Abstract:
A circulatory assist system has a pump with a motor coupled to rotate the pump at a selectable speed. A controller drives the motor at a target speed and collects blood flow measurements during operation of the pump. An impaired flow condition is identified when a plurality of successive blood flow measurements are between an expected minimum flow and a low flow threshold, such that the low flow would necessitate issuing an alert. During the impaired flow condition, it is detected whether an inflow obstruction exists by determining whether a reduction in speed of the pump is correlated with a predetermined increase in the blood flow measurements. If the inflow obstruction is detected, then the speed of the pump is further reduced to further increase the blood flow measurements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates in general to blood circulatory assist devices, and, more specifically, to autonomous control of a pump to maintain optimum blood flow under a variety of conditions including partial obstructions and low blood volume. 
         [0004]    Many types of circulatory assist devices are available to either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. One such system is the DuraHeart® LVAS system made by Terumo Heart, Inc., of Ann Arbor, Mich. One embodiment of the DuraHeart® system may employ a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. An electric motor magnetically coupled to the impeller is driven at a speed appropriate to obtain the desired blood flow through the pump. 
         [0005]    A typical cardiac assist system includes a pumping unit, electrical motor (e.g., a brushless DC motor integrated into the pump), drive electronics, microprocessor control unit, and an energy source such as rechargeable batteries. The system may be implantable, either fully or partially. The goal of the control unit is to autonomously control the pump performance to satisfy the physiologic needs of the patient while maintaining safe and reliable system operation. A control system for varying pump speed to achieve a target blood flow based on physiologic conditions is shown in U.S. Pat. No. 7,160,243, issued Jan. 9, 2007, which is incorporated herein by reference in its entirety. Thus, a target blood flow rate may be established based on the patient&#39;s heart rate so that the physiologic demand is met. The control unit may establish a speed setpoint for the pump motor to achieve the target flow. Whether the control unit controls the speed setpoint in order to achieve flow on demand or whether a pump speed is merely controlled to achieve a static flow or speed as determined separately by a physician, it is essential to automatically monitor pump performance to ensure that life support functions are maintained. 
         [0006]    The actual blood flow being delivered to the patient by the assist device can be monitored either directly by sensors or indirectly by inferring flow based on motor current and speed. Despite the attempt by the control unit to maintain a target flow, various conditions such as obstructions of the inflow conduit or outflow conduit from the pump, low blood volume due to dehydrations, or other problems may cause the blood flow to decrease. Low flow and no flow alarms are conventionally employed to indicate conditions when the blood flow through the pump has inadvertently fallen below a low flow threshold or a no flow threshold, respectively. The alarms may comprise warning sounds, lights, or messages to allow the patient or caregiver to take corrective action. In order to provide a greater safety margin, it would be desirable to identify and correct flow problems before the low flow or no flow thresholds are reached. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect of the invention, a method is provided for controlling a pump motor in an assist device for pumping blood of a patient. An actual pump flow value of the pump motor is monitored during pumping of the blood by the assist device. An expected minimum pump flow value is determined corresponding to nominal pump operation for the monitored speed and current flow. When the actual pump flow value is greater than the expected minimum pump flow value, a target speed of the pump motor is set according to predetermined criteria (which may comprise a predefined setpoint as determined by a physician, for example). When the actual pump flow value is less than the expected minimum pump flow value for at least a first diagnostic wait time, a pump flow diagnostic state is entered. 
         [0008]    In an embodiment, the pump flow diagnostic state comprises entering a low pump flow state if the actual pump flow value is less than a low flow threshold for at least a low flow wait time. The low flow threshold is less than the expected minimum pump flow value, and the low pump flow state includes generating a low flow warning. A no pump flow state is entered if the actual pump flow value is less than a no flow threshold for at least a no flow wait time. The no pump flow state includes generating a no flow warning, wherein the no flow threshold is less than the low flow threshold, and wherein the no flow wait time is less than the low flow wait time. An obstructed flow diagnostic state is entered if the actual pump flow value is less than the expected minimum pump flow value for at least an obstruction diagnostic wait time, wherein the obstruction diagnostic wait time is greater than the low flow wait time. 
         [0009]    In an embodiment, the obstructed flow diagnostic state comprises selectably modifying the target speed of the pump motor and monitoring the resultant actual pump flow value. An inflow obstruction is detected if a reduction in target speed is correlated with a predetermined increase in the resultant actual pump flow value. If an inflow obstruction is detected, then the target speed is selectably decreased to a new target that substantially maximizes the actual pump flow value. 
         [0010]    In an embodiment, the obstructed flow diagnostic state comprises detecting an outflow obstruction if a reduction in target speed is correlated with a predetermined decrease in the resultant actual pump flow value. If an outflow obstruction is detected, then the target speed is selectably increased to a new target until either a predetermined maximum speed or an actual pump flow value substantially equal to the expected minimum pump flow value is obtained. 
         [0011]    In an embodiment, changes in pulsatility associated with the modified speed of the pump motor are also used to detect an inflow or outflow obstruction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram of a circulatory assist system of a type employing the present invention. 
           [0013]      FIG. 2  is a graph showing changes in volumetric flow occurring during operation of a circulatory assist system. 
           [0014]      FIG. 3  is a flowchart showing one preferred method of the invention. 
           [0015]      FIG. 4  is a graph illustrating certain changes in flow and pulsatility that may be associated with changes in pump speed under certain conditions. 
           [0016]      FIGS. 5 and 6  are graphs showing large and small flow increases that may be associated with a reduction in pump speed. 
           [0017]      FIG. 7  is a matrix showing general correlations of pump speed, flow, and pulsatility with inflow and outflow obstructions. 
           [0018]      FIG. 8  is a more detailed decision matrix for one preferred embodiment. 
           [0019]      FIG. 9  is a graph showing pump speed adjustments and resultant changes in flow when correcting for a detected obstruction. 
           [0020]      FIG. 10  is a flowchart showing a further method of the invention. 
           [0021]      FIG. 11  is a state diagram corresponding to another preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    Referring to  FIG. 1 , a patient  10  is shown in fragmentary front elevational view. Surgically implanted into the patient&#39;s abdominal cavity  11  is the pumping portion  12  of a ventricular assist device. An inflow conduit  13  conveys blood from the patient&#39;s left ventricle into the pumping portion  12 , and an outflow conduit  14  conveys blood from the pumping portion  12  to the patient&#39;s ascending thoracic aorta. A power cable  15  extends from the pumping portion  12  outwardly of the patient&#39;s body via an incision to a compact controller  16 . A power source, such as a battery pack worn on a belt about the patient&#39;s waist, and generally referenced with the numeral  17 , is connected with controller  16 . 
         [0023]    Each of the conduits  13  and  14  may include a tubular metallic housing proximate the pumping portion  12  which may connect to elongated segments extending to the heart and ascending aorta, respectively. At the end of inflow conduit  13  connected to the patient&#39;s heart (preferably at the apex of the left ventricle), and at the end of outflow conduit  14  connected to the ascending thoracic aorta, the conduits are generally attached to the natural tissue by sutures through the use of a sewing ring or cuff so that blood flow communication is established and maintained. The distal end of the inflow conduit  13  is inserted through the ventricle wall and into the heart in order to establish blood flow from the heart to the pumping portion  12 . 
         [0024]      FIG. 2  illustrates a target flow Q Target  at  20  and an actual flow value  25  that varies over time. A no flow threshold  21  and a low flow threshold  22  define no flow region  23  and low flow region  24 , respectively, wherein appropriate alarms are generated by a pump control unit whenever actual flow dips into these regions. The trajectory of actual pump flow value  25  may fall to a value below an expected minimum flow threshold  26  into a respective diagnostic region  27 . Expected minimum flow threshold  26  may be obtained from a lookup table or a model based on empirically derived flow profiles that result from various inflow or outflow obstructions or various reductions in blood volume. The present invention is configured to detect operation in region  27  and to take steps to identify a potential cause and a remedy in order to increase flow if possible. 
         [0025]    When the actual flow falls below an expected minimum flow that should be present in view of the operating speed of the pump (i.e., assuming no obstructions and proper blood volume), the present invention enters a diagnostic state for identifying a potential cause of the impaired flow such as a partial or complete obstruction of the inflow conduit or the outflow conduit, or a condition wherein a flow is saturated for a given pump speed due to a limited blood volume resulting from dehydration, etc. 
         [0026]    As shown in  FIG. 3 , a method of the invention begins in step  30  wherein a physician or other medical practitioner configures target values and performance limits pertaining to blood flow rate and pump speed to be provided for a particular patient. The circulatory assist device then monitors for physiological conditions such as heart rate or pump pulse rate in step  31 . In step  32 , a target flow rate and a target speed (i.e., setpoint speed) are determined and used for controlling the system as known in the art. Alternatively, a speed setpoint may be determined according to other predetermined criteria such as a setpoint configured according to a static value chosen by a physician for the particular patient. A check is performed in step  33  to determine whether the actual (i.e., indirectly estimated) pump flow value (eLPM pump ) is less than an expected minimum pump flow value (LPM ExpMin ) for greater than a diagnostic wait time (T FlowDiagWait ). As mentioned above, eLPM pump  is an estimated average pump flow for a given pump speed. If not, then a return is made to step  31  and pump operation continues normally with the pump speed being determined by a target flow that is set according to physiological conditions. 
         [0027]    If the actual pump flow value is less than the expected minimum flow value in step  33 , then a check is made in step  34  to determine whether the actual flow is less than a low flow threshold (LPM LowFlow ). In particular, step  34  preferably requires that the actual flow value be less than LPM LowFlow  for greater than a predetermined low flow wait time (T LowFlowWait ). When eLPM pump &lt;LPM LowFlow  then a low flow warning is generated in step  35 . A low flow state is then entered while the low flow warning continues. Checks are made in step  36  to determine whether the actual flow value has risen above the low flow threshold for greater than the low flow wait time, and a check is made in step  37  to determine whether the actual flow value is less than a no flow threshold (LPM NoFlow ) for at least a no flow wait time (T NoFlowWait ). The value of T NoFlowWait  is less than the value of T LowFlowWait  so that detection of a no flow condition has priority. If the actual flow value rises above the low flow threshold, then the warning is turned off in step  38  and a return is made to step  34 . If an actual flow value falls below the no flow threshold for the no flow diagnostic wait time, then a no flow warning is generated in step  40  to indicate that a greater urgency of taking corrective action. While in a no flow warning state, a check is made in step  41  to determine whether the actual flow value rises above the no flow threshold for longer than the no flow wait time. When it does, the no flow warning is turned off in step  42 , the low flow warning is turned off in step  38 , and a return is made to step  34 . 
         [0028]    When step  34  determines that the actual flow value has not stayed below the low flow threshold for the low flow diagnostic wait time, then a check is made in step  43  to determine whether the actual flow value stays below the expected minimum flow value for at least an obstruction diagnostic wait time (T ObsDiagWait ) which is longer than both the low flow diagnostic wait time and the no flow diagnostic wait time. If not, then a check is made in step  44  to determine the actual flow value has recovered above the expected minimum flow value for at least the diagnostic wait time (T FlowDiagWait ), and if so, then a return is made to step  31  for nominal pump control. If the condition is not true in step  44 , then a return is made to step  34  for continuing to monitor for either a low flow condition or an obstructed condition. When the condition in step  43  is satisfied then the method proceeds to step  45  wherein a potential obstruction is diagnosed as described below. 
         [0029]    The present invention is based in part on an observation that a nominal reduction in pump speed generally results in an increase in flow if an inflow obstructions exists. As shown in  FIG. 4 , a pump is operating at a first speed at  50 , but then a speed reduction  51  to a lower speed  52  is deliberately introduced. After a sufficient time to allow flow to stabilize at a new value for measurement, speed then increases at  53  back to the original speed at  54 . An actual pump flow Q has an original value at  55  will rise to a higher flow at  56  during a reduced pump speed at  52  in the event that an inflow obstruction exists. If an outflow obstruction exists, then the actual flow instead decreases as shown at  57  during the time of reduced pump speed  52 . 
         [0030]    The change in pump speed may also affect the pulsatility index (e.g., the difference between the maximum and minimum flows divided by the average maximum flow) such that an initial pulsatility at  60  decreases to a value at  61  in the presence of an inflow obstruction when pump speed is reduced at  52 . On the other hand, in the presence of an outflow obstruction the pulsatility will increase at  62  during the speed reduction. Inspection of the change in flow resulting from a deliberate speed reduction may be sufficient to differentiate between an inflow obstruction and an outflow obstruction, but it may be coupled with an inspection of the change in pulsatility to potentially improve an identification. 
         [0031]    The diagnostic relationships employed by the present invention are shown in greater detail in  FIGS. 5 and 6 .  FIG. 5  shows an inflow obstruction wherein a pump speed RPM setpoint  and a pump flow eLPM pump  are measured at a first time t 1 . Pump speed is reduced by a predetermined speed of RPM ObsDiag  at a time t 2 . At time t 2 , the actual pump flow has stabilized at a new value representing an increase by more than a threshold designated LPM ObsDiag , which indicates the presence of the inflow obstruction. In a preferred embodiment, a plurality of speed modification trials of the type shown in  FIG. 5  are repeated in order to gather statistics for increasing a confidence level in detecting the inflow obstruction. 
         [0032]    In  FIG. 6 , the actual flow through the pump increases during the speed reduction by an incremental flow that is less than the value of LPM ObsDiag . In a preferred embodiment, the present invention does not detect an inflow obstruction based on only the smaller increase in pump flow, but may require simultaneous change in pulsatility index in order to decide on the presence or absence of an inflow obstruction. 
         [0033]    More specifically, an inflow or outflow obstruction may be determined as shown in  FIG. 7 . When pump speed is reduced and the resultant pump flow increases while pulsatility index decreases, then an inflow obstruction is detected. On the other hand, when the speed reduction creates a decreased resultant flow together with an increased pulsatility index, then an outflow obstruction is detected. 
         [0034]    The present invention may also distinguish between different levels of confidence in judging the presence of inflow and outflow obstructions for a saturated flow condition. For example, a large jump in flow being produced by a reduction in pump speed may always generate an indication of an inflow obstruction. Depending on whether pulsatility experiences a large drop or a small drop, the confidence of the inflow obstruction may be characterized as either probable or possible, respectively. As further shown in  FIG. 8 , a small jump in flow may correlate with a likely inflow obstruction if the pulsatility also experienced a large drop. If both the jump in pump flow and the drop in pulsatility are small (i.e., less than respective thresholds), then the diagnostic decision may correspond to a “no call” with respect to whether there is any obstruction or a saturated flow. 
         [0035]    When a reduced speed generates neither a large change in flow nor a large change in pulsatility, then a saturated flow may be detected. In the presence of a saturated flow, it may be desirable to reduce pump speed to the lowest value that maintains the current flow value. 
         [0036]    An outflow obstruction may be detected according to  FIG. 8  when a large drop in the flow is correlated with the reduction in pump speed. If the large drop in flow occurs with a large jump in pulsatility, then an outflow obstruction is probable. If associated with a small jump in pulsatility, then an outflow obstruction is classified as possible. When a small drop in pump flow occurs with a large jump in pulsatility, then an outflow obstruction is classified as likely, but if coupled with a small jump in pulsatility then no call is made. 
         [0037]    Based on the confidence with which either an inflow or an outflow obstruction is detected, corresponding measures can be taken to attempt to provide a greater flow or even restore the flow at least the expected minimum flow. As shown in  FIG. 9 , a plurality of speed modification trials including trials  65  and  66  are performed in order to assess the most likely obstruction. Prior to the corrective action, the pump speed has a setpoint  67  and a corresponding flow value  68 . When an inflow obstruction is present, corrective action comprises gradually decreasing the pump speed at  70  to produce a gradual increase in flow at  71 . A predetermined minimum speed  72  may preferably have been established by the physician based on the physiology of the patient, and if the speed reaches that minimum then no further changes would be made. As long as further decreases in speed along line  70  generate a corresponding increase in pump flow along  71 , then the speed continues to decrease. When the resultant flow reaches a peak at  73  and then decreases at  74 , the reduction in pump speed ceases at  75 . Then the speed achieving the highest flow is adopted at  76 . 
         [0038]    In the case of a detected outflow obstruction, corrective action comprises increasing the pump speed at  80  which results in an increased pump flow at  81 . The increase may continue until either reaching a maximum pump speed  82  as previously determined by a physician or until pump flow reaches the expected minimum flow. 
         [0039]    The plurality of trials and the corrective actions are further described in the method of  FIG. 10 . In step  85 , an actual flow value and a pulsatility index are measured at the current speed setpoint. In step  86 , the pump speed is reduced by a preset amount. In step  87 , a new flow value and pulsatility index are measured at the reduced speed. A check is made in step  88  to determine whether a predetermined number of trials have been obtained. If not, speed is increased back to the original setpoint in step  89  and a return is made to step  85 . 
         [0040]    Once sufficient trials have been conducted, the trials are classified in step  90 . Classification of each trial is performed in accordance with  FIG. 8 , for example. The classified trials are then examined statistically in order to ensure that sufficient data is present to indicate either an inflow obstruction, outflow obstruction, or saturated flow. In a preferred embodiment, a majority of trials must indicate a respective condition. In step  91 , a check is made to determine whether a majority of trials indicate that an inflow obstruction is either likely, possible, or probable. If so, then corrective action to increase pump flow begins at step  92  by dropping the pump speed by a predetermined amount. A check is performed in step  93  to determine whether the speed has been reduced to a predetermined minimum speed. If not, then a check is performed in step  94  to determine whether the latest drop in speed has instead caused a flow decrease. If not, then a return is made to step  92  to drop the speed once again. If a minimum speed is reached in step  93 , then the minimum speed is set as a new speed setpoint and the method returns to point A in  FIG. 3 . In  FIG. 3 , the method waits during a predetermined wait time (T EndDiagWait ) in step  110  before returning to normal operation. This periodic return to normal operation ensures that nominal operation is utilized whenever possible. 
         [0041]    Returning to  FIG. 10 , in the event that a flow decrease is detected in step  94  then the speed setpoint is set to the last speed that obtained a flow increase in step  96  and a return is made to point A. 
         [0042]    If there are not a majority of trials detecting an inflow obstruction in step  91 , then a check is made in step  97  to determine whether a majority of trials indicate a saturated flow. If they do, then pump speed is dropped by a predetermined amount in step  98 . A check is performed in step  99  to determine whether a minimum speed has been reached. If not, then a check is made in step  100  to determine whether a predetermined flow decrease has occurred (i.e., whether the flow has become unsaturated). If not, then a return is made to step  98  to drop speed once again. If a minimum speed is reached in step  99 , then the minimum speed is adopted as a new speed setpoint and the method returns to point A. If a flow decrease is detected in step  100 , then the current speed is used as a new speed setpoint and a return is made to point A. 
         [0043]    If a majority of trials do not indicate a saturated flow condition in step  97 , then a check is made in step  103  to determine whether a majority of trials indicated that an outflow obstruction is likely, possible, or probable. If not, then the flow problem has not been properly diagnosed and the method may retry to diagnose the obstruction in step  104  (e.g., by repeating a new plurality of trials at step  85 ). If a majority of trials indicate an outflow obstruction, then pump speed is increased by a set amount in step  105 . A check is made in step  106  to determine whether a maximum speed has been reached. If not, then a check is made in step  107  to determine whether the result flow has reached the expected minimum flow value. If not, then a return is made to step  105  to further increase the speed. If a maximum speed is detected in step  106 , then the maximum speed is adopted as a new speed setpoint in step  108  and a return is made to point A. If the flow reaches the expected minimum flow value in step  107 , then the current speed is used as a new speed setpoint in step  109  and a return is made to point A. 
         [0044]    The present invention can also be understood using a state diagram as shown in  FIG. 11 . State  115  is a normal pump control state wherein pump control may be implemented as according to U.S. Pat. No. 7,160,243, for example. As long as an actual flow remains greater than the expected minimum flow, operation continues to remain in state  115 . When pump flow falls below the expected minimum flow for greater than time T FlowDiagWait , then a transition is made from state  115  to a flow diagnostic state  116 . A transition is made back from state  116  to state  115  when the flow value remains above the expected minimum flow for greater than T FlowDiagWait . State  116  also checks for low flow. Thus, if actual flow falls below the low flow threshold for greater than a time T LowFlow  then a transition is made to a low flow alarm state  117 . A transition would be made back from state  117  to  116  whenever the actual flow remains greater than the low flow threshold for greater than T LowFlow . State  117  monitors for a no flow condition by comparing actual flow with a no flow threshold. If actual flow is less than the no flow threshold for at least time T NoFlow  then a transition is made to a no flow alarm state  118 . Flow continues to be compared with the no flow threshold and if it remains above the no flow threshold for at least T NoFlow  then a transition is made back to flow diagnostic state  116 . 
         [0045]    While in state  116 , actual flow continues to be compared to the expected minimum flow value and if it remains below it for greater than a time T ObsDiagWait , then a transition is made to diagnose obstruction state  120 . While in state  120 , a plurality of trials are conducted by modifying the pump speed in order to attempt to classify either an inflow obstruction, outflow obstruction, or saturated flow condition. When an inflow obstruction is detected, a transition is made to state  121  for executing a speed reduction action. When an outflow obstruction is detected, then a transition is made to state  123  for executing a speed increase action. When a saturated flow condition is detected, a transition is made to state  122  for executing a speed reduction action. After the actions in states  121 - 123 , transitions are made to wait state  124  wherein the pump continues to operate at a new speed setpoint, thus achieving the best flow results obtainable under current conditions. After a wait time (T EndDiagWait ) corresponding to an expected time in which conditions may eventually change, a transition is made back to normal pump control state  115  with a possible reintroduction of corrective speed changes if flow again does not exceed the expected minimum flow.