Abstract:
An external defibrillator includes a detector used to detect a life threatening condition of a patient, a controller operating the defibrillator automatically and a therapy delivery circuit that delivers appropriate therapy. The defibrillator is attached to a patient and is adapted to monitor the patient and when a life threatening condition is detected, to apply therapy automatically. An averaging scheme is used to determine a current cardiac rate by taking a first average of the intervals between a preset number of successive cardiac events, establishing a differential between this average and the intervals, dropping the interval corresponding to the largest differential. In this manner, the effects of over- and undersensing are eliminated or at least reduced.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of Invention 
     This invention pertains to an external defibrillator arranged and constructed to provide anti-tachyarrhythmia therapy to a patient on demand. In particular, an automatic external cardioverter/defibrillator is described which has several operational modes including a fully automatic mode in which shocks are delivered without any manual intervention, an advisory mode and a manual mode. Moreover, the invention pertains to a defibrillator with an integral tachyarrhythmia detector which detects an abnormal heart beat and determines whether this abnormal heart beat is amenable to shock therapy. 
     B. Description of the Prior Art 
     Defibrillators are devices which apply electric therapy to cardiac patients having an abnormally high heart rhythm. Two kinds of defibrillators are presently available: internal defibrillators which are implanted subcutaneously in a patient together with leads extending through the veins into the cardiac chambers, and external defibrillators which are attached (usually temporarily) to the patient. External defibrillators are used in most instances in case of an emergency, for example, when a patient has either suffered cardiac arrest or where a cardiac arrest is imminent. Typically therefore external defibrillators are manual devices which must be triggered by a physician or other trained personal. Internal or implantable defibrillators (and cardioverters) are implanted as a permanent solution for patients having specific well defined cardiac deficiencies which cannot be treated successfully by other means. They generally operate in an automatic mode. 
     However, there are some instances where an external defibrillator would be very advantageous which could be operated in both automatic and manual modes. For example, presently, it is well known that after a cardiac episode, such as a sudden cardiac arrest, many patients frequently suffer a second episode of similar nature. Therefore, cardiac patients are kept in a hospital under observation. While in the hospital, the patient is attached to a monitor which indicates the patient&#39;s heart rate, temperature, respiration rate and other vital signs. Many monitors are provided with an alarm function which is activated when these vital signs fall outside a nominal range. The monitor then generates an audible and visual signal at the bed side of the patient and/or at the remote location such as a nurse station. However, if a cardiac episode does occur, the attending staff has to examine the patient to determine that the patient needs electrical therapy, and then set up and use a manual defibrillator. All these steps are inherently time-consuming. 
     Some attempt has been made to overcome some of these problems. For example, some external defibrillators are available which can verify that a patient is suffering from ventricular fibrillation VF and prompt an attendant to activate the defibrillator for the delivery of therapy. However, the algorithms used by these defibrillators to detect VF are very limited. For example, some of defibrillators utilize an algorithm in which the electrocardiogram ECG can be verified only while the patient is unconscious, has no pulse and does not breath. Obviously, these algorithms are not satisfactory since it is important to detect when those conditions happen and apply therapy as soon as those conditions are detected. 
     Commonly-owned U.S. Pat. No. 5,474,574 discloses an external defibrillator. Commonly-owned U.S. Pat. No. 4,576,170 discloses an external defibrillator that can be worn by a patient. 
     OBJECTIVES AND SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide an automatic external cardioverter/defibrillator which is capable of detecting a current cardiac rate of a patient accurately by eliminating the effects of over- and under-sensing. 
     A further objective is to provide a defibrillator capable of performing amplitude and/or frequency analyses to detect a shockable rhythm based on a patient&#39;s ECG and to use the results of these analyses to categorize or recognize the current condition of the patient and to apply appropriate therapy for reverting the same. 
     Yet another objective is to provide an external defibrillator with several modes of operation, including an automatic mode in which shocks are applied on demand in accordance with preprogrammed shock parameters and without any prompting from an attendant, an advisory mode in which an attendant is alerted to a shockable rhythm however the application of shocks must be initiated by the attendant and a manual mode in which the attendant determines how and when shocks should be applied and the preprogrammed shock parameters are ignored. 
     Other objectives and advantages of the invention will become apparent from the following description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a front view of an automatic external cardioverter/defibrillator constructed in accordance with the subject invention; 
     FIGS. 2 and 3 show an orthogonal view of the defibrillator of FIG. 1 with details of the printer on the side of the housing; 
     FIG. 4 shows the defibrillation and sensing electrodes used with the defibrillator of FIG. 1; 
     FIG. 5 shows a block diagram of the circuitry for the defibrillator of FIG. 1; 
     FIG. 6 shows a flow chart illustrating the steps required to initialize the defibrillator of FIG. 1; 
     FIG. 7 shows diagrammatically the placement of the electrodes of FIG. 4 on a patient; 
     FIG. 8 shows a view of the display while an ECG is acquired during the initialization process of step  6 ; 
     FIG. 9 shows a flow chart for the operation of the defibrillator of FIG. 1; 
     FIG. 9A shows details of a rate detecting circuit for the defibrillator of FIGS. 1-5; 
     FIG. 9B shows a flow chart for the operation of the circuit of FIG. 5A; 
     FIG. 10 shows the screen displayed during automatic operation when the defibrillator is ready to apply anti-tachycardia therapy; 
     FIG. 11 shows a flow chart illustrating the operation of the device in the advisory mode; 
     FIG. 12 shows a block diagram for the tachycardia detector circuit of FIG. 5; 
     FIG. 13 shows a flow chart illustrating the operation of the circuit of FIG. 12; 
     FIG. 14 shows a flowchart illustrating the operation of the therapy selector of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a defibrillator  10  having a housing  12  with a front face display  14  on which a plurality of controls and indicating elements are provided, as described in more detail below. The defibrillator no further includes electrode assembly shown in FIG.  4  and described in detail below. As seen in FIGS. 2 and 3, one side  16  of the housing  12  is provided with a cavity  18 . A printer (not shown) is mounted in cavity  18 . A roll of paper  20  is mounted on shaft  22  in a manner which allows the printer to print alphanumeric characters and graphics on paper  20 . 
     The housing  12  can be positioned on a rack, or other support means so that it can be disposed adjacent to the patient. 
     Referring back to FIG. 1, a screen display  24  is mounted on the front face  14  so that it is clearly visible. The display is used to provide information to the clinician related to the operation of the defibrillator  10 , the status of the patient, etc. Disposed around the display  24 , there are other indicator and control elements, such as the menu selection control knob  26 , selector knob  28 , charge button  92 , shock buttons  30 ,  31  with built-in lights  30 A,  31 A respectively, and indicator lights  32 ,  34  and  36 . 
     The menu selection control knob  26  is used in combination with the display  24  to select various operational parameters or operations for the defibrillator  10 . 
     The knob  28  has several positions defining modes of operation, such as: Off, Auto/advisory, Disarm and Energy Selections. In the Off position, the defibrillator is deactivated. In the auto/advisory position, the defibrillator monitors the patient and can apply shocks using a preselected therapy. In the disarm position, an internal capacitor (not shown) is discharged to ensure that the defibrillator does not apply a shock accidentally. Finally, in the energy selection position, the defibrillator may be used to apply a shock to a patient at the selected energy level. 
     A socket  38  is provided for mating the housing  12  to the electrode assembly of FIG.  4 . Near the top of the face  14 , the housing  12  is provided with an additional illuminated indicator  33 . 
     FIG. 4 shows details of the electrode assembly  40 . The assembly  40  includes a first plug  42  constructed and arranged to mate with the jack  38  (FIG.  1 ), a connector  44  and a cable  46  extending between the plug  42  and the connector  44 . 
     The assembly  40  further includes a pair of defibrillator pads  48  and  50  coupled to the connector  44  by an adapter  52 , and two leads  54 ,  56  connected respectively to sensor electrode pairs  60  and  62 . To ensure the defibrillation pads  48  and  50  are approved pads and they are used within their specified or approved time limit, e.g. 24 hours, a pad identification (pad ID) may be embedded in either the pads  48  and  58 , cables  54 ,  56 , and  46 , or connectors  42  and  44 . The defibrillator includes a capability to verify the pad ID and time it out after specified or approved time limit of usage. 
     The defibrillator  10  also includes electronic circuitry disposed in housing  12  and used to operate the defibrillator and to generate the required electrical therapy. Referring to FIG. 5, the circuitry includes a microprocessor  70  which is associated with a memory  72  for storing programs and data logging information. The defibrillator further includes a sensing circuit  74 , an ECG detecting circuit  76 , a rate detector circuit  78 , an MDF circuit  80 , a therapy selector circuit  82 , a defibrillator shock generator  84 , an external interface  86 , and a user interface  88 . The microprocessor  70  receives commands from an attendant and other control signals through the various knobs, and push buttons shown in FIG. 1 via the analog interface  88 . The microprocessor also activates various visual indicators and a speaker  92  through the same interface  88 . The circuits shown in FIG. 5 can be implemented by software in RAM  72  however are shown as discrete circuits for the sake of clarity. Energy for the shocks is derived from a capacitor  84 A associated with the generator  84 . 
     As can be seen in FIG. 5, the electrode pairs  60 ,  62  and pads  48 ,  50  are connected through the jack  38  to a sensing circuit  74 . This circuit  74  senses the intrinsic signals detected from the heart of the patient through the electrodes or pads, filters the same, converts them into digital signals at a sampling rate of, for example, 512 samples per second. Of course the filtering can be performed on the digital signals as well. The circuit  72  further includes an impedance measuring element (not shown) which measures the impedance of between the pads  48 ,  50 . This impedance is provided to the microprocessor  70  so that the latter can determine if the pads and sensors are properly attached to the patient. The sensing circuit also detects if the sensing electrodes are connected properly. 
     The sampled digital signals from circuit  74  are fed to the microprocessor  70 . The ECG detector circuit  76  and the rate detector circuit  78 . The ECG detector  76  detects the ECG complex of the patient. The rate detector circuit  78  detects the current cardiac rate of the patient. 
     The microprocessor  70  analyzes the signals received from circuits  74 ,  76  and  78  and operates the other elements of defibrillator in accordance with these signals as discussed in more details below. In addition, the microprocessor  70  also sends information requested by an attendant to printer  80  when a pushbutton  90  (FIG. 1) is activated. In some cases, the microprocessor  70  activates the printer  80  automatically, for example, to display an ECG during defibrillation therapy. 
     The microprocessor can also exchange information with other devices or display a current ECG through an external interface  86 . 
     Before turning on the defibrillator  10 , the pads  48 ,  50  and electrodes  60 ,  62  are positioned on the patient. FIG. 7 shows one possible positioning for these elements. 
     The operation of the defibrillator  10  and its microprocessor  70  is now described in conjunction with the flow charts of FIGS. 6 and 9. Before the defibrillator  10  can be operated, it must be initialized. This stage of initialization may be performed whenever the defibrillator  10  is set up for a particular patient. In one embodiment of the invention, the defibrillator  10  can be set up for only one patient at a time. In another embodiment, the defibrillator may be set up to provide therapy selectively to one of several patients, in which case, operational parameters unique for each patient are stored in its memory  88 . 
     The first step in the initialization stage, step  100 , the defibrillator  10  is turned on. This may be performed, for example, by turning the selector knob  28  to the auto/advisory position. 
     Once the defibrillator  10  is activated, it goes into a self-test mode (step  102 ) during which various internal functions and components are tested. During this step  102 , the indicator light  32  illuminates to indicate that the defibrillator is currently unable to apply shocks and various sounds are emitted from the speaker (not shown) as well. 
     If the self-test fails in step  102 , then in step  104 , an error message is shown on display  24  and the initiation process is aborted. 
     If the self-test passes, then in the next step  106 , the ID of the patient to be treated is obtained. For example, instructions may be shown on display  24  requesting the name and/or a unique number for the patient. The requested information can be entered by manipulating the knob  26  or by using a keyboard (not shown). The patient ID may be an optional field. 
     Next, in step  108 , the ECG signals are analyzed to determine the best channel for the ECG acquisition. More particularly, the two pairs of sensing electrodes and the pads (which are also used in this instances as a sensing electrode pair) define three separate detection channels. The detection channel is selectable by an attendant. 
     Referring to FIG. 8, a portion  96  of display  24  identifies three detecting channels as channels  1 ,  2  and  3  respectively. In step  108 , each of these channels is selected by manipulating knob  26 . As each channel is selected, the ECG sensed through the corresponding pair of electrodes, the electrode pair impedance of the defibrillation pads, and current heart beat sensed through the electrodes is shown on the display  24 . For example, in FIG. 8, an ECG is shown as it is sensed from channel  2  (which may correspond to electrode pair  60 ), with an electrode impedance of 50 ohms and a heart rate of 72 beats. These measurements are derived by the microprocessor  70  from the signals sensed through the sensing electrode pairs, the sensing circuit, and the ECG detection circuit  76 . The attendant setting up the defibrillator examines the ECG and other parameters for each channel and based on attendant&#39;s observations and experience, the attendant then selects the best or optimal channel by manipulating knob  26 . 
     Next, in step  110 , the parameters for a particular therapy are selected by the attendant, including a cardiac rate Rmin. The range of Rmin is about 120-240 BPM. Another parameter set during step  110  is the rate Rmdf. Generally, the rate Rmdf is higher than Rmin. 
     The defibrillation therapy delivered by defibrillator  10  consists of one or more shocks. More particularly, the defibrillator  10  can be set to deliver a number of sequential shocks, for example, one to nine; each having an energy level in the range of 5 to 360 joules. The interval or delay between shocks can also be set from 10 to 600 seconds in either 5 or 10 second increments. These parameters are all selected in step  110 . Moreover, if multiple shocks are used, the energy level and or delay of each shock may be constant or can be separately programmed to predetermined levels. 
     After the operational parameters of the defibrillator  10  have been set (or programmed) in step  110 , the defibrillator proceeds to learn to recognize the ECG of the patient in step  112 . During this step, the microprocessor  70  monitors the signals sensed on the channel designated in step  108  for a predetermined time period (for example, 20 seconds). In step  114 , a test is performed to determine if the ECG signal recognition was successful. For example, during this period the heart rate is determined from the ECG by determining the time interval between successive R-waves, and compared to the Rmin. In addition the amplitude of the ECG signal is compared to a threshold value (such as 0.7 mv). If the heart rate is found to be below the rate Rmin and the amplitude is found to exceed the threshold then the recognition step is successful. If the recognition process is not successful, then in step  116 , a message is displayed to indicate failure and the process is aborted. In step  116  suggestions may also be made to the attendant which may cure the problem. For example, the attendant may be asked to reposition the electrode pairs, and/or select a different sensing channel. 
     If the learning process is found to be successful, then in step  118 , the ECG is shown on the display  24  together with the pertinent parameters and the attendant is requested to verify these parameters. In step  120 , the attendant is given the choice of accepting the ECG or to reject it. If the attendant rejects the ECG, the process is aborted. If the attendant accepts the ECG, then in step  122 , the attendant is asked to select a mode of operation (i.e., automatic or advisory). In step  124 , the choices made during the initialization process are displayed to the attendant. The attendant can request that the selected parameters and mode of operation be printed out during this step. 
     In step  126 , the attendant is given the choice of accepting the parameters as they were set in steps  106 - 122 . If the parameters are accepted, then in step  128 , the initialization process is completed, the defibrillator automatically print the parameters, and the operation starts its normal operation mode. 
     If in step  126 , the attendant does not approve the parameters but instead selects to edit them then the process goes back to step  106 . 
     If the attendant decides to cancel the selected parameters (step  130 ), the process is aborted. 
     Once the defibrillator  10  has been properly initialized, it is ready for operation. As described above, the mode of operation of the defibrillator  10  is determined by position of the selector switch  28 . If this switch is in the auto/advisory position, and it has been previously set to the automatic mode, then it operates as described in the flow chart of FIG.  9 . Starting with step  150 , the defibrillator first monitors the condition of the patient&#39;s heart. During this time, the display  24  is used to show the following information: the mode of operation (in this case AUTOMATIC), the current ECG, the current heart rate Rcur and the selected Rmin. 
     The current heart Rcur is determined using the circuit  78  as shown in FIGS. 9A and 9B. The circuit  78  includes a comparator  78 A, a threshold selector circuit  78 B and a rate calculator circuit  78 C. The comparator  78 A and the threshold selector circuit  78 B cooperate to detect the intrinsic ventricular rate in an adaptive manner. That is, prior to the acquisition of any signals, the circuit  78 B selects a low threshold level Tr which may in the order of 0.2 millivotes. Once a sensed signal exceeding this level is detected by comparator  78 A, the signal is identified as a potential R-wave. Thereafter. for a predetermined time period for all future incoming signals, the threshold level is increased slightly until a maximum threshold level Tm is reached. In this manner, a multiple digital signal processing method is used to detect the intrinsic cardiac signals using an adaptive threshold. 
     Next, the signals detected by comparator  78 A are fed to a rate calculator circuit  78 C. This circuit also receives a signal indicative of whether the the electrodes currently being used to detect the ECG complex are connected properly. This circuit  78 C measures the interval between consecutive signals and generates the corresponding a ventricular rate, using a special averaging technique. This technique from comparable  78 A has been selected to eliminate the adverse effects of over-sensing and under-sensing the cardiac signals. More particularly, referring to FIG. 9B in step  400 , the impedance signal is detected and analyzed. In step  402 , a test is performed to determine if this signal is abnormally high, indicating that a lead (or electrode) is off. If a lead is off, then in step  404 , an alarm is generated and the rate calculation process is terminated. As part of step  404 , a message is shown on display  24  with instructions to the attendant for correcting the problem. 
     If, in step  402 , it is determined that the electrode impedance is acceptable, then in step  406 , N intervals between sequential events from comparator  78 A are measured. In step  408 , a first average AI 1  is taken of the N intervals. In step  410 , the absolute difference is determined between the average AI 1  and each of the intervals. The interval with the largest difference is discarded. 
     Next, in step  412 , a new average AI 2  is generated using the remaining N−1 intervals. In step  414 , again, the absolute difference between each of the remaining intervals and the average AI 2  is determined and the interval corresponding to the largest difference is discarded. In step  416 , an average is taken of the remaining N−2 intervals and this average, or more properly, its inverse, is designated as the current cardiac rate Rcur for the patient&#39;s heart. This process (steps  400 - 416 ) is repeated for each new electric event from the comparator  78 A. 
     Back to FIG. 9, in step  152  at regular intervals, a check is performed to determine if the current rate Rcur (as determined in FIG. 9B) is indicative of a shockable cardiac rhythm is detected. The method of detecting such a shockable rhythm and of determining the corresponding therapy is discussed below, in conjunction with FIG.  14 . If a shockable rhythm is detected then in step  154 , the arrhythmia is categorized (i.e., as a tachycardia or fibrillation). In step  156 , the display  24  is used to show, as indicated in FIG. 10 the current ECG of the patient, at 91, patient&#39;s heart rate at 93, and the therapy parameters selected, including the selected energy level,  95 A, the number of shocks delivered  95 B the total number of shocks programmed  95 C and the total number of shocks that remain to be delivered,  95 D. Next, in step  158 , the defibrillator issues visual and audible warnings to the attendant indicating that the defibrillator is preparing to deliver shocks to the patient and the patient should not be touched. The visual warnings include turning light  33  on (FIG. 1) and the audible signals including voice signals are generated through the speaker (Not shown). 
     Next, in step  160 , the defibrillation pulse generator  84  is activated to start charging its capacitor  84 A. As shown in FIG. 10, the display  24  shows during this time the selected or targeted energy level which was set during the initialization mode. The display also shows at  95 A the current charge level within the defibrillation pulse generator. As the capacitor within the generator is charged up, this level is increasing, and a beeping signal is emitted to indicate this gradual charging process. 
     In step  162 , the charge level of the capacitor is tested to determine if the set energy level has been reached. If this level has not been reached, the charging process continues. 
     When the selected charging level is complete, the defibrillator  10  prepares to apply shocks. In step  164 , the indicator  34  is lit to indicate that the defibrillator  10  is ready to apply therapy. 
     At any time during the process described so far, an attendant can disable the automatic or advisory mode by moving the knob  28  to the disarm or energy selection position. In FIG. 9, in step  166 , a check is performed to determine if the knob  28  has been shifted to these positions. 
     If the defibrillator has not been disarmed, then for all shocks, except the first shock of a treatment in step  168 , a delay is imposed to conform to the delay programmed between shocks as discussed above. Once the delay is complete in step  170 , an attempt is made to synchronize the shock to the ECG. More particularly in step  172 , the ECG is analyzed and an attempt is made to detect an R-wave. If an R-wave is detected, then in step  174 , a pulse of predetermined duration and energy level is applied to the patient within a predetermined interval, for example 60 milliseconds, after the R-wave. 
     As step  172  is initiated, a timer (not shown) is also activated. This timer waits for a predetermined time (for example, 2.5 seconds) for synchronization to be achieved. If no synchronization is achieved in that time period, then in step  178 , a shock is applied asynchronously. 
     The defibrillation shock of step  174 ,  178  is delivered to the patient by the pads  48 ,  50  (FIG.  4 ). 
     After the deliver of the shock in step  180 , the heart rate of the patient is determined. If a non-shockable rhythm is detected (step  182 ), then no more shocks are applied and the heart monitoring is continued in step  150 . 
     If the shockable rhythm continues, then the process of steps  152 - 180  is repeated thereby delivering the next level of predetermined therapy. 
     This process continues until all the predetermined number of shocks are delivered, the system returns to step  150  and continues monitoring the patient. 
     Preferably, after the predetermined number of shocks is delivered, the heart is monitored in step  150  but no other steps are taken even if a shockable rhythm is detected in step  152  unless the therapy sequence resets after a predetermined period of non-shockable has been detected or the defibrillator has been reset. 
     If the process described above is halted at any time, for example, by turning knob  28  to the disarm or manual position as set forth above in step  166 , then the capacitor  84 A associated with the defibrillation pulse generator  84  is discharged internally. 
     In the above description, it was assumed that a ventricular tachyarrhythmia has been detected in step  154 . The process may be modified to suit other types of arrhythmias as well. For example, if a fine fibrillation is detected, the steps  170 ,  172  are omitted since no synchronization may be achieved. 
     In the automatic mode, when the peak to peak amplitude of ECG signal is greater than a threshold, e.g. 0.2 milli-volts, to ensure a shock is delivered to a shockable condition, the last intervals immediately before the shock is delivered need to be less than the shockable interval corresponding to the Rmin. In this particular application, two intervals immediately prior to the shock are required to be less than the shockable interval. 
     As previously mentioned, one of the operational modes of the defibrillator  10  is an advisory mode. This mode is now described in conjunction with FIG.  11 . In this mode the defibrillator performs the same functions that are performed in the automatic mode starting from step  150  through step  166  (FIG.  9 ). However, after step  166 , instead proceeding with the delivery of shock therapy, the lights  30 A,  31 A associated with pushbuttons  30  and  31  respectively are activated and indicating to an attendant that the defibrillator is ready to apply a shock. The attendant can then elect to apply a shock by depressing pushbuttons  30 ,  31  simultaneously. A check is performed in step  202  to determine if the pushbuttons have been depressed. If they have not been depressed, then in step  204 , a check is performed to determine if the shockable rhythm is still present. If the rhythm is still present, then the lights  30 A and  31 A remain activated in step  200  and the system continues to wait for the activation of buttons  30 ,  31 . If, in step  204 , it is found that a shockable rhythm is no longer present, then the system is reset in step  206 . 
     If the buttons  30 ,  31  are found activated in step  202 , then in step  208 , an attempt is made to synchronize with the R wave. In step  210 , a check is performed to determine if synchronization was achieved. If synchronization is achieved then a shock is applied in step  212 . In step  224 , a check is performed to determine if all the prescribed shock pulses have been applied. If shocks still remain, the system returns to step  200 . Otherwise, it resets itself. 
     If no synchronization is achieved in step  210 , then in step  214 , a check is performed to determine if the pushbuttons  30 ,  31  are still pressed. If they are not pressed, the system resets in step  216 . If the pushbuttons  33 ,  35  are pressed, then in step  220 , a check is performed to determine if a 2.5 second timer has elapsed. If it has not elapsed then the system returns to step  208  and tries to achieve synchronization again. If the timer has elapsed, as indicated in step  220 , then in step  222 , a shock is applied and the system continues with step  224 . 
     The defibrillator  10  can also be used as a standard manual defibrillator by setting the knob  28  to an energy selection position. In this position, the knob  28  can be used to select the level of energy for the defibrillation shock. In the manual mode, when the pushbutton  92  is activated, the pulse generator  84  charges its capacitor  84 A to the level designated by the knob  28 . When the desired level is reached, the lights  30 A,  31 A are illuminated and the shock can be applied by depressing the pushbuttons  30 ,  31 . 
     An important part of the subject invention is the detection of a shockable rhythm (step  152  in FIG.  9 ). Primarily, this determination is made from the patient&#39;s cardiac rate. When selected, the MDF function is performed by the MDF circuit  80  by analyzing the ECG signal. However, simply setting a rate threshold to detect tachyarrhythmias is insufficient in some cases because an abnormally high rate (above the threshold) may not be ventricular tachycardia origin but from other causes such as sinus tachycardia, SVT (supra-ventricular tachycardia), or atrial fibrillation. Shock therapy is generally not indicated for these latter arrhythymias and may even be harmful to the patient. In the present invention, the ECG is analyzed and both its magnitude and frequency characteristics are taken into account to distinguish, if possible, VT from other SVT arrhythmias including atrial fibrillation/flutter as well as sinus tachycardia. 
     More specifically, the present inventors have analyzed and compared the morphologies of VT and SVT rhythms in order to discriminate them. It should be noted that ventricular tachyarrhythmias are characterized by relatively low frequency components, as compared to SVT arrhythmias. Frequency alone may not be adequate for the purposes of this invention. Amplitude must also be taken into account because it fluctuates widely during an arrhythmia episode. However, an amplitude consideration alone (for example, measuring the duration during which the subject wave-shape is above a baseline) has been found to be unsatisfactory because of the inability to detect VT accurately. 
     Therefore, in the present invention, a procedure has been found which takes both frequency and amplitude into consideration and hence it is referred to as MDF or modulation domain function. The method and apparatus for detecting a shockable event herein has been designed to reduce the probability of delivering therapy for an SVT condition even if its characterized by a rate which is higher than the threshold value Rmin. 
     Referring now to FIG. 12, the MDFr circuit  80  includes a clipping circuit  300 , a first summing circuit  302 , a comparator  304 , a differential normalizing element  306 , a second summer  308 , and a comparator  310 . 
     The operation of the circuit shown in FIG. 12 will now be described in conjunction with the flow chart of FIG.  13 . The circuit  78  receives from circuit  76  a stream of digital signals Ai representative of the current ECG. As each digital signal is received (step  342 ), it is first clipped by clipping circuit  300  so that it does not exceed a predetermined maximum value (step  302 ). This step insures that abnormally large values do not unbalance the evaluation performed by the circuit  80 . After clipping, the signals Ai are fed to the summer  302 . The summer  302  generates a running sum S0 (step  344 ) of all the digital signals received over the period T. Typically, T may be about 64 milliseconds. 
     Next, in step  306  the running sum S0 is compared to a threshold value L0 by comparator  304 . If S0 is below threshold value L0 then so is set to zero (step  348 ) to insure that any baseline noise existing in the ECG signal does not contribute to the summation. 
     Next, a normalized differential parameter X is determined by element  308  as follows. First a differential parameter D is determined using the relation: 
     
       
           D=|S 0 −SP|   
       
     
     where S0 is the current sum from summer  302  and SP is the immediate previous sum, i.e., before the current digital signal Ai has been processed by the summer  302 . The parameter D is then normalized in step  352  by dividing it by the digital signal Ai to obtain the parameter X (i.e., X=D/Ai). The purpose of this step is to reduce the effect of any sudden amplitude changes in the signals Ai. 
     The parameter X is then fed to filter  308  which is a non-linear filter that uses four preselected parameters to perform a specific filtering function (step  354 ) to generate a filtered parameter Y. This parameter Y is related to X as follows:            X       Y             X   ≤   B0         0             B0   &lt;   X   ≤   B1         X             B1   &lt;   X   ≤   B2         B1               B2   &lt;   X   ≤   B3                        B1   *       (     B3   -   X     )     /     (     B3   -   B2     )                   B3   &lt;   X         0                              
     where B0&lt;B1&lt;B2&lt;B3. Typical values for these constants may be 10, 50, 160, and 220 respectively. 
     The parameter Y is fed to the summer  310 . The summer  310  in step  356  generates a running sum of all the values of Y received for the last N seconds. For instance, N may be 4 seconds. The resulting parameter Mf is fed to a comparator  312 . This comparator  312  which generates a parameter MDFi as follows. In step  356  the parameter Mf is compared to the threshold Ts. If Mf is greater than Ts then the comparator  312  generates an MDFi which is true. Otherwise MDFi is false. 
     One of the programmable options of the defibrillator  10  is the selective enablement of the MDF circuit  80 . That is, during the initialization of the defibrillator  10 , the attendant has the choice of activating the circuit  80 , in which care the parameter MDFi is determined as described above, or the function can be disabled, in which care the MDFi is ignored. 
     Referring back to FIG. 5, the parameter MDFi is fed to the therapy selector  82 . This selector  82  monitors the current cardiac rate Rcur and parameter MDFi (if applicable) and determines whether therapy is required, and if so, then what kind of therapy should be applied. 
     Referring to FIG. 14, in step  350 , a check is performed from step  150  to determine if the peak to peak amplitude of the signal is less than 0.2 milli-volts for the previous seconds. If it does, the algorithm checks to see if a shockable rhythm has been detected prior to this latter period  368 . If a shockable rhythm has been detected, the algorithm classifies the rhythm as Fine VF  370  which is shockable  372 . If a shockable rhythm has not been detected, the algorithm classifies the rhythm as a systole  376  which is not considered a shockable rhythm  362 . 
     Referring to FIG. 14, in step  352 , the current rate Rcur from step  350  is first checked to see if it exceeds the minimum rate Rmin. If it does not, then the rhythm is classified as non-shockable  362  and monitoring of the heart continues in step  150  without any therapy. In step  354 , a check is performed to determine if the MDF mode has been activated. If it is not, it continues to step  366 . If this mode has been activated in step  354 , a check is performed to determine if Rcur is greater than Rmdf  356 . If it is, it continues to step  366 . In step  358 , a check is performed to determine if the parameter MDFi is true. If it is true, it continues to step  366 . If MDFi is not true, the rhythm is classified as non-shockable  362  and no therapy is performed at this time. In step  366 , a P of R test is performed during which P of the last R intervals must have corresponded to a rate higher than Rmin. For example, P could be 4 and R could be 6. If the test failed, the rhythm is classified as non-shockable  362 , and it returns to step  150 . 
     If the P of R test is passed, then the current rate Rcur is designated as a shockable rhythm corresponding to ventricular tachyarrhytbmias and the process continues to step  156  in FIG.  9 . 
     In summary, the microprocessor  70 , rate detector  78 , and MDF circuit  80  and the therapy selector  82  cooperate to determine if the current cardiac condition of the patient should be classified as a shockable rhythm or not based on the current rate Rcur as well as the amplitude and frequency of the ECG signals. If the rate Rcur is below the threshold Rmin, no therapy is applied. If the rate is above Rmin, a determination is made as to whether the rhythm is shockable or not, based the parameters and modes described. Since VT and ventricular fibrillation could be life-threatening, it is preferably that a conservative approach be taken when selecting these parameters. 
     The process and apparatus described above and in FIGS. 12 and 13 is primarily designed for use in the automatic external cardioverter/defibrillator, however it may also be used in internal defibrillator/cardioverter devices and other cardiac devices as well. 
     Obviously, numerous modifications may be made to this invention without departing from its scope as defined in the appended claims.