The analysis and control subsystem contains signal processing hardware and software to analyze heart rhythm, a microprocessor to manage all operations of the device, memory for storage of data (ECG measurements, diagnosis and therapy delivery all vs. The sensing subsystem contains the following to detect ECG data from the patient: two conductive wire leads attached to two separate electrodes (pads), analog electronics (usually consisting of a low-noise low-frequency amplifier and bandpass filter of less than 20 Hz), a high-voltage protection device, and an analog-to-digital (A-to-D) converter. Īn AED's circuitry consists of three major subsystems: 1) Sensing, 2) Analysis and control, and 3) Shocking. Sudden cardiac arrest from V-Fib causes several hundred thousand deaths per year in the United States alone. Due to the lack of blood flow in the brain, irreversible brain damage and death can occur in just 5 minutes. This chaotic electrical activity leads to inefficient contraction of the heart and a loss of blood flow to the brain and the rest of the body. Ventricular fibrillation is a condition in which there is uncoordinated electrical propagation in the cardiac muscle of the ventricles. The AED is able to identify and treat some arrhythmias by signaling the operator of the AED to initiate a high-voltage shock (therapy) to the patient by pressing one or more buttons. Analysis of arrhythmias is done by monitoring the millivolt-level electrocardiographic (ECG) voltage on the patient's chest with two external electrodes (pads). This plus problems with the standard's' prescribed patient load make changes to the standard necessary.Īn AED is a portable, battery-powered electronic device that automatically diagnoses potentially life threatening irregular cardiac activity (arrhythmias) in a patient, such as ventricular fibrillation (V-Fib) and ventricular tachycardia (VT). Field strengths causing failures were at levels as low as 3 V/m at frequencies below 80 MHz where resonance of the patient leads and the AED input circuitry occurred. ConclusionsĪEDs experienced potentially life-threatening false-negative failures from radiated RF, primarily below the lower frequency limit of present AED standards. Also, we found it is not possible to test modern battery-only-operated AEDs for EMI using a patient simulator if the IEC 6 defibrillator standard's simulated patient load is used. These occurred when the patient simulator was delivering a V-Fib waveform to the AED. Failures occurred in some AEDs exposed to E fields between 3 V/m and 20 V/m, in the 38 - 50 MHz range. Some induced errors could cause AEDs to malfunction and effectively inhibit operator prompts to deliver a shock to a patient experiencing lethal fibrillation. Resultsįive of the seven AEDs tested were susceptible to RF interference, primarily at frequencies below 80 MHz. We developed a technique to screen ECG waveforms stored in each AED for electromagnetic interference at all frequencies without waiting for the long cycle times between analyses (normally 20 to over 200 s). An AED patient simulator was housed in a shielded box and delivered normal and fibrillation waveforms to the AED's patient leads. We performed radiated exposures in a 10 meter anechoic chamber using two broadband antennas to generate E fields in the 30 - 2500 MHz frequency range at 1% frequency steps. We tested AEDs with ventricular fibrillation (V-Fib) and normal sinus rhythm signals on the patient leads to enable testing for false negatives (inappropriate "no shock advised" by the AED). Deviations from the IEC standard were a lower frequency limit of 30 MHz to explore frequencies where the patient-connected leads could resonate. Square wave modulation was used to mimic cardiac physiological frequencies of 1 - 3 Hz. We studied the worst-case radiated radiofrequency (RF) susceptibility of automated external defibrillators (AEDs) based on the electromagnetic compatibility (EMC) requirements of a current standard for cardiac defibrillators, IEC 6.
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