Abstract:
A vascular access monitoring device for monitoring characteristics of a skin piercing vascular access device comprising: an acoustic sensor operable to detect an acoustic emission created by flow through said vascular access device and produce an electrical signal indicative of said emission; electronics processing means for processing and monitoring said electrical signal, and to determine, by identifying a change of characteristic of said signal, the possibility that the vascular access device has become dislodged from its normal working position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from PCT/GB/2010/001201 filed on Jun. 18, 2010 and from GB 0910478.7 filed June 18, 2009, which are hereby incorporated by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to vascular access monitoring, in particular it relates to a device and method for monitoring for the presence of a needle or cannular to determine if it becomes dislodged from a patient. 
         [0004]    2. State of the Art 
         [0005]    In a number of medical treatment regimes it is necessary to have access to the vascular system through a skin piercing, for example in drug infusion or blood processing treatments like dialysis. 
         [0006]    It can be very important to know if a needle becomes dislodged from a patient, and in some cases a dislodged needle can cause serious medical consequences, for example dislodgement of a needle providing life supporting drugs, if not detected could even result in death, as could dislodgement of a blood return needle in a hemodialysis treatment whereby the dialyser can pull blood from the body but not return it. This is especially dangerous in overnight dialysis while the patent is asleep as they would not see the blood escaping. 
         [0007]    Several devices have been proposed for detecting if a needle becomes dislodged from a patient, for example in hemodialysis sensors have been used which look at the effect of the presence of a needle on an electric circuit, for example as described in US2005/0038325. Another known device is described in DE 198 02 985 which discloses the use of an optical reflected signal to detect if the needle has become dislodged. The needle has a reflector mounted thereon to reflect an infra red signal from a transmitter back to a receiver. In this device the adaptation of the needle is required which is highly undesirable as it requires a change to a cheap part which is an accepted standard part across the industry. 
       SUMMARY OF THE INVENTION 
       [0008]    It is the purpose of the present invention to provide an alternative to the known sensors that can be used with a standard needle or cannular. 
         [0009]    According to a first aspect of the invention there is provided a vascular access monitoring device for monitoring characteristics of a skin piercing vascular access device comprising:
       an acoustic sensor operable to detect an acoustic emission created by flow through said vascular access device and produce an electrical signal indicative of said emission; electronics processing means for processing and monitoring said electrical signal, and to determine, by identifying a change of characteristic of said signal, the possibility that the vascular access device has become dislodged from its normal working position.       
 
         [0011]    In one preferred arrangement the acoustic sensor is adapted for location on the patient&#39;s skin adjacent a vascular access point. 
         [0012]    In an alternative preferred arrangement the acoustic sensor is adapted for location on a flow conduit attached to the vascular access device. 
         [0013]    In a preferred arrangement the device is adapted to monitor the flow of blood through a vascular access device, the flow of blood generated by an extra corporeal blood pump. 
         [0014]    Preferably the electronics processing means comprises an electrical input means for receiving a signal from an extra corporeal blood pump. More preferably the extra corporeal blood pump is arranged for intermittent pumping and the electronics processing means receives a control signal indicative of the pumping cycle of said extra corporeal blood pump. The electronics processing means is preferably phase locked with the pump cycle of the extracorporeal blood pump and wherein said electronics processing means compares the acoustic signals in either the amplitude or frequency domain, on successive cycles of the extracorporeal pump as it induces flow through said vascular access device. 
         [0015]    Preferably the device further monitors background noise and wherein the electronics processing means subtracts the background noise from the signal indicative of the acoustic emissions. Preferably the background noise includes noise created by a patient&#39;s heartbeat. 
         [0016]    In a preferred arrangement the device monitors background noise in between pump cycles of the extracorporeal blood pump. 
         [0017]    In one preferred arrangement the device comprises a second acoustic sensor located on the patients body, remote from said vascular access device, for measuring background noise. In an alternative preferred arrangement a single acoustic sensor is used for creating a signal indicative of acoustic noise due to flow through the vascular access device and indicative of background noise. 
         [0018]    As the background noise has its own characteristic pulse, at a rate equal to that of the heartbeat, and has its own range of broadband frequencies, substantially below 5 KHz, it can easily be identified and electronically separated from the total signal received via the sensor monitoring the presence of the vascular access device which will be a combination of the background noise and the nose related to flow through the vascular access device, the nose related to flow through the vascular access device having its own characteristic pulse at a frequency fixed by the blood pump which itself. 
         [0019]    Preferably the monitoring device further comprises a second monitoring device for monitoring the presence of the skin piercing vascular access device, the second monitoring device comprising:
       a mount having an ultrasonic transmitter and an ultrasonic receiver therein; attachment means for attaching the mount to a patient adjacent a vascular access point such that, in use, a vascular access device entering the patient at the vascular access point, passes underneath a sensing section of the base;   control electronics to monitor the signal received at the receiver, and wherein when the vascular access device is underneath said sensing section, ultrasound produced by said transmitter passes through the patient skin and is reflected by the patients body tissues and by the vascular access device, and   when the vascular access device is not underneath said sensing section, ultrasound produced by said transmitter passes through the patients skin and is reflected by the patients body tissue only, and wherein   the control electronics detects the presence of a vascular access device by performing a comparison on the received ultrasonic signal.       
 
         [0024]    Preferably the transmitter of the second monitoring device periodically emits an ultrasonic pulse and the receiver receives a reflected ultrasonic pulse. 
         [0025]    Preferably the control electronics monitor the received signal from the receiver of the second monitoring device received within in a particular time window, that time window being based on a maximum and minimum expected time of flight of the ultrasonic pulse from the transmitter, to the needle and back to the receiver. Preferably the time window exceeds the expected window by a factor relating to a margin of error. 
         [0026]    Preferably the sensor of the second monitoring device is configured to identify ultrasonic reflections from first and second surfaces, one of which is the vascular access device, and the sensor detects a change in the received signals. 
         [0027]    Preferably the vascular access device is located beneath the sensing section of the second monitoring device, the reflected signal shows altered characteristics. More preferably, by comparing subsequent received signals received from the receiver of the second monitoring device, a change in either amplitude or frequency content of the received signal can be identified by the control electronics and a warning can be raised that the vascular access device has become dislodged. Alternatively, by comparing the received signal to the transmitted signal, characteristics of the received signal can be identified by the control electronics of the second monitoring device and a warning can be raised when the vascular access device becomes dislodged. As a further alternative, by comparing the received signal from the receiver of the second monitoring device to a model received signal, characteristics of the received signal can be identified by the control electronics of the second monitoring device and if they deviate from the model signal more than a prescribed amount, a warning can be raised that the vascular access device has become dislodged. 
         [0028]    Preferably the amplitude, or frequency, of at least a part of the reflected signal received by the receiver of the second monitoring device, changes dependant on the presence of the vascular access device. 
         [0029]    Preferably the device further comprises an ultrasonic coupling means for, in use, ultrasonically coupling the second monitoring device to a patients skin. 
         [0030]    Preferably the device further comprises means for connection to a medical apparatus. 
         [0031]    Preferably, when dislodgement of the vascular access device is sensed, he control electronics sends a signal indicative of that to a medical apparatus, the signal forming a part of the control algorithm for the medical apparatus. 
         [0032]    Preferably the control electronics are configured to periodically receive an interrogation from a medical apparatus to perform a self diagnosis check, to carry out a self diagnosis check and to send a signal to the medical apparatus indicative that the diagnosis check was successful and the device is working properly. 
         [0033]    The invention will now be described, by way of example, with reference to the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a perspective view of one embodiment of the device in use; 
           [0035]      FIG. 2  is a cross section through same embodiment of the device in use; 
           [0036]      FIG. 3  is a perspective view of an alternative embodiment of the device having two acoustic sensors; 
           [0037]      FIG. 4  is a cross section of an embodiment of the device also having ultrasonic sensing; 
           [0038]      FIGS. 5 and 6  are diagrams of ultrasonic signals. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    Referring to  FIGS. 1 and 2  the device  1  is shown attached to a patient&#39;s arm  2  by means of an attachment strap  3 . The device  1  is located adjacent the entry point of a needle  4  which is connected to a medical apparatus  5  by means of a conduit  6 . The medical apparatus  5  can be used to either add fluid to or remove fluid from the patient via the needle  4 . 
         [0040]    Referring to  FIG. 2  a cross section through the device in use is shown. The device  1  is positioned adjacent a patients arm. In use an acoustic sensor  8  picks up acoustic signals generated by the flow of fluid through the needle  4  and also background noise generated, at least in part, by the flow of blood through he veins of the patient. The acoustic sensor  8  can be any type of sensor commonly used in electronic stethoscopes and functions in a similar manner. The sensor  8  generates an electrical signal indicative of the sensed sound and transmits the signal to control electronics  7 . 
         [0041]    The control electronics  7  contain filters to filter the received signal to remove the majority of unwanted background noise. This may be done with passive low pass electronic filters or a more complex active filtering may be used. 
         [0042]    Where active filtering is required the sensor monitors the background noise and identifies, within the background noise, noise created by the beat of the heart. This noise can then be subtracted from the signal to give a signal representative of flow through the needle. Where the needle  4  is being used to remove or to return blood from or to the patient the flow through the needle  4  will be cyclic dependant on the pump being used. The sensor  8  measures the background noise between cycles of the blood pump and the control electronics  7  subtract the background noise from the signal created during the pump cycle. Optionally the control electronics  7  can constantly monitor the signal. Within the signal they can identify the timing of the sound pulse created by the beat of the heart and the sound pulse created by the cyclic flow driven by the blood pump. In this configuration the control electronics  7  receive a signal from the blood pump identifying when it is cycling enabling the electronics to identify the signal coinciding with the pumping. As the pump cycle and be heartbeat will not be synchronised, and as the sound pulse associated with the pump cycle can easily be identified the remaining sound pulse will be indicative of the heart beat. In the simplest monitoring device  1  these signals are monitored and, as the sensor  8  is adjacent the patients skin above the needle  4 , if the needle  4  becomes dislodged then the pulse identified as coming from flow induced by the blood pump will be lost and an alarm can be raised. The alarm can be raised by any one of a number of known techniques, for example an audible alarm or visual alarm may be incorporated into the device  1 , or alternatively where a wireless or wired communication is included as described below the alarm may be raised on a separate medical apparatus or a dedicated alarm unit. 
         [0043]    In a more complex device, once the signal representative of the flow through the needle  4  has been identified, it can be monitored for changes in amplitude or frequency which may indicate an occlusion or partial occlusion, the occlusion or partial occlusion resulting in a reduced flow through the needle  4  and a corresponding change in signal. In this configuration the device  1  is powered by a battery  11  which is encapsulated within the device  1 . The battery may be recharged by an inductive recharger. In this way the battery can be recharged without needing to open the device to remove the battery. Furthermore, as the device is totally encapsulated it is easy to clean and/or sanitise between uses. 
         [0044]    The device  1  further comprises a wireless communications module  10  by which the device can wirelessly communication with the control system of a medical apparatus. In this manner it can receive the signals from the blood pump such that it can perform the identification and separation of noise associated with flow through the needle  4  and background noise described above. Furthermore, in this way, if the needle  4  becomes dislodged from the patient, the device  1  senses it and sends a signal to the medical apparatus  5  to which the needle  4  is connected. The apparatus  5  can then take appropriate action, for example it may sound a warning and/or may stop the medical process using the needle  4 . For example, if the needle  4  is being used to remove blood from the patient then if the needle  4  becomes dislodged then the medical apparatus can sound a warning to alert a nurse and can also stop the medical process. 
         [0045]    Referring to  FIG. 3  an alternative arrangement of a device according to the invention is shown. In this arrangement two devices  1   a ,  1   b  are used. Device  1   a  is positioned on the patients arm adjacent the vascular access point  12  and sensor  1   b  is positioned adjacent the patients arm at a position discreet from the vascular access point. Both devices are substantially as described with reference to  FIG. 2 , except in so far as they communicate wirelessly with one another so that the signals can be processed together. In addition wireless communication between the sensors can be used periodically or constantly to ensure that the system is functioning correctly. In this arrangement the signal generated by device  1   a  will contain two signals superimposed on top of one another, the signal representative of sound caused by the flow of fluid through the needle, and the signal representative of background noise including that associated with heartbeat. The signal generated by device  1   b  will contain the signal representative of background noise. The signal from device  1   b  is communicated to the control electronics of device  1   a  by wireless communication means. By subtracting one signal from the other a clean signal representative of the noise created by flow through the needle  4  can be achieved. This signal can then be monitored for any changes in amplitude, frequency or frequency distribution that may indicate that the needle  4  has become dislodged from its position, or that the flow path from the needle  4  has become at least partially occluded. In this arrangement as a cleaned signal is being monitored for changes it is not necessary for the control electronics of the sensor to receive data from the pump. Depending on the characteristic of the signal being monitored, the control electronics  7  may perform functions commonly used in the field of signal processing, for example Fourier transforms, to the signal to best isolate and identify changes. Such signal processing is common to those skilled in the art of signal electronics. 
         [0046]    Referring to  FIG. 4  an arrangement of the apparatus is shown providing the arrangement described in relation to  FIGS. 1 and 2  in combination with an ultrasonic needle monitoring system. By this method having two independent needle monitoring methods the two systems can be verified against one another to confirm the presence or displacement of the needle. In addition to the details described above, the device further includes a thin gel pack  13  which comprises an ultrasonic coupling gel in a thin flexible plastic pouch. The gel pack  13  conforms to the surface of the patient&#39;s arm  2  and to the device  1  such that the space between the patients arm  2  and the device  1  is substantially filled with ultrasonic coupling gel. In use an ultrasonic transceiver  14  transmits an ultrasonic pulse through the gel pack and into the patient&#39;s arm  2 . The gel pack  13  has walls made of a flexible membrane that has a thickness less than one third of the wavelength of the ultrasound transmitted from the device and the composition of the gel is such that it has an optimum transmission frequency substantially that of the transmission frequency of the device. The patient&#39;s arm will absorb some of the ultrasonic signal and reflect some of the signal. The transceiver also receives the reflected signal. The transceiver is connected to control electronics  7   a  which control the transmitter to transmit ultrasound and receive signals indicative of the received reflected signal from the transceiver  14 . The control electronics then compare the received signal, either to the transmitted signal or to a model signal stores in storage means  15  connected to the control electronics  7   a  to detect if it shows characteristics relating to absorption and reflection by the patient&#39;s arm  2 , or if it also shows characteristics relating to reflection by a needle  4 . Signals relating to a needle  4  will be clearly distinguishable from signals relating to just the patients body as a needle  4 , in particular a metal needle as is commonly used, will reflect a much greater amount of ultrasound. The acoustic sensor  8 , is designed not to receive sound frequencies in the ultrasound spectrum such that the ultrasonic transducer does not interfere with the acoustic needle monitoring. 
         [0047]    Referring to  FIGS. 5 and 6 , signal comparisons typical of those use in the device are shown.  FIG. 5  shows a typical ultrasonic pulse emitted by the device and  FIG. 6  shows two signals, one relating to the reflection of ultrasound by the patients body and the other showing how this changes when a metal needle is present. The control electronics can compare the shape of the received signals and easily identify whether there is a needle present or not. 
         [0048]    Other embodiments will be obvious to the person skilled in the art and are included in this invention, for example, the wireless connection could be replaced with a wires connection and the battery could be replaced with a connection to a remote power source.