Abstract:
A method and apparatus in which the blood flow in a fistula is determined by comparing the property of blood entering the fistula to the property of blood being withdrawn from the fistula as the blood in the fistula flows in the direction which is the reverse of the predominant flow. A property marker fluid is injected into the blood entering the fistula in order to alter the property of the blood. Aseptic conditions are maintained during the reversal of the blood flow by maintaining a closed blood loop exterior to the patient. The blood loop closure is maintained by reversing the action of the pump which induces blood flow in the closed blood loop.

Description:
FIELD OF THE INVENTION 
     This invention relates to measurement of recirculation rate and flow rate in a fistula. More particularly, this invention relates to measurement of the fistula fluid flow rate and recirculation rate during a medical procedure without opening a closed aseptic blood loop. 
     BACKGROUND OF THE INVENTION 
     In many medical situations it is desirable to quantitatively determine, or measure, the recirculation rate or the flow rate of a biological or medical fluid to increase the benefits of a therapeutic treatment, or alternatively, to decrease the time required for the treatment. This information is also useful for diagnostic purposes. For example, hemodialysis (herein “dialysis”) is an uncomfortable medical procedure. It is, therefore, widely recognized as desirable for therapy to minimize the amount of time required to complete the dialysis procedure while still achieving a desired level of treatment. 
     In dialysis, a joint is typically surgically created between a vein and artery of a patient. This joint provides a blood access site where a blood loop, comprising an inlet or arterial line to a dialysis apparatus and an outlet or venous line from the dialysis apparatus, may be connected to the patient. The patient&#39;s blood flows through the joint from the artery to the vein. The inlet line draws blood to be treated from the patient through a first cannula inserted into the joint, while the outlet line returns treated blood to the patient through a first cannula inserted into the joint, while the outlet line returns treated blood (i.e., after dialysis), to the patient through a second cannula inserted into the joint between the first cannula and the vein. The joint may be an arteriovenous fistula, which is a direct connection from one of the patient&#39;s arteries to one of the patient&#39;s veins. Alternatively, the joint may be a synthetic or animal organ graft connecting the artery to the vein. As used herein, the term “fistula” refers to any surgically created or implanted joint between one of the patient&#39;s veins and one of the patient&#39;s arteries, however created. More generally, the terms “shunt” or “access” refer to any similar joint, either in a hemodialysis patient or in another area. 
     A portion of the treated blood, after being returned to the patient by the outlet line, may recirculate within the fistula and commingle with untreated blood being withdrawn from the patient by the inlet line. The result is a decrease in the efficiency of the therapy, as some of the volume of blood in the inlet line has already been treated. The inefficiency in turn requires a longer treatment period with negative effects upon the patient. This recirculation and the resulting commingling is dependent in part on the rate at which the blood is withdrawn from and returned to the patient. In order to select the most efficient flow rate and thus achieve the quickest possible treatment time, it is desirable to know the proportion of recirculated treated blood in the blood being withdrawn from the patient by the inlet line. 
     A method and apparatus for quantitatively determining the degree of recirculation in a fistula is described in U.S. Pat. No. 5,510,717, incorporated herein by reference in its entirety. In the method disclosed in the patent, a bolus of marker fluid having a conductivity different from that of blood is injected through an injection site in the outlet line. A differential conductivity monitor quantitatively measures the degree of recirculation in a fistula by comparing the conductivity of blood entering the fistula to the conductivity of blood being withdrawn from the fistula. This measurement is made as blood flows in the predominant direction from artery to vein. Conductivity is not the only property used in the prior art for sensing recirculated blood. Among other fluid properties utilized for the measurement of recirculated blood are temperature of the fluid and speed of conduction of sound. Some of these measurement techniques also involve injection of a bolus of marker material into the blood flow. For all the above methods of detecting recirculating blood, the techniques for handling the blood loop remain essentially the same. 
     Measurement of the fistula&#39;s flow rate is also desirable. In the patient, the fistula gradually loses its ability to efficiently transport blood from artery to vein. Fat and other deposits build up within the fistula and flow is gradually reduced. Eventually, the fistula must be replaced. This repetitious replacement can account for half the long term cost of dialysis treatment. In the interest of making these replacements as infrequent as possible, it is necessary to know whether the fistula flow rate is adequate. It can be readily appreciated that ascertaining the flow rate through the fistula is very important to long term care of the patient. 
     The flow rate through the fistula or the access flow rate (“Q-access”) can be determined utilizing the same equipment and procedures used for monitoring of the recirculation rate, but with a reversed flow of blood to the fistula. To measure Q-access the inlet line is disconnected from the first line and connected to the second cannula while the outlet line is disconnected from the second cannula and connected to the first cannula. Blood is withdrawn from the fistula at a location downstream of the location at which it is returned to the fistula. By injecting a bolus of a marker fluid having a property such as conductivity different from that of blood, and measuring and comparing the values of that property in the inlet and outlet lines, the Q-access or access flow rate of the dialysis apparatus may be determined. 
     This reversal of the blood flow is accomplished by manually switching the tubing and cannula which connect the dialysis apparatus to the patient, or switching the lines at a point removed from the patient. This switching of the lines is well known in the prior art, regardless of the equipment or exact blood property being used to measure the recirculation rate and access flow rate. 
     Even in situations where it is desired to measure Q-access without measuring recirculation flow, this manual switching is required as the desirable flow of blood into and out of the fistula for dialysis treatment is the opposite of that required for measuring Q-access. 
     The switch of tubing and cannulae is a source of danger to the patient and to health care personnel. Normally, the dialysis machine, fistula, and associated tubing provide a closed loop or closed aseptic system. Switching of the tubing presents risks that aseptic techniques may be compromised by the operator. Ultrasonic air bubble protection may be lost. Blood may be sprayed or spilled onto the operator, raising the risk of blood borne infection. Finally, when dealing with open blood lines, there is a chance of exsanguination of the patient. 
     It is against this background that the significant advances of the present invention developed. 
     SUMMARY OF THE INVENTION 
     A significant aspect of the present invention is a method and apparatus that permits monitoring access flow while avoiding the disconnection and reconnection of blood lines. 
     In accordance with this aspect of the invention, the blood lines remain attached in the normal configuration, as for blood flow in a first direction throughout the dialysis procedure. The blood flow is reversed within the blood loop without compromising the aseptic integrity of the blood loop. Blood is flowed though the blood loop in the first direction by the action of a pump, such as a peristaltic pump. By reversing the action of the peristaltic pump, the flow of blood in the blood loop is reversed and the blood flows in a second direction. With the blood flow reversed, blood flows through the normal blood outlet line into the fistula and through the normal blood inlet line out from the fistula. 
     An injection site is provided in the blood inlet line, if needed, to inject a bolus of marker fluid into the inlet line. 
     A further aspect of the present invention is a method and apparatus whereby blood is flowed through the blood loop in the second direction by action of a valve assembly. 
     A further aspect of the present invention is a method and apparatus which allows automatic monitoring and/or control of the injection of the marker fluid. 
     A further aspect of the invention is a method and apparatus which detects air bubbles in the inlet or arterial line. 
     Further aspects of the present invention will be apparent from the detailed description of the preferred embodiment and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram not to scale of the blood loop of a dialysis system in accordance with the prior art. 
     FIG. 2 is a schematic diagram not to scale of the blood loop of a dialysis system using a prior art configuration to measure Q-access. 
     FIG. 3 is a schematic diagram not to scale of the blood loop of a dialysis system in accordance with the present invention. 
     FIG. 4 is a schematic diagram not to scale of the blood loop of a dialysis system of the present invention configured to measure Q-access. 
     FIG. 5 is a schematic diagram not to scale of the present invention configured for automatic control, showing both the blood loop and the control circuitry of the controller for automatic control. 
     FIG. 6 is a schematic diagram not to scale of a portion of the blood loop shown in FIGS. 3-5, illustrating a valve assembly to reverse the blood flow. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described by reference to FIGS. 1 to  6 . Like reference numerals in the drawings denote like elements. 
     FIG. 1 illustrates a dialysis system blood loop  20  in accordance with the prior art. The blood loop  20  is located outside a body of a dialysis patient (not shown). A fistula  22  having an arterial end  24  at an artery  34  and a venous end  26  at a vein  30  is surgically formed in the patient. The predominant fistula  22  blood flow direction is shown by fistula blood flow arrow  28 . Over time the fistula  22  normally suffers from increasing recirculation of flow indicated by recirculation flow arrow  29  and a gradual decrease in the fistula blood flow  28  through it. Both of these values may be ascertained in order to maximize the efficiency of treatment. Recirculation flow  29  causes some treated blood to flow with the untreated blood through the blood loop  20 , diluting the untreated blood flow through the apparatus and increasing the time of treatment. Decreased blood flow in the fistula also increases the time needed for treatment of the blood. Blood flow in the vein  30  is indicated by venous blood flow arrow  32  and blood flow in the artery  34  is indicated by arterial blood flow arrow  36 . 
     The terms “inlet ” and “outlet ” used hereafter are taken as defined for the blood loop  20  during the time the blood is being dialyzed, i.e., while the patient is being treated. The blood loop  20  comprises an inlet cannula  35  connected to an inlet or arterial line  38  of the blood loop  20  by an inlet connector  37  which draws fluids from near the arterial end  24  of the fistula  22 , and an outlet cannula  39  connected to an outlet or venous line  40  by an outlet connector  41  which returns treated blood to near the venous end  26  of the fistula  22 . The blood loop  20  also comprises an inlet monitoring device  42 , and outlet monitoring device  44 , a bolus outlet injection point  48 , a bubble trap  50 , a fluid treatment device such as a dialyzer  52  and a peristaltic pump header  55 . The blood loop  20  may also be comprised of other elements (not shown) as desired for the particular procedure. 
     Inlet and outlet monitoring devices  42  and  44  may be a differential conductivity monitor as described in U.S. Pat. No. 5,510,717 or may comprise a temperature, sound transmission (including ultrasonic) or other physical or chemical property monitor. 
     In a dialysis treatment, blood is drawn by a peristaltic pump  54  of a dialysis apparatus from the fistula  22  through the inlet cannula  35 , inlet connector  37  and inlet line  38 , through the inlet monitoring device  42 , through a pump header  55 , through the dialyzer  52 , where it is treated, and returned through the bubble trap  50 , the outlet bolus injection point  48 , the outlet monitoring device  44 , the outlet line  40 , the outlet connector  41  and outlet cannula  39  to the fistula  22 . 
     To measure recirculation, a bolus of a marker fluid  49  is injected into the treated blood through the outlet bolus injection point  48 . The marker fluid is selected to alter one or more physical, optical, electrical or chemical properties of the blood. Outlet monitoring device  44  measures the value of the property in the outlet line  40  after the injection of a bolus of marker fluid. The inlet monitoring device  42  measures the value of the property in the inlet line. Comparison of the two values measured by the two monitoring devices  42  and  44  allows quantitative determination of the degree of recirculation occurring in the fistula  22  by known techniques while blood flows through the blood loop  20  in the course of being dialyzed. 
     FIG. 2 illustrates the prior art method of measuring access flow or Q-access through the fistula. In the prior art method, dialysis treatment is stopped for the measurement of Q-access. The outlet line  40  is disconnected from the outlet connector  41  and the inlet line  38  is disconnected from the inlet connector  37 . The outlet line  40  is then connected to the inlet connector  37  and the inlet line  38  is connected to the outlet connector  41 . The peristaltic pump  54  is then started to draw blood from the venous end  26  of the fistula  22  through the outlet cannula  39 , through the outlet connector  41 , inlet line  38 , inlet monitoring device  42  and pump header  55 , to the dialyzer  52 , returning the blood through the bubble trap  50 , the outlet bolus injection point  48 , the outlet line  40 , the inlet connector  37 , and the inlet cannula  35  to the arterial end  24  of the fistula  22 . A bolus of marker fluid  49  is injected into the blood through the outlet bolus injection point  48 . The marker fluid alters a property of the blood in the outlet line  40 . Outlet monitoring device  44  and inlet monitoring device  42  measure the values of the property. By comparing the values of the property, a measurement of Q-access in the fistula may be made using known techniques. After Q-access is determined, blood flow in the blood loop  20  is again stopped and the sequence of connections and disconnections reversed to resume dialysis as shown in FIG.  1 . 
     The present invention will now be described by particular reference to FIGS. 3 and 4. 
     Referring to FIG. 3, the blood loop  20 ′ is similar to the blood loop  20  of the prior art with the addition of an inlet bolus injection site  46  and ultrasonic bubble detectors  57  and  58 . Dialysis treatment and measurement of a degree of recirculation occur as described above with reference to FIG. 1 with the pump operating in a first (for example, clockwise) direction. The blood flow during dialysis treatment and measurement of recirculation is in a first direction. 
     Measurement of Q-access or access flow is accomplished without opening the closed loop by reversing the direction of operation of the pump  54  (for example, in the counter-clockwise direction). Blood flow, therefore, reverses its direction in the blood loop  20 ′, and flows in a second direction opposite the first direction of blood flow for dialysis and recirculation. Blood is drawn from the venous end  26  of the fistula  22  through outlet cannula  39 , the outlet connector  41  and into the outlet line  40 , which now functions as a line leading into the blood loop  20  and through the outlet monitoring device  44  and bubble trap  50  to the dialyzer. Blood then flows from the dialyzer  52  through the pump header  55 , the inlet bolus injection point  46 , the inlet monitoring device  42 , the inlet line  38 , the inlet connector  37  and the inlet cannula  35  to the arterial end  24  of the fistula  22 . A bolus of marker fluid  49 ,(see FIG. 4) is injected at the inlet bolus injection point  46 . Comparison of the readings taken by the inlet monitoring device  42  and the outlet monitoring device  44  allows measurement of the fistula blood flow  28  by known techniques. The inlet line  38  remains throughout the procedure connected to the inlet cannula  35  at the arterial end  24  of the fistula  20 , and the outlet line  40  remains throughout the procedure connected to the outlet cannula  39  at the venous end  26  of the fistula  20 . 
     The inlet and outlet bolus injection sites may be any of several known types such as needle-pierceable self-sealing septa, needle-less split septa, or three way stopcocks, and of which may be adapted to accept a conventional syringe  49  for manual injection of the bolus of marker fluid. Alternatively, a syringe  49  may be connected to the blood loop at the bolus injection site and automatically actuated by a syringe pump of a known type. 
     FIG. 5 illustrates an alternative embodiment of the present invention in which the measurement of recirculation flow  28  and Q-access  29  are more fully automated. A controller  100  is provided to control and interpret the measurements. The controller  100  may also perform other functions associated with the operation of the dialysis apparatus. The bolus injection sites  46 ′ and  48 ′ are “t ” tubing connections. An inlet bolus syringe  102  is connected to the inlet bolus injection site  46  and an outlet bolus syringe  104  is connected to the outlet bolus injection site  48 ′. Each syringe  102  and  104  is loaded into a corresponding syringe pump  106  and  108  of the dialysis apparatus  21 . The syringe pumps  106  and  108  may be of known types. The controller  100  is configured and programmed to control the action of the peristaltic pump  54  and the two syringe pumps  106  and  108  to initiate and perform the measurement of degree of recirculation flow and Q-access as described above. The controller  100  may also receive information signals from the property monitoring devices  42  and  44 . The above discussed operations of the dialysis machine may be automated or semi-automated. The semi-automated modes could allow operator input at decision-making times, or could automate certain operations and not others. 
     An ultrasonic bubble detector  57  shown in FIGS. 3-5 can also be placed in the arterial or inlet line  38 . The ultrasonic bubble detector is to detect foam or air in the inlet line. Similarly, an ultrasonic bubble detector  58  may also be used in the venous or outlet line. The location of the ultrasonic bubble detector may be varied, however, it is preferable that it be after the injection site in the direction of flow for measurement of access flow. That is the ultrasonic bubble detector should be between the injection site  46 ,  46 ′ and the fistula connection  35 ,  37 . The ultrasonic bubble detector may be that shown in U.S. Pat. No. 5,394,732, incorporated herein in its entirety by reference, or of another well known type. 
     It will be apparent to those skilled in the art that the present invention will function in substantially the same way regardless of the known physical blood property being measured, the marker fluid used to alter that property, or the type of measuring devices used to measure the values of the property. For example, the known physical property to be measured may include, but is not limited to, temperature, sound transmission including ultrasonic transmission. 
     In the preferred embodiment, a second ultrasonic bubble trap, (not shown), may also be placed on the inlet line  38  if necessary to provide bubble protection for flow in either direction. Other known equipment can also be added to inlet line  38  or outlet line  40  as necessary. 
     It will be apparent to those skilled in the art that there are other ways available to cause blood flow in the second direction without disconnection of the inlet and outlet lines. 
     FIG. 6 illustrates one such method of reversing blood flow without reversal of the pump. The valve assembly of FIG. 6 allows for reversal of the blood flow without compromising the closed aseptic blood loop. The valve assembly may be placed between the monitoring devices  42 ,  44  and the respective inlet and outlet connectors  37  and  41 . A portion of the tubing loop showing the valve assembly is illustrated in FIG.  6 . 
     The tubing set in this embodiment further comprises an inlet bypass  122  from the inlet line  38  to the outlet line  40 , and an outlet bypass  124  from the outlet line  40  to the inlet line  38 . An inlet bypass pinch valve  130  keeps the inlet bypass  122  closed during operation with blood flow in the first direction. An outlet bypass pinch valve  132  keeps the outlet bypass  124  closed during operations with blood flow in the first direction. An inlet pinch valve  126  and an outlet pinch valve  128  remain open during operations with blood flow in the first direction. 
     When it is desired to reverse blood flow, inlet pinch valve  126  and outlet pinch valve  128  are closed and inlet bypass pinch valve  130  and outlet bypass pinch valve  132  are opened, causing the flow in the second direction as discussed above. 
     As will be apparent to one skilled in the art, FIG. 6 depicts only one possible arrangement of tubing and valves which would allow reversal of blood flow. Other equivalents can be substituted within the scope of the present invention. It is further apparent that other known valves other than pinch valves can be used. 
     The preferred embodiment of the present invention has been described by reference to determination of flow characteristics in a surgically created fistula during or in conjunction with a hemodialysis procedure. It should be understood that the present invention is not so limited. The present invention may be used in a variety of medical and non-medical circumstances where it is desirable to determine flow characteristics without compromising the integrity of the fluid loop. Presently preferred embodiments of the present invention and many of its aspects, features and advantages have been described with a degree of particularity. It should be understood that this description has been made by way of preferred embodiment, and that the invention is defined by the scope of the following claims.