Abstract:
A continuous, noninvasive fetal heart rate measurement is produced using one or more ultrasonic transducer adhered to the abdomen of the mother. Each ultrasound transducer generates an ultrasound beam having a signal strength. The signal strength is determined by an excitation voltage applied to the ultrasound transducer. An excitation voltage adjustment device is positioned between an excitation voltage generator and the ultrasound transducer to selectively control the strength of the ultrasound beam. A user input device allows an operator to control the ultrasound signal strength to vary the depth of viewing of the fetal heart rate monitor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present disclosure generally relates to methods and apparatus for determining the heart rate of a subject. More specifically, the present disclosure particularly relates to a method and apparatus for determining the beat-to-beat heart rate of a fetus. 
         [0002]    Fetal monitoring (i.e., monitoring of the fetal condition during gestation and at birth) usually comprises monitoring uterine activity and the fetal beat-to-beat heart rate. The fetal heart rate, which provides an indication of whether the fetus is sufficiently supplied with oxygen, is preferably calculated from beat to beat. 
         [0003]    To obtain a signal indicative of the fetal heart rate prior to rupture of the membranes, a noninvasive monitoring technique must be used. The most widely adopted measurement technique involves measuring the Doppler shift of an ultrasound signal reflected by the moving fetal heart. 
         [0004]    In accordance with a known ultrasonic detection technique, an ultrasound transducer is placed externally on the pregnant woman&#39;s abdomen and oriented such that the transmitted ultrasound waves impinge upon the fetal heart. The reflected ultrasound waves are received either by the same or by a different ultrasound transducer. The Doppler shift of the reflected ultrasound wave is directly related to the speed of the moving parts of the heart, e.g., the heart valves and the heart walls. 
         [0005]    Although the Doppler ultrasound is widely accepted and generally accepted method of monitoring fetal heart rate, ultrasound fetal heart rate monitoring has several drawbacks. One of these drawbacks is that the ultrasound fetal monitor transducer may not be able to monitor the fetal heart rate of a fetus in the case of an obese mother since the distance from the mother&#39;s skin surface to the fetal heart may be greater than the monitoring depth of the fetal heart rate monitor. Alternatively, ultrasonic fetal heart rate monitors that use a higher dose of ultrasound energy to increase the depth of sensing expose normal or underweight patients to a higher degree of ultrasonic energy than may be otherwise required. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    The present disclosure relates to a method and apparatus for determining the beat-to-beat heart rate of a fetus. In a disclosed embodiment, the continuous, non-invasive fetal heart rate measurement is produced using one or more ultrasonic transducers that are adhered or attached to the abdomen of a pregnant patient. Each ultrasound transducer generates an ultrasound beam that is reflected by the fetal heart and received by one or more of the ultrasound transducers. Based upon the received signal, the fetal heart rate monitor generates the heart rate of the fetus. 
         [0007]    The fetal heart rate monitor of the present disclosure includes an excitation voltage generator that generates a standard excitation voltage. The excitation voltage from the excitation voltage generator is received by an excitation voltage adjustment device. The excitation voltage adjustment device, in turn, is connected to a controller that is operable to control the operation of the excitation voltage adjustment device. 
         [0008]    During operation of the fetal heart rate monitor, an excitation voltage is initially applied to the ultrasound transducer. The signal strength of the ultrasound beam from each of the transducers is directly related to the excitation voltage. 
         [0009]    If the strength of the ultrasound beam is insufficient to detect the fetal heart rate, a user can operate a user input device to indicate that the strength of the ultrasound beam needs to be increased. When the controller of the fetal heart rate monitor receives such a signal from the input device, the controller provides a signal to the excitation voltage device to increase the excitation voltage. 
         [0010]    When the excitation voltage is increased by the excitation voltage adjustment device, the strength of the ultrasound beam from the ultrasound transducers increases, thereby increasing the depth of viewing for the fetal heart rate monitor. The controller operates a power level display to graphically illustrate to the operator the current signal strength from the ultrasound transducers relative to a maximum level. 
         [0011]    The user can continue to increase the signal strength of the ultrasound beam until the fetal heart rate is detected. Once the fetal heart rate is detected, the heart rate is displayed and the user can allow the signal strength to remain at the current level. In this manner, the signal strength of the ultrasound beam is optimized for each individual patient such that each patient receives only the required ultrasound level needed to detect the fetal heart rate. 
         [0012]    Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The drawings illustrate the best mode presently contemplated of carrying out the disclosure. In the Figures: 
           [0014]      FIG. 1  depicts a pregnant patient utilizing fetal heart rate monitor; 
           [0015]      FIG. 2  is a schematic illustration of the ultrasound power control system of the present disclosure; 
           [0016]      FIG. 3  is one embodiment of the excitation voltage adjustment device; 
           [0017]      FIG. 4  is a second embodiment of the excitation voltage adjustment device; and 
           [0018]      FIG. 5  is a graphic display of the power level of the ultrasound beam. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  illustrates a fetal heart rate monitor  10  that can be used to monitor the heart rate of the fetus of a pregnant patient  12 . Although the fetal heart rate monitor  10  is shown in  FIG. 1  in one exemplary form, it should be understood that the fetal heart rate monitor  10  could take many other forms while operating within the scope of the present disclosure. 
         [0020]    In the embodiment of  FIG. 1 , the fetal heart rate monitor  10  includes an ultrasound probe  14  that is secured to the patient&#39;s abdomen  16  by a strap  18 . The ultrasound probe  14  is shown in the embodiment of  FIG. 1  as being coupled to the fetal heart rate monitor  10  by cable  20 . However, it is contemplated that the fetal heart rate monitor  10  could communicate with the ultrasound probe  14  using a wireless communication technique. 
         [0021]    The fetal heart rate monitor  10  is shown in  FIG. 1  as including a display screen  22  that typically displays the monitored heart rate of the fetus. The display screen  22  can be configured to display other monitored signals obtained from the patient  12 . 
         [0022]    During operation, when the fetal heart rate monitor  10  is powered on, one or more ultrasound transducers contained within the ultrasound probe  14  each generate an ultrasound beam directed into the patient  12  through the skin of the abdomen. The fetal heart rate monitor  10  monitors the ultrasound signal returned to either the same or a different ultrasound transducer contained within the ultrasound probe  14  to detect the beating of the fetal heart. Based upon data acquired from the ultrasound probe  14 , the fetal heart rate monitor  10  calculates the fetal heart rate and displays the calculated fetal heart rate on the display  22  in a known manner. 
         [0023]    Referring now to  FIG. 2 , the detailed operation of the fetal heart rate monitor  10  will now be described. As illustrated in  FIG. 2 , the ultrasound probe  14  is positioned on the exterior surface of the patient&#39;s abdomen  16 . In the embodiment shown in  FIG. 2 , the ultrasound probe  14  includes multiple ultrasound transducers  24 . Each transducer  24  is operable to both generate an ultrasound beam  26  and receive reflected ultrasound energy from the fetal heart. In one embodiment of the disclosure, each of the ultrasound transducers  24  is a piezoelectric crystal that vibrates to create the ultrasound beam  26  emanating from the ultrasound transducer. The vibration of the piezoelectric crystal is created by an excitation voltage applied to the piezoelectric crystal through a voltage supply line  28 . 
         [0024]    Although in the embodiment shown in  FIG. 2  each of the ultrasound transducers  24  is able to both transmit the ultrasound beam and receive the reflected ultrasound energy, the ultrasound probe  14  could utilize separate transducers for transmitting and receiving the ultrasound energy. 
         [0025]    During operation of the fetal heart rate monitor  10 , the ultrasound transducers  24  generate the ultrasound beam  26  that penetrates the patient&#39;s abdomen  16  and travels into the pregnant patient until the ultrasound signal is reflected by the beating fetal heart  30 . As illustrated in  FIG. 2 , the distance A from the outer surface of the abdomen  16  to the fetal heart  30  must fall within the range of detection for the ultrasound transducers  24 . The range of detection of the ultrasound transducers  24  is directly related to the signal strength of the ultrasound beam  26 . In turn, the strength of the ultrasound beam  26  is directly related to the voltage level of the excitation voltage applied to the ultrasound transducers  24  along the voltage supply line  28 . If the position of the fetal heart  30  is outside of the detection range of the ultrasound transducers  24 , the fetal heart rate monitor  10  is unable to detect the heart rate of the fetus. In currently available fetal heart rate monitors, the value of the excitation voltage is selected such that the sensing distance of the ultrasound probe is sufficient to detect the fetal heart rate in a normal pregnant patient. 
         [0026]    When the fetal heart rate monitor  10  is used with an obese patient, the distance A from the patient&#39;s abdomen  16  to the fetal heart  30  can be much greater than with a relatively thin or normal patient. 
         [0027]    Referring now to  FIG. 2 , the fetal heart rate monitor  10  of the present disclosure includes circuitry that allows the power output, and thus the monitoring depth, of the ultrasound probe  14  to be selectively modified by a user. The selective modification of the power output of the ultrasound probe  14  allows the ultrasound probe  14  to detect the fetal heart rate at varying distances from the patient&#39;s abdomen  16 . Further, the fetal heart rate monitor  10  of the present disclosure allows an operator to control the amount of ultrasound power delivered to the pregnant patient. 
         [0028]    As illustrated in  FIG. 2 , the fetal heart rate monitor  10  includes an ultrasound excitation voltage generator  32 . The excitation voltage generator  32  generates the typical excitation voltage that is used to drive the piezoelectric crystals that are incorporated into the ultrasound transducer  24 . The excitation voltage is sinusoidal voltage that is generated along voltage line  34 . In prior fetal heart rate monitoring systems, the excitation voltage along voltage line  34  is applied directly to the ultrasound transducers  24 . In such a prior art system, the excitation voltage level is fixed and cannot be modified by the user of the fetal heart rate monitor. 
         [0029]    In the embodiment shown in  FIG. 2 , an excitation voltage adjustment device  36  is positioned between the excitation voltage generator  32  and the ultrasound transducers  24 . The excitation voltage adjustment device  36  receives the excitation voltage along line  34  and is operable to selectively amplify or reduce the excitation voltage as desired. The excitation voltage adjustment device  36  receives a voltage adjustment control signal from a controller  38  along a control line  40 . In the embodiment illustrated, the controller  38  generates a control signal along line  40  that controls the voltage adjustment device  36  to selectively increase or decrease the excitation voltage from the excitation voltage generator  32 . The modified excitation voltage from the voltage adjustment device  36  is provided to the ultrasound transducer  24  along the voltage supply line  42 . 
         [0030]    In the embodiment of the disclosure shown in  FIG. 2 , the controller  38  is a microprocessor that can generate digital signals along the control line  40  to the excitation voltage adjustment device  36 . Although the controller  38  is shown as a microprocessor, the controller  38  could be a microcontroller, FPGA and CPLD while operating within the scope of the disclosure. In the embodiment of  FIG. 2 , a user input device  44  is coupled to the controller  38  such that a user, such as a clinician, can control the modification of the excitation voltage by the excitation voltage adjustment device  36 . In one embodiment of the disclosure, the input device  44  is a track ball. The controller  38  senses the movement of the track ball that forms the input device  44  and generates a control signal to the excitation voltage adjustment device  36  to either increase or decrease the excitation voltage. Although the input device  44  is contemplated as being a track ball, the input device  44  could take various other forms while operating within the scope of the present disclosure. As an example, the input device  44  could be an adjustable dial slide switch or a touch screen incorporated as part of the display screen for the fetal heart rate monitor  10 . 
         [0031]    As discussed previously, the value of the excitation voltage directly impacts the signal strength of the ultrasound beam  26 . Thus, if the strength of the ultrasound beams  26  needs to be increased to increase the depth of viewing, the operator moves the input device  44  in the direction to increase the ultrasound signal strength. The controller  38  provides a control signal along line  40  to the excitation voltage adjustment device  36  to increase the excitation voltage. The user can continue to increase the strength of the excitation voltage until the fetal heart rate is detected and displayed on the heart rate display  22 . Once the fetal heart rate has been detected, the clinician can discontinue the increase in the excitation voltage, and thus the ultrasound signal strength. In this manner, the clinician, through the fetal heart rate monitor  10 , utilizes only the required ultrasound signal required to detect the fetal heart rate. 
         [0032]    As the input device  44  is activated to increase the signal strength of the ultrasound beam, the controller  38  can generate a feedback signal along line  48  to a power level display  50 . The power level display  50  allows the user to visually determine the signal strength of the ultrasound beam on a visual display.  FIG. 5  illustrates the power level display  50  in accordance with one embodiment. In the embodiment of  FIG. 5 , the power level display  50  is a bar having demarcations between 0 and 100% of the signal strength. A moving indicator line  52  indicates the current signal strength. 
         [0033]    Although the power level display  50  and the heart rate display  22  are shown separate in  FIG. 2 , it should be understood that the two displays could be shown on the same display screen, as is illustrated in  FIG. 1 . Further, in the embodiment illustrated in  FIG. 1 , the input device  44  is shown as being incorporated directly into the heart rate monitor  10 . Additionally, the controller  38  shown in  FIG. 2  as controlling the excitation voltage adjustment device  36  could either be separate or integrated into the controller or the entire fetal heart rate monitor  10 . 
         [0034]    Referring back to  FIG. 2 , the controller  38  can preferably include a feedback line  54  such that the controller  38  can monitor the modified excitation voltage present on the voltage supply line  42 . Through the feedback line  54 , the controller can monitor the modified excitation voltage and limit the maximum value of the excitation voltage supplied to the ultrasound transducers  24 . In this manner, the controller  38  can limit the maximum strength of the ultrasound signal supplied to the pregnant patient. 
         [0035]    Referring now to  FIG. 3 , a first embodiment of the excitation voltage adjustment device  36  is illustrated. In this embodiment, the excitation voltage present along line  34  is lower than a desired value to be fed to the ultrasound transducers. In the voltage adjustment device  36  shown in  FIG. 3 , an amplifier  56  receives the excitation voltage from line  34  and amplifies the voltage, which is then output along the voltage supply line  42 . In the simplified embodiment shown in  FIG. 3 , a variable resistor  58  is connected to the controller  38 . The controller  38  can adjust the value of the resistor  58  to control the gain of the amplifier  56 . It should be understood that the embodiment shown in  FIG. 3  is a schematic illustration only and could take many different forms while operating within the scope of the present disclosure. However, the embodiment of  FIG. 3  illustrates that the excitation voltage adjustment device  36  could be an amplification circuit that amplifies the excitation voltage on line  34  to generate the modified excitation voltage along the voltage supply line  42 . 
         [0036]    Referring now to  FIG. 4 , an alternate embodiment of the voltage adjustment device  36  is illustrated. In this embodiment, the excitation voltage along line  34  is fed into a voltage reduction circuit  60 . The voltage reduction circuit  60  is a voltage divider including a variable resistor  62  that forms one-half of a simple voltage divider. The variable resistor  62  is coupled to the controller  38  through the control line  40 . The controller  38  is able to control the value of the resistor  62  to modify the excitation voltage that is present along the voltage supply line  42 . Once again, the circuitry of the embodiment shown in  FIG. 4  is simplified for illustrative purposes only. However, it should be understood that the voltage adjustment device  36  shown in  FIG. 4  reduces the excitation voltage from an elevated value to the desired value supplied to the ultrasound transducer. 
         [0037]    In an alternate embodiment of the disclosure, the controller  38  can monitor the ultrasound signal received from the ultrasound probe  14  and provide a control signal along line  40  to the excitation voltage adjustment device  36  to either increase or decrease the excitation voltage based upon the received signal. In such an example, the controller  38  determines the strength of the ultrasound signal received and, if the signal strength is below a predetermined threshold, the controller  38  increases the excitation voltage. This process continues until the received ultrasound signal reaches the predetermined threshold. Likewise, if the ultrasound signal received from the probe  14  exceeds the predetermined threshold, the controller  38  can automatically decrease the excitation voltage until the received signal drops to the predetermined threshold. In such a manner, the controller  38  can automatically control the excitation voltage based upon a feedback signal received from the probe  14 . It is contemplated that the fetal heart rate monitor  10  could include some type of input device that allows the monitor to toggle between either a manual mode or a servo mode depending upon specific requirements from the operator. 
         [0038]    As can be understood by the previous description, the fetal heart rate monitor  10  of the present disclosure allows an operator to adjust the signal strength of the ultrasound beams such that only the required dose of ultrasound energy is supplied to the patient to detect the fetal heart rate. When the fetal heart rate monitor  10  is utilized with a small, underweight patient, the signal strength can be significantly reduced. Likewise, when the fetal heart rate monitor is utilized with an obese patient, the signal strength can be greatly increased to increase the depth of viewing to detect the fetal heart rate. In this manner, the fetal heart rate monitor  10  of the present disclosure can be utilized with a larger variety of pregnant patients as compared to currently available devices. 
         [0039]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.