Abstract:
An acoustic access disconnect detection system is useful for detecting when an access needle has become dislodged or when blood is leaking. The acoustic disconnect detection system includes an acoustic transmitter and one or more acoustic sensors placed upstream of an access site of a patient, the sensors suitable for generating and detecting an acoustic signal that is intended to pass unobstructed through the access site. The acoustic transmitter may be placed on a therapy machine for generating an acoustic signal. The acoustic sensor is mounted downstream of the transmitter, such as on the therapy machine where blood is pumped to or returned from the patient, on the patient, or on the therapy machine where blood is entering the therapy machine. The therapy machine, such as a dialysis machine, may be programmed not to start or continue operation unless the acoustic signal is within certain parameters.

Description:
BACKGROUND 
       [0001]    The field of the invention is medical treatments generally and patient vascular access systems. The present invention relates to embodiments of a method and a system for detecting disconnection of an access needle or catheter while receiving medical treatment. 
         [0002]    The maxim of “first, do no harm,” may be a good summary of the Hippocratic oath required of doctors and practiced by medical professionals. Nowhere is this principle required more than in modem medicine. With patients living longer, there are more extended treatments and more frail patients than ever. Such patients are in danger from a number of complications that can arise from continuing therapeutic procedures, and even from diagnostic procedures, that are necessary for their continued care. Treatments involving extra-corporeal blood treatment are clear examples. 
         [0003]    The most obvious danger is infection, but the harm caused by infection can be overcome by not re-using even supposedly-sterile devices, by diligent attention by the patient himself or herself, and by the careful attention of care givers attending the patient. Other problems also arise, but, like infections, have been difficult to eradicate. One of the problems arises in blood treatment procedures in which the patient&#39;s blood is physically removed for treatment and then returned, all in the same procedure. Removal and return of blood is practiced in hemodialysis, for those persons whose kidneys do not function well. Other procedures, such as apheresis, involve removing blood from a patient or a donor to separate blood platelets or plasma from the red blood cells, and then returning the red blood cells to the patient or donor, as described in U.S. Pat. Nos. 5,427,695 and 6,071,421. 
         [0004]    The extracorporeal medical treatments described above require that the blood be removed for treatment and then returned. This requires access to the patient&#39;s vascular system, from which blood is removed and to which blood is then returned. If a “batch” treatment is used, that is, a quantity of blood is withdrawn, treated and returned, only a single needle is used. Each batch treatment is typically short, and the treatment is attended by a medical professional at a clinic or hospital. Other treatments are continuous, such as the platelet separation discussed above, or dialysis treatment, and may require a duration of several hours or even overnight. Yet other treatments use a “batch” continuous method in which only a single needle is used. There are distinct withdraw and return phases in a batch continuous process. During the draw phase, blood is processed and additional blood is sent to a holding container to be processed during the return phase. In the return phase, blood is processed from the holding container and then returned to the patient or donor through the single needle. 
         [0005]    Continuous treatments require two needles, or access points, one for withdrawal of blood and one for return. The withdrawal site is normally an artery, and a needle and a pump are used to provide the blood to the therapeutic machine, but in some treatments, such as apheresis, blood is withdrawn from and returned to veins. It is relatively simple to detect a problem with withdrawal, for instance, if the withdrawal needle is dislodged, using conventional air sensor technology. Detecting a problem in the return of the blood to the patient is more difficult. The return line typically includes a needle with venous access. If the return line is dislodged, the blood is not returned to the patient, but may continue to be pumped and may accumulate near the patient, but not returned to the patient&#39;s vascular system. Depending on the pumping rate of the blood and the time for treatment, this could have life-threatening effects on the patient within a very short period time. 
         [0006]    Accordingly, a number of apparatuses have been devised for detecting needle dislodgement, especially venous dislodgement. Many of these techniques use pressure monitoring of the venous access line. One example is U.S. Pat. No. 6,077,443. This patent uses a pressure sensor mounted near a drip chamber to monitor pressure pulses from a blood pump. There appears to be very little difference between the signals when the access needle is properly in place and the signals when the access needle has been removed. In another example, U.S. Pat. No. 6,221,040, pressure-sensing equipment is made more sensitive, but this case results in a higher rate of false positives, i.e., false alarms. 
         [0007]    Another method is disclosed in U.S. Pat. No. 6,572,576. This patent discloses periodically generating a negative pressure in the return line. This causes air to be drawn into the line, which can then be detected by a standard air sensor. This also has some negative aspects, since no air can be allowed in blood returned to the patient. Any mishandling in this area, such as that resulting from worn tubing, could result in blood in the air line with disastrous consequences. What is needed is an access disconnect device that overcomes these difficulties while providing a safe and quick indication to the patient or caregiver that a disconnect or a leak has occurred. 
       SUMMARY 
       [0008]    One embodiment is an access disconnect detector. The access disconnect detector includes an acoustic transmitter for producing an acoustic signal and configured for mounting upstream of a venous access site, an acoustic sensor for sensing a signal from the acoustic transmitter, the acoustic sensor mounted downstream from the transmitter, and a controller configured for sensing signals from the acoustic transmitter and the acoustic sensor and for sending an alert upon a change in the signals detected from the acoustic transmitter or the acoustic sensor, wherein the controller is in communication with or is part of a therapy machine for receiving blood and returning blood to the venous access site. 
         [0009]    Another embodiment is an acoustic access disconnect detector. The acoustic access disconnect detector includes an acoustic transmitter for producing an acoustic signal and configured for mounting on a therapy machine, at least one acoustic sensor for sensing a signal from the acoustic transmitter, and a controller configured for sensing signals from the acoustic transmitter and the acoustic sensor and for sending an alert upon a change in the signals from the acoustic transmitter or the acoustic sensor, wherein the controller is in communication with or is part of a therapy machine for receiving blood and returning blood to a patient. 
         [0010]    Another embodiment is a method for detecting an access disconnection. The method includes steps of sending an acoustic signal into a venous access device, detecting the acoustic signal downstream of a point of origin of the acoustic signal, comparing the detected acoustic signal with baseline detected acoustic signals, deciding whether the detected acoustic signal is significantly different from the baseline detected acoustic signals, and sending an alert if the detected acoustic signal is significantly different from the baseline acoustic signals. 
         [0011]    Another embodiment is a method for detecting an access disconnect. The method includes steps of placing an acoustic sensor upstream of a venous access site, detecting a first heart beat of a patient, determining a first baseline signal from the first heart beat, sensing a second heart beat of the patient, determining a second baseline signal from the second heart beat, comparing the second baseline signal to the first baseline signal, and sending an alert if the step of comparing indicates that the access disconnect or a leak has occurred. 
         [0012]    Additional features and advantages are described herein, and will be apparent from, the following Detailed Description and the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0013]      FIG. 1  depicts a schematic view of an embodiment of a hemodialysis machine using acoustics to detect a venous disconnect; 
           [0014]      FIG. 2  depicts a plan view of a cassette useful in a hemodialysis machine; 
           [0015]      FIG. 3  depicts a method of detecting a venous disconnect using acoustic signals; 
           [0016]      FIG. 4  presents results of acoustic signature testing; 
           [0017]      FIGS. 5-6  depict results of testing using an impedance ratio as a detecting parameter; 
           [0018]      FIGS. 7-8  depict results of testing using a reflection coefficient as a detecting parameter; 
           [0019]      FIGS. 9-10  depict testing results using impedance ratio to detect access site leaks; 
           [0020]      FIG. 11  depicts a test result using a patient&#39;s heartbeat to detect a disconnect at the venous access site; and 
           [0021]      FIG. 12  depicts a hemodialysis machine showing the mounting of the acoustic transmitter and sensors within the machine. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    It is important that venous disconnects should be detected quickly and therapy stopped without delay when a disconnect occurs. As noted above, sending acoustic signals from the therapy machine and insuring that the signals arrive at the venous access site in sufficient magnitude is one way to insure patient safety. The goal of an access disconnect detector is to insure that the needle or other access device is continuously and firmly lodged in its correct location. Acoustics provide a unique, non-invasive way to accomplish this. Once it is decided to use this method, attention then focuses on the location of the transducer or other device to generate an acoustic signal, and also on the location of the sensor or other device to receive the signal. 
         [0023]    The theory is that if access disconnect occurs, the signals will not continue into the access site and there will be a break in the transmission, greater reflection of the signals, and other acoustic events. In any event there should be a significant change in the signal detected by the acoustic sensor. There will also be a significant change in the phases, i.e., in the timing of the acoustic signals as the sensor sees them. 
         [0024]    This patent will discuss several ways to use acoustics to detect access disconnects and leaks in the venous access site, both of which, in theory, should cause a change in the acoustic transmission medium, and therefore a change in the signal received. The methods discussed will include acoustic generation and reception (“pitch and catch”), also known as the acoustic signature method. Another method is to calculate a reflection coefficient of the media, which uses a ratio of the reflected and incident waves to more readily detect a discontinuity or change. Another method is acoustic impedance, which is based on the fact that when the transmission medium is disturbed, there will be a difference in the impedance of a first medium and a second medium, such as air and water. Finally, it is also possible to use the patient&#39;s own heart beat to detect a discontinuity in the venous or arterial access site. 
         [0025]    Acoustic Signature 
         [0026]    In  FIG. 1 , a patient P is connected to a therapy machine, such as hemodialysis machine  10 , and cassette  11 , via an arterial access A and a venous access V. The therapy machine includes a renal failure therapy fluid pumping cassette  11 , a twin-chamber dialyzer  12 , one or more blood pumps  13 , and arterial and venous drip chambers  14   a ,  14   b . Arterial access A is connected to the hemodialysis machine  10  via inlet tubing  16  and arterial access needle An, and venous access V is connected to the hemodialysis machine via outlet tubing  17  and venous access needle V n . An acoustic transmitter  18 , used to induce or transmit an acoustic signal, is mounted on the hemodialysis machine, and on the cassette in particular, and an acoustic sensor  19   a  is mounted adjacent acoustic transmitter  18 . A second acoustic sensor  19   b  may also be mounted adjacent acoustic sensor  19   a . An additional acoustic sensor  19   c  may also be mounted on the arterial input portion of the cassette  11 . As shown below, the acoustic sensors are mounted inside the hemodialysis machine to interface with the cassette. 
         [0027]    Acoustic transducer  18  is configured for generating and sending an acoustic signal into tubing  17  so that the signal is transmitted through the tubing, into venous access needle V n  and into the patient P. Acoustic sensor  19   a  is mounted adjacent the acoustic transmitter for detecting the signal sent by transducer  18  and also signals returned from the downstream tubing and connections. It is clear that the signal generated by transducer  18  will be attenuated by its path through tubing  17 , venous access needle V n , and the patient. Thus, the signal received by sensor  19   a  will likely be much diminished in magnitude, and there is also a time delay from the generation and sending of the signal until its receipt back from the access site. The acoustic transmitter and sensors in this embodiment are mounted on the cassette of the therapy machine, such as a hemodialysis machine, and in particular to the flexible membrane of the cassette. 
         [0028]    The transmitter may be mounted on the therapy machine and in one embodiment is mounted near the blood return line or output line of cassette  11 . Depending on the frequency and amplitude of the acoustic signal needed, different methods are used to induce the signal. A piezo-electric acoustic transducer or actuator may be used. These devices are commercially available from such companies as PI GmbH, Karlsruhe, Germany, and from Ceratec, Inc., Santa Clara, Calif., U.S.A. For larger displacements, an acoustic generator may be made from a moving coil, much like an acoustic speaker. These are available from BEI Kimco Magnetic, Vista, Calif., U.S.A. Other devices may also be used, such as a small motor with a cam or other mechanical device. 
         [0029]    The acoustic sensor itself is typically a very small electronic device with a membrane intended to interface with the surface or fluid to be monitored or measured. Thus, the sensor itself will typically be mounted in a small plastic or metallic housing, with an interface or membrane exposed for the measurement surface. When this patent refers to a sensor, it is intended that the term includes both the sensor and the necessary housing. For invasive applications, the acoustic transmitter and sensors may be mounted so that their interfaces are within the fluid lines. The sensors may include threads or quick-disconnects for such mounting. For non-invasive applications, the sensor, or more accurately, the sensor in sensor housing, will then be mounted to the membrane of the cassette very near the outlet of the cassette. 
         [0030]    Characteristics of the signal sent by transmitter  18 , including its timing, will be controlled and detected by controller  20  of the dialysis machine. In the same manner, the characteristics of the signal detected by sensor  19   a  will be sent to controller  20 . It will be recognized that one or more amplifications, conversions, or transformations will be accomplished by signal processing circuitry in one or more of a multiplexer, the transducer, the sensor, and the controller. For example, transmitter  18  may include an analog to digital converter (ADC) for converting an indication to the controller of the magnitude of the signal that was generated. Sensors  19   a ,  19   b ,  19   c  may include a pre-amplifier and an ADC for amplifying the attenuated signal and for converting the analog signal detected to a digital value to send to controller  20 . 
         [0031]    Controller  20  is connected to the transducer and sensors via signal and power lines  25 . In testing conducted, a piezoresistive Entran EPX-V01-50P transducer, from Entran Devices, Fairfield, N.J., was coupled invasively to the system and used as a transmitter. For measuring signals and coupling through the membrane, or membrane portion, a model 1865 piezoresistive transducer was used, from Honeywell, Inc., Automation and Control Solutions, Freeport, Ill., U.S.A. Piezoresistive sensors are generally good at capturing both static and dynamic acoustic measurements, while piezoelectric sensors are better at dynamic only, and thus may be used as acoustic sensors in a cassette or hemodialysis machine. 
         [0032]    Controller  20  may have a digital signal processor  21  for further processing or comparing of signal values. Controller  20  may be a controller of the therapy machine, such as the hemodialysis machine or other therapy machine, or may be a stand-alone controller. The controller also includes a microprocessor  22 , memory  23 , and a local output device  24 . The local output device  24  may be a screen, a printer, or a sound-type alarm. The output will alert the patient or a caregiver to take action, such as ceasing therapy, replacing the disconnected venous access needle, and so forth. The controller may also be programmed to stop blood pumping from the patient to the therapy machine, or from the therapy machine to the patient, or both. 
         [0033]    A closer look at cassette  11  is disclosed in  FIG. 2 , and discloses placement of the transmitter and sensors in this embodiment. Blood from the patient input line  16  and the cassette  11  circulates into dialyzer  12 , and is routed back to the patient through venous output line  17 . On the side of the cassette depicted, a flexible membrane  15  and valves  15   a  control the flow of blood and dialysate through the cassette and through the dialyzer. Pumps  13  and drip chambers  14   a ,  14   b  are also part of the cassette. The transducer  18  is mounted near the blood return line, as are sensors  19   a ,  19   b . Sensor  19   c  is located near the blood inlet line. The routing of fluids in the cassette is determined by the positions of valves  15   a . The actual positions of the acoustic transmitter and sensors with respect to blood flow are therefore better represented in  FIG. 1 , while  FIG. 2  depicts their positions on a working cassette. 
         [0034]      FIG. 12  depicts a hemodialysis machine from a top front perspective. Hemodialysis machine  100  has a door  102  which opens to admit the cassette discussed above. The cassette has dialyzer  106  attached. In this view, the front face  108  of the inside of the hemodialysis machine is visible. The transmitter and sensors are mounted on or behind this face, so that their interfaces protrude and are available for mating with cassette  102 , and in particular with the flexible membrane, as also discussed above. In this view, acoustic transmitter  110  is mounted lowest, and acoustic sensors  112 ,  114 , and  116  are also mounted within the panel for interfacing with the cassette. 
         [0035]    In some testing, an electrodynamic shaker was used to generate a signal for detection downstream. A signal may also be generated by an acoustic transmitter  18 , and the signal will be attenuated as it proceeds from the transducer, through the tubing, through the access site and access needle, and into the patient. The circuitry described above for alerting the patient and the caregiver takes account of this attenuation. In testing with 15 ga and 17 ga needles, access disconnects could easily be detected, as well as leaks of 10% and 50% of the fluid being tested, a water-glycerol mixture to approximate the viscosity of blood, about 3 cP. 
         [0036]    In addition to the acoustic sensor  19   a  adjacent the return line, discussed above, there are alternative or additional locations for sensors for detecting the acoustic signal. For instance, an additional acoustic sensor  19   c  may be located on the therapy machine, in this instance adjacent the input line. The rationale is to minimize discomfort to the patient by keeping the sensors away from the patient. This also tends to reduce interaction between the patient and the sensor, thus removing user error from the procedure. If the sensor is mounted adjacent the therapy machine blood input line, there are two paths that the signal may take from the acoustic transducer (original signal) to the detecting sensor. The first path is a backward path through the therapy machine. The signal will be highly attenuated in this path. For example, and with respect to  FIG. 1 , a portion of the signal originating from transducer  18  will travel backwards through blood drip  14   a , dialyzer  12 , blood drip  14   b , and pump  13 . The principal attenuation is caused by the pump and the blood drips. The signal will also be attenuated by lengths of tubing or connections between these devices. Thus, the acoustic signal through the therapy machine is expected to be very small. 
         [0037]    The signal will also travel in a forward path through the venous access site V n , through the patient P, through the arterial access site A and arterial needle A n , and then to the tubing connecting the arterial access site to the therapy machine. The venous and arterial access sites are typically separated on a patient by several inches, assuming that one arm is used for both arterial and venous access. This method will be difficult to use if the access sites are more widely separated, e.g., an arm and a leg, or two arms. 
         [0038]    This method was tried in laboratory testing and was successful. A 12 Hz signal was generated. The signal was measured in the venous sensor  19   a  as 80 mm Hg. The signal transmitted to the arterial sensor  19   c  on a hemodialysis machine arterial tubing, as depicted in  FIG. 1 , was about 2 mm Hg. The signal difference between transmission and receipt was thus 32 dB. Upon disconnection, a further 10 dB loss was detected. The testing could not detect at the arterial input line the portion of the signal that traveled through the hemodialysis machine. In general, with 15-17 ga needles, a reduction of about 30 dB, with a range from about 20-40 dB, occurs between transmission and detection when both the transmitter and the acoustical receiver are on the hemodialysis machine and when the sound path includes both access sites and the patient. After an access disconnect, an additional signal loss is detected. 
         [0039]    Accordingly, each application should account for these differences by running an initial setup, also known as a baseline or initialization. The set-up should insure that the acoustic transmission is detectable by one or more sensors in the particular setting at hand. One method for accomplishing a set up is depicted in  FIG. 3 . In a first step  31  of the method, the caregiver or patient mounts the acoustic transmitter and acoustic sensor or sensors as desired. In a second step  32 , the access needle or needles are then attached to tubing connecting them to the hemodialysis machine, or other therapy machine, and are placed into the patient. The tubing and needle or needles are then primed, that is, filled with blood from the patient. Using the controller from the therapy machine, or other controller, the transmitter and sensor are then tested  33 , to insure that the desired signal and amplitude are transmitted and are also received. 
         [0040]    It is known that a good deal of attenuation occurs between the transmission and the sensing of the signal, thus the transmitted signal amplitude should ordinarily be at least measureably greater than the detected signal amplitude. The signals should also differ in phase, i.e., the timing of the sending and receipt of the acoustic signal. These differences are sufficient to insure that the signals will indeed change markedly when a disconnect occurs, without having to induce a fault or a disconnect as part of the setup or initialization. 
         [0041]    When the signals transmitted and received are as desired, the controller settings and instrument parameter settings are noted and locked or secured in place  34 , per the protocol of the clinic or hospital setting. For home settings, the recommended procedures are followed. The therapy, such as hemodialysis, is then begun, and signals are noted. If necessary, the settings and parameters, such as signal amplitude, may be adjusted and again noted and locked or secured in place per the appropriate protocol or procedure followed. Thus, in one embodiment, the baseline may change over time, consistent with the tubing, the sensors, the room temperature, and so on, so that the baseline changes as necessary to insure patient safety while avoiding false alarms. Once therapy has begun, the controller monitors the transmitted and sensed signals and sends a warning signal  35  if either changes more than a previously determined amount, such as a sudden percentage change or sudden dB level change. Monitoring the transmitted level as well as the sensed level is recommended, since a failure or dislodgement of the acoustic transmitter will also result in a change of the signal, and fault analysis or failure resolution will be easier for an operator or the patient if this parameter is tracked as well. Using predetermined criteria based on the signal change or changes, the controller can then cease therapy, send an alert or alarm through a local output device, or take other action to safeguard the patient. 
         [0042]    It has been found that continuously sending and receiving acoustic signals, as described above, is not necessary. It is possible to periodically send an acoustic signal and to then periodically detect the signal received. For example, one or more cycles of a 30 Hz sine wave may be sent each second or other time period, such as twice per second. This schedule makes for a repeatable and reliable method for checking the integrity of the access connection. Other periodic checks may be made, for example, from about every one-tenth of a second to about every 1 second, i.e., about 1 to 10 Hz, or from about half-second to about every 2 seconds, i.e., 0.5 to 2 Hz. 
         [0043]    In determining whether an access disconnect, a leak, or other event has occurred, baseline readings and the particular application will determine the appropriate signal change needed. As will be seen below, events may cause a change in the signal anywhere in the range from a 100% loss of signal to a 100% gain, and many points in-between that are much more subtle. Each application, each tubing length and arrangement may be different, and these differences may each have an effect on acoustic transmission. Accordingly, the decision points on when the signals are significantly different from the baseline or previous signals so as to suggest a leak or an access disconnect, and thus whether to send a signal or sound an alarm, will best be determined for each site individually. As will be seen in the testing data below, a sudden change in acoustic data is a good indicator of a leak or of needle dislodgement. 
         [0044]    Results of one series of tests are depicted in  FIG. 4 . This testing was conducted with a simulated hemodialysis treatment, with 15 ga needles on both the arterial and venous access sites. A 12 Hz signal was used, and blood flow was set at 50 ml/min., 250 ml/min., 450 ml/min., and 650 ml/min. A sound pressure level of about 69 mm Hg was generated and transmitted through the blood. The sound pressure level was increasingly attenuated with increasing blood flow levels. As seen in  FIG. 4 , the acoustic signal was very detectable at the access site. A venous disconnect was readily detected at all four flow rates used. When the needle was re-attached, the signal also returned and the testing continued. The tester also moved the tubing, as indicated in the graph, to determine whether the system was sensitive to patient movement, and determined that the acoustics used were indeed sensitive. 
         [0045]    Acoustic Impedance and Reflection Coefficients 
         [0046]    In another way to detect a venous access disconnect, acoustic sensor  19   a  is placed downstream of acoustic transmitter  18 , adjacent the blood return line. Downstream, in this context, means in the direction of the flow of blood. The patient&#39;s blood here is flowing from the therapy machine back to the patient, along the path from blood drip  14   b , through tubing  17  and to the access site V n . Thus, an acoustic signal is generated by transducer  18  and travels with the blood to the access site. Acoustic sensor  19   a  is placed between the transducer  18  and the access site, with both transducer  18  and sensor  19   a  on the therapy machine. In the same vein, upstream means opposite the flow of the blood. In the example above, if the acoustic sensor  19   a  is downstream of acoustic transmitter  18 , as shown in  FIG. 1 , both the transmitter  18  and sensor  19   a  are upstream from venous access site V, that is, the blood flows to access site V only after the blood has passed transmitter  18  and sensor  19   a . In general, movement in the cycle depicted in  FIG. 1  in a clockwise direction is upstream movement, while movement in a counter-clockwise direction is downstream. If an additional acoustic sensor  19   b  is placed adjacent sensor  19   a , the acoustic impedance and reflection coefficient methods may be used to monitor signals continuously. 
         [0047]    In one method, a signal is generated by the transmitter, and is picked up by the sensor. Of course, the signal that is received is not only the signal directly from the transducer, but also signals reflected back from the needle, the access site, and so forth. If the acoustic signal travels in two media, a first medium, such as water or blood, and a second medium, such as water or blood mixed with air, there will be a difference in the transmission rates of sound through the media. Thus, if a leak or if dislodgement occurs, the normally-occurring reflection caused by the interface, will change. The reflection coefficient is defined as the reflected acoustic pressure divided by the incident acoustic pressure, Pr/Pi. The impedance ratio is then defined as the ratio of 1+the reflection coefficient divided by 1−the reflection coefficient. It is recognized that the reflection coefficient is a complex value, allowing calculation of both magnitude of the reflection coefficient and change of phase. 
         [0048]    The impedance ratio and the reflection coefficient are both useful in detecting access disconnects and leaks. In experimental work with the impedance ratio, it was discovered that not only is the magnitude of the impedance ratio useful, but also the differences in phase of the reflected wave, that is, the phase of the impedance ratio, and its timing shift upon the occurrence of a leak or a discontinuity.  FIGS. 5-6  depict results of testing using 2.7 cP simulated blood, 17 ga needles, and a 12 Hz acoustic signal. Flow rates of 50 ml/min., 150 ml/min., and 300 ml/min. were used. In  FIG. 5 , the magnitude of the impedance ratio was quiescent at about 0.8 (arbitrary units) at all three flow rates. Rearranging the tubing from the blood pump to the access site, as depicted on the graphs, changes the impedance ratio a little, after which the ratio resumes a relatively continuous value. 
         [0049]    When the needle is disconnected from the access site, a great change is observed, an increase in the magnitude of the impedance ratio, which suggests greater impedance, additional reflected signals, and a higher impedance ratio. As also seen in  FIG. 6 , the phase of the impedance ratio also changes. The phase is simply the difference in timing between the incident wave and the reflected wave. As seen in  FIG. 6 , the quiescent phase is different at each flow rate, and the phase difference increases with increasing flow rate, suggesting a greater phase difference as the flow rate increases. There is little effect from moving or adjusting the tubing, but a very noticeable effect when a discontinuity occurs. This testing was also conducted with 17 ga needles and a 12 Hz acoustic signal. Very similar results were also seen with 15 ga needles. 
         [0050]    Additional testing was also conducted to determine whether the reflection coefficient would be a suitable parameter for detecting access disconnection or leaks. In  FIGS. 7-8 , testing was conducted using the same simulated blood, but with larger 15 ga needles and using a 20 Hz acoustic signal. The reflection coefficients were calculated as discussed above and were plotted, as seen in  FIG. 7 , against time at four flow rates, 50 ml/min., 250 ml/min., 450 ml/min., and 650 ml/min. As seen in  FIG. 7 , the magnitude of the reflection coefficient is relatively quiescent at all four flow rates, until an access disconnect was induced. The effect on the magnitude of the reflection coefficient is immediate, within seconds, and dramatic, in that a very large change is observed.  FIG. 8  depicts the changes from the same access disconnect while recording the phase of the reflection coefficient. The effect there is also immediate and dramatic, as the phase, or timing, of the reflected waves changes dramatically. 
         [0051]    Leakage detection 
         [0052]    Impedance ratios and reflection coefficients are also useful for detecting leaks in the access site. A leak will at least cause loss of blood or fluid and may also cause infiltration of air. Thus, the transmission medium will change, and in theory, should show a difference in acoustic impedance, impedance ratio and reflection coefficients.  FIGS. 9-10  depict the use of the impedance ratios and reflection coefficients mentioned above to detect not only access disconnects but also leaks from the access site. In these tests, an orifice was drilled in the proximal end of a 17 ga needle used in the testing. The leak was calculated at about half the flow rate of blood through the needle. As seen in  FIG. 9 , the leak is readily detectable using the magnitude of the impedance ratio. After an initial signal change, the acoustic signal adjusts to a new and distinct level as the leak continues. When the needle is disconnected, another very distinct change takes place, as discussed above.  FIG. 10  depicts the phase of the impedance ratio in this series of tests. The phase also shows dramatic differences both when a leak occurs and when the access needle is disconnected. This testing was conducted with a 20 Hz acoustic signal. 
         [0053]    Heart Beat Acoustic Detection 
         [0054]    The heart beat of the patient can also be used to transmit an acoustic signal useful for detecting access disconnections. With most heart beats ranging from 50 to 85 beats per minute, a rate of about 1-2 Hz is the expected value of the signal. In this testing, a simulated heart beat of about 75 beats per minutes was used, with a 15 ga needle in the access site. Blood flow rates from about 100 to 400 ml/min. were used, and as seen in  FIG. 11 , access disconnect was readily detectable. The acoustic sensor was placed on the venous side. The sensor may be placed at any convenient location on the hemodialysis or other therapy machine, such as just downstream of the drip chamber or, if there is a return pump, between the return pump and the access site. 
         [0055]    The signal processing circuitry used for detection of the heart beat may also be used for signal detection and processing of the other methods discussed above. Among many other known methods, four quantization methods are pertinent. The method known as peak detection searches for and identifies the peak value of the magnitude of the venous acoustic signals within a prescribed frequency band. The program may be instructed to search for the largest peak within a particular period of time. The controller may be “tuned” by segmenting into larger or smaller periods of time, usually defined in milliseconds. For example, if a 30 Hz acoustic signal is used, searching for the largest peak in every 30 or 40 msec band may be appropriate. If a heartbeat is used, about 50 to 85 beats per minute, about 1-2 Hz, a much larger band would be better suited to this technique. 
         [0056]    The technique of power in band measures the spectral power of venous acoustic events. Using this technique, the spectral power within a prescribed frequency band is calculated and recorded, and used to characterize the acoustic signature. A cross spectrum or cross spectral technique, also known as a cross correlation technique, calculates the peak value of the magnitude of the cross spectra of the venous and arterial acoustic events. The values are calculated and recorded. Finally, an auto spectrum technique calculates the peak value of the magnitude of the auto power spectrum for venous acoustic activity. All four techniques were tested and worked well in using the patient&#39;s heartbeat for detecting venous access disconnect, but the cross spectrum and auto spectrum methods worked better. In addition, these processing techniques may also be used to process acoustic signatures. Software packages with these techniques may be purchased commercially from many companies. Examples are the AutoDAQ2 software from InterAC, L&#39;Union, France and the LabVIEW software from National Instruments, Santa Clara, Calif., U.S.A. 
         [0057]    It will be recognized that the transmission and detection of an acoustic signal through several media, such as access tubing, an access needle, a patient, and so forth, is not completely a straightforward task. The many variables that will attend each situation include the length or lengths of tubing, the mounting of the transducer and sensor or sensors, the length and gauge of the needle or needles, and the separation between the arterial and venous needle. This suggests that each application of acoustic technology for detecting access disconnect will be at least slightly different. 
         [0058]    It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.