Abstract:
An automatic external defibrillator is described that includes a high voltage delivery circuit for producing an electrical pulse to defibrillate a patient. In a preferred embodiment the electrical pulse is a biphasic or multiphasic electrical pulse. In one embodiment, the delivery circuit includes a high voltage capacitor coupled with a bridge circuit. The capacitor stores electrical energy for delivery to the patient, and the bridge circuit has four switching elements that are selectively switched to steer the current through the patient. A disarm circuit shunts the bridge circuit and operates to route energy away from the bridge circuit in the event a fault condition is detected, such as a short circuit at the patient electrodes. An example disarm circuit is a series-connected SCR and resistor. Also, a limiting circuit element (such as a resistor or an inductor) is provided in series with the capacitor. Together with the disarm circuit, the limiting circuit element reduces the voltage experienced by the bridge circuit switching elements when switched off in response to the detected fault condition. Consequently simpler, more robust, and less expensive high voltage delivery circuits are provided, as compared to conventional defibrillator circuit designs. A snubber circuit is also provided to prevent voltage from reaching the patient when the device is in standby mode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method and apparatus for delivering electrical energy produced by a defibrillator to a patient experiencing ventricular fibrillation (“VF”), and more particularly to a method and apparatus for controlling the delivery of electrical energy produced by an external defibrillator. The circuit of this invention allows for active and passive protection of the high energy delivery circuit in the event of a fault condition. The circuit also enables the patient to be protected from high voltage when the device is in standby or monitoring mode. The circuit provides a reliable and safe means of protecting the H-bridge from an over-current condition while increasing patient and operator safety. The circuit also has the advantage of being simple and inexpensive while maintaining a high degree of effectiveness. 
     2. Description of the Prior Art 
     Each day thousands of Americans are victims of cardiac emergencies. Cardiac emergencies typically strike without warning, oftentimes striking people with no history of heart disease. The most common cardiac emergency is sudden cardiac arrest (“SCA”). It is estimated that more than 1000 people per day are victims of SCA in the United States alone; this translates into one death every two minutes. 
     SCA occurs when the heart stops pumping blood. Usually SCA is due to abnormal electrical activity in the heart, resulting in an abnormal rhythm (arrhythmia). One such abnormal rhythm, VF, is caused by abnormal and very chaotic electrical activity in the heart. During VF the heart cannot pump blood effectively. VF may be treated by applying an electric shock to the patient&#39;s heart through the use of a defibrillator. The shock clears the heart of the abnormal electrical activity (in a process called “defibrillation”) by depolarizing a critical mass of myocardial cells to allow spontaneous organized depolarization to resume, thus restoring normal function. Because blood may no longer be pumping effectively during VF, the chances of surviving decrease with time after the onset of the emergency. Brain damage can occur after the brain is deprived of oxygen for four to six minutes. 
     External defibrillators send electrical pulses to the patient&#39;s heart through electrodes applied to the patient&#39;s torso. External defibrillators are typically located and used in hospital emergency rooms, operating rooms, and emergency medical vehicles. Of the wide variety of external defibrillators currently available, automatic and semi-automatic external defibrillators (AEDs) are becoming increasingly popular because they can be used by relatively inexperienced personnel. Such defibrillators can also be especially lightweight, compact, and portable. 
     AEDs must include circuitry capable of handling the high voltages and high currents associated with electrical defibrillation. In some instances, suitable components with the required electrical characteristics are not readily available, and the AED designer must instead rely on multiple component configurations where, functionally, a single component would suffice. 
     Additionally, AEDs require monitoring and control circuitry to protect the patient, as well as the AED circuitry itself, in the event of a fault condition. One common fault condition occurs as a result of variations in load impedances, such as those resulting from short circuits or open circuit conditions. The high voltages applied to patients may also create situations, such as arcing between electrodes or arcing between patient wires, that could also lead to failure of the therapy electronics if not properly protected. Such monitor and control circuitry is made increasingly complex by the multiple component configurations included in currently available AEDs. 
     One method employed by currently available defibrillators to solve this problem is by measuring patient impedance using a low-level signal prior to delivering a shock. The disadvantage of this method is that it relies heavily on the accuracy of the low-level signal measurement relative to the actual impedance (i.e., impedance detected during the high voltage pulse delivered during defibrillation). As will be appreciated by those of skill in the art, the low-level signal cannot predict all behaviors of the external circuit during defibrillation. An example of a condition that cannot be predicted is arcing. 
     Another method, employed by the ForeRunner® (manufactured by Heartstream, Inc., Seattle, Wash.), is to measure impedance during the initial portion of the waveform and to allow the circuit to continue if impedance is within tolerable limits. Toward that end a 20Ω resistor is placed in series for the first 100 μs that the voltage is delivered. During that time, the resistance across the electrodes is tested to ensure that the connection has not been shorted by monitoring the voltage across a 0.05Ω current sense resistor. Providing a resistance in series during the initial voltage delivery, ensures that the circuit will not be subjected to excessive current in the event that there is a short condition. However, if a fault occurs after the first 100 the circuit could be exposed to excessive currents. 
     What is needed, therefore, is an AED with a fault protection circuit that is capable of actively protecting the high voltage H-bridge. Protection of the H bridge can be accomplished by switching the bridge off during a fault condition, and/or passively protecting the high voltage bridge, e.g. by allowing the circuit to tolerate the fault condition. Further what is needed is a way to protect the H-bridge from valid load conditions while minimizing the exposure of the patient, or patient simulated load, to the energy stored in the AED. Finally what is needed is a way to protect the operator and/or patient in the event of a discharge to an abnormally high patient load. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an apparatus and method is provided for producing and controlling a high energy pulse for application to a patient experiencing VF. A storage circuit stores electrical energy and a steering circuit delivers the electrical energy from the storage circuit to the patient. A protection circuit is coupled with the storage circuit and with the steering circuit. The protection circuit selectively controls the delivery of the electrical energy from the storage circuit to the steering circuit. The protection circuit may include a disarm circuit that selectively shunts the electrical energy way from the steering circuit and patient. The protection circuit may include a limit circuit that limits the rate of delivery of the electrical energy from the storage circuit to the steering circuit. The rate at which electrical energy is delivered is measured and compared to a predetermined range of acceptable rates. If the rate falls within the acceptable range, then electrical energy continues to be delivered. If, however, the rate does not fall within the accepted range, the disarm circuit is enabled and the delivery of the electrical energy to the steering circuit is interrupted. Determination of whether the rate falls within a predetermined acceptable range occurs all the time, thus the disarm circuit can be enabled at any time the rate falls outside the accepted range. The limit circuit and disarm circuit together may limit a maximum voltage applied across the steering circuit when the delivery of electrical energy is interrupted. 
     This invention provides the advantage of limiting the exposure of the external circuit to high voltage/energy in the event of an over-current load condition. These advantages are achieved with the use of lower cost, readily available components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram depicting a defibrillator according to an embodiment of the present invention. 
     FIG. 2 is a functional block diagram depicting a high-voltage delivery circuit included in the defibrillator of FIG.  1 . 
     FIG. 3 is a schematic diagram depicting certain details of a first embodiment of the high-voltage delivery circuit of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Currently available external defibrillators provide either a monophasic or biphasic electrical pulse to a patient through electrodes applied to the chest. Monophasic defibrillators deliver an electrical pulse of current in one direction. Biphasic defibrillators deliver an electrical pulse of current first in one direction and then in the opposite direction. When delivered external to the patient, these electrical pulses are high energy (typically in the range of 30 J to 360 J). This invention may be employed by defibrillators that generate monophasic, biphasic or multiphasic waveforms. Additionally, this invention may be employed by defibrillators that allow the user to select the waveform type. 
     Defibrillators employing a monophasic waveform are well known in the art. While this invention may be used with a defibrillator employing a monophasic waveform, it is believed that the solution described herein is primarily beneficial for defibrillators that deliver biphasic or multiphasic waveforms. 
     An example of an AED employing a biphasic waveform is described in U.S. Pat. No. 5,607,454, entitled “Electrotherapy Method and Apparatus,” the disclosure of which is incorporated herein by reference. Such defibrillators employ a high voltage bridge circuit for steering the biphasic pulse applied to the patient. The energy delivered to the patient is first stored in an energy storage circuit such as a capacitor, with associated voltages commonly in the range of 1000-2500 V. Prior to delivery of the electrical energy to the patient, one or more of the components of the bridge circuit must withstand this voltage without significant leakage. 
     Should energy delivery via the bridge circuit be halted due to a fault condition, the corresponding currents and voltages handled by the bridge components are quite high. Given the components currently available to the AED designer, today&#39;s bridge circuits commonly include as many as eight to ten distinct switching elements. Correspondingly, the control circuitry associated with switching these elements is relatively complex. The circuit component numbers and complexity required by such AEDs can result in increased expense, potentially lowered reliability, and reduced portability. 
     In accordance with the present invention, embodiments of an external defibrillator are provided that have a high voltage bridge circuit using only five switching elements to steer the biphasic or multiphasic pulse. In an electrical path separate from the bridge circuit, a sixth switching element is provided for discharging/disarming the energy storage capacitor in the event of a fault. An additional switching element or elements can be provided for current initiation and commutation control. In the following description, certain specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be clear, however, to one skilled in the art, that the present invention can be practiced without these details. In other instances, well-known circuits have not been shown in detail in order to avoid unnecessarily obscuring the description of the various embodiments of the invention. Also not presented in any great detail are those well-known control signals and signal timing protocols associated with the internal operation of defibrillators. 
     FIG. 1 is a functional block diagram depicting a defibrillator or AED  10  having a high-voltage delivery circuit  12  in accordance with an embodiment of the present invention. The AED  10  includes a power supply  14 , which is powered by an energy source such as a removable battery  16  and provides power to other components of the AED. A microcontroller or processor  18  controls the operation of the various components of the AED  10 . The high-voltage delivery circuit  12  delivers a pulse of electrical energy to a patient via an electrode connector or interface  20  and electrodes  22 . 
     An electrocardiogram (ECG) circuit  24  acquires and processes the patient&#39;s ECG signals through the electrodes  22  and sends the signals to the processor  18  via a system gate array  26 . The system gate array  26  is preferably a custom application-specific integrated circuit (ASIC) integrating many of the defibrillator functions (including user interface control and many of the internal functions) and interfacing the processor  18  with other components of the AED  10 . Providing the separate system gate array or ASIC  26  allows the processor  18  to focus on other tasks. Of course, the functionality of the ASIC  26  could be included within the operations performed by the processor  18 , or could be replaced by discrete logic circuit components or a separately dedicated processor. 
     The AED  10  also includes a memory device  30 . As depicted in FIG. 1, memory device  30  is a removable PCMCIA card or magnetic tape. AED  10  also includes user interface components such as a microphone  32 , an audio speaker  34 , an LCD display panel  36 , and a set of push-button controls  38 . Those skilled in the art will understand that a number of other components are included within the AED  10  (e.g., a system monitor and associated status indicators), but are not shown in order to avoid unnecessarily obscuring the description of embodiments of the invention. 
     As shown in FIG. 2, the high-voltage delivery circuit  12  includes a number of functional circuit blocks which are both monitored and controlled by the ASIC  26 . A high-voltage charging circuit  40 , such as a flyback power supply, responds to one or more control signals issued by the ASIC  26  and generates electrical energy for provision to a capacitor  42 . By controlling the high voltage charger  40 , the ASIC can correct an over voltage condition as a result of measuring voltage on the capacitor  60  while it is charging. 
     The capacitor  42 , which could be an energy storage circuit (“ESC”), stores the electrical energy for delivery to the patient. The electrical energy is delivered to an energy transfer or steering circuit  46  (comprising four silicon controlled rectifier switches, SCR UL , SCR UR , SCR LL , and SCR LR ) through drive electronics  52 . The steering circuit  46  in turn delivers the electrical energy to the patient via the connector  20  and electrodes  22  (shown in FIG.  1 ). 
     The protection circuit  48  (shown in FIG. 3) functions to limit energy delivery from the ESC  42  to the steering circuit  46  (and hence to the patient) and to discharge or otherwise disarm the ESC  42  in the event of a fault condition. A monitor circuit  50  senses operations of both the protection circuit  48  and the steering circuit  46  and reports the results of such monitoring to the ASIC  26 . ASIC  26  provides instructions to the monitor circuit  50  which controls the disarm drive and IGBT drive of the circuit shown in FIG. 3 to prevent an over-current condition on the bridge. The above-described operations of the steering circuit  46  and the protection circuit  48  are controlled by a drive circuit  52  issuing a plurality of drive signals. Operation of the drive circuit  52  is, in turn, controlled by one or more control signals provided by the ASIC  26  and the microprocessor  18 . 
     FIG. 3 is a more detailed depiction of the invention shown in FIG.  2 . 
     An ESC  42  is provided which is a capacitor (or multiple capacitor unit)  60 . Suitable capacitance is approximately 100 μF; the ESC  42  is capable of regularly and reliably storing energy up to approximately 220 J (which corresponds to a voltage of approximately 2100 VDC). The capacitor  60  has a positive terminal or node  62 . The energy storage circuit provides energy to a steering circuit  46  which, in turn, controls the delivery of energy to a patient  104 . The steering circuit  46  enables the circuit to deliver either a biphasic or multiphasic energy pulse to the patient  104 . 
     The steering circuit  46  is configured as an “H-bridge”, with four switching elements. The steering circuit  46  includes an upper-left (UL) switching element, such as SCR UL    90 , and an upper-right (UR) switching element, such as SCR UR    92 . The anode of each of the SCRs  90 ,  92  is connected to an upper node  76 , and the cathode of each of the SCRs is connected to a respective one of two patient terminals  96  (which, in turn, are coupled with the connector  20  and respective ones of the electrodes  22  of FIG.  1 ). The control terminal or gate of each of the SCRs  90 ,  92  receives a respective UL or UR drive signal produced by the drive circuit  52  (shown in FIG. 2) to selectively switch the SCRs on. A patient  104  is represented by a resistor, shown in the electrical location of the patient during defibrillator operations. 
     The steering circuit  46  also includes a lower-left (LL) switching element, such as SCR LL    98 , and a lower-right (LR) switching element, such as a SCR LR    94 . The anode of the SCR LL    98  and the anode of the SCR LR    94  are each connected to a respective one of the patient terminals  96 . The cathode of the SCR LL    98  and the cathode of SCR LR    94  are each connected to a lower terminal or node  104  of the steering circuit  46 . The control terminal or gate of the SCR LL    98  receives an LL drive signal from the drive circuit  52  (shown in FIG. 2) to selectively switch the respective SCR on. The control terminal or gate of SCR LR    94  receives a LR drive signal from the drive circuit  52  to selectively switch the SCR on and off. 
     A high-voltage diode D 2  is connected in parallel to SCR UL    90  and SCR LL    98  at nodes  76  and  104 . Diode D 2  operates to snub inductance in the patient load when the bridge is turning off the current, for example during the commutation interval. 
     A sense resistor  106  is connected in series with the steering circuit  46 , between lower terminal  104  of the steering circuit  46  and the negative terminal of the energy steering circuit  42  at node  102 . A suitable resistance value for the sense resistor  106  is approximately 50 mΩ, and is preferably of the low-value precision resistor type commonly used as an electric shunt in ammeters. In a preferred embodiment, the monitor circuitry  50  (shown in FIG. 2) is an over-current/waveform abort control logic (OIWAL). If an over-current detection is “TRUE,” or the switch disarm signal is asserted, the OIWAL performs the necessary steps to shut down the patient load current and safely disarm the ESC  42 . Further details of how the drive is disarmed is discussed below. 
     Additionally, the information from the sense resistor  106  can be provided to the microprocessor  18 . The microprocessor can then perform time-integration calculations to obtain information concerning the voltage across the capacitor  60  during defibrillation energy delivery operations. 
     A limit circuit includes an inductor  64  is connected in series between the positive terminal of the energy storage circuit  42  at node  62  and resistor  68 . A suitable value for the inductor  64  is between 100 and 200 μH, preferably 150 μH. A high voltage diode D 1  is connected in parallel to the inductor  64  at nodes  62  and  80 . The inductor  64  controls the rate at which the current that is delivered to the steering circuit  46  by the ESC  42  can increase. The advantage of providing the inductor  64  in series with the ESC  42  is that by slowing the rate at which the current through the steering circuit  46  can ramp up, additional time is provided for the monitor circuit  50  to instruct a disarm circuit to disconnect the bridge in the event of an over-current situation. Further, the inductor  64  controls dl/dt such that a fixed current threshold can be used for over-current detection. The advantage of providing diode D 1  in parallel to the inductor  64  is that the diode functions to snub the inductor during current interruption. 
     The protection circuit  48  of FIG. 2 is shown in FIG. 3 as two distinct subcircuits—namely, a current limit resistor  68  and a disarm circuit  66 . 
     The current limit resistor  68  is connected in series between inductor  64  (which is connected to the positive terminal  62  of the capacitor  60 ) and the upper node  76  of the steering circuit  46 . The limit resistor  68  limits maximum current flow from the inductor  64  through the steering circuit; a suitable resistance value for the limit resistor  68  is between approximately 3-7Ω, more preferably 5Ω. 
     The disarm circuit  66  includes a disarm resistor  72  (with a suitable resistance value being between approximately 3-7Ω, more preferably 5Ω) and an SCR  74 . The disarm resistor  72  and SCR  74  are connected in series between the upper terminal  76  of the steering circuit  46  and the negative terminal  102  of the capacitor  60 , thereby providing an electrical path shunting the steering circuit. If a fault condition is detected (such as an over-current condition), the disarm SCR  74  is switched on and the energy stored in the capacitor  60  substantially dissipated in the disarm resistor  72  and the limit resistor  68 . The disarm SCR  74  is selectively switched on by a disarm drive signal provided by the drive circuitry  52  shown in FIG.  2 . 
     Another aspect of the invention is that it provides a mechanism to isolate the patient from the high voltages when the defibrillator is in monitoring mode, thus keeping current from leaking onto the patient  104  prior to delivery of the therapeutic energy pulse. Resistors  152  and  172  function to ground the patient and the ECG circuitry  24 , thus preventing current leakage during standby operations. Resistors  152  and  172  are selected to drain the upper SCR leakage currents when the ESC  42  is charging or charged in normal operation. IGBT  100  is left on (during the monitoring mode) to facilitate bleed off of the leakage through resistors  152  and  172 . The impedance of resistors  152  and  172  is selected so that under worst case operating conditions a minimal voltage is present at the isolation relay  200  contacts. A suitable value for resistors  152  and  172  is between approximately 5-10Ω, more preferably 9.4 kΩ. 
     An important aspect of this invention is that leakage resistors  152  and  172  are returned to the collector of IGBT  100 . This allows the resistors  152 ,  172  to have a low resistance value without compromising commutation of the H-bridge. For example, if the resistors were returned to ground, the current flowing through the upper SCRs might exceed the hold current and the SCRs would stay on between phases. If the SCRs stayed on, a cross-conduction of the H-bridge would occur. These resistors also serve as a path to remove charge from a snubber network  150  during the commutation interval. 
     Additionally resistor R x  and high voltage diode D 3  are provided in parallel to IGBT  100  collector-emitter at nodes  104  and  82 . Diode D 3  prevents a negative voltage across IGBT  100  during high impedance aborts. A high impedance abort occurs, for example, when the patient impedance at  104  is greater than 200Ω. Typically when patient impedance exceeds 200Ω the shock is aborted because it is not possible to complete the therapeutic shock without resulting in an over-voltage condition on the IGBT  100  during the commutation interval. 
     Resistor R x  bleeds off IGBT collector-emitter capacitance during commutation interval. This results in a reduction or elimination of residual voltage at V A  or V S  prior to initiation of the next phase of the shock. Where V A  is the voltage at the apex of the patient; V S  is the voltage at the sternum of the patient. 
     The snubber network  150  has a capacitor  154  and a resistor  156 . The capacitor  154  and resistor  156  are connected in series. Capacitor  154 , resistor  156  and inductor  64  function to limit the rate of change of voltage across the SCR LL    98  when patient impedance is high. By controlling the rate of change of voltage (dV/dT), SCR LL    98  will not accidentally turn on when current is flowing from SCR UL    90  to SCR LR    94  during the first phase of the energy delivery, which might otherwise occur as a result of the voltage change at node  148 . Suitable values for capacitor  154  is from 0.007 to 0.03 μF, preferably 0.01 μF; suitable values for resistor  156  is from 25 to 100Ω, preferably 50Ω. 
     Isolation relay  200 , comprising switches  202  and  204 , is provided respectively between nodes  20  and  22  and patient  104 . The isolation relay  200  is used to prevent leakage, impedances or voltages from interfering with the ECG acquisition function during monitoring and charging activities. 
     Like resistor  152 , resistor  172  is provided on the other side of the H-bridge to isolate the patient  104  and the ECG circuitry  24 , thus preventing current leakage during standby operations. 
     The above-described control signals may be provided by any of a wide variety of suitable drive circuits known to those skilled in the art. For example, the control signals applied to the gates of the bridge SCR UL    90 , SCR UR    92 , SCR LR    94 , SCR LL    98 , may each be suitably provided by a corresponding pulse transformer. The secondary coil of each of the transformers may be tied directly to the corresponding SCR gate, with the SCRs designed so that, once triggered and conducting, they will tolerate the short-circuit on the gate-cathode junction that occurs with transformer saturation. Because of the more precise timing requirements for defibrillator disarm operations, the disarm SCR  74  may, for example, be suitably controlled by a logic-level MOSFET switching a bipolar pull-up transistor (not shown). A switching circuit is also provided, shown as IGBT  100 . The control signal applied to turn IGBT  100  on and off may, for example, be provided by bipolar pull-up and pull-down transistors (not shown), respectively, which may themselves be triggered by logic-level MOSFET devices (not shown). 
     The operation of the circuit structure shown in FIG. 3 will now be described. The capacitor  60  is charged by the charging circuitry  40  (shown in FIG. 2) to approximately 2000-2400 V, with the positive terminal  64  having a positive voltage relative to the negative terminal  102 . During monitoring operations, the capacitor  60  is fully charged, but no defibrillation energy is delivered to the patient pending completion of ECG monitoring by the ECG circuit  24  (shown in FIG.  1 ). During standby operation IGBT  100  is on. If the results of the ECG monitoring indicate that defibrillation energy should be delivered to the patient the isolation relay  200  is closed. After an appropriate settling time, SCR UL    90  and SCR LR    94  are turned on and conduction is initiated. During the first phase of the biphasic pulse delivery, current flows from the positive terminal  62  of the capacitor  60  through the inductor  64 , limit resistor  68 , SCR UL    90 , the patient, SCR LR    94 , IGBT  100  and the sense resistor  106 . When the microprocessor  18  has determined that phase  1  of the waveform is nearing completion, it signals the ASIC  26  to terminate phase  1 . Following a brief pause of approximately 400 μs, known as the commutation interval (or interphase delay), IGBT  100  is turned on and the approximately 10 μs later SCR UR    92  and SCR LL    98  are turned on, and electrical energy is further discharged through the patient in the second phase of the biphasic pulse applied to the patient. As will be appreciated by those of skill in the art, delivery of a multiphasic pulse would require these steps to be repeated until the desired number of phases had been achieved. Thus, no specific description of how to deliver a multiphasic pulse is provided. 
     SCRs of the type suitable for use in the steering circuit  46  and as the disarm SCR  74  are currently readily available. These SCRs can withstand the high voltage and currents occurring during defibrillation operations, and can also survive relatively intense transient effects, such as might occur due to a short circuit or when energy delivery operations are interrupted. 
     As is well known in the art, one disadvantage of SCRs is that, once turned on, they are not easily turned off absent a forced current commutation. Thus, the energy steering circuit  46  requires at least one switching element that can be turned off for purposes of current commutation and reversing polarity during biphasic energy delivery. Switching elements that can withstand the high voltages and currents that may occur during defibrillation operations are not readily or cost-effectively available. For example, readily available IGBTs can safely withstand a voltage of 1200 V applied across the collector and emitter. In the past IGBTs have been stacked in an effort to overcome limitations on voltage tolerances. However, this solution involves unnecessary complications to the bridge design. Those skilled in the art will appreciate that, if the above-described IGBT  100  were itself to “open” the steering circuit  46  to interrupt delivery of electrical energy from the capacitor  60  (when fully or near-fully charged), the voltage experienced by the IGBT  100  would significantly exceed the rated 1200 V limit thereby damaging the circuit. 
     In accordance with the embodiment of the invention depicted in FIG. 3, the IGBT  100  is protected from elevated voltages and currents. In the event of an over-current condition (caused, for example, by a short-circuit at the patient electrodes  22 ), the disarm SCR  74  is first switched on to begin discharging the capacitor  60  through the limit resistor  68  and the disarm resistor  72 . Because the resistors  68  and  72  form a voltage divider, the IGBT  100  can then be shut off at a lower collector-to-emitter voltage than would otherwise be the case. Thus, a single IGBT  100  may be employed, rather than the conventional multiple component approach found in current AED designs. Further, because the disarm circuit is external to the H-bridge, the ESC  42  can be safely disarmed without exposing the patient  104  to high voltages. In the event of a high impedance load fault, the microprocessor  18  can signal the OIWAL to protect the H-bridge and the patient load in a similar fashion. As will be appreciated by those of skill in the art a high impedance load fault can occur at several times during operation of the bridge. Initially, a high impedance load fault can occur during the initial voltage delivery (for example where the electrode pads are shorted out). Additionally, a high impedance load fault can occur at the end of phase one, where, for example, more than 1200 V remains on the capacitor. In either situation, the microprocessor signals the OIWAL to protect the patient and the H-bridge by aborting the shock. However, where the load fault is detected at the end of a phase, the result is that the shock delivered comprises only the phases delivered. Specifically, where the fault occurs at the end of phase one, the result is that a monophasic shock to the patient. 
     The embodiment of the present invention shown in FIG. 3 provides a relatively inexpensive and robust defibrillation energy delivery circuit. In contrast with currently available designs, the provision of the disarm circuit  66  allows a bridge circuit design comprised of four individual switching elements, which are readily available and low cost SCRs. In the event of a fault condition, such as an over-voltage condition, the energy stored in the capacitor  60  can be similarly discharged safely through the disarm circuit  48 . 
     In operation, the disarm circuit  66  is triggered in response to an over-current condition. Approximately 1 μs later, the IBGT  100  is turned off by OIWAL. In a preferred embodiment, the over-current trip point is set at approximately 80 Amps. At the maximum voltage of the capacitor the dl/dt of the inductor is approximately 14 A/μs. When the SCR is fired the resistors form a dividing network (with a ratio of approximately 2:1), where the top of the H-bridge is at the center point. When the IGBT  100  is turned off, the maximum voltage at the collector is V CAP /2. More importantly, as the IGBT  100  is turning off there is effectively a 5Ω snubber resitance across the collector-emitter junction. This provides a high degree of margin for RBSOA, which is the safe operating area of the IGBT  100  during turn-off. 
     Those skilled in the art will understand that certain of the circuits and components shown in FIGS. 1-3 have not been described in particular detail. In such case, the circuits and components are the type whose function and interconnection is well known in the art, and one skilled in the art would be able to use such circuits and components in the described combination to practice the present invention. The internal details of these particular circuits are not critical to the invention, and a detailed description of such internal circuit operation is therefore not required. 
     It will be appreciated that, while specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Those skilled in the art will appreciate that many of the advantages associated with the circuits described above in connection with FIG. 3 may be provided by other circuit configurations. Those skilled in the art will also understand that a number of suitable circuit components, other than those particular ones described above, can be adapted and combined in a variety of circuit topologies to implement a high voltage delivery circuit in accordance with the present invention. Accordingly, the invention is not limited by the disclosed embodiments, but instead the scope of the invention is determined by the following claims.