Current-based defibrillator

• US 4,840,177 A
• Filed: 11/03/1987
• Issued: 06/20/1989
• Est. Priority Date: 11/03/1987
• Status: Expired due to Term

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Benvegar, Carl E., Rockwell, Martin G., Charbonnier, Francis M.
• Primary Examiner(s): Smith, Ruth S.
• Application Number:
  US07/117,125
• Time in Patent Office: 595 Days
• Field of Search: 128/419 D, 128/734
• US Class Current: 607/8
• CPC Class Codes: A61N 1/3904 External heart defibrillators [EHD]
• CPC Class Codes: A61N 1/3937 Monitoring output parameters

Abstract

A method and apparatus is disclosed for providing to the body of a patient a defibrillation pulse having a magnitude calculated to induce in that particular patient a desired defibrillation current. The patient'"'"'s response to a defibrillation pulse is characterized in advance of defibrillation by repeatedly sampling the patient'"'"'s transthoracic impedance to a small alternating current excitation signal. The sample having the lowest magnitude is taken to represent the patient'"'"'s true transthoracic impedance. This lowest sampled impedance value is normalized to take into account the shape of the expected waveform and is then multiplied by the desired current to yield the target charge level to which an energy storage device must be charged to induce the desired current to flow in the patient on discharge. Charging of the energy storage device desirably begins before measurement of transthoracic impedance is completed and continues until the target charge is reached. Discharge of an energy storage device so charged causes the desired current to flow through the patient.

Claims

1. A defibrillator system for delivering an electrical shock to a patient to restore normal cardiac rhythm comprising;

a capacitor for accumulating an electrical charge;

first and second electrodes for applying an electrical shock from the capacitor to the patient;

charging means for charging the capacitor;

detector means for detecting the charge accumulated by the capacitor;

impedance measuring means for determining a transthoracic impedance of the patient, said impedance measuring means including means for sampling the patient'"'"'s transthoracic impedance periodically;

current establishing means for establishing a fixed magnitude of current which is desired to be delivered to the patient irrespective of the patient'"'"'s transthoracic impedance; and

circuit means cooperating with the charging means, detector means, impedance measuring means and current establishing means for discontinuing the charging of the capacitor when the accumulated charge is sufficient to deliver the selected current to a patient having the measured transthoracic impedance, said circuit means including processing means for determining the charge accumulation required to deliver the desired current to a patient having a transthoracic impedance equal to the lowest value of transthoracic impedance sampled.

2. A defibrillator comprising:

storage means for storing energy;

first and second measuring electrodes;

impedance sensing means for applying a low amplitude current from said measuring electrodes to the patient'"'"'s chest during a measurement interval to develop thereby a first signal related to the impedance presented across said electrodes by the patient'"'"'s chest as a function of time during said interval;

first processing means for processing said first signal to produce a second signal related to the minimum impedance presented across said electrodes during said measurement interval;

second processing means for determining from said second signal the magnitude of voltage to which the storage means must be charged to deliver to said patient a desired defibrillating current;

means for providing the storage means with the voltage charge determined by the second processing means; and

means for discharging said storage means through said patient.

3. The defibrillator of claim 2 in which said first processing means comprises:

sampling means for periodically sampling said first signal to obtain a plurality of instantaneous signal values, each of said values being indicative of the impedance across the measuring electrodes at a particular instant during the measurement interval; and

examining means for examining each said sample value to determine whether it is indicative of an impedance lower than the other the sample values.

4. The defibrillator of claim 3 in which the sampling means includes means for producing a digital representation of each sample value and in which the examining means includes means for comparing each digital representation of a sample value against a current-minimum sample value stored in a digital memory.

5. A defibrillator comprising:

storage means for storing energy;

first and second measuring electrodes;

impedance sensing means for applying a low amplitude current from said electrodes to the patient'"'"'s chest to develop thereby a signal related to the impedance presented across said electrodes;

first processing means including an analog-to-digital converter for converting the signal developed by the impedance sensing means into digital form, said first processing means further including means for amplifying said signal prior to digitizing if the amplitude of said signal is below a predetermined threshold, said amplification increasing thereby the digital resolution of low amplitude signals provided by said analog to digital converter;

second processing means for determining from the digitized signal the magnitude of voltage required on the storage means to deliver a desired defibrillating current to the patient;

means for providing the storage means with the voltage charge determined by the second processing means; and

means for discharging the energy stored in said storage means through said patient.

6. A defibrillator comprising:

storage means for storing energy;

sensing means for applying a low amplitude current to the chest of the patient and for determining from the response of the patient thereto the magnitude of voltage required on the storage means to deliver a desired current to the patient, said sensing means operating during an initial, measurement phase of the defibrillator'"'"'s operation;

means for setting a desired initial voltage limit towards which the storage means can be charged during said measurement phase;

means for initiating the charging of the storage means during said measurement phase;

means for discontinuing the charging of the storage means during said measurement phase if the voltage on the storage means reaches said desired initial voltage limit; and

means for discharging the storage means through the patient.

7. The defibrillator of claim 6 which further includes means for reinitiating the charging of the storage means at the conclusion of the measurement phase if the magnitude of voltage determined by the sensing means to be required on the storage means exceeds the initial voltage to which the storage means has been charged.

8. The defibrillator of claim 6 which further includes means for bleeding charge from the storage means at the conclusion of the measurement phase if the magnitude of voltage determined by the sensing means to be required on the storage means is less than the initial voltage to which the storage means has been charged.

9. In a defibrillation method that includes the steps of detecting a patient'"'"'s transthoracic impedance and providing a corresponding, "target" electrical charge to a storage medium, said target charge being selected in response to the detected transthoracic impedance so as to provide a desired defibrillation current to the patient upon discharge of the storage medium through the patient, an improvement comprising the steps:

charging the storage medium to a charge greater than the target charge; and

bleeding charge from the storage medium until the target charge is reached.