Medical Applications Guide (Rev. B - Mouser Electronics

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40➔Diagnostic, Patient Monitoring and TherapyAutomatic External Defibrillator (AED)The automated external defibrillator (AED) is a highly sophisticatedmicroprocessor-based device that monitors, assesses and automaticallytreats patients with life-threatening heart rhythms. It captures ECGsignals from the therapy electrodes, runs an ECG-analysis algorithm toidentify shockable rhythms, and then advises the operator aboutwhether defibrillation is necessary. A basic defibrillator contains ahigh-voltage power supply, storage capacitor, optional inductor andpatient electrodes (see block diagram). It develops an electrical chargein the capacitor to a certain voltage, creating the potential for currentflow. The higher the voltage, the more current can potentially flow. TheAED outputs audio instructions and visual prompts to guide the operatorthrough the defibrillation procedure. In a typical defibrillationsequence, the AED provides voice prompts to instruct the user toattach the patient electrodes and starts acquiring ECG data. If the AEDanalyzes the patient’s ECG and detects a shockable rhythm, the capacitoris charged according to energy stored in the capacitor, W c = 1 / 2 CV 2 c;and capacitor voltage, V c(t) = V c(0) e –t/RC , where R = R(lead)
Diagnostic, Patient Monitoring and TherapyAutomatic External Defibrillator (AED)41➔The other front-end signal of the AED is the microphone input forrecording the audio from the scene of a cardiac arrest. Both ECG andmicrophone input are digitized and processed by a DSP. Most AEDdesigns use a 16-bit processor and therefore work well with 16-bitADCs to digitize ECG and voice input. The amplified ECG signal has abandwidth of 0.1Hz to 100Hz and requires a minimum SNR of 50dB.The audio recording/playback signal typically has a bandwidth of 8kHzand requires a minimum SNR of 65dB. The microphone input alsoneeds to be amplified with a maximum programmable gain of 40dB.The AED can have synthesized audio instruction with volume controloutput to either the headphone speaker or the 8Ω speaker. Systemdesigners will find that the TLV320AIC20 makes the AED front-enddigitization very easy and simple because it integrates two ADCs, twoDACs, a microphone amplifier, a headphone driver and an 8Ω driverwith volume control; and it can be gluelessly interfaced to a DSP.Current (A)403020100–10–200Typical AED drive current.200J Monophasic200J Biphasic130J Biphasic5 10 15 20Time (ms)AEDs can deliver either monophasic or biphasic defibrillation waveformsto the heart. Monophasic delivers a current that travels in one directionthroughout the shock. Newer biphasic technology allows the current to bereversed partway through the shock thus potentially lessening the risk ofburns and myocardial damage.DisplayCompactFlashMemoryCardKeypadRS232/USBInterfaceOMAP59xx ProcessorARM®Memory1.8V/3.3VTPS61042LED DriverSerial PortMcBSPDSPMemory5VTPS61045LCD BiasTPS3836SupervisorRSH-BridgeTiming/CapacitorCharge ControlADS8321TLV320AIC12/20DACSerial PortDACADCADCECGInputTPA2005D11-Wire12 to 18V2Ah Battery PackProtectionTPS5130BuckControllerS 1 S 2CINA326 orINA118OPA335 orOPA130ChargerFuel GaugeUCC38C42CurrentController2000VTransformerCircuitS 3 S 4Instrumentation/Operational AmplifiersTI ComponentsAED block diagram.Texas Instruments 2Q 2007Medical Applications Guide
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