Abstract:
A defibrillator circuit that delivers high voltage electrical defibrillation pulses and lower energy pacing pulses to a patient. The circuit includes a current regulator which is operative to control the current level applied to the patient. Control circuitry is provided which permits the current regulator to be disabled in the defibrillation mode of operation prior to application of defibrillation pulses. An H-bridge supplies pulses to the patient and the current regulation is provided serially with the H-bridge such that current is regulated regardless of the polarity of pulses applied to the patient.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a method and apparatus for delivering electrical energy to a patient for maintaining and/or restoring the pumping rhythm of a heart, and more particularly to a method and apparatus for regulating a current flow to the patent to conform with a predetermined maximum value and/or according to a predetermined waveform. A single power source is provided to deliver current with either pacing or defibrillation energy.  
           [0004]    2. Description of the Related Art  
           [0005]    The frequent occurrence of sudden cardiac arrest (SCA) is well known. SCA occurs when the heart stops pumping blood, usually due to abnormal electrical activity in the heart, such as for example, ventricular fibrillation, which is caused very fast electrical activity in the heart. Ventricular fibrillation is treated by applying an electric shock to the patient&#39;s heart through the use of a defibrillator. Other forms of abnormal cardiac rhythms, such as bradycardia (slow heart rate) and tachycardia (rapid heart rate) may be treated with a low voltage pacing pulse, which assists the heart&#39;s natural pacemakers. Devices which accomplish both defibrillation and pacing typically include two power sources, two capacitors, and two control mechanisms, one for the high energy defibrillation pulse and one for the lower energy pacing pulses. This double circuitry ultimately increases the weight size, and cost of the device.  
           [0006]    There is a need for an apparatus which performs both defibrillation and pacing and which uses a single power source, a single high energy capacitor and a single control mechanism to deliver either defibrillation or pacing energy.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is a defibrillator-pacer for delivering electrical energy to a patient as either defibrillation pulses or pacing pulses. The defibrillator-pacer comprises a power source, an H-bridge comprising a plurality of switches which control the delivery of the electrical energy from the power source to the patient; and a current control circuit which regulates electrical current supplied to the patient, wherein the current control circuit is serially connected with the H-bridge. The current control circuit regulates the current in response to a current control signal as determined by a controller. The current control signal may be a constant value or a time varying signal having a predetermined or varying waveform. The current control circuit is operable such that the current control signal has a first value where a first pair of switches of the H-bridge is operated and a second value where a second pair of the switches of the H-bridge is operated. The circuit is also operable wherein the current control signal has a waveform of a first shape where a first pair of the switches of the H-bridge is operated and a waveform of a second shape where a second pair of the switches of the H-bridge is operated. Various shapes of the control signal waveform may include sinusoidal, rectangular and truncated exponential waveforms. The current control circuit is also operable as a switch which may be operated prior to the operation of either pair of H-bridge switches. The H-bridge switches are operable in conjunction with the current control circuit to deliver a controlled current biphasic pulse to the patient.  
           [0008]    A defibrillator-pacer incorporating the inventive circuit comprises a storage circuit having first and second terminals, the storage circuit operable to store electrical energy; an H-bridge circuit, coupled to the first terminal of the storage circuit, adapted to couple with the patient and operable to deliver electrical current from the stored electrical energy to the patient; and a current control circuit, coupled with the H-bridge circuit and operable to electrically connect the H-bridge circuit with the second terminal of the storage circuit, to regulate the delivery of the electrical current to the patient. The current control circuit, which is operable in a linear mode and responsive to a control voltage, comprises an amplifier, a transistor and a resistor arranged as a voltage to current follower. A controller controls the H-bridge to control the polarity of the electrical current delivered to the patient and determines the control voltage as a fixed value or a time varying waveform, which may be, for example, a sinusoidal waveform, a half-sinusoidal waveform, a rectangular waveform or a decaying exponential waveform.  
           [0009]    In a first alternate embodiment of the current control circuit, a first transistor, an amplifier and a resistor are arranged as a voltage to current follower to control the current through the patient according to a predetermined scale factor determined by the resistor and a first control voltage, and a second transistor is arranged to selectively bypass the first resistor in response to a second control voltage to change the scale factor of the voltage to current follower.  
           [0010]    In a second alternate embodiment of the control circuit, a first resistor is connected in series with the H-bridge to passively limit the current through the patient. A first transistor, an amplifier and a second resistor are arranged as a voltage to current follower to control the current through the patient according to a predetermined scale factor determined by the first resistor, the second resistor and a first control voltage. A second transistor bypasses the first resistor in response to a second control voltage to change the scale factor of the voltage to current follower and a third transistor bypasses the second resistor in response to a third control voltage to further change the scale factor of the voltage to current follower. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The present invention will become more apparent and more readily appreciated from the following description of the various embodiments, taken in conjunction with the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a functional block diagram illustrating a defibrillator-pacer circuit according to an embodiment of the present invention.  
         [0013]    FIGS.  2 A- 2 F are waveforms illustrating delivery of a current where the circuit shown in FIG. 1 is operated in an unregulated mode.  
         [0014]    FIGS.  3 A- 3 F are waveforms illustrating delivery of a current where the circuit shown in FIG. 1 is operated in a current regulated mode.  
         [0015]    FIGS.  4 A- 4 K are waveforms illustrating delivery of a current where the circuit shown in FIG. 1 is operated in a current regulated mode from a power source having a constant value.  
         [0016]    [0016]FIG. 5 is an schematic diagram of an alternate construction of a current regulating portion of the defibrillator-pacer circuit shown in FIG. 1.  
         [0017]    [0017]FIG. 6 is an schematic diagram of another alternate construction of a current regulating portion of the defibrillator-pacer circuit shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.  
         [0019]    The present invention is operable in a first mode which provides either a high energy monophasic or a biphasic electrical pulse to a patient via electrodes applied to the chest of a patient and operable in a second mode which provides lower energy pacing pulses to the patient. Monophasic defibrillators deliver an electrical current pulse of a single polarity. Biphasic defibrillators deliver an electrical current pulse of a first polarity followed by an electrical current pulse of the opposite polarity. Where delivered external to the patient, these electrical pulses are high energy (typically in the range of 30 J to 360 J). The present invention may be employed by defibrillators or pacers intended to generate monophasic, biphasic or multiphasic waveforms and may be employed by defibrillators that allow the user to select the waveform type. Finally, the present invention may be employed in either external or implantable defibrillators or pacers. It is believed that the invention described herein is primarily beneficial for defibrillators that combine defibrillation with pacing.  
         [0020]    [0020]FIG. 1 is a functional block diagram illustrating a defibrillator-pacer  20  comprising a charging circuit  22 , a storage capacitor  24 , a controller  26 , such as for example a micro-controller or a processor (hereinafter controller  26 ), a user input section  28 , output electrodes  32  and  34 , an H-bridge  40  and a current control circuit  60 . The defibrillator-pacer  20  is powered by an energy source such as a removable battery (not shown). The controller  26  controls overall operation of the various components of the defibrillator-pacer  20 . The H-bridge  40  delivers a pulse of electrical energy to a patient via electrodes  32  and  34  under the control of controller  26 . The defibrillator-pacer  20  comprises other well known components which contribute to the overall operation of the defibrillator-pacer  20 , such as for example, a patient monitor  27 , which acquires and processes the patient&#39;s electrocardiogram (ECG) signals, and sends the patient&#39;s ECG signals to the controller  26 . The controller  26  controls at least one of timing, amplitude, polarity and shape of energy pulses applied to the patient according to the various modes of operation of the defibrillator-pacer  20 . Components which are well known and which are not necessary to the understanding of the present invention are not shown in order to avoid obscuring the embodiments of the invention.  
         [0021]    The H-bridge  40  comprises switches  42 ,  44 ,  46  and  48  which are driven by switch control circuits  52 ,  54 ,  56  and  58 , respectively. The switch control circuits  52 ,  54 ,  56  and  58  are controlled in turn by the controller  26 . The switches  42  and  46  have a common connection at a node  43  which is connected to a positive (+) terminal  25  of storage capacitor  24 . The storage capacitor  24  has a negative (−) terminal connected to circuit common  29 . The switches  44  and  48  have a common connection at a node  45  which is also a first node of the current control circuit  60 . A second node of the current control circuit  60  is connected to the circuit common  29 . The current control circuit  60  receives input commands from the controller  26 . The switches  44  and  46  have a common connection at a node  47  which is also connected with the electrode  32  and the switches  42  and  48  have a common connection at a node  49  which is also connected to the electrode  34 . A resistor RP shown between electrodes  32  and  34  represents the effective resistance of the patient. Switch contacts  33  and  35  are operable to connect the defibrillator-pacer to the patient. The switch contacts  33  sand  35  are preferably contacts of a relay which operates under control of the controller  26 .  
         [0022]    Referring now to FIGS.  2 A- 2 F, one mode of operation of the circuit shown in FIG. 1 will be described. The controller  26  controls the charging circuit  22  in response to an input from the patient monitor  27  or the user input  28 , causing the charger to operate for a time interval t 1 -t 2  as shown in FIG. 2A. The charging circuit  22  charges the storage capacitor  24  during the time interval t 1 -t 2  as shown in FIG. 2B to a relative high value, such as for example, 1500 to 2000 volts DC. At the time t 2 , the charging circuit  22  discontinues charging the storage capacitor  24  and the voltage of capacitor  24  with respect to circuit common  29  remains at a constant value during a time interval t 2 -t 3 .  
         [0023]    Where the defibrillator-pacer  20  is operated as a defibrillator in a biphasic mode, the controller  26  instructs switch control circuits  52  and  54  to turn on switches  42  and  44 , respectively, and instructs the current control circuit  60  to connect node  45  to node  29  either via a fixed low impedance path or to regulate the current passing through the patient. Although current regulation presently has little known value at high energy defibrillation discharges, and may be impractical dissipation wise with current known technology, current regulation of low energies of up to 2 joules is believed to be both practical and beneficial. Switches  42  and  44  are turned on for the time interval t 3 -t 4  as shown in FIG. 2C and where the current control circuit  60  is operating as a fixed low impedance path, current is delivered to the patient R P  via the electrodes  32  and  34  as a decaying exponential waveform according to the patient resistance R P  during the time interval t 3 -t 4  as shown in FIG. 2E.  
         [0024]    During a time interval t 4 -t 5 , the voltage across the storage capacitor remains at a relatively constant reduced value. At the time t 5 , the controller  26  again instructs the current control circuit  60  to connect node  45  to node  29  either via the fixed low impedance path or to regulate the current passing through the patient and instructs switch control circuits  56  and  58  to turn on switches  46  and  48 , respectively, during a time interval t 5 -t 6  as shown in FIG. 2D. During the time interval t 5 -t 6 , the direction of the current applied to the patient via the electrodes  32  and  34  is reversed from the direction previously applied during the time interval t 3 -t 4 . Where the current control circuit  60  connects node  45  to node  29  via the fixed low impedance path, current is applied to the patient R P  at electrodes  32  and  34  as a decaying exponential waveform during the interval t 5 -t 6  according to the patient resistance R P  as shown in FIG. 2E.  
         [0025]    Alternately, the current control circuit  60  may be commanded to connect node  45  to node  29  at a time to, such as for example, before the beginning of the charging of the capacitor  24  (before the time t 1 ) or before switches  42  and  44  are turned on at the time t 3 . Where the control circuit  60  connects nodes  45  and  29  before the voltage across the capacitor  24  exceeds a predetermined value, the stress on components of the current control circuit  60  is significantly reduced, contributing to increased reliability and where a high voltage insulated gate bipolar transistor (IGBT) is used in the H-bridge for commutation, parts with lower voltage ratings may be used in the current control circuit  60 , thus again contributing to lower cost.  
         [0026]    Where the current regulating circuit  60  is operated in the fixed low impedance mode, the maximum current applied to the patient is determined by the amplitude of the voltage across the capacitor  24 , the effective resistance R P  of the patient and the effective impedance of the fixed low impedance path of the current control circuit  60 .  
         [0027]    Referring now to FIGS.  3 A- 3 D, the operation of the circuit shown in FIG. 1 will be further described relative to operation in a pacing mode. The controller  26  controls the charging circuit  22  in response to an input from the patient monitor  27  and the charging circuit  22  charges the storage capacitor  24  during a time interval t 1 -t 2  as shown in FIGS. 3A and 2C to a relative low value, such as for example, 75 to 200 volts DC, which may vary dynamically in order to reduce power consumption. At the time t 2 , the charging circuit  22  discontinues charging the storage capacitor  24  and the voltage at node  25  of capacitor  24  with respect to circuit common  29  remains at a constant value during an interval t 2 -t 3 .  
         [0028]    Where the defibrillator-pacer is operated as a pacer in a biphasic mode, the controller  26  instructs switch control circuits  52  and  54  to turn on switches  42  and  44 , respectively, and instructs the current control circuit  60  to connect node  45  to node  29  and to regulate the current passing through the patient so as not to exceed a predetermined maximum value. Switches  42  and  44  are turned on for a time interval t 3 -t 4  as shown in FIG. 3C and current is delivered to the patient R P  at electrodes  32  and  34  according to the control of the current control circuit  60  and in a first polarity during the interval t 3 -t 4  as shown in FIG. 3E. At a time t 5 , the controller  26  again instructs the current control circuit  60  to connect node  45  to node  29  and to regulate the current passing through the patient and instructs switch control circuits  56  and  58  to turn on switches  46  and  48 , respectively, during a time interval t 5 -t 6  as shown in FIG. 3D, reversing the direction of current applied to the patient from the direction previously applied during the time interval t 3 -t 4 . The reversed polarity waveform is indicated during the time interval t 5 -t 6  as shown in FIG. 3E. In FIG. 3E, the current waveforms applied to the patient for the time intervals t 3 -t 4  and t 5 -t 6  are indicated as broken lines, since the shape of the waveform will depend on the character of the current control command, as will be explained more fully below.  
         [0029]    Referring again to FIG. 1, the current control circuit  60  comprises a transistor Q 1 , a resistor R R , an amplifier U 1 , and a current command circuit  62 . Resistor R R  has a first end connected to circuit common  29  and a second end connected to the emitter of transistor Q 1 . The collector of transistor Q 1  is connected to the H-bridge at the node  45 . The gate of transistor Q 1  is connected to the output of amplifier U 1  and the non-inverting input (+) of amplifier U 1  is connected to the second end of resistor R R  so as to form a current follower circuit wherein the current through resistor R R  follows the voltage applied at the inverting input (−) of amplifier U 1 , thus the patient current I P  becomes V C  divided by R P , where V C  is the voltage at the inverting input of amplifier U 1 . Thus, the current through the patient is regulated according to the voltage V C  applied at the inverting input of amplifier U 1 . The current command interface circuit  62  receives commands from the controller  26  and outputs either a fixed value as V C  or a time variant value of V C  according to a desired mode of operation of the defibrillator-pacer  20 . During operation of the defibrillator-pacer where patient current is being controlled by the current control circuit  60 , the charging circuit  22  continuously or periodically replenishes the charge on capacitor  24  to ensure that the voltage at node  25  is sufficiently high to maintain the voltage of the waveform commanded by the time variant V C . Where it is desired to operate the defibrillator-pacer  20  wherein node  45  is connected to node  29  via the fixed low impedance path, the value V C  is determined so that transistor Q 1  effectively operates as a switch. Where it is desired to operate the defibrillator-pacer  20  so that the current regulating circuit  60  controls the current passing between nodes  45  and  29 , the values of resistor R R  and input voltage V C  are determined so that Q 1  remains in an active region for desired values of patient current. Where Q 1  remains in the active region, the polarity of current applied to the patient is controlled by the H-bridge and the amplitude and waveform of current applied to the patient are controlled by the current regulating circuit  60 .  
         [0030]    Referring to FIGS.  4 A- 4 E, an example of operation of the pacer-defibrillator  20  in a current regulation mode is illustrated. For simplicity of explanation, the voltage across capacitor  24  is assumed to be charged and held at a constant value. It will be readily recognized by persons skilled in the art that if the voltage across capacitor  24  is not maintained at a constant value, that is, allowed to decay as a function of current delivered, the waveform applied to the patient may vary, however, the maximum amplitude of current applied to the patient will still be controlled by the current regulator  60 .  
         [0031]    As shown in FIG. 4A, the voltage across capacitor  24  is charged to a constant value at a time t 1 . At a time interval t 3 -t 4  indicated in FIG. 4B, the controller  26  causes the switches  42  and  44  to close, determining the polarity of a pulse to be delivered to the patient. The controller  26  also causes the current command circuit  62  to provide the waveform indicated in FIG. 4D at the time interval t 3 -t 4  to be applied to the inverting input of amplifier U 1 , thus applying a current to the patient R P  during the time interval t 3 -t 4  of the waveform as shown in FIG. 4E. Next, at a time interval t 5 -t 6  as indicated in FIG. 4C, the controller  26  causes the switches  46  and  48  to close, determining the polarity of the pulse to be delivered to the patient to be opposite the polarity of the pulse previously applied during the time interval t 3 -t 4 . The controller  26  concurrently causes the current command circuit  62  to provide the waveform indicated in FIG. 4D at the time interval t 5 -t 6  to be applied to the inverting input of amplifier U 1 , thus applying a current of the waveform shown in FIG. 4E at the time interval t 5 -t 6  to the patient who is represented by the resistance R P . Where Q 1  is operated so as to remain in a linear region, the current waveform I P  through the patient R P  will follow the voltage waveform V C . Thus, current waveforms, such as for example, a one-half sine waveform, FIG. 4F, a truncated exponential waveform, FIG. 4G, a damped sine waveform, FIG. 4H, a rectangular waveform, FIG. 4J, and a rounded rectangle waveform, FIG. 4K may be generated. A person skilled in the art will recognize that where a controlled current mono-phasic waveform is desired, the controller  26  may be appropriately programmed to cause only one of switch pair  42  and  44  or switch pair  46  and  48  to close and to cause an appropriate voltage waveform to be applied as V C . Thus, both monophasic and biphasic pulses are regulated by the present invention.  
         [0032]    Transistor Q 1  is preferably an insulated gate bipolar transistor (IGBT). A transistor having a lower voltage rating, such as for example a field effect transistor (FET) or a junction type bipolar transistor, may be used where it is not desired to regulate the current in the defibrillation mode or where a high voltage IGBT is used in the H-bridge for commutation. Where a transistor having a lower voltage rating is used, transistor Q 1  is preferably operated as a switch and fully turned on before the capacitor  24  is charged to high voltages and remain fully turned on until the capacitor has been discharged.  
         [0033]    Preferably, the sense resistor R R  in the current regulating circuit  60  of FIG. 1 is selected to optimize regulation of low level pacing pulses. In the fixed low impedance mode, it is advantageous to bypass the resistor R R  in the current regulating circuit  60  shown in FIG. 1. Referring now to FIG. 5, a current regulating circuit  60 - 1  which includes a circuit arrangement for bypassing the resistor R R  is illustrated. The current regulating circuit  60 - 1  operates in a similar manner as the current regulating circuit  60 , however the current regulating circuit comprises a transistor Q 2  which performs a bypass of the resistor R R  and an additional node  65  which receives an input voltage V S1  to operate the bypass. The transistor Q 2  is preferably a power field effect transistor having a source and a drain connected across the resistor R R  and a gate connected to the node  65 . A voltage rating in the 20 to 50 volt range is adequate for the transistor Q 2 .  
         [0034]    Referring now to FIG. 6, a current regulating circuit  60 - 2  includes a circuit arrangement for bypassing the resistor R R  and a series limiting resistor is illustrated. The current regulating circuit  60 - 2  operates in a similar manner as the current regulating circuit  60 , however the current regulating circuit  60 - 2  further comprises a transistor Q 2  which performs a bypass of the resistor R R , an additional node  65  which receives an input voltage V S1  to operate the bypass of R R , a series limiting resistor R L , a transistor Q 3  which bypasses the resistor R L  and a node  67  which receives a voltage V S2  which operates the bypass of R L . The transistor Q 2  is preferably a power field effect transistor having a source and a drain connected across the resistor R R  and a gate connected to the node  65 . A voltage rating of transistor Q 2  in the 20 to 50 volt range is adequate. The series limiting resistor R L  is inserted between the collector of Q 1  and the node  45 . The series resistor R L  is used where it is desirable to have additional limiting, such as for example, in the pacer mode. The transistor Q 3  performs a bypass of the resistor R L  in response to an input voltage V S2  received at node  67 . The transistor Q 3  is preferably a power field effect transistor having a source and a drain connected across the resistor R L  and a gate connected to the node  67 . A voltage rating of transistor Q 3  should be in the 50 to 200 volt range or higher. Where it is desirable to bypass both the resistors R L  and R R  simultaneously, the voltage V S1  should be applied before the voltage V S2  so that a ground reference is established for the transistor Q 3 . Otherwise, a more complex gate drive for the transistor Q 3  is required. Where the circuit configuration of FIG. 6 is employed, switches  44  and  48  (FIG. 1) are high voltage switching components. Where the switches  44  and  48  of the H-bridge are high voltage switching components, Q 1  may be a MOSFET having a voltage range of 200 volts or greater.  
         [0035]    Defibrillator-pacers incorporating the present invention may also incorporate additional circuitry (not shown) which avoids application of the capacitor voltage to the H-bridge components during standby operations or which disconnects the patient from the outputs of the H-bridge during standby operations, such as for example, a relay may be incorporated between the patient and the H-bridge.  
         [0036]    Although a few embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.