METHOD FOR DETERMINING A FLOW SPEED OF A FLUID FLOWING THROUGH AN IMPLANTED, VASCULAR ASSISTANCE SYSTEM AND IMPLANTABLE, VASCULAR ASSISTANCE SYSTEM

Abstract

The invention relates to a method for determining at least a flow velocity or a fluid volume flow (5) of a fluid flowing through an implanted vascular support system (1), comprising the following steps: a) carrying out a pulsed Doppler measurement by means of an ultrasonic sensor (2) of the support system (1), b) evaluating a measurement result from step a), which has a possible ambiguity, c) providing at least one operating parameter of a flow machine (3) of the support system (1), d) determining at least the flow velocity or the fluid volume flow (5) using the measurement result evaluated in step b), wherein the possible ambiguity of the measurement result is corrected using the operating parameter.

Claims

1-10. (canceled)

11. A method for determining a flow velocity of blood flowing through a cardiac support system, the method comprising: carrying out a pulsed Doppler measurement using an ultrasonic sensor of the cardiac support system to determine a measurement result; evaluating the measurement result to generate an evaluated measurement result, wherein the measurement result comprises at least one possible ambiguity; determining at least one operating parameter of a flow machine of the cardiac support system; determining the flow velocity based on the evaluated measurement result; and correcting the possible ambiguity of the measurement result using the operating parameter.

12. The method of claim 11, wherein carrying out the pulse Doppler measurement comprises emitting a second ultrasonic pulse after an echo of a prior first ultrasonic pulse has died away.

13. The method of claim 11, wherein evaluating the measurement result comprises determining that the pulsed Doppler measurement has a pulse repetition rate of less than two times a maximum Doppler frequency shift of the blood flowing through the cardiac support system.

14. The method of claim 11, wherein the at least one operating parameter is based on at least one of the following: a rotational speed of a drive of the flow machine, an electrical current of the flow machine, an electrical power of the flow machine, and a differential pressure across the flow machine.

15. The method of claim 14, wherein determining the flow velocity is further based on the at least one operating parameter.

16. The method of claim 11, further comprising determining a plausible range in which plausible measurement results can be located based on the at least one operating parameter.

17. The method of claim 11, further comprising determining a fluid volume flow of the blood flowing through the cardiac support system based on the flow velocity.

18. The method of claim 11, wherein the at least one possible ambiguity comprises several possible flow velocities of the flowing blood.

19. A cardiac support system comprising: a flow machine; an ultrasonic sensor configured to carry out a pulsed Doppler measurement of blood flowing through the flow machine; and a processing unit configured to correct a possible ambiguity of a measurement result of the ultrasonic sensor based on at least one operating parameter of the flow machine to determine at least the blood flow velocity or the blood volume flow.

20. The support system of claim 19, wherein, to correct the possible ambiguity, the processing unit is further configured to: cause the ultrasonic sensor to carry out a pulsed Doppler measurement using an ultrasonic sensor of the cardiac support system to determine the measurement result; determine the at least one operating parameter of the flow machine of the cardiac support system; and determine at least the blood flow velocity or the blood volume flow based on the measurement result, wherein the possible ambiguity of the measurement result is corrected using the operating parameter.

21. The cardiac support system of claim 19, wherein the ultrasonic sensor is further configured to carry out the pulsed Doppler measurement by emitting a new ultrasonic pulse only if an echo of an ultrasonic pulse sent out immediately beforehand has died away.

22. The cardiac support system of claim 19, wherein the measurement result comprises the possible ambiguity based on the pulsed Doppler measurement having a maximum pulse repetition rate of less than two times a maximum occurring Doppler shift of the flowing blood.

23. The cardiac support system of claim 19, wherein the at least one operating parameter comprises at least one of the following: a rotational speed of a drive of the flow machine, an electrical current consumption of the flow machine, an electrical power consumption of the flow machine, and a differential pressure across the flow machine, a derived parameter of the rotational speed of a drive of the flow machine, a derived parameter of the electrical current consumption of the flow machine, a derived parameter of the electrical power consumption of the flow machine, and a derived parameter of the differential pressure across the flow machine.

24. The cardiac support system of claim 23, wherein to determining at least the flow velocity or the fluid volume flow is further based on the at least one operating parameter.

25. The cardiac support system of claim 19, wherein to determine at least the blood flow velocity or the blood volume flow, the processing unit is configured to determine a plausible range in which plausible measurement results can be located based on the at least one operating parameter.

26. The cardiac support system of claim 19, wherein the blood volume flow through the cardiac support system is determined based on the blood flow velocity.

27. A method for determining a flow velocity of blood flowing through a cardiac support system, the method comprising: determining that a pulse repetition rate sent out by an ultrasonic sensor of the cardiac support system is less than twice a maximum Doppler frequency shift of the blood flowing through the cardiac support system; determining a range of possible velocities for the blood flowing through the cardiac support system; determining at least one operating parameter of a flow machine of the cardiac support system; and determining the correct flow velocity from the range of possible velocities of the blood flowing through a cardiac support system based on the operating parameter.

28. The method of claim 27, wherein the at least one operating parameter is based on at least one of the following: a rotational speed of a drive of the flow machine, an electrical current of the flow machine, an electrical power of the flow machine, and a differential pressure across the flow machine.

29. The method of claim 27, further comprising determining a volume flow of the blood flowing through the cardiac support system based on the flow velocity.

30. A cardiac support system comprising: a flow machine; an ultrasonic sensor; and a processing unit configured to: determine that a pulse repetition rate of the ultrasonic sensor is less than twice a maximum Doppler frequency shift of blood flowing through the cardiac support system; determine possible velocities for the blood flowing through the cardiac support system; determine at least one operating parameter of the flow machine; and determine the correct flow velocity among the possible flow velocities based on the operating parameter.

31. The cardiac support system of claim 30, wherein the at least one operating parameter is based on at least one of the following: a rotational speed of a drive of the flow machine, an electrical current of the flow machine, an electrical power of the flow machine, and a differential pressure across the flow machine.

32. The cardiac support system of claim 30, wherein the processing unit is further configured to determine a blood volume flow of the blood flowing through the cardiac support system based on the flow velocity.

Description

[0049] The solution presented here as well as its technical environment are explained in more detail below with reference to the figures. It should be pointed out that the invention is not to be limited by the exemplary embodiments shown. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and to combine them with other components and/or insights from other figures and/or the present description. The following are shown schematically:

[0050] FIG. 1 an implantable vascular support system,

[0051] FIG. 2 the support system according to FIG. 1 implanted in a heart,

[0052] FIG. 3 a further implantable vascular support system,

[0053] FIG. 4 the support system according to FIG. 3 implanted in a heart,

[0054] FIG. 5 an exemplary illustration of a Doppler measurement,

[0055] FIG. 6 a sequence of a method presented here in a normal operating procedure,

[0056] FIG. 7 an exemplary Doppler frequency spectrum,

[0057] FIG. 8 a further exemplary Doppler frequency spectrum, and

[0058] FIG. 9 a functional illustration of a possible embodiment of the method presented here.

[0059] The vascular support system is preferably a ventricular and/or cardiac support system or a heart support system. Two particularly advantageous forms of heart support systems are systems positioned in the aorta according to FIG. 2 and systems positioned apically according to FIG. 4. The respective systems are explained in more detail in connection with FIG. 1 (aortic) and FIG. 3 (apical).

[0060] FIG. 1 schematically shows an implantable vascular support system 1. FIG. 1 illustrates an embodiment of an aortically positioned (cf. FIG. 2) or positionable support system 1. The support system 1 comprises an ultrasonic sensor 2 configured to carry out a pulsed Doppler measurement, a flow machine 3, and a processing unit 6 configured to correct a possible ambiguity of a measurement result of the ultrasonic sensor 2 using the operating parameter of the flow machine 3. The ultrasonic sensor 2 in this case comprises by way of example precisely one ultrasound (transducer) element 19.

[0061] In this case, the support system 1 according to FIG. 1 furthermore comprises, by way of example, a distal part with inlet openings 7 through which the blood can be drawn into the interior of the system, and an inlet tube 8 (which is formed in the manner of an inlet cannula in the aortic embodiment according to FIG. 1). In addition, the flow machine 3 is equipped by way of example with an impeller 9. In this case, a supply cable 10 is positioned by way of example proximally to the drive (e.g., electric motor; not shown here) of the flow machine 3. In the region of the impeller 9, there are also outlet openings 11, through which the blood can be discharged. During operation, a fluid volume flow 5 flows through the inlet tube 8, said fluid volume flow entering the support system 1 via the inlet openings 7 and exiting again via the outlet openings 11. This fluid volume flow 5 can also be referred to as a so-called pump volume flow.

[0062] FIG. 2 schematically shows the support system 1 according to FIG. 1 implanted in a heart 15. The reference signs are used uniformly so that reference can be made to the above explanations.

[0063] The inlet openings 7 are located in the implanted state, for example, in the region of the ventricle 12, while the outlet openings are located in the implanted state in the region of the aorta 13. This orientation of the support system 1 is merely exemplary here and not mandatory; rather, the support system can be oriented in the reverse direction, for example. In this case, the system is furthermore implanted by way of example in such a way that it passes through the aortic valve 14. Such an arrangement can also be referred to as a so-called aortic valve position.

[0064] FIG. 3 schematically shows a further implantable vascular support system 1. FIG. 3 illustrates an embodiment of an apically positioned (cf. FIG. 4) or positionable support system 1. The functioning of an apically implanted system is in principle comparable so that uniform reference signs can be used for all components in this case. Reference is therefore made here to the above explanations regarding FIG. 1.

[0065] FIG. 4 schematically shows the support system 1 according to FIG. 3 implanted in a heart 15. The reference signs are used uniformly so that reference can also be made here to the above explanations.

[0066] FIG. 5 schematically shows an exemplary illustration of a Doppler measurement. For this purpose, the ultrasonic sensor 2 of the support system 1 according to FIG. 1 is used by way of example in order to carry out a measurement in an inlet tube 8 of the support system 1 according to FIG. 1.

[0067] The measurement window, also referred to as the observation window and/or measurement range, for the ultrasound measurement is marked in FIGS. 1, 3, and 5 with reference sign 16. The selection of the measurement window 16 depends on the specific design of the (heart) support system 1 and should in principle be positioned where suitable flow conditions prevail. For example, FIG. 5 shows a simplified sectional view of the distal end of the embodiment of FIG. 1. In this case, it is shown schematically that no parallel flow lines prevail to the left of the measurement window 16 in the range 17. Since the Doppler effect is also a function of the cos(α) between the main beam direction of the ultrasound transducer and the main flow direction, it is advantageous to measure in a range of parallel flow lines. Although a measurement window (e.g., range 18) positioned too far away is possible in principle, it can intensify the aliasing effect explained below and/or provide strong attenuation of the ultrasound signal.

[0068] The ultrasonic sensor 2 is configured to carry out a pulsed Doppler measurement. The pulsed Doppler (pulsed wave Doppler; in short: PWD) method is basically used for ultrasound measurement in this case. The ultrasonic sensor 2 and the processing unit 6 can therefore also be referred to below as a so-called PWD system.

[0069] The measurement window 16 can typically be selected electronically in the PWD system so that a statement about the flow conditions in different regions of the flow guidance can also advantageously be made by means of measurement windows 16 of different depths.

[0070] In the (apical) embodiment according to FIG. 4, the blood flows toward the ultrasound element 19 in the opposite direction. The rotating impeller 9 is located between ultrasound element 19 and inlet tube 8. In this case, strong turbulence in the blood flow is to be expected so that it is also particularly advantageous here to position the measurement window 16 in front of the impeller 9, approximately in the region of the inlet tube 8.

[0071] The relatively high flow velocities in the range of the measurement window 16 in relation to the distance of the ultrasound (transducer) element 19 from the measurement window 16 have a great influence on the PWD application in both (heart) support system variants, predominantly in the (aortic) variant according to FIG. 1.

[0072] FIG. 6 schematically shows a sequence of a method presented here in a normal operating procedure. The method is used to determine at least a flow velocity or a fluid volume flow of a fluid flowing through an implanted vascular support system 1 (cf. FIGS. 1 to 5). The illustrated order of the method steps a), b), c), and d) with the blocks 110, 120, 130, and 140 is only exemplary. In block 110, a pulsed Doppler measurement is carried out by means of an ultrasonic sensor 2 of the support system 1. In block 120, a measurement result from step a) which has a possible ambiguity is evaluated. In block 130, at least one operating parameter of a flow machine 3 of the support system 1 is provided. In block 140, at least the flow velocity or the fluid volume flow is determined using the measurement result evaluated in step b). In the method, the possible ambiguity of the measurement result is corrected using the operating parameter.

[0073] For an exemplary illustration of the method, a system according to FIG. 1 with the following parameters is assumed: [0074] inner diameter of the inlet tube: 5 mm [0075] maximum blood flow to be measured: 9 liters/minute [0076] resulting maximum flow velocity: 7.64 meters/second [0077] sound speed in blood: 1540 m/s [0078] frequency of ultrasound: 6 MHz [0079] distance of the ultrasound transducer from the start of the measurement window: 25 mm [0080] ultrasound oscillation cycles per sent-out ultrasonic PVD pulse: 10 [0081] resulting burst length (10 oscillations at 1540 m/s): 2.57 mm

[0082] An ultrasonic pulse is sent out at the ultrasound element 19 and propagates in the direction of the measurement window 16. After sending out the pulse, the PWD system switches to the receiving direction and receives the portions that are continuously scattered back by scattering bodies in the blood, for example. The transit time of the pulse from the ultrasound element to the measurement window and from the measurement window back to the ultrasound element is taken into account in the process. In the case shown, the total relevant propagation path is thus 55.13 mm long (ultrasound element 19 to start of measurement window 16 plus burst length×2). The PWD system is switched back to transmission mode and the next pulse is sent out at the earliest when the last echo from the range of the measurement window 16 has arrived. In the specifically considered case, the pulse transit time limits the maximum pulse repetition rate to 27.93 kHz.

[0083] On the other hand, the maximum Doppler shift occurring in the case shown is 59.53 kHz. In a complex-value evaluation (10 demodulation), this leads to a minimum pulse repetition rate of 59.53 kHz, in which the present Doppler shift can be interpreted without ambiguity. However, since the measurement is carried out with a maximum of 27.93 kHz (maximum pulse repetition rate; see above), the Nyquist sampling theorem is violated in this case and ambiguities generally occur in the resulting Doppler spectrum. In this case, these ambiguities are resolved using an operating parameter of the flow machine of the support system in order to be able to make a clear statement about the main flow velocity in the observation window.

[0084] FIG. 7 schematically shows an exemplary Doppler frequency spectrum. Here, a schematic illustration of the previously presented relationships in the frequency spectrum is shown. A corresponding illustration of the shown relationships is also illustrated in FIG. 8.

[0085] FIG. 7 shows the amplitude 32 of the Doppler signal over the (averaged) frequency 33 with fixed pulse repetition rate 34 (PRF). The (fixed) pulse repetition rate 34 in the example considered here is 27 kHz. Simplified spectra for various flow velocities of the fluid (here: the blood) are shown in FIG. 7. A first flow velocity 20 is less than a second flow velocity 21, which in turn is less than a third flow velocity 22, which in turn is less than a fourth flow velocity 23, which in turn is less than a fifth flow velocity 24.

[0086] It can be seen that at the third flow velocity 22, there is already a violation of the Nyquist theorem, i.e., the Doppler frequency is in the range of the pulse repetition rate (PRF; here 27 kHz by way of example). With further increasing flow velocity of the blood, the spectrum moves from the negative frequency range to the coordinate origin. Here, there is already ambiguity about the direction of flow, i.e., either a fast flow toward the ultrasound element or a slower flow away from the ultrasound element. With further increasing flow velocity, the spectrum of the fifth flow velocity 24 appears in the ambiguity range of high or low flow velocity.

[0087] The solution presented here advantageously allows a resolution of such ambiguities. In principle, a comparatively rough range estimation can contribute to this purpose since the ultrasound method still works with high precision (resolution to 1-2 decimal places of the flow velocity in meters/second or of the volume flow in liters/minute), but ambiguity about the range of several meters/second or liters/minute is present.

[0088] FIG. 8 schematically shows another exemplary Doppler frequency spectrum. FIG. 8 illustrates the problem and the resolution of ambiguity once again using the example with the parameters used above f.sub.us=6 MHz, PRF=27.93 kHz, C.sub.blood=1540 m/s, and a windowing (window function) with a so-called Hamming window 25. The figure shows the frequency behavior at the following flow velocities: [0089] first flow velocity 20=−1 m/s, [0090] second flow velocity 21=+1 m/s, [0091] third flow velocity 22=2 m/s, [0092] fourth flow velocity 23=3 m/s, [0093] fifth flow velocity 24=4.5 m/s.

[0094] In this context, a negative velocity means blood flowing toward the ultrasound element and appears in a frequency shift with a positive sign.

[0095] This example shows that the measured frequency peaks are very close to one another at flow velocities of 1 m/s and 4.5 m/s. This ambiguity can be resolved by the (a priori) knowledge of the approximate velocity on the basis of the operating parameter of the flow machine.

[0096] This approximate velocity interval v.sub.int (plausible range of the flow velocity) can be resolved with the aid of the following formula to a corresponding Doppler shift or a Doppler shift interval fd.int.

[00001]$f_{d,\mathrm{int}}=\frac{2.Math.f_0}{c_{\mathrm{Blut}}}.Math.v_{\mathrm{int}}$

[0097] In the example, the corresponding Doppler shift interval is 31.95 kHz to 35.84 kHz. In order to shift the corresponding frequency interval into the frequency range that can be represented with the PRF used, the determined non-representable frequencies can be converted into the representable frequency range using the following formula (for positive flow velocities).

[00002]$f_{d,\mathrm{int},PRF}=\left(f_{d,\mathrm{int}}\mathrm{mod}PRF\right)-\frac{PRF}{2}$

[0098] For the values shown in the example, the frequency interval that can be predicted by means of the operating parameter thus includes all frequencies between −9.95 kHz and −6.05 kHz. All frequencies measured in this interval correspond to velocities in the range of 4.1 m/s to 4.6 m/s.

[0099] The exact velocity (actual flow velocity) can be determined by a calculation from the number of spectral “wraps” with the aid of the operating parameter interval (frequency interval that can be predicted by means of the operating parameter) and successive back calculation from the measured frequency using the formulas already shown. A “wrap” here refers to the jump of a signal from the greatest positive representable frequency (f.sub.pRF/2) to the representable negative frequency of highest magnitude (−f.sub.pRF/2). The true frequency is determined according to the formula

f.sub.d=nf.sub.PRF=f.sub.meas

[0100] where the parameter n denotes the number of spectral “wraps.” For low flow velocities, f.sub.d=f.sub.meas; at higher velocities, ambiguities occur with regard to the value of n, which can be resolved according to the solution proposed here by additional knowledge (the operating parameter(s) of the flow machine).

[0101] FIG. 9 schematically shows a functional illustration of a possible embodiment of the method presented here. The method in accordance with the illustration according to FIG. 9 serves to resolve the ambiguities. A PWD volume flow measurement 26 and a motor characteristic map-based volume flow measurement 27 take place in parallel or sequentially. The PWD volume flow measurement 26 can, for example, be carried out during step a). The motor characteristic map-based volume flow measurement 27 can be carried out, for example, between steps c) and d) or during step d). The PWD volume flow measurement 26 provides a Doppler spectrum 28. This can occur during step b), for example. The motor characteristic map-based volume flow measurement 27 provides an estimated (rough) fluid volume flow 4. This can occur, for example, between steps c) and d) or during step d). The Doppler spectrum 28 and the estimated fluid volume flow 4 are sent to an anti-aliasing unit 29. The anti-aliasing unit 29 determines from the estimated fluid volume flow 4 the (plausible) range in which the (actual) flow velocity is located, and from the Doppler spectrum 28 and the (plausible) volume flow range the corrected flow velocity 30, which is here also referred to as (actual) flow velocity through the support system. The anti-aliasing unit 29 can, for example, be a component of the processing unit also described here. A volume flow calculation unit 31 combines the known cross-sectional geometry and the known flow profiles determined in a construction type-specific and flow velocity-dependent manner for the (actual) fluid volume flow 5.

[0102] The PWD volume flow measurement 26 can comprise the following steps: [0103] sending out an ultrasonic pulse, [0104] waiting until the relevant echo of the measurement window, [0105] receiving the echo of the measurement window, [0106] possibly waiting for another waiting period until distant echoes die away, [0107] sending out the next ultrasonic pulse.

[0108] The generated data can be stored temporarily in a memory for later evaluation or (e.g., with parallel implementation in programmable logic) can be further processed directly. While the ultrasonic pulse from the desired measurement window arrives (time limitation), the received echo sequence with the known ultrasonic pulse frequency is generally demodulated (“downmixing into the baseband”). Subsequently, the obtained baseband signal is generally transformed into the frequency range (transformation from time to frequency range for calculating the Doppler spectrum).

[0109] The motor characteristic map-based volume flow measurement 27 (rough volume flow measurement) can comprise the following steps: [0110] determining a pump operating parameter, such as rotation rate (revolutions per minute, in short: RPM), power consumption, current consumption, and/or pressure difference across the flow machine (e.g., pump), [0111] calculating the estimated fluid volume flow via a relationship determined in a type-specific manner or via interpolation (e.g., table-based interpolation) from a motor characteristic map determined in a type-specific manner.

[0112] The volume flow calculation unit 31, for example, carries out the following: multiplication of the known cross section in the range of the observation window 16 (formula symbol: A) with the flow velocity 30 (formula symbol: v), and a flow velocity-dependent flow profile correction parameter (formula symbol f(v)). In this case, the (actual) fluid volume flow (formula symbol Q.sub.p) can result according to the following formula:

Q.sub.p=f(v)×v×A

[0113] The anti-aliasing unit 29 and the volume flow calculation unit 31 can also be combined into one unit. In addition, the Doppler spectrum can be mapped directly to the volume flow Q.sub.p, for example.

[0114] The solution presented here allows in particular one or more of the following advantages: [0115] highly accurate calculation of the pump volume flow by means of a Doppler ultrasonic sensor. [0116] combination of a high-precision Doppler ultrasound measurement and a rough estimation on the basis of motor operating parameters (e.g., one or more of rotational speed, current, power, built-up pressure) allows the operation of the ultrasound measurement with violation of the Nyquist theorem (necessity arising from geometric conditions) and a subsequent resolution of arisen ambiguities with the aid of the rough estimate.