Abstract:
Pulsatile blood flow in a heart-lung machine is accomplished by pumping blood into a bladder at a constant rate, and cyclically emptying the bladder into aortic line of the heart-lung machine either by means of the bladder&#39;s own elasticity or by a mechanical compression mechanism which can be programmed to simulate the human heartbeat.

Description:
This is a divisional application of prior application Srr. No. 08/643,123 filed on Apr. 30, 1996 now U.S. Pat. No. 5,916,191 and entitled PULSATILE FLOW GENERATION IN HEART-LUNG MACHINES. 
    
    
     FIELD OF INVENTION 
     This invention relates to heart-lung machines, and more particularly to a method and apparatus for simulating the natural heartbeat&#39;s pressure pattern in the blood output of the heart-lung machine. 
     BACKGROUND OF THE INVENTION 
     The natural human heart provides the body with a Pulsatile flow of blood corresponding to the filling and emptying (beating) of the various chambers of the heart. The instantaneous blood flow rate varies in a complex cyclical manner from near zero to some maximum rate, with the overall blood flow rate being a time weighted average. 
     The cardiopulmonary bypass circuits of heart-lung machines used in open-heart surgery typically utilize centrifugal or positive displacement (i.e. roller type) pumps to provide the motive power for circulation of the blood. These pumps provide an essentially constant flow rate of blood through the circuit at all times, the instantaneous rate and the average rate being nearly identical. 
     Medical studies have suggested that Pulsatile flow, being more physiologically correct than constant flow, may have a beneficial impact on the efficacy of the extracorporeal perfusion. This can result in improved patient outcomes following cardiac bypass surgery. 
     Various ways have been proposed to mimic in a heart-lung machine the natural Pulsatile flow of the heart, but none of them have so far been satisfactory. The simplest way of providing a pulsed flow is to cyclically clamp and unclamp the inlet or outlet line of the heart-lung machine&#39;s arterial pump. Clamping the pump inlet is not desirable since it can create very high suction pressures in the inlet which can damage the red blood cells, or in some cases even cause cavitation which can potentially release gas bubbles into the blood stream. Further, during the low flow or rest periods, the pump rotors spin on a stagnant volume of fluid, which may result in mechanical trauma to the blood cells. Clamping the pump outlet is not desirable in a centrifugal pump due to this mechanical trauma. Clamping the pump outlet is not desirable in a positive displacement pump since the rapid buildup of pressure in the lines can rupture the connections or tubing, potentially resulting in a catastrophic event. 
     A more acceptable way of creating Pulsatile flow is to vary the speed of the pump in a cyclical manner. This is easily accomplished electronically by the pump controller. However, the inertia of the spinning elements of the pump tends to render the resulting waveform more sinusoidal than the natural heartbeat waveform and forces the wave period to be longer than the natural period. In addition, the components of the bypass circuit downstream of the pump, such as the oxygenator and arterial filter, also damp the pulses due to their volumetric holdup. 
     SUMMARY OF THE INVENTION 
     The present invention creates a Pulsatile flow by causing a continously running pump to fill an elastic bladder with blood through a check valve. The bladder is connected to a blood outlet through an intermittently operable outlet valve. The outlet valve is triggered by the expansion and contraction of the elastic bladder and preferably operates with a hysteresis, i.e. it fully opens when the expansion of the bladder exceeds a predetermined volume, and closes fully when the contraction of the bladder reduces the volume below another, substantially smaller, volume. 
     In a first embodiment of the invention, a mechanical outlet valve with an appropriate toggle mechanism is directly connected to the bladder so as to snap open when the bladder is fully expanded, and to snap closed when it is nearly empty, the elasticity of the bladder driving the blood through the outlet valve. In a second embodiment, the expansion of the bladder trips a switch or sensor which causes a controller to open the outlet valve and actuate a pressure mechanism which mechanically compresses the bladder to eject the blood. Other known mechanisms with a similar action may also be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a heart lung machine using the invention; 
     FIGS. 2 a  through  2   d  are schematic views illustrating four phases in the operation of a first embodiment of the invention; 
     FIGS. 3 a  through  3   d  are schematic views illustrating the same four phases in the operation of a second embodiment of the invention; and 
     FIG. 4 is a schematic view illustrating an alternative version of the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates in simplified form, the general environment in which the invention is useful. During coronary bypass surgery, a heart-lung machine  10  diverts the blood of patient  12  from the vena cava  14  into a reservoir  16 . From there, a pump  18  pumps the blood through an oxygenator  20  and filter  21  back into the aorta  22  of patient  12 . 
     The pump  18  is typically a roller pump running at a substantially steady speed, which is desirable for the proper functioning of the oxygenator  20 . The Pulsatile action of the natural heart  24  can be simulated, an accordance with the present invention, by interposing between the oxygenator  20  and the aorta  22  a bladder assembly  26  shown in more detail in FIGS. 2 a  through  4 . 
     FIGS. 2 a  through  2   d  show the operation of a first preferred embodiment of the invention. An elastic bladder  28  receives blood at a steady rate from the oxygenator  20  of FIG.  1 . As the blood flows into bladder  28 , the bladder  28  expands and pressure builds up in it. 
     When the pressure reaches a predetermined amount, it forces the toggled outlet valve  30  open (arrows  32  in FIG. 2 b ) allowing the elastic bias of bladder  28  to propel a bolus of blood  33  into the aortic line  34  (FIG. 2 c ). Due to its toggle action, the outlet valve  30  remains open until the bladder  28  has essentially relaxed. The valve  30  then closes (arrows  36  in FIG. 2 d ), and the cycle repeats. It will be understood that the flow  33  in aortic line  34  during the phase illustrated in FIG. 2 c  is considerably greater than the flow being pumped into the bladder  28  by the pump  18 . 
     FIGS. 3 a-d  illustrate a more complex embodiment of the invention. In that embodiment, the bladder  28  is disposed on a fixed plate  38  and receives blood from oxygenator  20  through a check valve  40 . A servo-type valve  42  which may be electrically, pneumatically or mechanically controlled normally blocks the aortic line  34 . 
     When the bladder  28  expands from its rest condition of FIG. 3 a , it eventually reaches the position of FIG. 3 b  where it contacts a movable plate  44  which is actuated by a controller  46 . In the position of FIG. 3 b , an appropriate limit switch or sensor (not shown) activates the controller  46 . This causes the controller  46  to open the valve  42  and drive the movable plate  44  downward to squeeze the bladder  28  (arrow  48  in FIG. 3 b ). After the movable plate  44  travels a predetermined distance downward, the controller  46  shuts the outlet valve  42  and retracts the movable plate  44  to its original position (arrow  50  in FIG. 3 c ) to repeat the cycle (FIG. 3 d ). 
     The advantage of the embodiment of FIGS. 3 a-d  is that the rate of descent of the movable plate  44  is variable as desired by appropriately programming a microprocessor which controls whatever conventional mechanism drives the plate  44 . FIG. 4 illustrates such an arrangement. In FIG. 4, a pressure transducer  52  is provided in the aortic line  34 . A signal representative of the blood pressure in the aortic line  34  is applied to the microprocessor  54  which compares the sensed pressure in a feedback loop to a preprogrammed time-amplitude pattern and operates the plate drive  56  to follow the preprogrammed pattern. In this manner, the apparatus of FIG. 4 (unlike the apparatus of FIGS. 2 a-d  which is less expensive and more reliable but in which the blood output always follows a decreasing exponential curve) can be programmed to simulate the natural heartbeat as closely as the mechanical inertia of the mechanism of plate  44  will allow. 
     It is understood that the exemplary apparatus for producing Pulsatile blood flow in a heart-lung machine described herein and shown in the drawings represents only a presently preferred embodiment of the invention. Indeed, various modifications and additions may be made to such embodiment without departing from the spirit and scope of the invention. Thus, other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of difference applications.