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Published in 2014-10-05 10:58:24       

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Design Considerations

Automated External Defibrillator


Most automated external defibrillators (AED) are highly sophisticated embedded processor-based devices that monitor, assess and automatically treat patients with life-threatening heart rhythms. They capture ECG signals from the electrodes, runs an ECG-analysis algorithm to identify shockable rhythms, and advises the operator about whether defibrillation is necessary. A basic defibrillator contains a high-voltage power supply, storage capacitor, optional inductor and patient electrodes.  It develops an electrical charge in the storage capacitor, creating the potential for current flow. The higher the voltage, the more current can potentially flow. The AED outputs audio instructions and visual prompts. In a typical defibrillation sequence, the AED provides voice prompts to instruct the user to attach the patient electrodes and starts acquiring ECG data. If the AED analyzes the patient’s ECG and detects a shockable rhythm, the capacitor is charged, Where Wc = 1/2CV^2c; and capacitor voltage, Vc(t) = Vc(0)e–t/RC, where R = R(lead) << R(chest). When the shock button is pressed to deliver the high-voltage pulse; current begins flowing through the body to depolarize most of the heart cells, to re-establish coordinated contractions and normal rhythm. The amount of flowing current is determined by the capacitor and body impedance. Many jurisdictions require that the AED record the audio from the scene of a cardiac arrest for post-event analysis. All AEDs include a means to store and retrieve patient ECG patterns. The front-end signals of the AED come from the ECG electrodes placed on the patient, which requires an instrumentation amplifier to amplify its very small amplitude (<10mV). Targeted instrumentation amplifiers would be designed to have:



  • Capability to sense low-amplitude signals from 0.1mV to 10mV

  • Very high input impedance (>5MO)

  • Very low input leakage current (<1µA)

  • Flat frequency response of 0.1Hz to 100Hz

  • High common-mode rejection ratio (CMRR) (>100dB)

Another input of the AED is the microphone for recording the audio from the scene. Both ECG and microphone inputs are digitized and processed by a DSP. Most AED designs use a 16-bit processor and therefore work well with 16-bit ADCs to digitize ECG and voice input. The amplified ECG signal has a bandwidth of 0.1Hz to 100Hz and requires a minimum SNR of 50dB. The audio recording/playback signal typically has a bandwidth of 8kHz and requires a minimum SNR of 65dB. The microphone input also needs to be amplified with a maximum programmable gain of 40dB. The AED can have synthesized audio instruction with volume control output to either the headphone speaker or the 8Ω speaker. The TLV320AIC20 makes the front-end digitization very simple because it integrates two ADCs, two DACs, a microphone amplifier, a headphone driver and an 8Ω driver with volume control; and it can be gluelessly interfaced to a DSP.



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