprises a processing circuit, for example a software programmable microprocessor, configured to control the controllable voltage regulator. The processing circuit may be coupled to the controllable voltage regulator via a data bus or data wire. The processing circuit may be configured to control the waveform of at least the first drive signal such that respective power dissipations in the first and second resistive segments are controlled to achieve target temperature(s) of the intravenous fluid at one of more points in the fluid channel such as the fluid outlet.

[0019] The support structure of the resistive heating element may comprise various types of carrier substrates such as a printed circuit board (PCB) or ceramics substrate that comprises the resistive heating element formed as a plurality of separate resistors soldered to a surface of the carrier substrate. The surface of the carrier substrate may for example comprise a surface facing the fluid channel or an opposing surface facing away from the fluid channel.

[0020] According to another embodiment of the fluid warmer the support structure and the resistive heating element are formed as a single integrally formed structure such a printed circuit board or ceramics substrate. The printed circuit board or ceramics substrate comprises a plurality of electrical traces at least partly defining the resistive heating element.

[0021] In yet another embodiment of the fluid warmer the support structure and the resistive heating element are formed by a single uniform and thermally conductive material, such as metal such as steel, machined from uniform piece of raw material for example as described below in additional detail.

[0022] The resistive heating element, and hence the first and second resistive segments, may be arranged on a surface of the carrier substrate for example a surface facing the fluid channel. The latter arrangement achieves high thermal conductivity from the resistive heating element to intravenous fluid in the fluid channel. The support structure and resistive heating element may be separate elements in some embodiments of the fluid warmer. The resistive heating element may comprise a coil or solenoid that is mounted to the support structure and surrounds a fluid channel that comprises straight tube or meandering tube extending from the fluid inlet to the fluid outlet.

[0023] The resistive heating element may comprise a meandering pattern arranged on the carrier substrate and running in parallel with, or orthogonal to, the fluid channel. The skilled person will understand that resistive patterns can be manufactured using ordinary wire or trace layout techniques of PCBs. Therefore, leading to a reliable and low-cost embodiment of the support structure, e.g. PCB, and the resistive heating element.

[0024] In one embodiment of the fluid warmer shapes and dimensions of the first resistive segment are substantially identical to shapes and dimensions of the second resistive segment. Thereby, the first intermediate node may be arranged proximately in the middle between the proximal electrical terminal and the distal electrical terminal. The latter embodiments may lead to uniform distribution of thermal energy to the intravenous fluid along fluid channel during operation of the fluid warmer. According to an embodiment of the fluid warmer, the surface of the support structure facing the fluid channel comprises an electrically insulating and thermally conductive coating, layer or member interposed between the fluid channel and the surface of the support structure facing the fluid channel. The resistive heating element may be arranged on, e.g. bonded to or etched on, the surface of the support structure facing the fluid channel.

[0025] One embodiment of the fluid warmer comprises a heat exchanger which preferably comprises a highly thermally conductive material such as metal such as aluminium, sliver or copper. The heat exchanger may be interposed between the surface of the support structure facing the fluid channel and the fluid channel. A lower surface of the support structure, or lower surface of the heat exchanger, may at least partly define an upper wall the fluid channel. The fluid channel may further comprise a lower member which defines a lower wall of the fluid channel. The electrically insulating and thermally conductive layer may comprise an Aluminium oxide coating deposited on surfaces of the upper and lower surfaces of heat exchanger that define the fluid channel.

[0026] The fluid channel may comprise a straight central section arranged between the fluid inlet and the fluid outlet wherein the straight central section comprises a rectangular cross-sectional shape with a width to height ratio larger than 50 or larger than 100. The fluid channel may comprise an outlet transition zone where the rectangular cross- sectional shape gradually transitions to a circular cross section at the fluid outlet. The fluid channel may comprise a similar inlet transition zone where the rectangular cross- sectional shape gradually transitions to a circular cross section at the fluid inlet.

[0027] The fluid channel may alternatively comprise a meandering section comprising a width wise pattern extending orthogonally to the flow of intravenous fluid at the fluid inlet and fluid outlet. The meandering section may comprise a tube with an essentially circular cross-section.

[0028] The skilled person will understand the resistive heating element may comprise one of more resistive segments in addition to the first and second resistive segments. The resistive heating element may for example further comprise

[0029] - a third resistive segment arranged between a second intermediate node and the first intermediate node; and

[0030] - a fourth resistive segment connected between the first intermediate node and a third intermediate node. The fluid warmer further comprises a second voltage source configured to supply of a third drive voltage to the third resistive segment to dissipate power therein and supply of a fourth drive voltage to the fourth resistive segment to dissipate power therein. The third and fourth resistive segments may have identical shapes and dimensions to the first and second resistive segments, respectively.

[0031] A second aspect of the invention relates to an intravenous fluid delivery system which comprises a fluid warmer according to any of the above-described embodiments thereof. The intravenous fluid delivery system further comprises a fluid delivery tube comprising a proximal connector for realisably connect, e.g. via a Luer lock connector, to the fluid outlet of the fluid warmer. The intravenous fluid delivery system further comprises a distal connector configured for realisably connect to a subcutaneous needle, e.g. butterfly needle, for delivery of the intravenous fluid into a blood vein of a patient. The intravenous fluid delivery system further comprises a fluid source tube which comprises a proximal connector configured to detachably connect to an outlet of an IV bag and a distal connector configured to detachably connect the fluid source tube to the fluid inlet of the fluid warmer.

[0032] A second aspect of the invention relates to a fluid warmer, such as an intravenous fluid warmer, comprising: a resistive heating element mounted to a support structure and extending from a proximal electrical terminal to a distant electrical terminal. The fluid warmer comprises a fluid channel comprising a fluid inlet and a fluid outlet. The fluid channel or fluid passage is thermally coupled to the resistive heating element e.g. via an interposed heat transfer member such as a heat exchanger. The resistive heating element comprises

[0033] - a first resistive segment extending from the proximal electrical terminal of the resistive heating element to a first intermediate node of the resistive heating element; and

[0034] - a second resistive segment extending from the first intermediate node to the distal electrical terminal; wherein

[0035] - the first resistive segment is arranged above at least a part of a first section of the fluid channel extending from the fluid inlet to an intermediate position of the fluid channel,

[0036] - the second resistive segment is arranged above at least a part of a second section of the fluid channel extending from the intermediate position of the fluid channel to the fluid outlet. The fluid warmer further comprises a first voltage source configured to generate and supply a first drive voltage to the first resistive segment to dissipate power therein. The first voltage source, e.g. power source, may be configured to generate and supply a second drive voltage to the second resistive segment to dissipate power therein. The first and second drive voltages are configured to cancel, e.g. eliminate or set to zero, voltage differences between the proximal electrical terminal and the distal electrical terminal.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The appended drawings illustrate the design and utility of embodiments of the fluid warmer in which similar elements are referred to by common reference numerals. These drawings are not necessarily drawn to scale. To better appreciate how the above-recited and other advantages and objects are obtained, a more particular description of the embodiments will be rendered, which are illustrated in the accompanying drawings. These drawings depict only typical embodiments and are not therefore to be considered limiting of the scope of the appended patent claims.

[0039] FIG. 1 shows a simplified schematic drawing of a prior art fluid warmer,

[0040] FIG. 2A shows a simplified schematic drawing of a first exemplary embodiment a fluid warmer according to the invention,

[0041] FIG. 2B shows a simplified schematic drawing of a second exemplary embodiment a fluid warmer according to the invention,

[0042] FIG. 20 shows an enlarged subsection of the first exemplary embodiment the fluid warmer,

[0043] FIG. 3A shows a simplified schematic drawing of a third exemplary embodiment of a fluid warmer according to the invention,

[0044] FIG. 3B shows a simplified schematic drawing of a fourth exemplary embodiment of a fluid warmer according to the invention,

[0045] FIG. 4 shows a simplified schematic drawing of a resistive heating element mounted on a support structure of an exemplary fluid warmer according to a first embodiment of the invention, FIG. 4A shows a simplified schematic drawing of a resistive heating element mounted on a support structure of an exemplary fluid warmer according to a second embodiment of the invention,

[0046] FIG. 5 shows a simplified schematic of a controllable voltage source for generation of one or more drive voltages to the respective resistive segments of the resistive heating element,

[0047] FIG. 6 shows a schematic drawing an intravenous fluid delivery system which comprises a fluid warmer according to the prior art coupled to a patient’s body via a fluid delivery tube,

[0048] FIG. 7 shows a schematic drawing an intravenous fluid delivery system which comprises a fluid warmer according to any of the first, second, third and fourth embodiments thereof and coupled to a patient’s body via a fluid delivery tube.

[0049] DETAILED DESCRIPTION OF THE DRAWINGS

[0050] The skilled person will understand that the accompanying drawings are schematic and simplified for clarity and may in some instances merely show details which are essential to the understanding of the exemplary embodiments of the fluid warmer and corresponding intravenous fluid delivery systems while other details have been left out.

[0051] FIG. 1 shows a simplified schematic drawing of a prior art fluid warmer 201 . The prior art fluid warmer 204 comprises a support structure 220 for a resistive heating element 230. The resistive heating element 230 is bonded to a proximal electrical terminal 160 and a distal electrical terminal 260 which terminals create electrical contact to the opposite terminations of the resistive heating element 230. Positive and negative outputs of a power source VD is coupled to the proximal electrical terminal 160 and distal electrical terminal 260, respectively, such that electric power is dissipated in the resistive heating element 230 to control its temperature. The power source V1 may deliver an AC line voltage, e.g. 110 - 230 V, as drive voltage of the resistive heating element 230. Power dissipated in the resistive heating element 230 is thermally conducted to intravenous fluid flowing in the fluid channel 205 during operation of the fluid warmer 204. The dissipated power heats the intravenous fluid to a target temperature. The support structure 220 may comprise a printed circuit board and the resistive heating element 230 may be embodied as conductive wire traces of the printed circuit board. The resistive heating element 230 is arranged above the fluid channel 205 and extends from the proximal electrical terminal 160 to the distal electrical terminal 260. The fluid channel 205 extends between a proximal fluid inlet 206 and a distal fluid outlet 208 where the proximal electrical terminal 160 is arranged above and at the fluid inlet 206 while the distal electrical terminal 260 is arranged above and at the fluid outlet 208.

[0052] The skilled person will understand that the fluid channel 205 is filled with intravenous fluid flowing from the proximal fluid inlet 206 to the distal fluid outlet 208 during operation of the prior art fluid warmer 204. This flow of intravenous fluid is schematically indicated by the arrow. The prior art fluid warmer 204 comprises an electrically insulating member 222 arranged between the support structure 220 and the fluid channel 205 to electrically insulate the support structure 220 and the resistive heating element 230 to the intravenous fluid. However, the skilled person will understand that the drive voltage such as AC line voltage is applied across the resistive heating element 30 by the power source VD which means that the voltage difference between the proximal electrical terminal 16 and distal electrical terminal 26 corresponds largely to the drive voltage. The arrangement of the proximal electrical terminal 160 close to the fluid inlet 160 and the distal electrical terminal 260 close to the fluid outlet 208 together with the voltage difference between the terminals have certain important short comings under certain important fault situations of the intravenous fluid delivery system. In particular, the formation of an undesired leakage current path through the patient. These shortcomings are discussed in additional detail below in connection with the disclosure of exemplary fluid warmers according to the invention.

[0053] FIG. 2A shows a simplified schematic drawing of a fluid warmer 1 along a vertical longitudinal cross-section according to a first exemplary embodiment of the invention.. The fluid warmer 1 comprises a housing 4 which may comprise a moulded elastomeric compound. The housing 4 comprises a proximal connector 44 such as a barbed fitting, configured to provide a leak tight connection to a fluid source tube (108 on FIG. 7). The fluid source tube 108 provides a fluid flow path from an IV bag or IV container (104 on FIG. 7) to the proximal connector 44 which may comprise serve as a fluid inlet 6, or comprise the fluid inlet 6, of the fluid warmer 1. The housing 4 further comprises a distal connector 42, such as a barbed fitting, configured to provide a leak tight connection to a fluid delivery tube (110 on FIG. 7). The fluid delivery tube 110 provides a fluid flow path from the fluid outlet 8 of the distal connector 42 of the fluid warmer 1 to the patient’s body. The distal connector 42 may comprise the fluid outlet 8. During normal operation of the fluid warmer 1 the intravenous fluid flows from the IV bag to the fluid inlet 6 and further through a fluid channel 5 to the fluid outlet 8. The intravenous fluid is heated to a target temperature by a resistive heating element 30 to make the intravenous fluid suitable for intravenous administration to patients. The target temperature of the intravenous fluid may for example lie between 39 degrees Celsius and 42 degrees Celsius. The skilled person will appreciate that the temperature of the intravenous fluid at the fluid inlet 6 may be significantly lower than the fluid temperature at the fluid outlet 8 for example less than room temperature for the reasons discussed above.

[0054] The fluid channel 5 may comprise a straight central section or meandering central section, channel or conduit running through the heat exchanger 1 to conduct the flow of intravenous fluid from the fluid inlet 6 to the fluid outlet 8. In some embodiments, the straight central section comprises a rectangular cross-sectional shape with a width to height ratio larger than 50. These dimensions lead to efficient thermal coupling of thermal power or energy to the intravenous fluid due to a large contact area between walls of the heat exchanger and the intravenous fluid. Where the fluid channel 5 comprises a meandering section, it may comprise a width wise channel shape or pattern extending orthogonally to the flow of intravenous fluid at the fluid inlet 6 and fluid outlet 8.

[0055] The resistive heating element 30 comprises a first resistive segment 31 extending from a proximal electrical terminal 16 of the resistive heating element to a first intermediate node 36 of the resistive heating element 30. The first resistive segment 31 is electrically connected between the proximal electrical terminal 16 and the first intermediate node 36. The resistive heating element 30 further comprises a second resistive segment 32 extending from the first intermediate node 36 to the distal electrical terminal 26 of the resistive heating element 30. The second resistive segment 32 is electrically connected between the distal electrical terminal 26 and the first intermediate node 36. The resistive heating element 30 may in certain embodiments be mounted on the support structure 20 such as a carrier substrate as schematically depicted. The first resistive segment 31 is arranged above at least a part of a first section 5A of the fluid channel 5. The first section 5A of the fluid channel 5 extends from the fluid inlet 6 to an intermediate position below the first intermediate node 36. The second resistive segment 32 is arranged above at least a part of a second section 5B of the fluid channel 5 extending from the intermediate position to the fluid outlet 8. As schematically illustrated, the first resistive segment 31 comprises the proximal electrical terminal 16 which has the smallest distance to the fluid inlet 6. Similarly, the second resistive segment 32 comprises the distal electrical terminal 26 which has the smallest distance to the fluid outlet 8. The distance between the fluid inlet 6 and proximal electrical terminal 16 is preferably less than 15 mm. The corresponding distance between the fluid outlet 8 and distal electrical terminal 26 is preferably less than 15 mm.

[0056] The first resistive segment 31 is mainly thermally coupled to the first section 5A of the fluid channel to heat intravenous fluid flowing through the first section 5A. The temperature of the intravenous fluid flowing through the first section 5A may be heated to a first target temperature during operation of the fluid warmer 1. The second resistive segment 32 is mainly thermally coupled to the second section 5B of the fluid channel 5 to heat intravenous fluid flowing through the second section 5B e.g. to a second target temperature during operation of the fluid warmer 1. The skilled person will understand that the first and second target temperatures may be identical or differ for example because intravenous at the fluid inlet 6 will often possess a lower temperature than intravenous fluid at the fluid outlet 8. The fluid warmer 1 may comprise a temperature sensor 28 arranged at the fluid outlet for the purpose of monitoring the temperature of the intravenous fluid at the fluid outlet. The skilled person will understand that additional or alternative temperature sensors may be arranged at other points of the fluid channel 5 to provide alternative reading or more accurate readings of intravenous fluid temperatures throughout the fluid channel 5.

[0057] The support structure 20 may comprise the carrier substrate which may comprise a printed circuit board (PCB) or a ceramics substrate. The resistive heating element 30 may be embodied as conductive wire traces of the PCB or ceramics substrate to provide an integrally formed structure. Such integrally formed structures may be manufactured at low costs by use of well-known fabrication methodologies and materials. The first intermediate node 36 and the proximal and distal electrical terminals 16, 26 may be formed as respective contacts, such as pads, on a surface of the printed circuit board (PCB) or ceramics substrate. The fluid channel 5 is thermally coupled to the resistive heating element 30 through an interposed electrically insulating and thermally conductive member 22 that may comprise an electrically insulating and thermally conductive coating or layer. In this manner, the electrically insulating and thermally conductive member 22 is arranged in abutment to the support structure 20 and may at least partly define an upper wall of the fluid channel 5. The fluid warmer 1 may comprise a member 24 that defines a lower wall of the fluid channel 5.

[0058] In one embodiment, the resistive heating element 30 is formed by wire traces on a PCB or similar substrate wherein the wire traces preferably face the fluid channel 5. The electrically insulating and thermally conductive member 22 electrically insulates the resistive heating element 30 and the intravenous fluid in the fluid channel 5 to prevent or reduce leakage current flowing to the patient during normal operation of the fluid warmer 1. Such flow of leakage current is hazardous to patient safety and must be reduced at least to various upper limits required by regulatory bodies. Other embodiments of the fluid warmer 1 may comprise a straight tube like or meandering tube-like fluid channel. The resistive heating element 30 may in certain embodiments be shaped as a coil or solenoid that surrounds the straight tube-like or meandering tube-like fluid channel. These embodiments may comprise a matingly shaped and sized support structure for the resistive heating element 30. These embodiments may optionally comprise a matingly shaped and sized electrically insulating and thermally conductive member arranged between the straight tube like or meandering tube-like fluid channel and the mating resistive heating element.

[0059] The fluid warmer 1 comprises a first voltage source V1 configured to generate and supply a first drive voltage to the first resistive segment 31 to dissipate power therein and thereby heat the first resistive segment 31. The fluid warmer 1 comprises a second voltage source V2 configured to generate and supply a second drive voltage to the second resistive segment 32 to dissipate power therein. The first and second drive voltages are generally configured to cancel, or at least minimize, voltage differences between the part of the first resistive segment 31 that is closest to the fluid inlet 6 and the part of the second resistive segment 32 that is closest to the fluid outlet 8. In the present embodiment that means cancelling the voltage difference between the proximal electrical terminal 16 and distal electrical terminal 26 preferably at least during normal operation of the fluid warmer 1. The first intermediate node 36 may be connected to a common electrical potential such as ground 40 of the fluid warmer 1. The skilled person will appreciate that the first and second voltage sources V1, V2 may be implemented as a common voltage source such that the first and second resistive segments 31, 32 effectively are connected in parallel to the common voltage source. Thereby, the voltage difference between the first and second resistive segments 31, 32 will be essentially zero. If the first and second voltage sources V1, V2 are independent voltage sources there may in practice be a minor voltage difference between the between the proximal electrical terminal 16 and distal electrical terminal 26 caused by component tolerances etc. However, the skilled person will understand even in the latter situation careful matching of the first and second voltage sources V1, V2 may lead to significant reduction of voltage differences between the proximal electrical terminal 16 and distal electrical terminal 26 compared to the above-outlined prior art connection. In the latter the first drive voltage is applied across the resistive heating element 30 to make the voltage difference between the proximal electrical terminal 16 and distal electrical terminal 26 equal to the first drive voltage.

[0060] The voltage source V1 may be configured to deliver the first and second drive voltages to the first and second resistive segments 31 , 32, respectively, AC line voltage, e.g. 110 - 230 V @50 Hz - 60 Hz. In alternative embodiments, the first voltage source VD may be configured to generate each of the first drive voltage and the second drive voltage as switched voltage waveforms such as pulse width modulation or pulse density modulation etc. The first drive voltage and the second drive voltage may each have an amplitude between 10 V and 48 V as discussed in further detail below. The first drive voltage and the second drive voltage may be derived from a DC input voltage supplied by a battery pack comprising a plurality of rechargeable battery cells or a plurality of supercapacitors.

[0061] FIG. 2B shows a simplified schematic drawing of a fluid warmer 1 according to a second exemplary embodiment of the invention along a vertical longitudinal crosssection. The fluid warmer comprises an integrally formed resistive heating element and support structure 30. The integrally formed resistive heating element and support structure 30 may comprise a single thermally conductive member for example comprising a metal such as steel, copper, aluminium etc. The integrally formed resistive heating element and support structure 30 may therefore be viewed as a combination of a resistive heating element, a heat exchanger and a support structure. The integrally formed resistive heating element and support structure 30 may be fabricated by machining, grinding or moulding a single uniform piece of raw material such as a metal slab into the desired shape. This may simplify the manufacturing process of the integrally formed resistive heating element and support structure 30 for example compared to a PCB where multiple different materials are used to form the carrier substrate, wire traces, pads and protective layers etc. The use of single uniform piece of raw material may likewise simplify the manufacturing process compared to fabrication of multi-component structures. In multi-component structures the resistive heating element will comprise a number of separate resistive components or resistors that are attached and electrically connected to a separate carrier substrate for example by soldering or glueing etc. The integrally formed resistive heating element and support structure 30 may be essentially plane and possibly shaped and sized to extend above, or below, or surround, a major part of the fluid channel 5. The fluid channel 5 may comprise a straight tube like or meandering tube-like shape. The integrally formed resistive heating element and support structure 30 may possess a solenoid shape that surrounds or forms a correspondingly shaped and sized fluid channel 5. A resistance of the integrally formed resistive heating element and support structure 30 may be less than 40 Q such as between 2 Q and 20 Q. The skilled person will appreciate that the resistance below 40 Q may be achieved by a suitable combination of material, shape and dimensions of the integrally formed resistive heating element and support structure 30. In some embodiments, the fluid warmer 1 may comprise an electrically insulating and thermally conductive member (not shown), such as an electrically insulating coating, interposed between a surface of the integrally formed resistive heating element and support structure 30 facing the fluid channel 5. The integrally formed resistive heating element and support structure 30 may possess a thermal conductivity of at least 20 W / (m K).

[0062] FIG. 2C shows an enlarged section of the fluid warmer 1 of FIG. 2A depicted at the fluid inlet 6. FIG. 2B illustrates schematically how the previously discussed undesired leakage currents flowing into the patient during operation of the fluid warmer 1 are generated. Arrow 27 symbolizes the leakage current. A capacitor 23 represents a distributed capacitance between the resistive heating element 30 and the fluid channel 5 holding the intravenous fluid. A resistor 21 is coupled in parallel with the capacitor 23 and represents a distributed resistance between the resistive heating element 30 and the fluid channel. The value of the distributed capacitance depends on various dimensions and construction details of the fluid warmer 1 such as the dimensions of the resistive heating element 30, dimensions of the fluid channel and distance between the resistive heating element 30 and the fluid channel 5. The distributed resistance, represented by resistor 21, depends on various dimensions and other construction details of the fluid warmer 1 such as the insulating performance of the electrically insulating and thermally conductive member 22.

[0063] FIG. 3A shows a simplified schematic drawing of a fluid warmer 1 according to a third exemplary embodiment thereof along a vertical longitudinal cross-section. The fluid warmer 1 according to this second exemplary embodiment comprises a heat exchanger comprising upper and lower walls 25, 24.

[0064] The fluid warmer 1 may comprise an electrically insulating and thermally conductive member 22 interposed between a downwardly facing surface of the support structure 22 and the upper wall 25 of the fluid channel 5. The heat exchanger 24, 25 may comprise a metallic material or compound for example aluminum, preferably possessing a thermal conductivity of at least 20 W / (m K). In an embodiment of the heat exchanger 24, 25 are made of a thermally conductive polymer having a thermal conductivity of at least 20 W / (m K). In certain embodiments, the electrically insulating and thermally conductive member 22 may discarded, or supplemented by, an Aluminium oxide coating such as AI2O3 or Aluminum Nitrate or Beryllium Oxide deposited on the upper and lower walls 25, 24 that are facing the fluid channel 5 to provide electric insulation. The latter embodiment may be advantageous to reduce thermal resistance between the resistive heating element 30 and the fluid channel 5.

[0065] FIG. 3B shows a simplified schematic drawing of a fluid warmer 1 according to a fourth exemplary embodiment along a vertical longitudinal cross-section. The fluid warmer 1 according to this third exemplary embodiment may be like the second exemplary embodiment except for the presence of four resistive segments 31 A, B and 32A, B of the resistive heating element 30. The third resistive segment 31 B is connected between a second intermediate node 36A and the first intermediate node 36. The fourth resistive segment 32B is connected between a second intermediate node 36B and the first intermediate node 36. A third voltage source V3 is configured to generate and supply a third drive voltage to the third resistive segment 31 B to dissipate power therein and thereby heat the third resistive segment 31 B. A fourth voltage source V4 is configured to generate and supply a fourth drive voltage to the fourth resistive segment 32B to dissipate power therein and thereby heat the fourth resistive segment 32B. The first, second, third and fourth drive voltages may be identical e.g. have the same amplitude and waveform. In particular, the respective drive voltages may be similar to those discussed above. The first, second, third and fourth drive voltages may be configured to cancel, or at least minimize, voltage differences between the proximal electrical terminal 16 and distal electrical terminal 26 during operation of the fluid warmer 1.

[0066] FIG. 4 shows a simplified schematic drawing of a resistive heating element 30 mounted on, or etched to, an underside 20a of the support structure 20 according to a first exemplary embodiment of the fluid warmer. The support structure 20 may comprise a printed circuit board (PCB) or a ceramics substrate as discussed above. The support structure 20 may be essentially plane and possibly shaped and sized to extend above, or below, or surround, a major part of the fluid channel 5. A resistance of the resistive heating element may be less than 40 Q such as between 2 Q and 20 Q.

[0067] The resistive heating element 30 comprises at least the first resistive segment 31 extending from, and electrically connected between, the proximal electrical terminal 16 of the resistive heating element 30 to a first intermediate node 36 of the resistive heating element 30. The resistive heating element 30 comprises a second resistive segment 32 extending from, and electrically connected between, the first intermediate node 36 and a distal electrical terminal 26 of the resistive heating element 30. The resistive heating element 30 may be embodied as conductive wire traces of the PCB or ceramics substrate to provide an integrally formed structure that may be manufactured at low costs by use of well-known fabrication methodologies and materials. The resistive heating element 30 may comprise a meandering pattern running widt lengthwise relative to the flow of intravenous fluid between the fluid inlet and outlet 6,8 that are indicated by dotted lines.

[0068] A resistance of each of the first and second resistive segments 31, 32 may be less than 20 Q such as between 1 and 10 Q. In some embodiments, shapes and dimensions of the first resistive segment 31 are substantially identical to shapes and dimensions of the second resistive segment 32 of the resistive heating element 30. Consequently, the first intermediate node 36 may arranged midways on the carrier substrate 20 between the proximal electrical terminal 16 and the distal electrical terminal 26. The skilled person will understand that the latter embodiment may provide an essentially uniform distribution of thermal energy to the intravenous fluid along fluid channel 5 provided that the first and second drive voltages are substantially equal. However, in alternative embodiments of the resistive heating element 30 the shapes and dimensions of the first resistive segment 31 may differ from the shapes and dimensions of the second resistive segment 32. In the latter embodiment, the resistance of the first resistive segment 31 may be smaller than the resistance of the second resistive segment 32 for example to provide more thermal energy to the intravenous fluid closest to the fluid inlet 6 where the intravenous fluid typically has the lowest temperature. The skilled person will appreciate that the part of the first resistive segment 32 that is closest to the fluid inlet 6 is the proximal electrical terminal 16. Likewise, the part of the second resistive segment 32 that is closest to the fluid outlet 8 is the distal electrical terminal 26. The first and second drive voltages, that are applied to the first resistive segment 31 and second resistive segment 32, respectively, are configured to cancel, or at least minimize, voltage differences between the proximal electrical terminal 16 and the distal electrical terminal 26. Hence, holding substantially equal voltage potentials at the fluid inlet 6 and fluid outlet 8 during normal operation of the fluid warmer. The skilled person will understand that the respective amplitudes and waveforms of the first and second drive voltages may be controlled directly or indirectly by a processing circuit 50 (Fig. 5) to achieve the desired cancellation, or at least minimization, of voltage differences between the proximal electrical terminal 16 and the distal electrical terminal 26 for example as discussed below with reference to Fig. 5.

[0069] FIG. 4A shows a simplified schematic drawing of a resistive heating element 30 mounted on, or etched to, e.g. e.g. the underside 20a of the support structure 20 according to a second exemplary embodiment of the fluid warmer. The skilled person will apricate that one or more features of the second embodiment of the resistive heating element 30 may be identical to those of the first embodiment discussed above except for the specifically described features. Hence, the features of the first embodiment will not be repeated here for the sake of brevity. The first resistive segment 31 comprises a first part 45 that is closest to the fluid inlet 6. In contrast to the first embodiment discussed above, the closest part is not the proximal terminal but an intermediate section of the first resistive segment 31. The second resistive segment 32 likewise comprises a first part 47 that is closest to the fluid outlet 8. In contrast to the first embodiment discussed above, the closest part of the first resistive segment 31 to the fluid inlet 6 is an intermediate section of the first resistive segment 31. Likewise, the closest part of the second resistive segment 32 to the fluid outlet 8 is an intermediate section of the second resistive segment 32.

[0070] A discussed above, the first and second drive voltages, that are applied to the first resistive segment 31 and second resistive segment 32, respectively, are configured to cancel, or at least minimize, voltage differences between the first part of the first resistive segment 31 that is closest to the fluid inlet 6 and the part of the second resistive segment 32 that is closest to the fluid outlet 8. In the present embodiment that means cancelling the voltage difference between the closest part 45 of the first second resistive segment and the closest part of the second resistive segment 47 such as to hold substantially equal voltage potentials at the fluid inlet 6 and fluid outlet 8.

[0071] FIG. 5 shows a simplified schematic of a controllable voltage regulator 52 of the fluid warmer and configured to generate at least the first drive voltage that may be applied to the first and second resistive segments 31, 32, respectively. The controllable voltage source 52 may be coupled to, and energized by, a battery or battery pack 60 which may comprise one or more rechargeable battery cells 63 for example Li-ion based battery cells. The battery 63 or battery pack 60 may be utilized to energize electrical circuits of the fluid warmer 1 , such as the controllable voltage source 52 and processing circuit 50 etc. Thereby, the electrical circuits remain electrically insulated from the AC mains voltage increasing patient safety because the AC mains voltage is not present anywhere in the fluid warmer 1. The battery or battery pack 60 may be encapsulated by battery housing 62 to protect e.g. internal mechanical and / or electrical components against impacts, environmental dust and moisture etc. The battery housing 60 may be a separate component from the housing 4 of the fluid warmer 1. The battery pack 60 may be electrically coupled to the controllable voltage source 52 via an output terminal 61 and via a power supply input terminal 65 of the controllable voltage source 52. The electrical connection between the output terminal 61 and power supply input terminal 65 may comprise a releasable or fixed cable 64. The battery pack 63 may be configured to deliver a DC voltage between 12 V and 48 V at the output terminal 61.

[0072] In alternative embodiments of the fluid warmer 1 , the battery pack 60 is replaced by an AC power supply, for example comprising a mains line connection. The AC mains voltage may be rectified and converted to a suitable DC voltage at the output terminal 61. This may for example be accomplished by an AC-DC converter mounted in the battery housing 60. The controllable voltage regulator 52 may be configured to convert the DC supply voltage at the input terminal 65 to a higher or lower DC voltage or keep the DC supply voltage to define an amplitude of the least the first drive voltage.

[0073] The controllable voltage regulator 52 may be configured to control the voltage waveform of at least the first drive voltage, and optionally the second drive voltage, such as one or more of its amplitude, waveform, frequency, modulation scheme etc. Hence, the first drive voltage may comprise a switched voltage waveform with an amplitude between 12 V and 48 V. The fluid warmer preferably comprises a processing circuit 50, for example a software programmable microprocessor or DSP, configured to control the controllable voltage regulator 52. In the latter embodiment, the controllable voltage regulator 52 is controlled by the processing circuit 50 which generates a control signal on a data bus or data wire 56. The processing circuit 50 is configured to control the waveform of the at least the first drive signal such that the respective power dissipations in the first and second resistive segments 31 , 32 are controlled for example to reach a target temperature of the intravenous fluid at the fluid outlet and or elsewhere in the fluid channel to control of the target temperature of the intravenous fluid. The previously discussed temperature sensor 28 may be configured to repeatedly deliver a temperature signal T1 to an input port of the processing circuit 50. The processing circuit 50 may be configured to control the respective waveforms of the first drive signal and / or second drive signal, and possibly any further drive voltages, by adaptation of the control signal supplied to the processing circuit 50 on the data bus 56. The processing circuit 50 may control the respective waveforms of the first and / or second drive signal by controlling the amplitude, pulse width or pulse density of the first and / or second drive signals and thereby control a corresponding dissipation of thermal energy or power in the first and second resistive segments 31 , 32 to reach the target temperature(s) of the intravenous fluid. The skilled person will understand that the controllable voltage regulator 52 and the processing circuit 50 may be configured as components of a feedback loop that is based on the instantaneous temperature of the intravenous fluid for example as measured by the temperature sensor 28.

[0074] FIG. 6 shows a schematic drawing an intravenous fluid delivery system 600 which comprises a fluid warmer according to the above-discussed prior art. Typical intravenous fluids comprise blood, electrolytes, such as sodium and potassium, and glucose in addition to water. These ingredients make the intravenous fluid electrically conductive which has certain consequences for patient safety as discussed below.

[0075] The prior art fluid warmer is coupled to a patient’s body 100 via a fluid delivery tube 110. The intravenous fluid delivery system comprises fluid bag or container 104 which comprises an outlet that is connected to a fluid source tube 108 via a releasable connector 106. The fluid source tube 108 is connected to the fluid inlet 6 of the fluid warmer 1 through a second connector 44. The second connector 44 may be configured to detachably connect the fluid source tube 108 to the fluid inlet 6. Heated intravenous fluid flows into a fluid delivery tube 110 and from there to a needle 102 inserted in the patient’s blood vessel to deliver appropriately warmed intravenous fluid to the patient 100. The skilled person will understand that this configuration of the intravenous fluid delivery system 600 represents a normal operational mode.

[0076] However, the intravenous fluid delivery system 600 is illustrated in a fault situation caused by a leak of intravenous fluid from the fluid inlet 6 and / or from a defective proximal connector 44 and / or from the releasable connector 106 of the IV bag 104. This undesired leak of the intravenous fluid is schematically indicated by a fluid leakage path 46. The leaking intravenous fluid may contact various body parts of the patient 100 in an uncontrolled manner such as leaking fluid to the opposite arm of the patient 110 to the one where the needle 102 is inserted, as schematically illustrated. The skilled person will understand that the leak of intravenous fluid via fluid leakage path 46 creates an electrical connection from the fluid channel 5 to the patient 100 due to the above-discussed electrically conductive property of intravenous fluid. Furthermore, the flow of intravenous fluid in the fluid delivery tube 110 creates another electrically conductive path to the patient’s left arm. The fluid delivery tube 110 is therefore capable of conducting leakage current, as shown by item 27 of FIG. 2B, into the patient’s body through the IV needle 102. Accordingly, the illustrated fault scenario comprises a closed leakage current loop 116 running from the distal electrical terminal 26 through the above-discussed parasitic impedance to the transfusion fluid in the fluid channel 5. This closed leakage current loop 116 further comprises a leakage current path into the patient’s body through the delivery tube 110 and through the IV needle 102. The closed leakage current loop 116 further comprises a conductive path running through the patient’s body according to the body resistance between the specific entry point and exit point on the patient’s body. The exit point may be the patient’s upper right arm as depicted. The closed leakage current loop 116 is finally closed to the intravenous fluid at the proximal electrical terminal 16 by the parasitic impedance of the latter to the fluid channel 5. Consequently, the undesired flow of leakage current in the fault situation is caused by the fluid leakage path 46 in combination with a voltage difference between the proximal and distal electrical terminals 16, 26 as the latter are closest to the fluid inlet 6 and fluid outlet 8, respectively, in the present embodiment. This voltage difference largely corresponds to the drive voltage supplied by the voltage source and applied across the resistive heating element to heat the latter. The undesirable implications of latter facts are discussed in additional detail below in connection with the disclosure of exemplary fluid warmers according to the invention.

[0077] FIG. 7 shows a schematic drawing an intravenous fluid delivery system 700 which comprises a fluid warmer 1 according to any of the above-discussed exemplary embodiments of the fluid warmer in accordance with the invention. The intravenous fluid delivery system 700 is illustrated in the above-discussed fault situation where the undesired leak of intravenous fluid flows from the fluid inlet 6 and / or from the proximal connector 44 and / or from the releasable connector 106 of the IV bag 104 to the patient’s body. This undesired leak of intravenous fluid is schematically indicated by fluid leakage path 46 as above.

[0078] Similar features are marked by corresponding reference numerals to those of the prior art intravenous fluid delivery system 600 discussed above. The closed leakage current loop 116 through the patient’s body that was illustrated in the fault scenario of the prior art intravenous fluid delivery system 600 is likewise illustrated by dotted line 116. However, the voltage difference between the proximal and distal electrical terminals 16, 26 arranged closets to the fluid inlet 6 and fluid outlet 8, respectively, of the fluid warmer 1 is eliminated, e.g. about zero, by the previously discussed configuration of the first and second drive voltages relative to the first and second resistive segments. The skilled person will understand that substantially zero voltage difference between the proximal and distal electrical terminals 16, 26, respectively, represents a marked improvement to the presence of the full drive voltage between the proximal and distal electrical terminals 16, 26 at the fluid inlet 206 and fluid outlet 208 in the prior art fluid warmer (FIG. 1 and 6). Consequently, in the intravenous fluid delivery system 700 the undesired flow of leakage current through the patient’s body, along the closed leakage current loop 116, is largely eliminated in the depicted fault situation e.g. fault state. This elimination of leakage current is caused by the absence of any voltage difference between the proximal and distal electrical terminals 16, 26. The inventor has calculated that typical parasitic capacitances from the resistive heating element to the fluid channel may lie between 1 nF and 20 nF such as about 5 nF, depending on dimensions, geometry and materials of each of the resistive heating element, the fluid channel and possibly of any intervening layers, coating etc. The level of leakage current may be further reduced compared to the prior art if there exists resistive components from the resistive heating element to the fluid channel. Such resistive components of the parasitic impedance may be caused by defects of the electrically insulating and thermally conductive coating, layer or member. The resistance of the intravenous fluid in the delivery tube 110, with an exemplary diameter of 4 mm and filled with 0.9% saline solution, may lie between 100 and 400 kQ such as about 200 kQ depending on the length and actual composition of the infusion fluid. The undesired fluid leakage path 46 may exhibit a similar resistance. The resistance of the patient’s body may lie between 500 Q and 2 kQ where the direct contact with the inner of the patient’s body through the IV needle 102 leads to a relatively small resistance. These impedance and resistance estimates are equally valid for the prior art fluid warmer and for the fluid warmer according to the present invention.

[0079] The skilled person will therefore appreciate that the level of leakage current through the patient’s body in the prior art intravenous fluid delivery system 600, i.e. using the prior art fluid warmer, inter alia depends on the amplitude and maximum frequency of the first and second drive voltages e.g. between 12 V and 48 V at 100 Hz - 500 Hz, or 230 V and 50 Hz. This dependency on the amplitude and maximum frequency of the first and second drive voltages is caused by the voltage difference between the proximal and distal electrical terminals 16, 26, respectively, that are closest to the fluid inlet 6 and fluid outlet 8, respectively, of the infusion fluid warmer. The voltage difference between the proximal and distal electrical terminals 16, 26, respectively is driving the level and flow of the leakage current. In contrast, the level of leakage current through the patient’s body in the intravenous fluid delivery system 700 is about zero under the same fault condition due to the absence of any voltage difference between the proximal and distal electrical terminals 16, 26, respectively.

Claims

CLAIMS1. A fluid warmer comprising: a resistive heating element mounted to a support structure or integrally formed with the support structure; said resistive heating element extending from a proximal electrical terminal of the resistive heating element to a distant electrical terminal of the resistive heating element, a fluid channel comprising a fluid inlet and a fluid outlet and thermally coupled to the resistive heating element, e.g. via an interposed heat transfer member such as a heat exchanger, wherein said resistive heating element comprises- a first resistive segment extending from the proximal electrical terminal to a first intermediate node of the resistive heating element; and- a second resistive segment extending from the first intermediate node to the distal electrical terminal; wherein- the first resistive segment is arranged above at least a first section of the fluid channel extending from the fluid inlet to an intermediate position of the fluid channel, wherein the first resistive segment comprises a first part that is closest to the fluid inlet,- the second resistive segment is arranged above at least a second section of the fluid channel extending from the intermediate position of the fluid channel to the fluid outlet, wherein the second resistive segment comprises a first part that is closest to the fluid outlet; said fluid warmer further comprising- a first voltage source configured to supply a first drive voltage to the first resistive segment to dissipate power therein and configured to supply a second drive voltage to the second resistive segment to dissipate power therein, wherein the first and second drive voltages are configured to cancel voltage differences between the first part of the first resistive segment and the first part of the second resistive segment.

2. A fluid warmer according to claim 1 , wherein each of the first drive voltage and the second drive voltage comprises any of: a sinusoidal waveform, a switched voltage waveform,AC line voltage.

3. A fluid warmer according to claim 2, wherein the first voltage source comprises a controllable voltage regulator configured to control the voltage waveform of at least the first drive voltage such as at least one of an amplitude, a frequency, a modulation scheme etc.

4. A fluid warmer according to any of the preceding claims, wherein the support structure of the resistive heating element comprises a carrier substrate such as a printed circuit board (PCB) or ceramics substrate; and wherein the resistive heating element comprises a plurality of separate resistors soldered to a surface of the carrier substrate for example a surface facing the fluid channel or an opposing surface facing away from the fluid channel.

5. A fluid warmer according to any claims 1-3, wherein the support structure and the resistive heating element are formed as a single integrally formed structure such a printed circuit board which comprises a plurality of electrical traces at least partly defining the resistive heating element.

6. A fluid warmer according to claim 5, wherein the support structure and the resistive heating element are formed by a single uniform and thermally conductive material such as metal, machined from uniform piece of raw material7. A fluid warmer according to any of the preceding claims, wherein the resistive heating element comprises a meandering pattern arranged in parallel with, or orthogonal to, the fluid channel.

8. A fluid warmer according to any of the preceding claims, wherein shapes and dimensions of the first resistive segment are substantially identical to shapes and dimensions of the second resistive segment.

9. A fluid warmer according to any of claims 4-8, wherein the surface of the support structure, or the surface of the integrally formed support structure and resistive heating element, facing the fluid channel comprises an electrically insulating and thermally conductive coating, layer or member interposed between the fluid channel and the resistive heating element.

10. A fluid warmer according to any of the preceding claims, further comprising a heat exchanger, preferably comprising a highly thermally conductive material such as a metal such as aluminium, sliver or copper, interposed between the surface of the support structure facing the fluid channel and the fluid channel.

11. A fluid warmer according to any of the preceding claims, wherein a lower surface of the support structure or lower surface of the heat exchanger at least partly defines an upper wall the fluid channel; and said fluid channel further comprising a lower member which defines a lower wall of the fluid channel.

12. A fluid warmer according to any of the preceding claims, wherein- the fluid channel comprises a straight central section arranged between the fluid inlet and the fluid outlet, wherein the straight central section comprises a rectangular cross-sectional shape with a width to height ratio larger than 50.

13. A fluid warmer according to any of the preceding claims, wherein the first part of the first resistive segment comprises the proximal electrical terminal and the first part of the second resistive segment comprises the distal electrical terminal.

14. A fluid warmer according to any of the preceding claims, wherein the resistive heating element further comprises:- a third resistive segment arranged between a second intermediate node and the first intermediate node; and- a fourth resistive segment connected between the first intermediate node and a third intermediate node; said fluid warmer further comprising- a second voltage source configured to supply of a third drive voltage to the third resistive segment to dissipate power therein and supply of a fourth drive voltage to the fourth resistive segment to dissipate power therein.

15. An intravenous fluid delivery system comprising:- a fluid warmer according to any of the preceding claims; and- a fluid delivery tube comprising a proximal connector for realisably connect to the fluid outlet of the fluid warmer; and- a distal connector configured to make a realisable connection to a subcutaneous needle, e.g. butterfly needle, for delivery of the intravenous fluid into a blood vein of a patient; and- a fluid source tube comprising a proximal connector configured to detachably connect to an outlet of a fluid bag; and- a distal connector configured to detachably connect the fluid source tube to the fluid inlet of the fluid warmer.

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