Abstract:
A blood pump is temporarily operated at a low rotational speed lying below a design rotational speed. This involves a risk of thrombogenesis since flow detachments may occur at blades of an impeller of the rotary blood pump. For eliminating deposits at said impeller, the rotational speed of said pump is temporarily increased to the design rotational speed. Alternatively, said pump alternately operates at said design rotational speed and a low rotational speed, and this pulsed operation is synchronized with the heart rate.

Description:
RELATED APPLICATIONS 
     This application is a U.S. national phase of PCT/EP06/061582, filed Apr. 13, 2006, which claims priority from German Application No. 102005017546.5, filed Apr. 16, 2005. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for controlling the rotational speed of a rotary blood pump having a design rotational speed at which minimal detachment and swirling of the blood flow occur. 
     In EP 0 961 621 B1 (Impella) an intravascular blood pump is described which is inserted through the vascular system into the body and placed in the heart or at any other location where blood is intended to be pumped. The blood pump comprises a housing with a diameter of 5.4 to 6.4 mm which defines the stator of a motor. A rotor is connected with an impeller which rotates in a pump ring. This very small blood pump has a relatively high rotational speed in the range of 30,000 rpm. 
     In EP 0 925 080 B1 (Impella) a method for controlling a rotational speed of a blood pump is described, wherein the blood pump comprises pressure sensors for determining the pressure difference prevailing at the blood pump. The rotational speed of a motor is controlled in accordance with the signals supplied by the pressure sensors. Thus, a desired volume flow is generated which ensures the desired pumping action. 
     Continuously delivering rotary pumps have a so-called design rotational speed. The impeller advancing the blood is shaped such that at the design rotational speed flow detachments at the impeller blades are minimized, or, ideally, no flow detachments occur at all. Thus, a low-turbulent flow is obtained. Flow detachments and turbulences involve the risk of thrombogenesis, wherein blood accumulates and gathers at the impeller or at other locations in the blood system. This impairs the operation of the impeller. Further, this involves the risk of obstructions in the blood system. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for controlling a rotational speed of a rotary blood pump, said method permanently ensuring highly efficient blood conveyance, and further allowing the blood pump to be temporarily operated below the design rotational speed. 
     The method according to the invention comprises the features of claim  1 . Accordingly, the blood pump is operated at alternating rotational speeds, wherein the blood pump is operated alternately at a low rotational speed and at the design speed. 
     The invention is based on the fact that a thrombogenesis at the impeller can be prevented by at least temporarily operating the blood pump at the design rotational speed. However, it is also possible to operate the blood pump at a lower rotational speed without any risk involved. By increasing the rotational speed to the design rotational speed after a given operational period, a beginning thrombogenesis at the impeller is eliminated. It has turned out that a short-time operation at the design rotational speed is sufficient for removing blood clots from the impeller such that a buildup of blood clots is prevented. The invention allows for selecting a volume flow and/or delivery rate below the design rotational speed, and to vary said volume flow and/or delivery rate, if necessary, without a continuous operation at the high design rotational speed being required. 
     An increase from the low rotational speed to the design rotational speed is preferably performed at an acceleration of more than 3,000 s −2 . Thus, a rapid short-time increase in the rotational speed of the pump is attained. Such an increase is possible due to the small mass of the blood pump which is configured as an intravascular or paracardiac blood pump. 
     The rotational speed of the blood pump is preferably controlled by presetting an excitation frequency. The rotor is a slip-ringless direct current motor provided with cyclically excited coils in the stator and permanent magnets in the rotor. A controller arranged remotely from the rotor supplies an operating frequency for the motor. The motor is connected with the controller via an electrical conduit which extends through a flexible catheter. 
     The deceleration of the rotational speed requires a supply of energy. Said deceleration preferably takes place by a non-braked natural slowdown (damping performed by the driving impeller), wherein the deceleration also takes place within more than 3,000 s −2  and thus within a very short time. 
     The invention allows the rotary blood pump to be operated according to a first variant of the inventive method, wherein pulses are periodically generated at the design rotational speed, and according to a second variant, wherein acceleration and deceleration are externally controlled. 
     In the first variant, the delivery rate is mainly determined by the low rotational speed, and the delivery volume is only marginally affected by the short-time increases to the design rotational speed. Consequently, the low rotational speed can be set according to a patient&#39;s need. When the heart of the patient has recovered due to the support provided by the blood pump and needs less support, a corresponding low rotational speed of the blood pump can be selected, wherein the thrombogenesis is prevented by short-time spike pulses which raise the rotational speed to the design rotational speed. These increases in the rotational speed only marginally affect the overall delivery rate of the blood pump. 
     In the second variant, the blood pump, which, in fact, is designed for continuous delivery, can be used for a pulsatile support of the heart. The pumping function of the blood pump is superimposed by the natural pumping function of the heart. This results in a periodically changing flow, wherein the periodic change is induced by the pumping function of the heart which temporarily supports the action of the blood pump, namely during the systole, and temporarily restrains the action of the blood pump, namely during the diastole. The invention allows, by monitoring of the motor current, the periods of high rotational speeds of the pump and the periods of low rotational speeds of the pump to be synchronized with the pumping function of the heart, and/or to be triggered in an out-of-phase relationship to the heart function without an ECG lead being required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is in particular suitable for right ventricular support, where the intake opening of the pump is placed in the right atrium, while the pump delivers into the pulmonary artery. 
       Embodiments of the invention will now be described in greater detail with reference to the drawings in which: 
         FIG. 1  shows a schematic representation of the configuration of the blood pump, 
         FIG. 2  shows, on an enlarged scale, the flow conditions at a blade of the pump impeller, 
         FIG. 3  shows the time profile of the pump rotational speed at periodic control, 
         FIG. 4  shows the time profile of the pump rotational speed at a pump operation synchronized with the heart function, and 
         FIG. 5  shows a schematic representation of the pump drive unit and the sensor for determining the motor current. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a rotary blood pump  10  for supporting the heart function. Here, the blood pump is an intravascular pump whose maximum outer diameter is at no place larger than 7 mm. The blood pump may also be configured as an intracardiac blood pump which is inserted through an incision of the heart. Such a blood pump may have a somewhat larger diameter. Further, the pump may be placed paracardiacally (around the heart) and take blood via two incisions from a location in front of the heart or from the heart, and return the blood via the second incision behind the heart. 
     The blood pump  10  comprises an elongate cylindrical motor  11  whose proximal end has connected therewith a flexible catheter  12 . Electrical conduits (not shown) extend through the catheter  12  to a controller. 
     A shaft  13  of the motor  11  carries an impeller  14  with a hub  15  tapering towards a distal end thereof, with helical blades  16  extending from said hub  15 . 
     The impeller  14  is surrounded by an elongate ring  17  which has approximately the same outer diameter as the motor  11 . Between the motor  11  and the ring  17  outflow openings  18  are arranged. When rotating, the impeller  14  axially takes in blood and conveys the blood towards the motor  11 . The blood leaves the blood pump through the openings  18  so as to subsequently flow along the motor  11 . The pump shown in  FIG. 1  may further have a fluidic design which allows a reverse direction of flow to be obtained. 
       FIG. 2  shows a schematic enlarged representation of a portion of the impeller  14 . A blade  16  extends from the hub  15 , said blade  16  being bent like a wing of an aircraft. The flow generates a positive pressure at the concave inside  16   a , and a vacuum or negative pressure is generated along the convex upper side  16   b . The blades  16  are configured such that at a design rotational speed a detachment-free flow along the blades is obtained. In the present embodiment, the design rotational speed is 30,000 rpm. At lower rotational speeds flow detachments and swirls  19  occur which may damage the blood due to thrombogenesis. Blood clots grow on the surface of the blade  16  and affect the blood flow, thereby promoting the thrombogenesis. 
       FIG. 3  shows a diagram of a time profile of the rotational speed n of the motor  11 , wherein along the abscissa the time t is plotted, and along the ordinate the rotational speed n per minute is plotted. The rotational speed can be varied by correspondingly adjusting the controller. In the present embodiment it is assumed that the rotational speed generally is 10,000 rpm. This value exemplifies a delivery rate required by a patient for supporting the heart function during a recovery phase. 
     The continuous rotational speed of 10,000 rpm is interrupted by periodically generated pulses I, wherein during the pulse duration the rotational speed temporarily increases to the design rotational speed of 30,000 rpm. The rising edge A 1  of the pulse I has a slope of more than 3,000 s −2 . Here, the rotational speed of the motor continuously increases from the low value of 10,000 rpm to the design rotational speed of 30,000 rpm. This rise in frequency is produced by the controller. When the design rotational speed has been reached, the rotational speed slows down in a trailing edge A 2 . Said slowdown takes place without supply of braking energy or any other external energy just by non-supply of driving energy. The slowdown, too, takes place with a delay of more than 3,000 s −2 . The slowdown is terminated when the rotational speed has returned to the lower value which is subsequently maintained. In the present embodiment, the duration of the pulse I is approximately 100 ms, and the pulse cycle time, i.e. the duration of a pulse and a subsequent interpulse period, is approximately 1,000 ms. 
     The pulses I are spike pulses with a small pulse width relative to the pulse cycle time. Therefore, the additional delivery rate produced by the pulses I is small relative to the basic delivery rate at 10,000 rpm. Thus, the pulses I influence said basic delivery rate only to a small extent. The pulses I have the effect that despite a small flow thrombogenesis at the impeller is prevented even over a period of several days. 
       FIG. 4  shows a second variant of the method according to the invention. In this variant, too, the rotational speed n is temporarily increased by pulses E, but said pulses E are substantially wider than the pulses I of the first method variant. The pulses E, during whose duration the rotational speed is temporarily increased, are produced synchronously with the heart rate, but out of phase relative thereto.  FIG. 5  shows the circuitry of the motor  11  and a controller  30  which supplies the frequency and the current for the motor  11 . The controller  30  is connected with the motor via a conduit  32 . A sensor  33  measures the current flowing in the conduit  32  and communicates the respective value to the controller  30 . The motor current depends on a load of the motor. The load changes over time in accordance with the natural pumping action of the heart which is superimposed on the continuous pumping action of the pump  10 . From the time profile of the current measured by the sensor  33  the timing for the generation of the pulses E of  FIG. 4  can be derived. Thus, an ECG lead comprising additional measuring electrodes is not needed. The pump does not require any external sensors. 
     As shown in  FIG. 4 , in this variant, too, the pump is operated such that either a low rotational speed n or the design rotational speed is set. Here, too, the acceleration of the pump and the deceleration take place within more than 3,000 s −2 . In this pump operation variant, the rotary blood pump generates a pulsating pump operation which is attuned to the natural pumping frequency of the heart.