Biomedical apparatus for pumping blood of a human or an animal patient through a secondary intra- or extracorporeal blood circuit

Abstract

A biomedical apparatus for pumping blood of a human or an animal patient through a secondary blood circuit is provided, including a blood pump (10), an inlet duct (11) and an outlet duct (12) for guiding blood of the patient to the blood pump (10) and back to the patient. The device further includes a measuring device (14) for measuring a physical parameter of the heart (2) or of a blood vessel, and a controller (13) for regulating the power of the blood pump (10). The measuring device (14) is adapted to be arranged inside the heart (2) or the blood vessel, and the controller is configured to determine an estimate for an inner volume of the heart (2) or of the blood vessel based on the physical parameter and is configured to regulate the power of the blood pump (10) depending on this estimate.

Claims

1. A biomedical apparatus for pumping blood of a human or an animal patient through a secondary intra- or extracorporeal blood circuit, comprising a blood pump for pumping blood, an inlet duct connected to the blood pump, for being inserted into a patient's circulatory system, in order to guide blood of the patient to the blood pump, an outlet duct connected to the blood pump, for being inserted into the patient's circulatory system, in order to guide blood from the blood pump back to the patient's circulatory system, a measuring device for measuring at least one physical parameter of the heart or of a blood vessel of the patient, the measuring device being adapted to be arranged inside the heart or the blood vessel and being adapted to send out an electromagnetic or a mechanical wave, and a controller being configured to determine an estimate for an end-diastolic inner volume of the heart based on the measured physical parameter and being configured to regulate the power of the blood pump based on the following linear function:
PW.sub.des(t)=(EDV(t)EDV.sub.0).Math.k.sub.prsw, in which PW.sub.des(t) denotes the desired pump work per heartbeat at a certain time t and EDV(t) the estimate for the end-diastolic volume of the heart determined based on the measured physical parameter, and in which the two parameters EDV.sub.0 and k.sub.prsw denote the end-diastolic volume at which the desired pump work is zero and the gain of the pump work relative to the estimated EDV(t), respectively.

2. The biomedical apparatus as claimed in claim 1, wherein the measuring device is arranged on the inlet duct.

3. The biomedical apparatus as claimed in claim 1, wherein the biomedical apparatus is a Ventricular Assist Device (VAD), a heart-lung machine or an extracorporeal membrane oxygenation (ECMO) apparatus.

4. The biomedical apparatus as claimed in claim 1, wherein the physical parameter is a distance between the measuring device and a ventricular wall of the heart or between the measuring device and a vascular wall of the blood vessel.

5. The biomedical apparatus as claimed in claim 4, wherein the measuring device is adapted for measuring a plurality of distances between the measuring device and the ventricular wall or between the measuring device and the vascular wall in differing directions.

6. The biomedical apparatus as claimed in claim 1, wherein the measuring device is an ultrasound measuring device suited for carrying out ultrasound measurements.

7. The biomedical apparatus as claimed in claim 1, wherein k.sub.prsw is in the range of 0.003 J/ml to 0.02 J/ml.

8. The biomedical apparatus as claimed in claim 7, wherein k.sub.prsw is in the range of 0.006 J/ml to 0.012 J/ml.

9. The biomedical apparatus as claimed in claim 1, wherein EDV.sub.0 is in the range of 10 ml to 150 ml.

10. The biomedical apparatus as claimed in claim 9, wherein EDV.sub.0 is in the range of 25 ml to 90 ml.

11. The biomedical apparatus as claimed in claim 1, wherein the controller is configured to determine an estimate for the heart rate of the patient based on the measured physical parameter.

12. The biomedical apparatus as claimed in claim 1, wherein the controller is configured to detect an asystolic cardiac motion and to regulate the power of the blood pump according to a special mode, if an asystolic cardiac motion has been detected.

13. A method for operating a biomedical apparatus for pumping blood of a human or an animal patient through a secondary intra- or extracorporeal blood circuit, wherein the biomedical apparatus comprises a blood pump for pumping blood, an inlet duct connected to the blood pump, for being inserted into a patient's circulatory system, in order to guide blood of the patient to the blood pump, and an outlet duct connected to the blood pump, for being inserted into the patient's circulatory system, in order to guide blood from the blood pump back to the patient's circulatory system, and wherein the method comprises at least the following steps: measuring at least one physical parameter of the heart or of a blood vessel of the patient by means of electromagnetic or mechanical waves sent out by a measuring device arranged inside the heart or the blood vessel, determining an estimate for an end-diastolic inner volume of the heart based on the measured physical parameter, and regulating the power of the blood pump based on the following linear function:
PW.sub.des(t)=(EDV(t)EDV.sub.0).Math.k.sub.prsw, in which PW.sub.des(t) denotes the desired pump work per heartbeat at a certain time t and EDV(t) the estimate for the end-diastolic volume of the heart determined based on the measured physical parameter, and in which the two parameters EDV.sub.0 and k.sub.prsw denote the end-diastolic volume at which the desired pump work is zero and the gain of the pump work relative to the estimated EDV(t), respectively.

14. The method as claimed in claim 13, wherein the physical parameter is measured by measuring at least one distance between the measuring device and a ventricular wall of the heart or between the measuring device and a vascular wall of the blood vessel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the invention are described in the following with reference to the drawings, which only serve for illustration purposes, but have no limiting effects. In the drawings it is shown:

(2) FIG. 1 a schematic view of a Ventricular Assist Device (VAD) according to the invention, implanted in the heart of a patient;

(3) FIG. 2 a graph illustrating the preload recruitable stroke work (PRSW): stroke work as a function of end diastolic volume;

(4) FIG. 3 a schematic representation of the main control structure of the controller of a VAD according to the invention;

(5) FIG. 4a a graph illustrating the raw and the filtered signals of the left ventricular volume (LVV) as well as a linear fit to the filtered LVV signal;

(6) FIG. 4b a graph illustrating the zeroed LVV signal; and

(7) FIG. 5 a graph illustrating the pump-power lookup-table (PPLUT): pump speed as a function of pump power.

DESCRIPTION OF PREFERRED EMBODIMENTS

(8) In FIG. 1, an inventive embodiment of a biomedical apparatus for pumping blood of a human or an animal patient through a secondary intra- or extracorporeal blood circuit is shown. The apparatus of this embodiment is a Ventricular Assist Device (VAD) 1 used to partially or completely replace the function of a heart 2 of a patient with heart failure.

(9) The VAD 1 comprises a blood pump 10, which can be a pneumatically or electrically actuated pulsatile volumetric pump, or an axial or centrifugal turbodynamic pump with classical contact bearings, with a magnetically levitated rotor or with a blood-immersed bearing. A large variety of pumps of these kinds and suited for being used in a VAD are known to the person skilled in the art.

(10) Connected to the blood pump 10 is an inlet duct or, here, an inlet cannula 11, which has a free end being inserted into the left ventricle 20 in the region of the apex of the heart 2. The inlet cannula 11 serves to guide blood from the inside of the left ventricle 20 to the blood pump 10. Due to the pumping action of the blood pump 10 the blood is drawn through an inlet opening located at the free end of the inlet cannula 11 into the inlet cannula 11 and to the blood pump 10.

(11) In direction of the blood stream, an outlet duct or, here, an outlet cannula 12 is connected to the blood pump 10 on the opposite side relative to the inlet cannula 11. The outlet cannula 12 serves to guide the blood from the blood pump 10 back to the patient's circulatory system. To this end, the outlet cannula 12 is inserted into the aorta 3 of the patient.

(12) The inlet cannula 11, the blood pump 10 and the outlet cannula 12 together constitute a secondary blood circuit, which is preferably located completely inside the body of the patient. The blood streaming through this secondary blood circuit originates from the left ventricle 20 and streams into the aorta 3. Within the secondary blood circuit, the blood is pumped by the blood pump 10 in the direction towards the aorta 3. Thus, the blood pump 10 supports the function of the (failed) heart 2.

(13) The output power by which the blood is pumped into the outlet cannula 12 by the blood pump 10 is regulated by a controller 13 which is able to communicate with a measuring device 14 attached to the inlet cannula 11 and the blood pump 10. The controller 13 can particularly be represented by an integral circuit and preferably comprises at least one data storage module. The regulation of the power of the blood pump 10 by the controller 13 is based on the determination of an estimate for the inner volume of the left ventricle 20 at end-diastole. In order to obtain an estimate of the inner ventricular volume for regulating the power of the blood pump 10, physical parameters in the form of a plurality of distances between the free end of the inlet cannula 11 and the inner surface of the ventricular wall 21 are measured in differing directions. These measurements are carried out by a plurality of ultrasound transceivers 14 being arranged on the free end of the inlet cannula 11.

(14) The ultrasound transceivers 14 are adapted both to send out ultrasound waves and to receive ultrasound waves. In order to measure a distance between an ultrasound transceiver 14 and the inner surface of the ventricular wall 21, the ultrasound transceiver 14 sends out an ultrasound wave (see arrows in FIG. 1), which is reflected on the tissue boundary at the surface of the ventricular wall 21. The reflected ultrasound wave is received by the same (and optionally also by other) ultrasound transceiver(s) 14. The time period measured for the ultrasound wave to travel from the transceiver 14 to the ventricular wall 21 and back to the transceiver 14 is used to calculate the distance between the transceiver 14 and the inner surface of the ventricular wall 21. This calculation is usually carried out in the controller 13. From the different measurements of the distances between the inlet cannula 11 and the ventricular wall 21 in differing directions, an estimate for the actual left ventricular volume LVV at the time of the measurement is calculated by the controller 13.

(15) The estimate for the left ventricular volume is used to determine an estimate for the end-diastolic volume (EDV) of the heart 2. Based on the estimate for the end-diastolic volume, the desired stroke work (PW.sub.des) of the blood pump 10 per heartbeat is calculated by the controller 13 according to the concept of preload recruitable stroke work (PRSW), as illustrated in FIG. 2. Thus, the regulation of the power of the blood pump 10 by means of the controller 13 is based on the theory of venous return (Guyton A C, Hall J E. Textbook of Medical Physiology, 12th Edition. Saunders W B Co, 2010.) and the Frank-Starling law of the heart. FIG. 2 shows a graphical interpretation of the PRSW. This model states that the stroke work of the left ventricle increases linearly with the end-diastolic volume EDV. The PRSW has been evaluated in a group of 60 patients with an average age of 59.48.7 years in: Takeuchi M, Odake M, Takaoka H, Hayashi Y, Yokoyama M. Comparison between preload recruitable stroke work and the end-systolic pressure-volume relationship in man. Eur Heart J 1992; 13:80-84. In baseline, the PRSW curves of these patients are described by the slope k.sub.prsw=0.008980.00299 J/ml and the x-axis intercept EDV.sub.0=38.027.0 ml. An increased contractility of the LV increases the slope of the PRSW. Since the output (stroke work) of the heart is proportional to the input (end-diastolic volume EDV), the PRSW can be viewed as a proportional controller of the heart. Hence, the controller 13 imitates the PRSW by proportionally adjusting the pump power based on the determined EDV.

(16) As shown in FIG. 3, in which the main control structure of the controller 13 is shown, the input of the main control structure of the controller 13 is the estimate for the actual left ventricular volume LVV; the output is the desired pump speed N.sub.des, which in turn is transmitted to a pump speed controller of the blood pump 10. The control structure according to the present embodiment is implemented as a discrete-time system with a sampling frequency of 100 Hz. The main control structure of the controller 13 can be subdivided into three stages 1)-3). In the first stage 1), signal processing algorithms are used to determine the end-diastolic volume EDV and the heart rate HR. In the second stage 2), the determined end-diastolic volume EDV and heart rate HR are used to calculate a desired pump power PP.sub.des. This part of the control structure imitates the PRSW. The third stage 3) of the algorithm uses the desired power to compute a desired pump speed that approximately achieves the desired power.

(17) The first stage 1) of the control structure takes the left ventricular volume LVV signal as an input and computes the end-diastolic volume EDV and the heart rate HR. FIGS. 4a and 4b illustrate the signal processing procedure carried out in this stage 1) for a simulated left ventricular volume LVV signal with subsequently added zero-mean white noise (=5 ml) and a sampling time of 10 ms. First, the left ventricular volume signal (LVV raw) is low-pass filtered using a second-order IIR filter. The resulting signal is denoted with LVV filt. in FIG. 4a. Then, using a sliding window, the last 3 s of the LVV signal are acquired at every time step. FIG. 4a shows the 3 s window with the raw and the filtered LVV signals. The instantaneous HR and EDV signals are calculated from this window as follows: First, an affine function is fitted to the filtered LVV signal using the least-squares method (linear fit in FIG. 4a). This affine function represents the offset and a linear trend of the LVV signal. Second, this affine function is subtracted from the filtered signal, which yields a zero-mean sine-like signal as shown in FIG. 4b. Third, all zero crossings and the direction of their crossing are detected. Fourth, the time intervals between all subsequent downward crossings and all subsequent upward crossings are calculated. Fifth, the heart rate HR is calculated by dividing 60 by the median of all previously calculated time intervals. And sixth, the estimate for the end-diastolic volume EDV is calculated by taking the maximum value of the filtered LVV signal between the last downward crossing and the last preceding upward crossing as indicated in FIG. 4a. All six steps of the algorithm are repeated at every time-step of the controller 13.

(18) If the heart is asystolic, the HR and EDV detection described above is not valid, since the LVV signal does not have a sine-like shape. Such a case is detected by the signal processing procedure, and an alternative algorithm is then used by the controller 13 for the HR and EDV detection. The detection of an asystolic heart is based on the pulsatility of the LVV signal. This implementation allows the controller 13 to be used in patients without a regular or an undetectable heart rate.

(19) The second stage 2) of the main control structure of the controller 13, as shown in FIG. 3, consists of one part describing the preload recruitable stroke work PRSW and an additional part to include the heart rate HR. The PRSW is implemented as a linear function of the EDV:
PW.sub.des(t)=(EDV(t)EDV.sub.0).Math.k.sub.prsw,
where EDV.sub.0 is the x-axis intercept and k.sub.prsw is the gain of the stroke work with respect to the EDV according to the PRSW (see FIG. 2). This function calculates the desired pump work per heartbeat PW.sub.des(t) at a time t. The two parameters EDV.sub.0 and k.sub.prsw can be adapted by the physician, by trained medical personnel or by the patient. The desired pump power PP.sub.des is calculated by:
PP.sub.des(t)=PW.sub.des(t).Math.HR(t)/60.

(20) The third stage 3) of the main control algorithm, as shown in FIG. 3, consists of a low-pass filter and a lookup-table. The low-pass element is a first-order IIR filter with a cut-off frequency of 1 rad/s. The pump-power lookup-table (PPLUT) is a mapping between the hydraulic pump power and the pump speed. The actual power delivered by the pump is not only a function of the pump speed, but also the resistance against the pump flow and the fluid viscosity, but for the implementation in the controller 13, these dependencies are neglected and the pump speed directly mapped to a pump power. FIG. 5 shows the pump-power lookup-table PPLUT implemented in the controller 13.

(21) The presented controller 13 according to the embodiment as illustrated FIGS. 1-5 adapts the cardiac output very similar to the physiological circulation, while effectively preventing overpumping and underpumping. The controller 13 shows a high sensitivity towards preload changes and a low sensitivity to afterload changes. The pump speed adaptation with regard to the blood pump 10 is fast and keeps the left ventricular volume in a safe range. The main control structure of the controller 13 is very simple and only two parameters need to be adapted, namely k.sub.prsw and EDV.sub.0. The adaptation of these two parameters could be made by the physician, trained medical personnel, or the patient in order to find a comfortable level of perfusion. As long as k.sub.prsw is kept between certain limits, the controller 13 retains its safe operation.

(22) The invention is of course not limited to the preceding presented embodiment and a plurality of modifications is possible. For example, the controller 13 with the main control structure as presented above could also be used for controlling the output power of a blood pump in an extracorporeal membrane oxygenation (ECMO) apparatus, a heart-lung machine or a dialysis apparatus. The measurement of the distances does not necessarily be carried out by means of ultrasound transceivers. Optical measuring devices, such as a laser, or an electrical conductance measuring device, such as an admittance or conductance catheter, could also be used for this purpose. Instead of determining an estimate for the inner volume of the left ventricle, it would also be conceivable to determine an estimate for the inner volume of the right ventricle, the left or the right atrium or of a certain section of a blood vessel. The inlet and the outlet cannulas do not necessarily be inserted into in the left ventricle 20 and the aorta 3, respectively, but could be inserted at any other location of the circulatory system, depending on the concrete purpose of the biomedical apparatus. It would also be conceivable to construct the measuring device as a separate part with respect to the inlet cannula and to even arrange the measuring device at a different location in the circulatory system than the inlet cannula. For example, the inlet cannula could be inserted into the aorta 3 or into any other blood vessel and the measuring device could be located in the left ventricle. It can be imagined that the controller 13 is used as the low-level controller in a hierarchical control system. A high-level control system could then influence the k.sub.prsw of the controller and influence the controller gain, while maintaining the safe operation of the controller. Such a high level controller could be based on an additional measurement, e.g. a measurement of the patient movement (acceleration sensor) or the blood oxygen concentration. A plurality of further modifications is possible.

(23) TABLE-US-00001 REFERENCE NUMERALS 1 Ventricular Assist Device 14 Ultrasound transceivers (VAD) 2 Heart 10 Blood pump 20 Left ventricle 11 Inlet cannula 21 Ventricular wall 12 Outlet cannula 3 Aorta 13 Controller