Abstract:
A defibrillator having a self test operation requiring reduced energy is provided. During a self test operation of the defibrillator, it is necessary to deliver a test pulse to a test load under appropriate levels of voltage and current stress to the HV circuits. In verifying the functionality of an HV switch under self test, several critical parameters must be evaluated. First, the voltage stress test is conducted at the maximum voltage level to ensure that the dielectric withstand voltage of the various components of the HV circuit are adequate under maximum voltage conditions. Second, the current stress test is conducted at the maximum current level but at a partial voltage which consumes substantially less energy. The maximum voltage and maximum current levels may be chosen to exceed the operating voltage and current levels encountered during normal operation of the defibrillator to more fully test the HV switch.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to electrotherapy circuits and in particular to a defibrillator having an improved self test operation that requires less energy. 
     Electro-chemical activity within a human heart normally causes the heart muscle fibers to contract and relax in a synchronized manner that results in the effective pumping of blood from the ventricles to the body&#39;s vital organs. Sudden cardiac death is often caused by ventricular fibrillation (VF) in which abnormal electrical activity within the heart causes the individual muscle fibers to contract in an unsynchronized and chaotic way. The only effective treatment for VF is electrical defibrillation in which an electrical shock is applied to the heart to allow the heart&#39;s electro-chemical system to re-synchronize itself. Once organized electrical activity is restored, synchronized muscle contractions usually follow, leading to the restoration of cardiac rhythm. 
     The necessity to apply defibrillation quickly after onset of VF has given rise to automatic external defibrillators (AEDs) which may be used by first responders and lay people. AEDs may remain unused for long periods of time and yet must be ready to operate reliably in an emergency situation. To ensure operational readiness, most AEDs employ a self test operation that is conducted at regular intervals. 
     The Heartstream Forerunner® AED, for example, employs a self test system that generates self test operations automatically in response to a predetermined schedule. The self test operation typically includes a number of different system checks including functional, calibration, and safety tests to verify that the defibrillator&#39;s components and operation are within predetermined specifications. The high voltage (HV) circuit is a critical component of the defibrillator that provides the defibrillation pulse. Verification of the proper functioning of the defibrillator is a typical part of any self test operation. 
     U.S. Pat. No. 5,591,213, “Defibrillator System Condition Indicator”, issued Jan. 7, 1997, to Morgan et al., describes a defibrillator system which includes means for periodically operating a high voltage (HV) circuit to discharge a test pulse to a test load. Such self tests may be done periodically or in response to changes in the defibrillator environment such as the ambient temperature as described in U.S. Pat. No. 5,868,792, issued Feb. 9, 1999, “Environment-Response Method for Maintaining Electronic Devices Such As An External Defibrillator”, issued Feb. 9, 1999, to Ochs et al. which is incorporated herein by reference. 
     U.S. Pat. No. 5,800,460, “Method for Performing Self-Test In A Defibrillator”, issued Sep. 1, 1998, to Powers et al., describes in detail the operation of a defibrillator self test system which is incorporated herein by reference. An energy storage capacitor is twice charged to full voltage and discharged, first to functionally verify operation of the HV circuit under combined maximum voltage and current conditions and second to calibrate the HV circuit to ensure that the amount of energy delivered in the defibrillation pulse is within specification limits. The test load is resistance typically in the range of 10 to 20 ohms. 
     Providing a test pulse to the test load at a combined maximum voltage and maximum current stress as taught in by Powers et al. results in a substantial amount of energy dissipated in the test load for each self test operation. It has been found that over time the periodic self test operations conducted by the AED substantially reduced its battery life. The self test of the HV circuit forms only a portion of the overall defibrillator self test but consumes the majority of total energy that is required. 
     U.S. Pat. No. 5,873,893, “Method and Apparatus for Verifying the Integrity of an Output Circuit Before and During Application of a Defibrillation Pulse”, to Sullivan et al. describes an external defibrillator capable of testing the high voltage output circuit for open and shorted switches by monitoring the energy storage capacitor voltage. The high voltage circuit, the well known H-bridge configuration, is tested first by sequentially turning on each of the switches in each leg of the H-bridge while the energy storage capacitor is charged up to a test voltage. No current conducting paths should appear through the H bridge. Next, current conducting paths through each side of the H bridge are created for a brief time and the energy storage capacitor voltage is monitored. The energy storage capacitor voltage should drop by a predetermined amount for each discharge. While it is mentioned by Sullivan et al. that the test voltage may be less than the maximum voltage to reduce energy consumption, there is no teaching on how the test voltage may differ between the open and short circuit tests to allow for full testing of HV circuit at both maximum current and maximum voltage levels. 
     It would therefore be desirable to provide a method and apparatus of reduced energy self test in a defibrillator which allows for testing of the HV circuit at both maximum current and maximum voltage. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a defibrillator having a self test operation requiring reduced energy is provided. During a self test operation of the defibrillator, it is necessary to deliver a test pulse to a test load under appropriate levels of voltage and current stress to the HV circuits. HV circuits will typically include an energy storage capacitor, a high voltage charger to charge the energy storage capacitor, and an HV switch which operates to deliver a defibrillation pulse to a patient in a desired polarity and for a desired pulse duration using the energy from the energy storage capacitor. In the preferred embodiment, the HV switch is an H-bridge circuit which is well known in the art to commutate the voltage from the energy storage capacitor to the patient first in one polarity and then in the other polarity to form a biphasic defibrillation pulse. 
     In verifying the functionality of an HV switch during the self test operation, several critical parameters must be evaluated. First, a voltage stress test is conducted to ensure that the dielectric withstand voltage of the various components of the HV switch are adequate at the maximum voltage level. In the preferred embodiment, the components of the HV switch are silicon controlled rectifiers (SCRs) and insulated gate bipolar transistors (IGBTs). 
     First, the voltage stress test is conducted by imposing the maximum voltage level which would be incurred during normal use, typically 2,000 volts, across each of the components in the HV switch and monitoring either for any current flow through each component or a significant voltage drop in the energy storage capacitor voltage. 
     Second, the current stress test is conducted to ensure that the current handling capability of each component is adequate to source the maximum current level which would be incurred during normal use, typically 100 amperes, to the test load. The test load is preferably a low impedance that less than the lowest impedance of the expected range of impedances spanning 20 to 200 ohms. The maximum voltage level and the maximum current level may be chosen to exceed the operating voltage level and operating current level which are the maximum values that occur during normal operation of the defibrillator in order to more fully stress the components of the HV switch and charging circuitry. 
     The present invention allows for testing of the HV circuit at the maximum current level and at the maximum voltage level with substantially reduced energy consumption and battery drain during the self test of the HV circuit from that of the prior art. The current stress test is conducted at substantially lower energy levels by coupling a low impedance test load across the H bridge in place of a patient impedance and with the energy storage capacitor charged to a partial voltage. The voltage stress test is done at the maximum voltage level and on its own consumes no energy because there is no path for the current to flow under normal conditions and is. The charge stored in the energy storage capacitor, which is still at the maximum voltage level following the voltage stress test, can then be used for other portions of the self test operation including generation of the calibration pulse. 
     One feature of the present invention is to provide a self test method for a defibrillator. 
     A further feature of the present invention is to provide a method of reduced energy self test in a defibrillator. 
     A further feature of the present invention is to provide a method of reduced energy self test in a defibrillator that provides for testing at maximum voltage and maximum current levels. 
     Another feature of the present invention is to provide an external defibrillator having a reduced energy self test circuit. 
    
    
     Other features, attainments, and advantages will become apparent to those skilled in the art upon a reading of the following description when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a defibrillator having an HV circuit that is tested according to the present invention; 
     FIG. 2 is a schematic diagram of the HV switch portion of the HV circuit of FIG. 1; 
     FIG. 3 is a graph of voltage versus current of a combined voltage stress and current stress test as conducted according to prior art self test methods; 
     FIG. 4 is a graph of voltage versus current showing the separate voltage stress and current stress tests as conducted according the self test method of the present invention; 
     FIG. 5 is a graph of voltage versus current showing an alternative embodiment of the present invention in which the maximum current and voltage exceed the voltage and current of the operating region; and 
     FIG. 6 is a flow chart of the self test method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram of a defibrillator  10  that incorporates a method of reduced energy self test according to the present invention. A pair of electrodes  12  are coupled across the chest of a patient (not shown). The pair of electrodes  12  are coupled to an ECG front end  18  which filters, amplifies, and digitizes the ECG signal from the patient to obtain ECG information which is provided to a controller  26 . A defibrillation pulse may be delivered to the patient by the defibrillator  10  upon detection of a shockable rhythm such as ventricular fibrillation (VF) that is detected in the ECG information by the controller  26  running a shock advisory algorithm. 
     An HV circuit  14  is coupled to the electrodes  12  and delivers a defibrillation pulse in a desired polarity and duration across the electrodes  12 . In the preferred embodiment, the defibrillation pulse is a biphasic truncated exponential waveform. The HV circuit  14  contains an HV switch  16  that is coupled to the pair of electrodes  12 , an energy storage circuit  20  coupled to the HV switch  16 , and a high voltage charger  22  for charging the energy storage circuit  20  to the desired charge level. In the preferred embodiment, the HV switch  16  is constructed as an H bridge as explained in more detail below. 
     The energy storage circuit  20  is coupled to the HV switch  16  to provide the high voltage, high current waveform necessary to develop the defibrillation pulse. The energy storage circuit  20  typically consists of at least one capacitor with a capacitance value in the range of 100 to 200 microFarads (uF) and which is charged to over 2000 volts at the maximum voltage. The high voltage charger  22  converts the relatively low battery voltage, typically 12 volts, from a battery  24  to the relatively high voltage levels required to charge the energy storage circuit  20 . 
     The controller  26 , implemented for example with a general purpose microprocessor, an embedded controller, or a state machine, operates to control the functions of the defibrillator  10  including the self test operation according to the present invention. The HV switch  16  develops the defibrillation pulse responsive to a switch control signal from the controller  26  in the desired polarity and pulse duration. The voltage of the energy storage circuit  20  is controlled through via the charge control signal to the high voltage charger  22  from the controller  26 . During the self test operation, a current signal from the HV switch  16  and a voltage signal from the energy storage circuit  20  may be fed back to the controller  26  which evaluates the results of the self test operation. 
     In FIG. 2, there is shown a simplified schematic drawing of the energy storage circuit  20  and the HV switch  16 . A capacitor  40  in the energy storage circuit  20  provides for storage of the energy required for the defibrillation pulse. The capacitor  40  is charged to a voltage level determined by the controller  26 . The voltage level is measured using a voltage measurement circuit  42  that provides the voltage signal back to the controller  26 . 
     The voltage measurement circuit  42  preferably consists of a voltage divider network and analog to digital converter that provides the voltage signal in the form of digital measurement values to the controller  26 . If the voltage signal from the energy storage circuit  20  has a value within specification limits based on the charge control signal to the high voltage charger, then the high voltage charger  22  is determined to be functioning normally. 
     SCRs  50 - 56  form an H bridge circuit coupled between the capacitor  40  and ground. The H bridge circuit, under the control of the controller  26  which generates the switch control signals A-F, operates to couple the voltage from the capacitor  40  to the patient in the desired polarity and for the desired time duration. The SCRs  50 - 56  are controlled via control inputs A-D which each receive the switch control signals A-D from the controller  26 . The voltage from the capacitor  40  is coupled to the H bridge circuit via a series resistor  60  and a parallel combination of an inductor  62  and clamping diode  64 . The inductor  62  has an inductance value chosen to limit the rate of current change through the H bridge in order to allow current limiting circuits (not shown) time to engage before excessive current flows through the H bridge. The series resistor  60  is in the range of 5 ohms in the preferred embodiment. 
     Pull down resistors  66  and  68  ensure that the cathode ends of the SCRs  50  and  52  are near ground potential when the SCRs  50  and  52  are off and the IGBT  70  is on. This ground potential is also the normal operating state of the HV switch  16  when the energy storage circuit  20  is charged up and ready to deliver the defibrillation pulse. The resistors  66  and  68  have resistance values that are high enough, for example in the 10,000 ohm range that they draw only nominal current when either of the SCRs  50  or  52  is on. 
     A snubber circuit  82 , consisting of a resistor  84  in series with a capacitor  86 , is coupled in shunt across the SCR  54 . The snubber circuit  82  is provided to prevent the inadvertent triggering of the SCR  54  responsive to a rapid rise in voltage between the anode and cathode, commonly known as dV/dt trigger. The ability of the snubber circuit  82  to prevent dV/dt trigger may be effectively tested during the voltage stress test by providing a rapidly changing voltage across the SCR  54  such as by turning on one of the SCRs  50  and  52  with the switch  80   a-b  in the short position. 
     Interposed between the H bridge and ground is an insulated gate bipolar transistor (IGBT)  70  having a control input F. The control input F receives a portion of the switch control signal from the controller  26 . The IGBT  70  operates as a switch to interrupt the flow of current through the H bridge to allow the SCRs  50 - 56  to turn off. A current measurement circuit  74  measures the current flowing through the H bridge by measuring the voltage across a resistor  72  which is interposed between the capacitor  40  and ground. The current measurement circuit  74  may alternatively consist of an over-current detector in the form of a comparator configured to provide an over-current signal when the current through the H bridge exceeds a predetermined trip level. 
     A disarm circuit  79  consisting of an SCR  76  with a control input E in series with a load resistor  78  operates in parallel with the H-bridge circuit. The control input E receives a portion E of the switch control signal from the controller  26 . The disarm circuit operates to safely discharge the capacitor  40  through the load resistor  78  and the resistor  60  during normal operation of the defibrillator  10  when a defibrillation pulse to the patient is not needed. The disarm circuit may also be used in the self test mode with the load resistor  78  and the resistor  60  operating as the test load to receive the calibration pulse generated by turning on the SCR  78  using the switch control signal E from the controller  26 . 
     The discharge time of the calibration pulse is measured by the controller  26  by monitoring the voltage or current signal in order to control for variations in the charge voltage and capacitance value of the capacitor  40 . Using the measured discharge times, the controller  26  may then compensate for variations in voltages and component values in order to deliver a defibrillation pulse to the patient within desired specification limits. Applying the disarm shock through the SCR  76  will also apply the maximum voltage level to the collector of the IGBT  70  which should be in the off state. 
     At the output of the H bridge is a switch  80   a-b  which operates as a ganged double pole, double throw switch with an upper and lower position controlled by a control input G. The control input G receives a switch control signal G from the controller  26 . In the lower position, the switch  80   a-b  couples the HV switch  16  to the electrodes  12  in order to deliver a defibrillation shock to the patient. In the upper position, the switch  80   a-b  couples the H bridge to a short circuit. In the preferred embodiment, the short circuit is an electrical connection such as a short piece of wire with a resistance substantially close to zero ohms and the resistor  60  operates as the test load. Alternatively, the switch  80   a-b  could couple the H bridge to a test load with the desired test load resistance. In another alternative, the switch  80   a-b  couples the H bridge to an external resistor (not shown) with a value chosen such that the series combination of the external resistor and the resistor  60  equal the desired test load resistance. 
     To deliver a biphasic defibrillation pulse to the patient, the capacitor  40  is charged to a desired voltage level and the switch  80   a-b  is placed in the lower position to couple the HV switch  16  to the pair of electrodes  12 . The IGBT  70  is then turned on followed by SCRs  50  and  56  to begin the first phase of the defibrillation pulse that is delivered to the patient. After a desired period of time, the IGBT  70  is turned off followed by the SCRs  50  and  56  to end the first phase. To begin the second phase, the IGBT  70  is again turned on, followed by the SCRs  52  and  54 . After a desired period of time, the IGBT  70  is again turned off followed by the SCRs  52  and  54 . If a no-shock decision is reached by the controller  26  after the capacitor  40  had been charged, the disarm circuit could be activated by turning on SCR  76  to safely discharge the capacitor  40  through the load resistor  78 . 
     During the self test operation of the defibrillator  10  according to the present invention, a series of tests of the H bridge are conducted in order to ensure its proper function within specified limits. Such tests include current stress, voltage stress, and calibration. The current stress test is conducted at the maximum current level and the voltage stress test is conducted at the maximum voltage level. The self test operation according to the prior art will be explained in more detail below according to FIG.  3 . The self test operation according to the present invention will be explained in more detail below according to FIGS. 4 and 6. In an alternative embodiment of the present invention, the maximum voltage and current levels may exceed the current and voltage levels encountered during normal operation of the defibrillator  10  as explained in more detail below according to FIG.  5 . 
     FIG. 3 is a graph of voltage versus current for the voltage and current stress tests and calibration tests as conducted according to the prior art. The current stress and voltage stress tests occur simultaneously as the maximum current and maximum voltage conditions are met with discharge into a 20 ohm load which includes 10 ohms of series resistance and 10 ohms test load. This first test corresponds to location 100 on the graph of FIG. 1 with a maximum voltage of 2,000 volts and a maximum current of 100 amperes (A). 
     Under the self test conditions given in this example, 10 ohms as the test load resistance is the worst case condition occurring below in the expected range of patient impedances which are generally believed to span 20 to 200 ohms. The full amount of energy stored in the energy storage capacitor would be discharged into the two 10 ohm resistors. For example, a 100 uF energy storage capacitor charged to 2,000 volts would have 200 joules (j) of energy according to the formula: 
     
       
         energy (j)=0.5·C·V 2   
       
     
     where: C is capacitance (in Farads) of the capacitor 
     V is the charge voltage across the capacitor 
     Next, the energy storage capacitor is again charged up to maximum voltage and a calibration pulse is discharged into the test load corresponding to location  102  on the graph. Other known resistor values may be substituted for the test load resistance corresponding to locations  104  and  106  so that the discharge time for each of the known load resistances may be measured. Using the discharge time measurements and the known resistances to solve a system of equations, the charge voltage and the capacitance of the energy storage capacitor can be determined and then controlled for by compensation in the pulse duration of the defibrillation pulse in order to deliver a desired amount of energy to the patient. While this self test method according to the prior art is effective, it results in two full discharges of the capacitor totaling 400 j using the energy value from the above example, resulting in substantial battery drain for each self test operation. 
     FIG. 4 is a graph of voltage versus current for the voltage and current stress tests and calibration tests as conducted according to the present invention. The current stress and voltage stress tests occur separately, along with the calibration portion. According to the present invention, the capacitor  40  is charged to the maximum voltage only once for the voltage stress test which occurs at location  200  on the graph. Since no current flows for a normal voltage stress test, no energy is dissipated and the capacitor  40  remains at the maximum voltage level. Calibration pulses may then be generated and their discharge times measured for calibration purposes at locations  202  and  204  on the graph. 
     Finally, the capacitor  40  is charged to the partial voltage, for example to 500 volts, to obtain the desired level of current for the current stress test of 100 A with a test load resistance of 5 ohms corresponding to location  206  on the graph. At the partial voltage of 500 volts, the capacitor  40  is storing only {fraction (1/16)} the amount of energy of the maximum voltage level of 2,000 volts, resulting in substantially reduced energy requirements for self test from that of the prior art. 
     FIG. 5 is a graph of voltage versus current for the voltage and current stress tests and calibration tests as conducted according to an alternative embodiment of the present invention. An operating region  206  is a region which defines as the range of voltages and currents that may be expected during the normal operation of the defibrillator  10  when delivering a defibrillation pulse. The maximum voltage and maximum current, instead of coinciding with the operation voltage and current as in the preferred embodiment, are now chosen to exceed the operating voltage and current. 
     In the example illustrated in FIG. 5, the maximum current is chosen to exceed the operating current by ten percent and the maximum voltage is chosen to exceed the operating voltage by ten percent. The benefit of exceeding the operating voltage and current during the voltage stress test and current stress test is achieving an added margin of safety in ensuring the proper function of the components of the HV switch  16 , energy storage circuit  20 , and the high voltage charger  22 . The benefits of present invention are realized in the alternative embodiment by obtaining a wider margin of safety in the self test operation while substantially reducing the energy requirements. 
     The capacitor  40  is charged to maximum voltage level only once for the voltage stress test which occurs at location  208  on the graph. Since no current flows for a normal voltage stress test, no energy is dissipated and the capacitor  40  remains at maximum voltage. The calibration pulses may then be generated and their discharge times measured for calibration purposes preferably at locations within or along the border of the operating region  206 . 
     Finally, the capacitor  40  is charged to the partial voltage, for example to 550 volts, to obtain the desired level of current for the current stress test of 100 A corresponding to location  206  on the graph. At the partial voltage of 550 volts, the capacitor  40  is storing only {fraction (1/16)} the amount of energy of its maximum voltage level of 2,200 volts, still resulting in substantially reduced energy requirements for self test from that of the prior art. 
     In FIG. 6, there is shown a flow chart of the self test method according to the present invention. In step  300  labeled CHARGE CAPACITOR TO MAXIMUM VOLTAGE, the capacitor  40  is charged by the high voltage charger  22  to the maximum voltage level. In the preferred embodiment, the maximum voltage is approximately 2,000 volts. The voltage level across the capacitor  40  is measured by the voltage measurement circuit  42  and fed back to the controller  26  as the voltage signal. If the voltage signal is within specification limits, the operation of the high voltage charger  22  and energy storage circuit  20  are determined to be normal by the controller  26 . 
     In step  302  labeled CONDUCT VOLTAGE STRESS TEST, the SCRs  50 - 56  are exposed to the maximum voltage present in the capacitor  40 . The switch  80   a-b  is placed in the upper position to short the cathodes of the SCRs  50  and  52  together and the IGBT  70  is turned on. The SCRs  50  and  52  form the top half of the H bridge and are tested first. The pull down resistors  66  and  68  operate to pull the junction to ground potential to impose the maximum voltage across each of the SCRs  50  and  52  presently in their off state and appearing as an open circuit. 
     Leakage current through the H bridge circuit is monitored during the voltage stress test. The voltage signal from the voltage measurement circuit  42  may alternatively be monitored for voltage drop in place of the current signal. If the SCRs  50  and  52  leak only negligible amounts of current, then they are deemed to have passed the voltage stress test. Next, one or both of the SCRs  50  and  52  is turned on, now imposing the maximum voltage across each of the SCRs  54  and  56  which form a bottom half of the H bridge and appear as open circuits. If the SCRs  54  and  56  leak only negligible amounts of current, they are deemed to have passed the voltage stress test. A failure of the voltage stress test occurs when any of the SCRs  50 - 56 , or any other element of the H bridge, breaks down and begins to pass substantial amounts of current. 
     In step  304  labeled GENERATE CALIBRATION SHOCK TO TEST LOAD, the capacitor  40  is still fully charged from the step  302  and can provide the calibration pulse to the test load. The test load consists of the load resistor  78  in series with the resistor  60  in the disarm circuit. The calibration pulse is delivered by turning on the SCR  76  which completely discharged the capacitor  40  before turning off again. Alternatively, the test load may consist of an external load resistor put in place of the pair of electrodes  12  which is delivered by turning on the IGBT  70  and either the SCRs  50  and  56  or the SCRs  52  and  54 . The discharge time across the test load which is a known resistance such as  10  ohms is measured by the controller  26  in order to calibrate the amount of energy delivered to the patient in the defibrillation pulse. 
     In step  306  labeled CHARGE CAPACITOR TO PARTIAL VOLTAGE, the capacitor  40  is charged to a partial voltage which is a fraction of the maximum voltage and is chosen so that the high voltage circuit can source the maximum current level to the short circuit given the series resistance of the resistor  60 . The switch  80   a-b  is moved into the shorting position, effectively shorting the HV switch  16 , with only the resistor  60  in the current path to obtain the desired test load resistance. As an example, to obtain a maximum current level of 100 A sourced from the HV circuit into the short during the current stress test with a series resistance for the resistor  60  chosen to be 5 ohms, the capacitor  40  would be charged to 500 volts. 
     In step  308  labeled CONDUCT CURRENT STRESS TEST, the current handling capability of the H bridge is tested at the maximum current level. Similar to the sequence for generating a biphasic defibrillation pulse, the current test pulse is created in both polarities. IGBT  70  is turned on followed by SCRs  50  and  56  during the first phase to source the maximum current through the short circuit test load. After a desired time duration, IGBT  70  is turned off followed by SCRs  50  and  56  to end the first phase. The IGBT  70  is again turned on, followed by SCRs  52  and  54  during the second phase to source the maximum current through the short circuit. After a desired time duration, IGBT  70  is again turned off followed by SCRs  52  and  54 . 
     Alternatively, IGBT  70  is turned on followed by SCRs  50  and  54  during the first phase to source the maximum current through one side of the H bridge circuit which acts as a short circuit. After a desired time duration, IGBT  70  is turned off followed by SCRs  50  and  54 . The IGBT  70  is again turned on, followed by SCRs  52  and  56  during the second phase to secure the maximum current through the other side of the H bridge circuit which acts as a short circuit. After a desired time duration, IGBT  70  is again turned off followed by SCRs  52  and  56 . 
     The time duration of the first phase is preferably very short to minimize the energy lost from the capacitor  40 . Following the first phase, the capacitor  40  is then charged back up to the partial voltage level. Since only a portion of the energy stored in the capacitor  40  has been discharged during the first phase depending on its duration, charging the capacitor  40  for the second phase should require a fraction of the energy needed for the first phase. A failure of the current stress test occurs when the H bridge fails to source the maximum current level for the desired time duration, either in the first or second phases. 
     An advantage of the current stress test according to the present invention is that the H bridge is exercised in the same manner as it would be during the delivery of a defibrillation pulse under normal operation to obtain a more realistic test. The same pairs of SCRs, either SCRs  50  and  56  or SCRs  52  and  54  are used to deliver the first and second phases of the current stress test as well as the first and second phases of the biphasic defibrillation pulse. 
     It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above-described preferred embodiments of the invention without departing from the spirit of the invention in its broader aspects. For example, the self test method may be readily applied to any configuration of the HV switch  16 , such as a simpler switch topology that supports only monophasic defibrillation pulses. The energy storage circuit  20  may also be constructed using multiple capacitors. The current stress test consisting of steps  306  and  308  may be conducted before the voltage stress test and calibration consisting of steps  300 - 304  during the self test operation. Therefore, the scope of the present invention should be determined by the following claims.