BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to medical methods, apparatus, and kits. More particularly, the present invention relates to methods, systems, and kits for pumping blood through extracorporeal processing units and returning the processed blood to patients.

[0004] A variety of extracorporeal blood therapies exist which require blood withdrawal, passage through processing equipment, and return of the processed blood to the patient. Examples of such extracorporeal blood therapies include hemodialysis, hemofiltration, hemodiafiltration, apheresis, and the like. Access to a patient's vasculature may be provided through implanted ports, transcutaneous catheters, direct needle access into blood vessels, and other approaches. Once blood withdrawal and blood return lines have been established, the blood is pumped through an appropriate processing unit, such as a dialysis unit, filtration unit, apheresis unit, or the like and the treated blood returned to the patient.

[0005] It is easy to appreciate that careful control and monitoring of the extracorporeal blood circulation is important to both successful blood treatment and patient safety. Important parameters and conditions to be monitored and controlled include blood flow rate, line pressures upstream and downstream of the pump, blockages in the blood draw line, blockages in the blood return line, air leakage into the recirculation blood stream, and the like. Previous extracorporeal blood circulation systems have often relied on setting the speed of a peristaltic pump to control the blood flow rate. Since peristaltic pumps operate by the positive displacement of blood, it has been assumed that the flow rate will be fixed by the pump speed.

[0006] As recognized by the inventors herein, however, that assumption is not warranted. Peristaltic pumps, also referred to tube or roller pumps, rely on moving rollers to progressively “pinch” a tube to advance a series of small blood volumes through the tube and out of the pump. So long as the inlet pressure to the pump tube is generally constant, the pump output will be a predictable function of pump speed. In the case of extracorporeal blood circulation, however, where blood is being drawn through a relatively small needle or other access tube, the inlet pressure of blood to the pump can vary significantly. Moreover, the flow characteristics of a peristaltic pump may vary over time so that the volumetric output will change even if the inlet pressure remains generally constant. While use of a peristaltic pump does have a number of advantages, e.g. there are much less likely to apply a deleterious negative pressure to the blood being circulated, calculating the flow rate based on pump speed alone is nonetheless problematic.

[0007] To help monitor whether the pump is starved of inlet blood flow (which can alter the flow rate as discussed above), some prior art systems have employed pressure monitors on the blood draw and/or return lines. A fall in pressure in the draw line indicates that a blockage or other failure has occurred in the draw line, that the access needle is too small and/or that the access vessel has undergone a partial or total collapse. In contrast, a rise in pressure in the return line indicates the occurrence of an occlusion or other problem in the return line and/or the occurrence of a blockage in the vessel, access device, or fistula. In order to help assure sterility, pressure measurement has usually been performed using drip chambers where the pressure is transmitted via an isolated air line and a transducer protector to the appropriate transducer. Such drip chambers, however, increase the cost of the catheters (blood lines) used for the draw and return lines and the air interface can cause clotting, air entrapment, and other flow problems in the blood recirculation.

[0008] For these reasons, it would be desirable to provide improved methods, systems, and kits for the extracorporeal recirculation and processing of blood. In particular, it would be desirable to provide extracorporeal blood flow systems having improved blood flow rate control as well as improved capability for monitoring proper operation of the blood circulation circuit. Such systems should permit monitoring with a reduced risk of contaminating the blood or causing clotting, air entrapment, or other degradation of the blood. Preferably, such improved systems and system components will permit relatively low cost operation, and specifically will permit implementation without the use of drip chambers as required by certain prior art systems. At least some of these objectives will be met by the invention described hereinafter.

[0009] 2. Description of the Background Art

[0010] U.S. Pat. No. 5,562,617 assigned to the assignee of the present application, describes a system of implantable ports and catheters for accessing a patient's vasculature, which system could be used together with the extracorporeal blood recirculation systems of the present invention. U.S. Pat. No. 4,181,132, describing an extracorporeal processing and blood circulation unit which is attached to a patient's vasculature through an implanted port. Co-pending applications assigned to the assignee of the present invention and including related subject matter include: These patents and pending applications are incorporated herein by reference.

SUMMARY OF THE INVENTION

[0011] The present invention provides improved methods, systems, and kits for the extracorporeal circulation and processing of blood for a variety of purposes including but not limited to hemodialysis, hemofiltration, hemodifiltration, apheresis and the like. Particular improvements provided by the present invention include non-contact measurement of the actual blood flow rate in the circuit, preferably at a location close or adjacent to the return access site on the patient. Based on such actual blood flow measurement, the speed of the blood pump in the flow system can be adjusted to maintain the measured blood flow rate at a control point. Thus, operation of the of the system does not rely on an inferred flow rate based on the operational speed of the pump. Moreover, by monitoring the pump operation characteristics, system failures can be detected. For example, a measured blood flow rate which is significantly below an expected blood flow rate calculated from the speed at which the pump is being driven and known pump characteristics indicates a system failure, most likely loss of blood flow in the return line. Actual power consumption by the pump which is significantly above an expected power consumption based on the measured blood flow rate indicates a failure in the blood circulation system, most likely a blockage in the return line or elsewhere distal to the pump. The non-contact flow measurement is preferably performed using an ultrasonic flow detector. Output of the ultrasonic flow detector is also useful for indicating the presence of air in the blood flow which can result from a leak anywhere proximal to (upstream of) the flow detector. Air leaks may also be detected by an actual pump speed which is higher than expected for the measured blood rate. The methods and systems for implementing these safety and monitoring features are described in more detail below.

[0012] Methods according to the present invention for extracorporeally processing blood comprise pumping blood with a pump having a nominal relationship between pump speed and flow rate, i.e., pump output may be approximated based on pump speed but will be variable due to the factors discussed above. Such pumps will usually be positive displacement pumps, typically being peristaltic pumps which are often preferred since they permit complete isolation of the blood and reduced risk of blood contamination. It would be possible, however, to utilize centrifical and other nonpositive displacement pumps so long as the pumps permit monitoring of the pump speed and prediction of an expected flow rate based on the pump speed.

[0013] The blood flow rate delivered by the pump is measured, and the pump speed is controlled to maintain the measured blood flow at a control point, typically in the range from 100 ml/min to 1000 ml/min, preferably from 250 min to 500 m/min. Pumped blood is processed in any desired manner, including dialysis, hemofiltration, hemodifiltration, apheresis, and the like, and then returned to the patient. Usually, the blood will be withdrawn from an artery and returned to a vein or will be withdrawn from a vein and returned to a vein. It is also possible, although generally less preferred, to both draw the blood from and return the blood to an artery.

[0014] The blood flow measuring step is preferably performed with a non-contact flow sensing device, such as an ultrasonic flow sensor. By “non-contact,” it is meant that no component of the measuring device need be immersed in or otherwise in contact with the flowing blood. Preferably, the flow sensors will be mounted or attached over the blood return line or other conduit of the system. Suitable ultrasonic flow sensors are commercially available from suppliers, such as Transonics, Ithaca, N.Y. Other suitable non-contact flow sensing devices include magnetic flow meters, optical flow detectors, electrical conductance flow detectors, and the like. The ultrasonic or other non-contact flow measuring device is preferably mounted over an exterior surface of a blood return line to the patient, more preferably being close to the blood return site on the patient so that the blood is monitored immediately prior to its return to the patient. Use of the ultrasonic flow sensing device also permits the detection of entrained air or other gases in the blood since the ultrasonic signal generated by air passing through the sensor will be immediately detectable i.e. the air will disrupt reflectance of the ultrasound signal which can be readily detected.

[0015] In a preferred aspect of the methods of the present invention, a failure in the extracorporeal blood flood flow circuit will be detected by calculating or otherwise determining an expected blood flow rate value based on the pump speed. Usually, such a determination can be made by a microprocessor or other controller which is controlling operation of the system as described in more detail below. The expected blood flow rate value is compared with the measured blood flow rate value (i.e. the value measured by the blood flow measurement device), and a difference is determined. If the difference exceeds a threshold value, typically about 5% of the measured flow rate, usually about 10% of the measured flow rate, then an alarm condition will be initiated. An alarm condition may comprise an audible, visual, or other signal being initiated to alert the system user, and/or may include system shut down, or preferably both.

[0016] In a still further preferred aspect of the method of the present invention, the blood flow status through the system may be monitored by measuring or otherwise determining the actual power being consumed by the pump while it is operating to establish extracorporeal blood flow. An expected value of the power consumption level can be determined by the system based on the pump speed and measured blood flow rate. Any differences between the actual power level being consumed and the expected power level can then be determined. If such a difference exceeds a threshold value, typically above 5% of the measured power consumption, usually above 10% of the measured power consumption, then an alarm condition can be initiated. The alarm conditions may be any of those set forth above.

[0017] As a further safety measure, pressure of the blood flow in the return line from the processor to the patient may also be detected and monitored. Preferably, the pressure is monitored externally on the blood return line, e.g. by placing a radially inward constriction on the return flow line. Radially outward forces on the constriction can then be monitored and will increase as the pressure within the flow line increases. Such a system can be calibrated to provide a rough estimation of pressure within the blood return line and alarm conditions can be initiated when threshold values are exceeded.

[0018] Optionally, a safety valve can be placed externally on the blood return line to positively stop blood flow from the system should an alarm condition occur.

[0019] Systems according to the present invention comprise a pump, a processing unit, a blood draw line, a blood return line, an external flow detector which may be positioned over an exterior surface of the blood return line, and a control unit. The pump is of a type generally described above, preferably being a positive displacement pump, and more preferably being a peristaltic pump. The processing unit may be a convention hemodialysis, hemofiltration, hemodifiltration, or apheresis unit. The blood draw and return lines will typically comprise catheters which are connectable in the system. In particular, the blood draw line will be connectable between the patient and the pump, while the blood return line will be connectable between the processing unit and the patient. The control unit is preferably a microprocessor and is connectable to both the pump and the flow detector so that the control unit can monitor flow and control pump speed according to the methods described above.

[0020] In particular aspects of the system, the control unit will be programmed to perform other functions as described in connection with the methods above. In particular, the control unit can monitor the actual pump speed and actual blood flow rate to determine if the expected blood flow rate based on pump speed is being achieved. Further, the control unit can monitor power consumption by the pump to determine if it is higher or lower than the expected value of power consumption based on the measured blood flow rate. Still further, the control unit can monitor the output of an ultrasonic flow detector to determine if there are air or other gas bubbles entrained in the flowing blood. Still further, the control unit may be programmed to monitor pressure in the blood return line from an external pressure detector.

[0021] The present invention will still further comprise kits including system components together with instructions for use. In a specific embodiment, the kit may comprise a blood draw catheter, a blood return catheter, and instructions for use setting forth any of the methods described above. System components will typically be packaged in a conventional medical device package, such as a pouch, tray, box, tube, or the like. Instructions may be printed on a separate sheet of paper or may be printed in whole or in part on part of the package materials. Usually, the system components will be maintained in a sterile condition within the packaging.

[0022] In an additional aspect of the present invention, a method for extracorporeally processing blood comprises drawing blood from the patient and pumping the drawn blood with a peristaltic pump at a predetermined stroke volume and rate corresponding to a theoretical pumped blood flow rate, i.e. a theoretical or expected value of blood flow rate that can be calculated based upon the known stroke volume and actual rate at which the peristaltic pump is being driven. An actual blood flow rate delivered by the pump is directly measured using any of the techniques described above, and the measured actual blood flow rate is compared with the theoretical or calculated blood flow rate. In a first instance, an alarm condition is signaled if the difference between the actual blood flow rate and the theoretical blood flow rate exceeds a predetermined minimum or threshold value. In a second instance, the rate at which the peristaltic pump is being driven is altered or varied in order to change the pumped blood flow rate to a desired value, e.g. one that more closely matches the theoretical pumped flow rate. It will be appreciated, of course, that the theoretical blood flow rate will vary over time as the pump speed is varied so that the theoretical flow rate and a target or control point flow rate will not always be precisely the same. It will further be appreciated that both the alarm and control aspects of this method may be employed or together.

[0023] In yet another aspect of the present invention, apparatus for extracorporeally processing blood comprises tubing having connectors for drawing blood from a patient and returning blood to the patient, where the tubing is connected to or comprises a section of peristaltic pump tubing. A controller operates the peristaltic pump at a desired stroke volume and rate which, at least at the outset, corresponds to a theoretical pumped blood flow rate. Apparatus directly measures the actual blood flow rate delivered by the pump through the tubing, and further apparatus compares the measured actual blood flow rate with the theoretical pumped blood flow rate. A signal is generated corresponding to a difference between the theoretical and actual blood flow rates. A signal may be used to initiate an alarm condition and/or control the actual pump speed in order to return the actual blood flow rate to a desired level or control point. Preferably, all tubing in the apparatus will be free of air-containing chambers, such as drip chambers.

[0024] In a still further aspect of the present invention, a blood processing system comprises a pump operable at different speeds to convey blood through a path. The system further includes a sensor which monitors blood flow rate and optionally detects the presence of air in the blood flow. A controller is coupled to both the sensor and the pump in order to control pump speed (and thus blood flow rate) as a function of monitored flow rate. Usually, the controller adjusts the flow rate as a function of deviation between the monitored blood flow rate and a desired (set point) flow rate. The control algorithm can be proportional, integral, derivative, or virtually any other known control algorithm. Usually, the sensor will be an ultrasonic sensor as described above.