METHOD FOR DETERMINING A FLUID TOTAL VOLUME FLOW IN THE REGION OF AN IMPLANTED VASCULAR SUPPORT SYSTEM AND IMPLANTABLE VASCULAR SUPPORT SYSTEM

Abstract

The invention relates to a method for determining a total fluid volume flow (1) in the region of an implanted vascular support system (2), comprising the following steps: a) determining a reference temperature (3) of the fluid, b) determining a motor temperature (4) of an electric motor (5) of the support system (2), c) determining the thermal dissipation loss (6) of the electric motor (5), d) ascertaining the total fluid volume flow (1) using the reference temperature (3), the motor temperature (4), and the thermal dissipation loss (6) of the electric motor (5).

Claims

1. A method for determining a total fluid volume flow of blood in a region of a cardiac support system, comprising: determining a reference temperature of the blood, determining a motor temperature of an electric motor of the cardiac support system, determining a thermal dissipation loss of the electric motor, and determining the total fluid volume flow based on the reference temperature, the motor temperature, and the thermal dissipation loss of the electric motor.

2-20. (canceled)

21. The method according to claim 1, further comprising heating the blood by the electric motor, wherein determining the reference temperature comprises measuring the reference temperature prior to heating the blood by the electric motor.

22. The method according to claim 1, wherein determining the motor temperature comprises measuring the motor temperature of the electric motor at a surface along which the blood flows.

23. The method according to claim 1, wherein determining the motor temperature comprises measuring the motor temperature of the electric motor inside the motor.

24. The method according to claim 1, further comprising determining a flow speed of the blood based on calibration data, the reference temperature, the motor temperature, and the thermal dissipation loss of the electric motor.

25. The method according to claim 1, wherein determining the total fluid volume flow is based in part on a cross-sectional geometry of an aorta in the region of the cardiac support system.

26. The method according to claim 1, further comprising determining a fluid volume flow of a portion of the blood flowing through the support system.

27. A computer readable storage medium storing therein computer-readable instructions that, when executed by a processing unit, cause the processing unit to: determine a reference temperature of blood flowing in a region of a cardiac support system, determine a motor temperature of an electric motor of the cardiac support system, determine a thermal dissipation loss of the electric motor, and determine a total fluid volume flow of the blood based on the reference temperature, the motor temperature, and the thermal dissipation loss of the electric motor.

28. A cardiac support system comprising: a reference temperature sensor configured to determine a reference temperature of blood, an electric motor, a motor temperature sensor configured to determine a motor temperature of the electric motor, and a current sensor configured to determine at least a current flow through the electric motor or a thermal dissipation loss of the electric motor.

29. The support system according to claim 28, further comprising a processing unit configured to determine a total fluid volume flow of the blood in the region of the cardiac support system using the reference temperature, the motor temperature, and the thermal dissipation loss of the electric motor.

30. The support system according to claim 28, comprising a flow machine configured to convey the blood and a cannula configured to guide the blood to the flow machine, wherein the electric motor is configured to guide the flow machine.

31. The support system according to claim 30, wherein the cannula is configured to guide the blood from a ventricle of a heart into an aorta.

32. The support system according to claim 30, wherein the reference temperature sensor is arranged on the cannula or near a region thereof at a distance from the flow machine.

33. The support system according to claim 30, wherein the reference temperature sensor is arranged on the cannula or near a region thereof facing away from the electric motor.

34. The support system according to claim 30, further comprising: a tubular elongated structure comprising a cannula section, the cannula section comprising the cannula, and a motor housing comprising a motor housing section configured to connect to the cannula section, wherein the electric motor is arranged in the motor housing.

35. The support system according to claim 34, wherein the reference temperature sensor is arranged in a region of the cannula section at a distance from the motor housing section.

36. The support system according to claim 28, wherein the electric motor is arranged in a motor housing, wherein the motor housing is configured to allow the blood to flow around the motor housing in the aorta.

37. The support system according to claim 34, wherein the motor housing is configured to allow the blood to flow around the motor housing in the aorta.

38. The support system according to claim 34, wherein the motor temperature sensor is configured to measure a surface temperature of the motor housing.

39. The support system according to claim 28, wherein the motor temperature sensor is configured to measure a temperature of a stator of the electric motor.

Description

[0045] The following are shown schematically:

[0046] FIG. 1a a percutaneous, minimally invasive left-heart support system,

[0047] FIG. 1b a left-heart support system invasively implanted under the chest opening,

[0048] FIG. 2 an implanted vascular support system,

[0049] FIG. 3 an arrangement of an implanted vascular support system,

[0050] FIG. 4 a component architecture of a support system,

[0051] FIG. 5 an illustration of a heat flow,

[0052] FIG. 6 an illustration of a temperature curve, and

[0053] FIG. 7 a further illustration of a temperature curve.

[0054] Implanted left-heart support systems (LVAD) exist mainly in two design variants, as shown in FIGS. 1a and 1b. FIG. 1a shows a (percutaneous) minimally invasive left-heart support system 16, while FIG. 1b shows a left-heart support system 17 invasively implanted under the chest opening. The variant according to FIG. 1a conveys blood directly from the left ventricle 18 into the aorta 9 since the (percutaneous) minimally invasive left-heart support system 16 is positioned centrally in the aortic valve 19. The variant according to FIG. 1b conveys the blood from the left ventricle 18 via a bypass tube 20 into the aorta 9.

[0055] Depending on the level of support, which describes the proportion of volume flow conveyed by a conveying means, such as a pump of the support system, to the total volume flow of blood from the ventricle 18 to the aorta 9, a certain volume flow reaches the aorta 9 via the physiological path through the aortic valve 19. The heart-time volume or the total volume flow (Q.sub.HTV) from the ventricle 18 to the aorta 9 is therefore usually the sum of the pump volume flow (Q.sub.p) and the aortic valve volume flow (Q.sub.a).

[0056] FIG. 2 schematically shows an implantable vascular support system 2 in the aortic valve position. For further illustration, reference is also made simultaneously to the schematic arrangement of the support system 2 according to FIG. 3, wherein the reference signs are used uniformly in all figures.

[0057] The support system 2 is here, by way of example, a left ventricular heart support system (LVAD).

[0058] The support system has a tubular elongated structure with a cannula section in which an inlet cannula 21 is formed as cannula, and comprises a motor housing section which is connected to the cannula section and in which an electric motor 5 is located in a motor housing 23.

[0059] The support system 2 protrudes from the aorta 9 through the aortic valves 19 distally into the ventricle 18. Here, the support system 2 has, by way of example, an inlet cannula 21 which protrudes into the ventricle 18. A fluid volume flow 10 is conveyed, e.g., pumped, through the inlet cannula 21 from the ventricle 18 into the aorta 9 using an electric motor 5 of the support system 2, which drives a flow machine in the form of a pump in the support system 2. Therefore, the fluid volume flow 10 is also referred to as the pump volume flow (Q.sub.p), which only quantifies the flow through the support system 2 itself.

[0060] In addition, it can be seen in FIG. 2 and FIG. 3 that a certain aortic valve volume flow 24 reaches the aorta 9 via the physiological path through the aortic valve 19. The heart-time volume or the total fluid volume flow 1 (Q.sub.HTV), passing through a cross-sectional geometry 8 of the aorta 9 in the region of the support system 2, from the ventricle 18 to the aorta 9 is therefore the sum of the fluid volume flow 10 (Q.sub.p) and the aortic valve volume flow 24 (Q.sub.a). This is described by the following equation (1).

Q.sub.HTV=Q.sub.p+Q.sub.a  (1)

[0061] The support system 2 comprises a reference temperature sensor 13 for determining a reference temperature 3 of a fluid, in this case blood by way of example. The support system 2 furthermore comprises an electric motor 5 and a motor temperature sensor 14 for determining a motor temperature 4 of the electric motor 5. In addition, the support system 2 has a current sensor (not shown here) for determining the thermal dissipation loss (not shown here) of the electric motor 5.

[0062] The motor temperature sensor 14 is, by way of example, integrated in a motor housing 23, in which the thermal dissipation loss of the electric motor 5 is dissipated to the surrounding fluid. The motor temperature sensor 14 is configured and arranged such that it can measure the motor temperature 4. For this purpose, the motor temperature sensor 14 can be configured and arranged such that it measures a surface temperature of the motor housing 23 or a temperature of the stator (not shown here) of the electric motor 5. In this case, the temperature of the stator can be approximated by an internal temperature in the motor housing 23 between the motor housing 23 and the coil package (not shown here). Alternatively, the temperature in the coil package can also be measured directly.

[0063] The reference temperature sensor 13 detects the reference temperature 3, which here is the background blood temperature by way of example. For this purpose, the reference temperature sensor 13 is positioned in the thermally uninfluenced blood flow upstream of the electric motor 5 representing the heat source; here, by way of example, in the region upstream of the electric motor 5. For this purpose, the reference temperature sensor 13, as shown in FIG. 2, is arranged in a region of the cannula section at a distance from the motor housing section at a distal end of the inlet cannula 21, i.e., where the blood flows from a ventricle into the inlet cannula 21.

[0064] FIG. 4 schematically shows a component architecture of a support system 2. The support system 2 comprises a reference temperature sensor 13 for determining a reference temperature 3 of a fluid, in this case blood by way of example. The support system 2 furthermore comprises an electric motor 5 and a motor temperature sensor 14 for determining a motor temperature 4 of the electric motor 5. In addition, the support system 2 has a current sensor 15 for determining the thermal dissipation loss 6 of the electric motor 5. For this purpose, the current sensor 15 ascertains, by way of example, the current flow (not shown here) through the motor 5 and converts it into the thermal dissipation loss 6. According to the illustration according to FIG. 4, the support system 2 furthermore comprises a processing unit 11 configured to determine a total fluid volume flow (not shown here) in the region of the support system 2 using the reference temperature 3, the motor temperature 4, and the thermal dissipation loss 6 of the electric motor 5. In addition, the support system 2 has an electronically readable memory 12 with calibration data 25.

[0065] The measurement data of the reference temperature sensor 13, the motor temperature sensor 14, and the current sensor 15 are transmitted to the processing unit 11. The processing unit 11 processes the measurement data with calibration data 25 from the memory 12 to form the blood flow speed or the (total) blood volume flow. The processing unit 11 furthermore comprises an output 26 to a communication unit (not shown here), an output 27 to a power supply (not shown here), and an output 28 to a motor control (not shown here).

[0066] FIG. 5 schematically shows an illustration of an exemplary heat flow (horizontal arrows) through the electric motor 5 to the fluid flow (vertical arrow) or the total fluid volume flow 1. The electric motor 5 in this case comprises, by way of example, a movably mounted rotor (not shown here) and a stationary coil package 22 which is offset by an air gap outside and which is connected to the stator 29. FIG. 5 thus schematically illustrates in other words the thermal conduction transitions from the coil package 22 of the electric motor 5 via the stator 29 to the blood flow. The loss mechanisms in the electric motor 5 primarily relate to the Joule current heat losses Pv (see equation (2) below).

P.sub.V=R.sub.TW.Math.I.sup.2  (2)

[0067] Here, R.sub.TW denotes the winding resistance of the coil package 22 at the operating temperature T.sub.W. The winding resistance R.sub.TW in the case of copper is a linear function of the winding temperature T.sub.W. This is described by equation (3) below:

R.sub.TW=R.sub.25.Math.(1+α.sub.Cu(T.sub.W−25))  (3)

with the winding resistance R.sub.25 at 25° C., the winding operating temperature T.sub.w, and the constant α.sub.cu=0.0039K.sup.−1.

[0068] In addition, iron losses also occur, e.g., magnetization losses according to the following equation (4):

P.sub.V,magn=π/30.Math.M.sub.Magn.Math.n  (4)

and eddy current losses in the back iron material of the stator according to the following equation (5):

P.sub.V,Eddy=const.Math.n.sup.2  (5)

with the number of revolutions n of the motor and the magnetic friction torque M.sub.Magn. In addition, bearing losses from the bearing of the motor occur, which are generally negligible.

[0069] The thermal resistance between a heat source and a heat sink is measured in Kelvin per watt (K/W). The determining thermal conduction mechanism between the coil package and the blood flow is thermal conduction through the layers of the motor to the outside, as shown in FIG. 5. In order to determine the temperatures, the heat capacities of the individual components traversed by the heat flow as well as the respective heat transfer resistances are required. Since it can be adequately assumed that the electric motor is in stationary operation and thus in thermal equilibrium, the heat capacities are negligible. All necessary parameters can be determined in advance and can be stored in a processing unit.

[0070] FIG. 6 schematically shows an illustration of a temperature curve along the material layer sequence from the coil package 22 via the stator 29 and the motor housing 23 to the total fluid volume flow 1. FIG. 6 shows a temperature distribution resulting in the thermal equilibrium for a heat flow according to FIG. 5. The highest temperature is present in the heat source, the coil package 22 through which the electrical current flows. The winding temperature 31 (formula symbol T.sub.W) of the coil package 22 is therefore the highest temperature in FIG. 6. For simplification, a constant heat distribution over the entire thickness of the coil package 22 was assumed here. Due to the finite thermal conductivity of the stator material and housing material, a linear temperature gradient results via the stator 29 and the motor housing 23, or a logarithmic temperature gradient in the non-simplified case of a cylindrical motor housing 23.

[0071] When considering the simplified principle, the winding temperature 31 arising in the coil package 23 (formula symbol T.sub.W) is:

[00001]$\begin{array}{cc}{T}_W=T_A+\left(R_{\mathrm{th}1}+R_{\mathrm{th}2}\right).Math.P_v& \left(6\right)\\ T_W=T_A+\left(R_{\mathrm{th}1}+R_{\mathrm{th}2}\right).Math.R_{\mathrm{TW}}.Math.I^2& \left(7\right)\\ T_W=T_A+\left(R_{\mathrm{th}1}+R_{\mathrm{th}2}\right).Math.R_{25}.Math.\left(1+{\alpha }_{\mathrm{Cu}}\left(T_W-25\mathrm{°C}.\right)\right)& \left(8\right)\\ T_W=T_A+\frac{\left(R_{\mathrm{th}1}+R_{\mathrm{th}2}\right).Math.R_{25}.Math.I^2}{1-{\alpha }_{\mathrm{Cu}}.Math.\left(R_{\mathrm{th}1}+R_{\mathrm{th}2}\right).Math.R_{25}.Math.I^2}& \left(9\right)\end{array}$

[0072] Here, the electrical current flow 30 (formula symbol I) and the surface temperature 32 (formula symbol T.sub.A) are the only variable parameters. R.sub.th1 describes the thermal resistance between the coil package 22 and the stator 29. R.sub.th2 describes the thermal resistance between the stator 29 and the fluid flow. The current flow 30 (formula symbol I) can be ascertained by measuring with the current sensor 15, for example, in a control device of the current sensor, and is thus precisely known. The surface temperature 32 (formula symbol T.sub.A) denotes the temperature on a surface 7 of the electric motor 5 along which the fluid flows. In other words, the surface 7 is in the blood stream.

[0073] FIG. 7 schematically shows a further illustration of a temperature curve. FIG. 7 shows a detailed view of the illustration according to FIG. 6 in the region of the surface 7 at two different flow speeds. In other words, FIG. 7 illustrates in printed form the dependence of the temperature(s) (surface temperature and thus also stator temperature and thus also coil package temperature) on the flow speed of the fluid flow or of the blood.

[0074] As shown in FIG. 7, a liquid film of thickness 33 is formed near the surface 7. The thickness 33 of the liquid film and the temperature difference T.sub.A−TB between the surface temperature 32 (formula symbol T.sub.A) and the reference temperature 3 (formula symbol T.sub.B), which represents the background temperature of the fluid (blood), is a function of the flow speed of the fluid, as illustrated in FIG. 7. According to the illustration in FIG. 7, a lower flow speed of the fluid along the surface 7 leads to a higher surface temperature 32′ than the surface temperature 32, which arises at a comparatively higher flow speed.

[0075] The heat flow through the liquid film is

{dot over (Q)}=α.sub.B(T.sub.B−T.sub.A)A  (10)

with the heat transfer coefficient α.sub.B from the top of the housing to the blood and the wetted surface A of surface 7. The heat transfer coefficient is defined as

[00002]$\begin{array}{cc}{\alpha }_B=\frac{\mathrm{Nu}\lambda }{L}& \left(11\right)\end{array}$

with the dimensionless Nusselt number Nu, the thermal conductivity λ of the fluid (here: blood), and a reference length L, which can be a tube diameter, for example. It furthermore applies to the Nusselt number averaged across the body surface that it is a function of the dimensionless Reynolds number Re and Prandtl number Pr:

Nu=f(Re,Pr)  (12)

[0076] These can each be calculated as a function of the geometry and the flow (Re and Pr) or as a function of the fluid properties (Pr) and stored in the calibration data memory. The Reynolds number is defined as

[00003]$\begin{array}{cc}\mathrm{Re}=\frac{\mathrm{uL}}{v}& \left(13\right)\end{array}$

with the characteristic length L (e.g., tube diameter), the kinematic viscosity of the fluid v, and the sought flow speed u. The Prandtl number is a pure substance variable and given by

[00004]$\begin{array}{cc}\mathrm{Pr}=\frac{v}{\alpha }& \left(14\right)\end{array}$

with the temperature conductivity a of the fluid. If the definitions are inserted into the convective heat flow through the liquid film (equation (10)), the relationship between the known heat flow {dot over (Q)} and the sought flow speed u is obtained. The result of this insertion is shown in equation (15) below. The heat flow {dot over (Q)} is known from an energy balance. It follows from the energy balance for the stationary case considered here that the heat flow {dot over (Q)} (in terms of magnitude) substantially corresponds to the thermal dissipation loss 6 (formula symbol P.sub.V).

[0077] The surface temperature 32 (formula symbol T.sub.A) can be measured here, for example, directly on the surface 7 by means of the motor temperature sensor 14, or the motor temperature sensor 14 can measure a temperature inside the motor and the surface temperature 32 (formula symbol T.sub.A) is ascertained from the logarithmic temperature relationship to the temperature distribution in the motor housing (cf. FIGS. 6 and 7). The reference temperature 3 (formula symbol T.sub.B) is determined by the reference temperature sensor 13. The parameters L, v, a, λ, and A are generally stored in the system as calibration data.

[00005]$\begin{array}{cc}Q=\frac{f\left(\frac{\mathrm{uL}}{v}.Math.\frac{v}{\alpha }\right)\lambda }{L}\left(T_A-T_B\right)A& \left(15\right)\end{array}$

[0078] With known cross-sectional geometry 8 of the aorta 9 of the patient in the region of the support system (ascertainable, for example, by ultrasound, computer tomography, or magnetic resonance tomography), the total fluid volume flow 1 (formula symbol Q.sub.HTV) can be determined from the flow speed u determined in this way. The corresponding relationship is specified in the following equation (16):

Q.sub.HZV=k(u)uO  (16)

[0079] Here, k(u) is a calibration factor dependent on the flow profile, u is the calculated flow speed, and O is the measured aortic cross section (cf. cross-sectional geometry 8).

[0080] The solution proposed here allows in particular one of the following advantages: [0081] Fully implanted, in particular pump-integrated and/or automatic determination of Q.sub.HTV instead of only Q.sub.p. [0082] Anemometric measuring methods using the waste heat of a VAD motor instead of an additional heating element do not result in an additional heat input into the organism. [0083] This also prevents additional current consumption, whereby the battery runtime of autonomous systems is extended.