Abstract:
An expert system-controlled defibrillator for delivering precise energy doses to a patient who&#39;s heart is in fibrillation. An energy source connects to the patient&#39;s chest (during emergency resuscitation) or directly to the heart (during open-heart surgery) and discharges energy in one or more pulses. The apparatus measures a patient-dependent parameter or parameters, and determines, from an expert based system, the waveform morphology and the precise amount of energy to deliver.

Description:
PRIORITY  
       [0001]    This application claims the benefit of U.S. Provisional Application Serial No. 60/309,294, filed on Aug. 1, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to the use of defibrillators to deliver energy to the heart for emergency resuscitation of a patient whose heart has gone into fibrillation. The method of delivering energy to the chest of such a patient is well established. An energy storing device, usually one or more capacitors, is coupled to two electrodes (usually called paddles or pads). The paddles are placed in contact with the chest of the patient (in the case of external defibrillation), or directly to the heart of the patient (in the case of internal defibrillation during open-heart surgery), to apply energy to the heart of the patient. The energy momentarily stops the heart so that fibrillation also stops. When the voltage gradient across the heart decays, the heart will begin contracting normally if the defibrillation event was successful. If a defibrillation pulse is applied to a heart in fibrillation within approximately two minutes of the onset of fibrillation, there is a good chance the heart will begin to contract normally.  
           [0003]    The graph of the current or voltage of the energy versus time shows the waveform of the energy delivered. The waveform of the energy delivered is characterized by shape, polarity, duration, and the number of phases. The shape includes the amplitude (voltage or current), the width (time), and the tilt (rate of decay). An exemplary waveform is illustrated in FIG. 2.  
           [0004]    Monophasic waveforms were initially used in defibrillation. The use of the application of energy in a biphasic waveform, using lower voltages and lower total energy than with a monophasic waveform, is well established.  
           [0005]    Prior art defibrillators measured charge delivered or time of delivery of energy. An improvement of this art was to utilize a patient-dependent parameter to determine the shape of the waveform. Some prior art defibrillators deliver a test pulse to the patient to determine the patient&#39;s impedance, which is then used to determine the shape of the waveform by accumulating charge or calculating the required time to deliver the selected energy. By shaping the waveform in this way, the defibrillator must know the exact capacitance of the energy storage device to deliver a precise amount of energy. The maker of the defibrillator accordingly must purchase expensive components in which the capacitance is known to a very high degree, or must utilize a calibration unit within the defibrillator, which adds to the cost and weight of the unit. Additionally, capacitors degrade with use, requiring either the replacement of the capacitor in the device or frequent calibration of the device.  
           [0006]    The present invention involves delivery of energy to the patient with a energy protocol and waveform shape determined by an expert system. It is an object of using an expert system to maximize the effectiveness of the defibrillation pulse based on various physical parameters of the patient as well as the patient&#39;s ECG morphology or cardiac electrical activity. The expert system used to determine the pulse shape can include knowledge gained in past episodes of defibrillation by using embedded algorithms to determine shock efficacy. The expert system can also be programmed using a rule-based look up table stored in memory using known or proven rules of defibrillation based on current state of the art as described in the preferred embodiment. The expert system can use one or many of the known algorithmic or other approaches known in the art such as look-up tables, neural networks, fuzzy-logic based systems, genetic algorithms and adaptive performance surface searching. The previous list is not all-inclusive and may be added to as technology progresses. The main feature of this invention is the use of an expert-based system.  
           [0007]    It is a further object of this invention to deliver energy to the patient on a per pulse basis as determined by the expert system. In the preferred embodiment, the unit measures a patient dependent parameter and uses a table generated from a rule-based expert system to determine the amount of energy to deliver on a per pulse basis. By measuring in real time the energy being delivered to the patient, the unit can compensate for small differences in the capacitor bank value to deliver an accurate amount of energy. Since the characteristics of the pulse are determined by energy, the capacitance of the energy storage unit need not be known with any great degree of certainty and less expensive components, in which the capacitors are not required to have a tight tolerance, so that the actual capacitance may vary from the nominal value, saving on component costs. Additionally, the characteristics of the delivered waveform can be predicted more accurately by the method of the present invention.  
           [0008]    By using a rule-based mechanism to choose waveform parameters, the defibrillator waveform can be chosen to maximize effectiveness based on set of patient parameters. This rule-based or expert system can be pre-programmed or programmed at the time of pulse delivery to deliver an appropriate energy and waveshape based on current defibrillation science. Since the rules are stored in memory in the unit, a user or the manufacturer can change the rules used by the expert system as medical studies indicate.  
           [0009]    It is a further object of the invention is to provide a precise energy dose to a patient in a monophasic or multiphasic defibrillation waveform by delivering controlled energy pulses, with the energy of each pulse retrieved from a table in memory, using a patient-dependent parameter-derived index.  
           [0010]    It is a further object of the invention to provide a defibrillator in which the rules can be changed upon further medical study, so that the device is adaptable to advances in medical research.  
           [0011]    It is a further object of the invention to provide a defibrillating apparatus in which the user or the manufacturer can select and edit the values in the tables in memory by modifying the rule base or expert system.  
           [0012]    It is a further object of the invention to provide a defibrillator using a large energy storage device, in order to decrease the tilt of the waveform allowing higher terminating currents. By maximizing the terminating current, or tilt, less voltage and current can be used to achieve effective defibrillation. Higher terminating current can also decrease post-shock arrhythmias necessitating further defibrillation events. In the preferred embodiment, a 500 microfarad electrolytic capacitor is used as the energy storage element. Having a capacitor above 300 microfarad allows tilt to be optimized for single phase or multiphasic defibrillation pulses. The tilt is defined as the starting voltage V s  minus the ending voltage V e  divided by the starting voltage V s  (multiply by 100 to get percent tilt).  
           tilt=( V   s   −V   e )÷ V   s    
         SUMMARY OF THE INVENTION  
         [0013]    The present invention delivers a truncated exponential pulse waveform to the patient, of one or more polarities, using a single capacitor as the energy storage device. The energy of the pulses is dependent on the desired total energy, a patient-dependent parameter or parameters, and pulse energies retrieved from a look-up table. In the preferred embodiment, the tilt of the waveform is kept low by using a large storage capacitor. The large capacitor allows the pulse length to be extended to accommodate patients with high impedance, and to prevent re-fibrillation or other complications, by maintaining a high terminating current.  
           [0014]    The desired total energy is based on a device-defined or user-defined energy index. The patient-dependent parameter of the preferred embodiment is patient resistance. The look-up table defines how much energy to deliver on each pulse. The table is created by a rule-based generator, using information defined prior to the creation of the table, which is then stored in memory. The user can edit the table, or the apparatus can be programmed to modify table entries based on effectiveness as recorded in past history.  
           [0015]    The apparatus  10 , by measuring the patient&#39;s ECG via the electrode  16 , detects that the heart has resumed normal electrical activity, and has potentially begun pumping blood again. The apparatus  10  can be programmed to record the success or failure of a delivered energy pulse, along with the characteristics of that pulse and measured or physical parameters of the patient. The patient&#39;s parameters can include weight, pulse, percentage of body fat, ECG or other physiological measurements, or any other parameters that medical studies indicate are relevant to re-fibrillation. The apparatus contains an expert system, which uses one or more of the following: look-up table, neural network, fuzzy-logic based system, genetic algorithm, adaptive performance measures, or error surface searching. The expert system can analyze past data and can adjust energies delivered and or the characteristics of the delivered energy pulses based on that data.  
           [0016]    In the preferred embodiment, the apparatus interpolates energy values if required. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a diagram of the apparatus of the preferred embodiment.  
         [0018]    [0018]FIG. 2 is a voltage versus time graph illustrating an exemplary biphasic waveform.  
         [0019]    [0019]FIG. 3 is an exemplary rule-based diagram showing the plot of total energy, patient resistance, and energy ratio.  
         [0020]    [0020]FIG. 4 a flowchart showing the pulse delivery sequence for a biphasic defibrillator.  
         [0021]    [0021]FIG. 5 is a flowchart of pulse delivery for a single pulse in the preferred embodiment.  
         [0022]    [0022]FIG. 6 is an exemplary energy table as implemented in the preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    The apparatus  10  is shown in FIG. 1. The apparatus consists of an energy storage capacitor  12 , a charger  13 , electrodes or paddles  14   a  and  14   b , a pulse delivery circuit  15 , electrodes  16  to determine the state of the patient&#39;s heart, a user interface display  18 , a power switch  19 , a charge button  20 , a fire switch  21 , a microprocessor containing an expert system  22 , a memory  24 , voltage sampling means  26 , current sampling means  28 , and a target energy selection control  30 .  
         [0024]    In a manual embodiment, the user interacts with the apparatus  10 . The user turns on the unit by the power switch  19 . The user assesses the patient&#39;s condition by connecting the ECG electrodes  16  to the patient&#39;s chest. If the apparatus  10  detects a shockable heart rhythm, i.e. that a shock is required, the user selects a target energy based on a predetermined protocol. That protocol is based on the American Heart Association/Advanced Cardiac Life Support Guidelines. The user places the paddles or disposable pads  14   a  and  14   b  of the apparatus  10  on the patient&#39;s bare chest, charges the apparatus  10  by pressing the charge button  20 , causing the charging means  13  to charge the capacitor  12 , and, when prompted by the apparatus  10 , depresses the fire button  21  to deliver the energy. In the preferred embodiment, the target energy selection control  30  has preselected target energy levels of 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 20.0, 30.0, 50.0, 70.0, 100.0, 150.0, 200.0, 300.0, and 360.0 joules, the actual values dependant on configuration of the apparatus  10  and current medical studies.  
         [0025]    In an automatic embodiment, the apparatus  10  chooses the energy to be delivered based on a user-defined energy protocol that can be programmed into the memory  24  at the time of purchase, or modified later by the user, or a value determined by the expert system  22 . The user places the electrodes  14   a  and  14   b  on the patient&#39;s bare chest and depresses the power switch  19 . The apparatus  10  analyzes the patient&#39;s ECG waveform via electrode  16  and determines whether a shock is required. If required, the apparatus  10  charges up and prompts the user to depress the fire button  21 . The apparatus  10  can also deliver the energy without user interaction in a fully automatic mode.  
         [0026]    In the preferred embodiment, a rule base is drawn up based on clinical data. The values from the rule base are entered into an expert-system program and an include file is generated containing a table used during operation. An example of a rule base is illustrated in FIG. 3 and a sample energy table is shown in FIG. 6. This table is then compiled into the code and stored in memory  24  for use. In other embodiments the expert system can be contained in the apparatus  10  itself and interacted with by the manufacturer or the user via the front panel, a connected PC or other computer, or remotely.  
         [0027]    The logic of the application of a biphasic application is shown in FIG. 4. The apparatus  10  begins with a first pulse, with a voltage (V) sufficient to discharge the target energy in 12 mSec into a 50 ohm load. These initial values can be changed as medical studies indicate. The apparatus  10  determines the required starting voltage using the standard equation for energy and solving for V s , the starting voltage:  
         V        (   E   )       :=         -   1       (       C   ·     exp        (       -   3       125   ·   Rp   ·   C       )         -   C     )       ·       [       -   2     ·   C   ·     (       exp        (       -   3       125   ·   Rp   ·   C       )       -   1     )     ·   E     ]     2                             
 
         [0028]    At the start of the first pulse, the apparatus  10  determines the resistance of the patient. The voltage and current (I) are determined continually by sampling the waveform. An exemplary biphasic waveform is shown in FIG. 3. At approximately 400 microsecond into the first pulse, the apparatus  10  takes the voltage and current readings and divides to determine resistance, using the standard equation for calculation of resistance, which equals voltage divided by current:  
         
       R=V/i  
     
         [0029]    The apparatus  10  then looks to the rule-based table, illustrated in FIG. 6 and stored in memory  24 , to determine how much energy to deliver on this first pulse. The apparatus  10  continues to discharge while integrating the sampled values until the desired energy value has been reached, or until a maximum time is reached in the case of a very highly resistive patient or open load. If a maximum time is reached, the microprocessor  22  signals the pulse delivery circuit  15  to terminate the current.  
         [0030]    The apparatus  10  uses the voltage and current of the discharge and integrates over time to determine energy delivered. Voltage readings and current readings are taken approximately every 400 microsecond and multiplied by time to determine energy, using the standard equation:  
         Σ E=ViΔt    
         [0031]    When the apparatus  10  has delivered the desired energy for the first pulse, it truncates the waveform by shutting off current flow, using the pulse delivery circuit  15 . The apparatus  10  then waits a predetermined amount of time and starts the delivery of the second pulse.  
         [0032]    The apparatus  10  then begins to deliver the second pulse, of opposite polarity, using the same logic as described for the first pulse: turning on the output, calculating the patient resistance by measurement of voltage and current, determining from the rule table the amount of energy to deliver, and discharging the capacitor until that desired energy is reached. Alternately, the second pulse energy could be determined using the patient-dependent parameter determined in the first pulse.  
         [0033]    The preferred embodiment as described herein applies to a biphasic waveform. The invention, however, can apply to a monophasic waveform or to multiphasic waveforms, such as triphasic, quadraphasic, etc.  
         [0034]    While preferred embodiments of the present invention are shown and described, it is envisioned that those skilled in the art may devise various modifications of the present invention without departing from the spirit and scope of the appended claims.