Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N1/00—Electrotherapy; Circuits therefor
  • A61N1/18—Applying electric currents by contact electrodes
  • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
  • A61N1/38—Applying electric currents by contact electrodes alternating or intermittent currents for producing shock effects
  • A61N1/39—Heart defibrillators
  • A61N1/3925—Monitoring; Protecting
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/02—Detecting, measuring or recording for evaluating the cardiovascular system, e.g. pulse, heart rate, blood pressure or blood flow
  • A61B5/026—Measuring blood flow
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/02—Detecting, measuring or recording for evaluating the cardiovascular system, e.g. pulse, heart rate, blood pressure or blood flow
  • A61B5/026—Measuring blood flow
  • A61B5/0295—Measuring blood flow using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N1/00—Electrotherapy; Circuits therefor
  • A61N1/18—Applying electric currents by contact electrodes
  • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
  • A61N1/38—Applying electric currents by contact electrodes alternating or intermittent currents for producing shock effects
  • A61N1/39—Heart defibrillators
  • A61N1/3904—External heart defibrillators [EHD]
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/02—Detecting, measuring or recording for evaluating the cardiovascular system, e.g. pulse, heart rate, blood pressure or blood flow
  • A61B5/026—Measuring blood flow
  • A61B5/029—Measuring blood output from the heart, e.g. minute volume
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/24—Detecting, measuring or recording bioelectric or biomagnetic signals of the body or parts thereof
  • A61B5/316—Modalities, i.e. specific diagnostic methods
  • A61B5/318—Heart-related electrical modalities, e.g. electrocardiography [ECG]
  • A61B5/346—Analysis of electrocardiograms
  • A61B5/349—Detecting specific parameters of the electrocardiograph cycle
  • A61B5/352—Detecting R peaks, e.g. for synchronising diagnostic apparatus; Estimating R-R interval
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
  • A61B5/7235—Details of waveform analysis
  • A61B5/7264—Classification of physiological signals or data, e.g. using neural networks, statistical classifiers, expert systems or fuzzy systems

Definitions

• This invention relates to a method of determining whether events in a heart's electrical activity result in corresponding blood flow events, and to an external defibrillator implementing such method.
• a modern AED is a compact portable unit which automatically analyses electrocardiograph (ECG) rhythms obtained from the human thorax via electrodes in contact with the skin.
• ECG electrocardiograph
• a user attempting to respond to a suspected SCD emergency would simply get the AED and press the "ON" button and then follow the voice and text prompts and instructions generated by the device.
• a typical sequence of events would be for the device to prompt the user to apply the pads to the patients chest and make sure that the electrodes are properly connected to the device.
• the device will then automatically analyse the patients heart rhythm and make a diagnosis. If it determines that a high voltage shock is required, it will automatically charge to a predetermined energy level and advise the user to press a clearly marked "SHOCK” button. Upon pressing this button, the energy is delivered to the patient through the same electrodes used to monitor the ECG. Whether or not the device advises shock, the user is informed continuously about the state of the patient and/or the actions and states of the device.
• ICG impedance cardiograph
• the bottom trace shows the corresponding time synchronised trace of that same individuals ICG differentiated with respect to (wrt) time, dz/dt.
• This dz/dt signal is more commonly used for analysis rather than the raw ICG itself and so for the rest of this document the term ICG will be used to actually denote the signal dz/dt.
• the raw ICG sensed from the patient electrodes is referenced it will be explicitly named as such.
• the ICG is sensed as a small change in value of a much larger baseline impedance.
• Other factors such as volume change due to inspiration and expiration also affect the measurement. Even very slight muscle activity and motion can affect the electrode contact impedance and considerably affect measurement. AU these factors result in the use of the ICG for measurement of cardiac output being restricted to laboratory or controlled conditions. There are many techniques known to those skilled in the art which can reduce the effects of these factors. Some methods involve filtering, signal averaging ⁇ , Fourier analysis and differential impedance measurement. There are also documented methods which analyse the ICG once it has been acquired. Typically these have involved the use of mathematical derivative and integrative processes attempting to glean measurement of cardiac output from the ICG.
• Figures 5 and 6 show the measurement of peak dz/dt and area under the C wave which are just two parameters reported to have a quantitative relationship to cardiac output.
• Figure 5 also shows how the various parts of the dz/dt waveform are labelled by conventional Unfortunately these parameters and techniques have been somewhat unreliable.
• the baseline impedance, the structure of the heart and the subjects vascular demands all vary from one individual to the next. These previous attempts all share the limitation that they are determining to measure blood flow through the heart while employing a signal source that contains much interference due to variable factors that are not desirable.
• a method of determining whether events in a heart's electrical activity result in corresponding blood flow events comprising: simultaneously taking an electrocardiograph (ECG) and an impedance cardiograph (ICG) of the heart; examining successive periods of the ECG to detect successive occurrences of at least one periodic waveform feature indicative of electrical activity of the heart; defining a period of time following each such detection; examining the ICG, or a signal derived therefrom, in each said search period to detect the occurrence of a waveform feature ("corresponding waveform feature") indicative of blood flow resulting from the detected occurrence in the ECG; and generating a signal indicative of the degree of concordance of the ICG with the ECG as a function of measurements made in respect of said detected waveform features.
• ECG electrocardiograph
• ICG impedance cardiograph
• the invention further provides an external defibrillator having circuitry arranged to implement the method specified above and to selectively advise a shock dependent upon the value of said signal indicative of the degree of blood flow.
• Figure 1 shows an example ECG signal from a subject in normal SR (top), together with corresponding time synchronised ICG signal (bottom).
• Figure 2 shows an example ECG signal from a subject in VF (top), together with corresponding time synchronised ICG signal (bottom).
• Figure 3 shows an example ECG signal from a subject in VT (top), together with corresponding time synchronised ICG signal (bottom) - this person requires alternate therapy and should not receive an unsychronised shock.
• Figure 4 shows an example ECG signal from a subject in VT (top), together with corresponding time synchronised ICG signal (bottom) - this person requires an emergency terminating shock.
• Figure 5 shows the measurement of the peak dz/dt parameter used in the determination of cardiac output.
• Figure 6 shows the measurement of the area under the C wave, another parameter used in the determination of cardiac output.
• Figure 7 shows the result of the wave detection process for both R waves in the ECG and C waves in the dz/dt ICG signal.
• Figure 8 shows an example comparison between the ECG and ICG.
• Figure 9 shows the a block diagram of an embodiment of the invention.
• ECG-ICG event gating has proved very effective in discriminating shockable from non-shockable VT's in both human and animal subjects, and it is likely that the approach is applicable to other conditions such as atrial flutter/fibrillation and VF. It must be appreciated that the gating can be applied to any rhythm and use a multitude of parameters measured from both ECG an ICG without departing from the scope of the invention;
• the gating uses ECG and ICG signals which have first been processed using methods well know to those skilled in the art.
• ICG these remove as much interference, contamination and variation as possible while leaving the fundamental ICG intact.
• this pre-processing can be performed much more aggressively.
• the reason for this is that the invention does not require an accurate quantitative measurement of the cardiac output.
• the pre-processing design has therefore a far larger degree of freedom to clean the signal, thereby almost totally removing interference due to movement, etc., a solution not previously possible using other approaches.
• the frequency bandwidth overlap of ECG content and interference has always been a troublesome dilemma for bio-medical engineers.
• FIG. 9 is a block diagram of an embodiment of the invention as implemented in an automatic external defibrillator (AED).
• AED automatic external defibrillator
• the AED comprises conventional patient electrodes 10 which are applied to the patient to obtain both the ECG and ICG waveform signals simultaneously.
• Signal conditioning circuitry 12 filters each signal to a bandwidth of 3-20Hz using analog filters with a Butterworth response.
• the circuitry 12 further differentiates the ICG to obtain dz/dt as the ICG signal for subsequent analysis, and individually but simultaneously digitises the ECG and differentiated ICG signals.
• the digitised ECG and ICG signals are passed to respective microprocessor-based feature extraction circuits 14, 16 respectively.
• the circuit 14 examines successive heartbeat cycles (periods) of the ECG to detect successive occurrences of each of a plurality of periodic waveform features Ei to EN indicative of electrical activity of the heart.
• One such feature is the R wave, shown at the top of Figure 7 for successive periods of the ECG.
• Other such features which may be detected, such as the Q wave, are well-known to those skilled in the art.
• the circuit 16 examines successive heartbeat cycles (periods) of the ICG to detect successive occurrences of each of a plurality of periodic waveform features Ij to IN indicative of blood flow in the heart.
• the ICG waveform features Ii to IN are not arbitrarily chosen, but each is paired with one of the ECG waveform features Ei to EN such that each pair E n , I n (0 ⁇ n ⁇ N+l) has an electromechanical relationship.
• the particular ECG waveform feature E n results (in a healthy heart) in the corresponding ICG waveform feature I n .
• the paired waveform feature in the ICG is the C wave, shown at the bottom of Figure 7 for successive periods of the ICG.
• a wave detector is separately used on both signals' ⁇ , the ECG detector being optimised for R wave detection and the ICG detector for C wave detection.
• the detected features are passed to a temporal comparator 18.
• This defines a period of time (search period) following the detection of each ECG waveform feature E n and examines the ICG waveform for the presence of the paired feature I n during that search period.
• the duration of the search period is calculated from heart-rate dependant measurements made on the ECG and is designed such that the paired feature (if present) should appear within that time period following the detected ECG feature.
• the search period duration will therfore depend on the heart rate and the particular waveform features selected. In this particular embodiment the search period was calculated as twice the period of the QRS interval in the ECG.
• a measurement (hereinafter referred to as a parameter) is made in respect of each feature of the pair E n , I n .
• the parameters are not chosen arbitrarily, but are equivalent for the feature E n and the feature I n .
• the parameter for E n is the width of the R wave
• the equivalent parameter for I n is the width of the C wave.
• parameter pairs are the height of R wave in the ECG and the height of C wave in the ICG, the ratio of the areas under the R and Q waves in the ECG and the ratio of the areas under the C and X waves in the ICG, and other parameter pairs which would be known to those skilled in the art.
• the parameter measured in respect of each R wave is the R-R interval 5R 1 , 6R 2 , etc. to the next R wave
• the equivalent parameter measured in respect of each C wave is the C-C interval 5C 1 , 5C 2 , etc.
• This process applied to all the features Ei to EN in each period of the ECG waveform, generates for each waveform period a respective set of parameter pairs X 1 to XN where src denotes the source of the parameter ECG or ICG.
• X 1 ⁇ 0 represents the value of parameter number one for the ECG and X 1 100 the value of its ICG pair.
• the parameter pairs X/ rc to XN" "C are passed to a concordance estimator 20.
• the estimator generates the linear sum of all the comparisons of all the parameter pairs :-
• the parameter pairs are chosen such that they should have very little difference during concordance, this means that the estimate 7 furnished a very low value when there was concordance between the ECG and ICG and a very large value when there was bad correlation or during dissassociation when a large error value contributed.
• the estimate 7 was selectively used by a diagnostic algorithm 22 which ignored its value for various rhythms but compared its value to a predetermined threshold when it had already classified the rhythm as VT in addition to several other criteria outside the scope of this specification. If the concordance estimate was below the threshold for the given VT then no shock was advised. Therefore, as a final qualification a shock was advised if:
• shock circuitry 24 of the AED gave a shock or no shock advisory according to the value of Y.
• This trechnique overcomes the problems described in relation to the prior art and provides a method whereby the ICG can be employed in a practical environment to reliably discern shockable from non-shockable cardiac rhythms.

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• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Cardiology (AREA)
• Public Health (AREA)
• Heart & Thoracic Surgery (AREA)
• Engineering & Computer Science (AREA)
• Biomedical Technology (AREA)
• Veterinary Medicine (AREA)
• Animal Behavior & Ethology (AREA)
• General Health & Medical Sciences (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Radiology & Medical Imaging (AREA)
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• Physiology (AREA)
• Physics & Mathematics (AREA)
• Biophysics (AREA)
• Pathology (AREA)
• Medical Informatics (AREA)
• Molecular Biology (AREA)
• Surgery (AREA)
• Electrotherapy Devices (AREA)
• Measurement And Recording Of Electrical Phenomena And Electrical Characteristics Of The Living Body (AREA)
• Measuring Pulse, Heart Rate, Blood Pressure Or Blood Flow (AREA)

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• 2005
  • 2005-07-07 EP EP05772643.2A patent/EP1778347B1/en not_active Expired - Lifetime
  • 2005-07-07 AU AU2005261908A patent/AU2005261908B2/en not_active Expired
  • 2005-07-07 US US11/571,804 patent/US20080306559A1/en not_active Abandoned
  • 2005-07-07 WO PCT/EP2005/007454 patent/WO2006005557A1/en not_active Ceased
  • 2005-07-07 JP JP2007519737A patent/JP4819045B2/ja not_active Expired - Lifetime

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