Patent Publication Number: US-2005119533-A1

Title: Radiofrequency adapter for medical monitoring equipment

Description:
TECHNICAL FIELD  
      The present invention relates to medical monitoring equipment. As used herein, the term “medical monitoring equipment” means equipment designed to receive and analyse signals from sensors connected to or associated with a patient, to monitor one or more medical conditions of that patient. A wide range of medical monitoring equipment is currently available, for monitoring one or more of a number of different conditions. Typically, medical monitoring equipment has the ability to record the analysis of signals from sensors, and often includes an alarm system to alert medical staff of undesirable or dangerous changes in the patient&#39;s condition.  
     BACKGROUND ART  
      In the most commonly used types of medical monitoring equipment, a sensor designed to sense the selected condition is attached to the patient and is connected to a monitor by a cable. In use, the sensor generates an analogue signal corresponding to the condition sensed in the patient, and transmits this analogue signal to the monitor via the cable. The monitor then processes/analyses/records readings obtained from the signal. This type of equipment is in widespread use in hospitals throughout the world, and is both reliable and efficient. However, it has the drawback of requiring a cable between the patient being monitored and the monitor. Whilst this is acceptable for short periods of monitoring, it causes numerous difficulties when used over longer periods, since it restricts the scope of the patient&#39;s movements. If the patient is asleep or unconscious, the patient may move so as to dislodge the monitor.  
      To overcome this problem, wireless monitors have been developed, but to date such monitors have not been widely adopted, partly because they are expensive and partly because most hospitals already have a considerable investment in the wired monitors.  
     DISCLOSURE OF INVENTION  
      It is therefore an object of the present invention to provide a radiofrequency adapter for medical monitoring equipment, the adapter permitting the use of known types of sensor and known types of monitor, but without requiring the use of connection cables between the sensor and the monitor. The adapter of the present invention thus permits hospitals to upgrade their existing equipment inexpensively, without any reduction in reliability.  
      The present invention provides a radiofrequency adapter for medical monitoring equipment which includes: a controller adapted to be physically connected to a sensor; and an integrator adapted to be physically connected to a medical monitor; the controller being physically separated from the integrator;  
      wherein the controller provides:  
     
         
         
           
              a) signal conditioning and digitising means adapted to receive, condition and digitise signals received from the sensor;  
              b) a radiofrequency transmitter adapted to receive digitised signals from said signal conditioning and digitising means and transmit said signals to said integrator by means of a wireless radiofrequency link;  
              c) a battery power supply for said signal conditioning and digitising means and radiofrequency transmitter, 
 
 and wherein said integrator provides: 
 
              d) a radiofrequency receiver adapted to receive digital radiofrequency transmissions from said radiofrequency transmitter;  
              e) converting means for converting digital signals received by said receiver to analogue signals;  
              f) means for transmitting the analogue signals to a monitor physically connected to said integrator.  
           
         
       
    
      Preferably, each of the radiofrequency transmitter and the radiofrequency receiver is a radiofrequency transceiver; and each of said signal conditioning and digitising means and said means for converting a digital signal is adapted to convert signals both from analogue to digital and from digital to analogue.  
      Preferably the integrator further provides sensor control signal sampling means adapted to receive sensor control signals from a monitor connected to the integrator and to transmit said sensor control signals as digitised signals to said controller via said converting means and said radiofrequency transceiver in the integrator; and the controller further provides sensor control signal reconstruction means adapted to receive said sensor control signals from the radiofrequency transceiver in the controller, converted to analogue signals by said signal conditioning and digitising means, and to pass said analogue sensor control signals to the sensor.  
      The adapter of the present invention may be used in combination with any of a wide range of known medical monitoring equipment and the corresponding sensor, for example: 
          a) pulse oximetry monitors for measuring the oxygenation levels of the blood.     b) electrocardiograph monitors for measuring cardiac activity.     c) respiration monitors for measuring respiration (e.g. via pressure transducers or airflow or inductance plethysmography or piezoelectric strain gauges).     d) capnography monitors for measuring the carbon dioxide content of breath.     e) blood pressure monitors for measuring blood pressure e.g. using an inflatable cuff.     f) temperature monitors the measuring the body temperature using a thermometer.        

      The sensors normally used in combination with the monitors listed above are of known type. It will be appreciated that the type of sensor will vary in accordance with the particular characteristic being sensed, but since all of the sensors designed for use with existing medical monitoring equipment operate by producing an analogue signal as described above, any of the sensors may be used in combination with the controller of the present invention, if necessary subject to suitable modification of the controller to receive the physical connection from the sensor and adjustment of the signal conditioning and digitising means as appropriate for the signals from the sensor.  
      The controller may be mounted in any convenient place on the patient e.g. on one of the patient&#39;s limbs, or round the patient&#39;s neck, or around the patient&#39;s waist. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      By way of example only, preferred embodiment of the present invention are described in detail, with reference to the accompanying drawings, in which:  
       FIG. 1  is a block diagram showing the apparatus and signal processing steps of the present invention;  
       FIG. 2  is a block diagram showing the apparatus and signal processing steps of the present invention as applied to a pulse oximeter monitor; and  
       FIG. 3  is a diagrammatic exploded view of the adapter of the present invention, as applied to a pulse oximeter monitor. 
    
    
      Referring to  FIG. 1 , an adapter in accordance with the present invention consists of a controller  52  and integrator  53 ; these two components are designed to be used together but are not physically connected to each other.  
      The controller  52  is mounted in a housing suitable for attaching to the patient, e.g. by means of a wrist or ankle strap, or by means of a neck strap. The controller is provided with a suitable port  54  through which one or more sensors  55  are physically connected to the controller. The housing of the controller  52  contains a power supply  56  which is electrically connected to a central processing unit (CPU)  57 , a sensor signal conditioner  58 , an analogue/digital (A/D.) converter  59 , a radiofrequency transmitter  60  and (optionally) a sensor control signal reconstruction unit  61 . These components are interconnected as shown in  FIG. 1 , but for clarity the electrical connections between the power supply  56  and the other components have been omitted.  
      The power supply  56  normally is a battery but, depending upon the intended use of the controller, it may be preferred to provide both a battery and a mains connection, so that the controller  52  can be directly mains powered if necessary. Obviously, using mains power to the controller  52  negates the principal advantage of the invention, and the controller would not normally be used in this manner. However, if the adapter is going to be used for long periods under conditions where accurate monitoring is vital (e.g. during a surgical operation), then the ability to use a mains connection is essential, in case the battery runs flat during the monitoring.  
      The integrator  53  is adapted to be physically connected to any of a range of known types of medical monitoring equipment, either by mounting the integrator  53  directly on the monitor, or by a cable connection. The integrator  53  may include a battery (not shown) but preferably would be powered by the monitor to which it was attached.  
      The integrator contains a radiofrequency receiver  82 , a digital/analogue (D/A) converter  63 , a sensor signal reconstruction unit  84 , a central processing unit (CPU)  65 , and (optionally) a sensor control signal sampler  66 . These components are interconnected as shown in  FIG. 1 .  
      The above described equipment operate as follows: the or each sensor  55  is attached to the patient in the appropriate manner, depending upon the nature of the sensor and the condition being sensed. The controller  52  is attached to the patient, and the or each sensor  55  is connected to the controller  52  via the port  54 . The integrator  53  is connected to the monitor  67  either directly or via a cable.  
      The signal from the or each sensor  55  is received by the sensor signal conditioner  58 , which manipulates the signal if necessary: typically, the sensor signal conditioner would receive the signal, which would then be amplified and filtered and passed to the A/D, converter  59 , which converts the analogue signal to a digital signal. The digital signal is then passed to the radiofrequency transmitter  60 , which transmits the digital signal via the wireless RF link to the radiofrequency receiver  62  in the integrator  53 . The CPU  57  in the controller  52  may be used to further condition the data contained in the digital signals, and the CPU also may store the data and control the intervals at which the transmitter  60  transmits the data.  
      The digital signal received by the receiver  82  is passed to the D./A. converter  63  and converted back to an analogue signal. Since the sampling rate of the controller  52  may not correspond exactly to the sampling rate of the monitor  67 , the digital signal received by the receiver  62  may be buffered by the CPU  65 , by transmitting the digital signal initially to the CPU so that the CPU can output the signal to the D./A. converter  53  at a rate suitable for reception by the monitor  67 . The sensor signal reconstruction unit  64  may be used to manipulate the signal for clarity before the signal is passed to the monitor  67 . When the signal is received by the monitor  67 , it is processed in known manner to get a standard reading as provided by that type of monitor.  
      Some types of sensor  55  need to be controlled by the monitor  67 . This is provided by sending control signals from the monitor  67  to the sensor specific control signal sampler  66  which then generates signals for controlling the sensors in response to instructions from the monitor  67 . Such signals ere passed to the CPU  65 , digitized by the D./A. converter  63 , and are then transmitted to the receiver  62 . If control signals of this type are needed, the transmitter  60  and receiver  62  in fact both are transceivers (i.e. transmitter/receivers), so that on receiving a sensor specific control signal, the transceiver  62  can transmit the signal back across the radiofrequency link to the transceiver  60 , from which the control signal is passed to the CPU  57 , converted to analogue by the A./D. converter  59 , passed to the sensor specific control signal reconstruction unit  81  and thus to the sensor  55 .  
      It should be noted that the D./A. and A./D. converters both can convert signals in either direction.  
      If the sensors  55  do not require control signals from the monitor, then the sensor control signal reconstruction unit  61  and the sensor control sampler  66  are not needed and may be omitted from the controller and integrator respectively.  
       FIGS. 2 and 3  illustrate an embodiment of the invention designed specifically for pulse oximetry.  
      Oximetry relies on the change in the absorption of electromagnetic energy with change in the percentage of oxygen bound to the haemoglobin molecule in blood. The pulse oximeter functions by comparing the light absorption of fully oxygenated and fully deoxygenated haemoglobin passed through a capillary bed. All currently available conventional pulse oximeters use a combination of two wavelengths, normally 660 nm (red) and 940 nm (near infrared), generated in the sensor by a pair of light emitting diodes. The light is measured with a miniature semiconductor photodetector also in the sensor. The signal in either the red or infrared channels is due to the absorption of some of the energy during its transit from light emitting diode to photodetector. The electronics in a pulse oximeter perform the following functions: 
          a. Amplification of the photodetector signal     b. Separation of the red and infrared plethysmograph signals     c. Switching and control of current through light emitting diodes     d. Adjustment of the gain of one of the two signals to make them equivalent     e. Separation of the pulsatile (or “arterial”) composition of the signal     f. Analogue to digital conversion of the red and infrared signals     g. Calculation of the red:infrared ratio     h. Calculation of the oxygen saturation (SpO 2 ) 
            i. Oxygen saturation=AR 2 +BR+C where: R=(AC R /AC IR )/(DC R /DC IR )     1. where AC R  and DC R  are respectively the AC and DC components of the red photodetector signal, AC IR  and DC IR  are respectively the AC and DC components of the near infrared photodetector signal, and A, B, and C are constants determined by curve fitting against the results of standard blood oxygen measurements.    
            i. Display: 
            i. SpO 2       i.i. Heart rate     i.i.i. Signal ‘strength’ or pulse detection    
            j. Control of alarms     k. Storage of trend of SpO 2  for averaging purposes        

      Pulse oximetry sensors consist of a transmission sensor, which is designed to be secured over a thin part of the body (generally a finger or toe), such that a portion of the sensor carrying red and infrared light sources lies on one side of the body part, and a portion of the sensor carrying red and infrared light photodetectors lies on the opposite side of the body part. The photo detectors sense light from the red and infrared light sources modulated by passing through the body part and generate a corresponding analogue signal.  
      Referring to  FIG. 3  of the drawings, an adapter in accordance with the present invention consists of a controller  2  and an integrator  3 ; these two components are designed to be used together but are not physically connected to each other.  
      The controller  2  consists of a housing  4  which is provided with a securing strap  5  (e.g. a wrist strap) to enable the controller to be attached to the patient, close to the part of the patient&#39;s body which is carrying the sensor  6 . The sensor  6  is a pulse oximeter sensor of known type, formed as a finger stall with infrared and red light emitting diodes (LEDs)  7  mounted on one side and photo detectors  8  mounted an the other side. The sensor  6  is connected by a shielded cable  9  to a plug  10  which is connectable to a port  11  on the controller  2 .  
      The housing  4  of the controller  2  optionally has a display panel  12  (e.g. an LCD display) on its upper surface. The housing  4  contains a power supply  13  which is electrically connected to a central processing unit (CPU)  14 , a sensor-signal conditioner  15 , an analogue/digital (A/D) converter  16 , a radiofrequency transceiver  17 , and an LED driver (reconstruction unit)  18 . The components are connected as shown in  FIG. 2 ; for clarity, the electrical connections between the power supply  13  and the other components have been omitted.  
      The power supply  13  normally would be a battery, but for safety reasons may also incorporate provision for a mains connection, so that the controller  2  can be directly mains powered if necessary.  
      The integrator  3  is adapted to be physically connected to any of a range of known types of pulse oximeter monitors  20 , either by mounting the integrator directly on the monitor or by a cable connection.  FIG. 3  depicts connection by a cable  20   a . The pulse oximeter monitor  20  is of known type and will not be described in detail; however, it should be noted that the pulse oximeter monitor  20  is of a type which is designed to be physically connected to the pulse oximeter sensor  6 .  
      The integrator  3  may include a battery (not shown) but preferably would be powered by the monitor  20 .  
      The integrator  3  contains a radiofrequency transceiver  21 , a digital/analogue (D./A.) converter  22 , a sensor signal reconstruction unit  23 , a central processing unit (CPU)  24  and an LED drive current sampler  25 . The outer housing of the integrator provides a feedback interface  26  (e.g. an LCD display)  
      The above described equipment operates as follows: the sensor  6  is secured to a patient&#39;s finger in the usual manner, and the controller  2  is secured around the patient&#39;s wrist using the strap  5 . The sensor  6  is connected to the controller by means of the cable  9 . The integrator  3  is connected to the pulse oximeter monitor  20  as described above.  
      The LED driver  18  in the controller is powered by the power source  13  and controlled by the CPU  14  to supply power to the LEDs  7 , the power supply being intermittent so that the red and infrared LEDs  7  pulse an end off in known manner. The switching frequency of the LEDs is selected to allow the adapter to reproduce the analogue signal in the integrator  3  in a form and at a strength suitable for processing by the monitor  20 .  
      The photo detectors  8  an the sensor  6  sense the light from the LEDs as modulated by a passing through the patent&#39;s finger, as described above, and generate a corresponding analogue signal which passes to the controller  2  by the cable  9 . The brightness of both the red and infrared LEDs can be altered by the CPU  14  and driver  18  to optimise the detection of the light by the photo detectors  8 .  
      The signal from the photo detectors  8  is received by the sensor signal conditioner  15 , where the signal is manipulated if necessary: typically the signal received from the sensor would be amplified, filtered, and passed to the A./D. converter  16  where the analogue signal is converted to a digital signal. To improve the accuracy of conversion, a known D.C. current may be subtracted from the signal from the sensor; this known current varies dynamically and it adjusts the sample signal to be within a predetermined band of values to give the best accuracy in the digitised signal. The digital signal is received by the radiofrequency transceiver  17  at intervals controlled by the CPU  14 , which may further condition the data and may store the data in a buffer to ensure that the data is transmitted at the correct timing. The data is then transmitted to the radiofrequency transceiver  21  in the integrator  3 .  
      The signal transmitted from the transceiver  17  to the transceiver  21  normally would include other components as well: for example, a controller identification signal (in case of more than one controller is being used in a given area) and information on the status of the power supply  13 . The number of times per minute that the digital signal is transmitted is selected to achieve an optimum balance between maintaining the data from the sensor  6  up-to-date, managing the use of the radio bandwidth, and economical use of the power from the power supply  13 .  
      It should be noted that multiple controllers can be used within radio range of each other by use of well-known techniques (e.g. narrowband frequency sharing or random time transmissions). Each controller&#39;s transmissions are kept separate by the incorporation of the controller identification signal in each transmission.  
      The digital signal received by the transceiver  21  is passed to the D./A. converter  22  and converted back to an analogue signal. The sampling rate of the controller  2  may not correspond exactly to the sampling rate of the oximeter monitor  20 , so the digital signal received by the transceiver  21  may be buffered by the CPU  24 , which receives the digital signal and outputs it to the D./A. converter  22  at a sampling rate suitable for reception by the monitor  20 . The analogue signal may be further manipulated for clarity by the sensor signal reconstruction unit  23 , before being passed to the monitor  20 . When the signal is received by the monitor  20 , it is processed in known manner to get a standard oximetry monitoring reading.  
      The signal received by the monitor  20  may include additional information generated by the sensor e.g. calibration resistor values, and light intensity correction factors.  
      The LED drive current sampler  25  monitors the control current generated by the monitor  20 ; this feature is necessary only for some designs of monitor.  
      For the types of monitor which require this, the sampler  25  passes a signal back to the sensors  6  via the CPU  24 , converter  22 , transceiver  21 , transceiver  17 , converter  16  and reconstructor  18 .  
      The above described equipment could be modified to allow information to be displayed, for example using a liquid crystal display panel. A further possible modification would be to add further inputs to the controller to allow the reception and processing of signals from other sensors e.g. cardiac monitoring electrodes.  
      The present invention has been described from the viewpoint of using a separate adapter for each different type of monitor, but it will be appreciated that it would be possible to use a single adapter for two or more different monitors.