Abstract:
Retrograde flow in an extracorporeal blood circuit using a centrifugal blood pump is prevented by passing the blood line through a normally-closed powered occlusion clamp, and opening the clamp in response to powering of the pump or, preferably, in response to the sensing of forward blood flow in the extracorporeal circuit.

Description:
FIELD OF THE INVENTION 
     This invention relates to heart-lung machines, and more particularly to an automatic occlusion clamp that prevents retrograde blood flow through a centrifugal blood pump. 
     BACKGROUND OF THE INVENTION 
     In a typical heart-lung machine of the type used in open-heart surgery, an extracorporeal blood circuit is established from the patient&#39;s venous system through a venous reservoir, a blood pump and an oxygenator to the patient&#39;s arterial system. This circuit takes over the function of the patient&#39;s heart and lungs while the patient&#39;s heart is stopped for the surgery. 
     The blood pump in the extracorporeal circuit is typically either a roller pump or a centrifugal pump. An advantage of the roller pump is that the blood line is always occluded at some point in the pump, so that there can be no retrograde blood flow in the circuit when the pump is stopped. A disadvantage is that a roller pump can convey air boluses through the line. Such air boluses need to be dealt with separately in order to avoid serious injury or death of the patient. 
     Centrifugal pumps, on the other hand, do not convey air boluses through the circuit. If an air bolus is applied to the intake of a centrifugal pump, the pump deprimes and needs to be reprimed and restarted. Because of this safety factor, centrifugal pumps are the preferred choice of many perfusionists. 
     A problem with centrifugal pumps is that they do not occlude the line, and that consequently, retrograde blood flow through the pump is possible when the pump is stopped, due to the pressure head in the patient&#39;s arterial system. Such a retrograde flow drains blood from the patient and is medically unacceptable. 
     Retrograde flow through a centrifugal pump has been averted in the prior art by inserting into the extracorporeal circuit a one-way duckbill valve. The use of such a valve, though effective, does have two disadvantages: first, the shape of a duckbill valve causes turbulence at the exit end of the valve and thereby causes some hemolysis; and second, the duckbill valve, because it is part of the extracorporeal circuit, is a single-use item and therefore adds to the cost of patient disposables. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described deficiencies of the prior art by providing an electrically or pneumatically operated occlusion clamp external to the blood line that is arranged to occlude the line, preferably between the centrifugal pump and the oxygenator, whenever the forward blood flow in the line essentially ceases. The clamp of this invention is designed to operate in a fail-safe mode, i.e. it will clamp the line shut in the event of a power failure. In combination with conventional devices that shut down the pump in the event of a low blood level condition in the venous reservoir and/or of the detection of an air bolus, the present invention provides a triple-action safety system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an extracorporeal circuit using the invention; 
     FIG. 2 is a partial block diagram of the perfusionist&#39;s console; and 
     FIG. 3 is a schematic diagram of the clamp of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates the environment in which the invention is useful. A patient  10  undergoing open heart surgery is connected to the extracorporeal circuit  12  by a venous line  14  which discharges venous blood by gravity into a venous reservoir  16 . Filtered and defoamed cardiotomy blood may also be introduced into the reservoir  16  by appropriate equipment (not shown). The venous reservoir  16  may be equipped with a conventional level sensor  15 . 
     Blood is pumped by a pump  17  from the venous reservoir  16  into the oxygenator  18  and the arterial line  20  connected to the patient  10 . A conventional air sensor  22  may be connected in the line  24  between the reservoir  16  and the pump  17 . The level sensor  15  and the air sensor  22  provide conventional inputs to the perfusionist&#39;s control console  26 . A conventional flow sensor  27  preferably connected between the pump  17  and the oxygenator  18  also provides an input to the console  26 . 
     Although the blood pump  17  may be of any suitable type, the present invention is useful specifically with centrifugal pumps. These pumps have certain advantages that make them desirable in extracorporeal blood circuits, but they permit retrograde blood flow through the pump when the pump is stopped, and they can be deprimed by an air bolus in the blood line. 
     A problem thus arises when a centrifugal pump  17  is stopped during open-heart surgery. This occurs from time to time, for example when the surgeon wants to stop blood flow into the surgical field for a few seconds to enhance visibility. At such times, the pressure head of about 200-300 mmHg in the patient&#39;s arterial system drives oxygenated blood backward through the extracorporeal circuit  12  unless it is restrained from doing so. 
     With a roller pump, this restraint is automatic because a roller pump inherently occludes the line when it is stopped. With a centrifugal pump, the prior art typically restrained retrograde blood flow by inserting a one-way valve, such as a duckbill valve, in the extracorporeal line between the pump  17  and the oxygenator  18 . The disadvantage of this approach was, first, that the valve, being part of the blood path in the extracorporeal circuit  12 , had to be disposable; and second, that the valve created turbulence at its outlet and thereby caused some hemolysis. 
     In accordance with the invention, the in-line duckbill valve of the prior art is replaced by a mechanical clamp  30  external of the flexible tubing which constitutes the blood line  32 . The clamp  30 , when actuated, is arranged to pinch or clamp the line  32  so as to preclude blood flow therethrough. The clamp  30 , shown in more detail in FIG. 3, is preferably operated electrically but may be operated hydraulically or pneumatically. As shown in FIG. 3, the clamp  30  basically consists of a plunger  34  which carries a clamping head  36 . The clamping head  36  is biased toward a platen  38  by a spring  40 . The blood line  32  is positioned between the head  36  and the platen  38 , so that the spring  40  biases the head  36  into the position of FIG. 3 in which it occludes the flexible tubing of line  32 . 
     In the preferred embodiment, the plunger  34  is mounted in a solenoid  42  so that the head  36  is lifted off the line  32  whenever the solenoid  42  is energized. The clamp  30  is thus fail-safe in that it occludes the line  32  in the event of a power failure. Also, the positive action of the clamp  30  makes it possible, by appropriate control circuitry, to occlude the line  32  under selectable conditions other than the onset of retrograde blood flow. 
     The functioning of the inventive apparatus is shown schematically in FIG.  2 . The perfusionist&#39;s control console  26  contains the pump  17  and a pump control  50  which responds to signals from the low level shutoff circuit  52 , the air sensor  22 , and the low flow shutoff circuit  54 , as well as to manual commands from the operator keyboard  56 . A level indicator  58 , a flow indicator  60 , and an air warning light  62  are provided. 
     When the pump  17  is to be started, appropriate commands are entered on the keyboard  56 . These commands set the pump speed and momentarily override the low flow shutoff circuit  54  so that the pump  17  can start and the clamp  30  releases the blood line  32 . As the flow sensor  27  now senses blood flow, the low flow shutoff  54  is deactivated, and the clamp  30  remains retracted and clear of the line  32 . 
     If positive blood flow in the line  32  now ceases, either because the pump  17  has been stopped by the perfusionist or by the action of low level shutoff  52  or air sensor  22 , or because a blockage has occurred downstream, the low flow shutoff circuit  54  becomes activated and cuts the power to both the pump (if it is still on) and to the solenoid  42 . This causes the clamp head  36  to move against the platen  38  under the action of spring  40 . Thus, the line  32  is squeezed between the head  36  and the platen  38 , and the line  32  is occluded against retrograde blood flow into the pump  17 . 
     Because the components of clamp  30  are entirely outside of the blood path in line  32 , the clamp  30  need not be disposable. Furthermore, while the pump  17  operates, clamp  30  does not constrict nor otherwise affect the flexible line  32 , so that the blood path remains clear and free of hemolysis-promoting obstructions. 
     It is understood that the exemplary automated occlusion clamp for use with centrifugal blood pumps 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 different applications.