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• Used to overcome ventricular fibrillation – may be due to coronary occlusion, shock, or abnormalities in blood chemistry • Main problem: heart muscle fibers are continuously stimulated by adjacent muscles – no synchronization succession of events • Results in steep fall in cardiac output • Defibrillator – high energy shock – causes all muscle fibers to contract simultaneously • Energy storage capacitor charged at slow rate (few seconds) • Discharged in few milliseconds • Charging – Ac Mains and rectifier (or) Battery and DC to DC converter • Some shaping of discharge current is needed • Simplest – discharge through patient’s R – exponential decay typical of RC circuit • Truncated – Ratio of duration of shock to RC is small – Pulse is rectangular shape • Truncated – Ratio of duration of shock to RC is larger – current appears trapezoidal Vacuum type high voltage change-over switch Current limiting, 100 mH, R = 20W Variable Autotransformer 4000 V, 20mS Oil-filled 16 mF • Patient – pure resistance – 50-100W for 80cm 2 electrode area • RLC circuit – underdamped, damping factor <1 • Pulse duration: 5 ms or 2.5 ms • Damped sinusoidal waveforms (DSW) • • • • • Used in over 90% of devices C = 30-50mF R = 40-70W Overdamped Energy = 360 J for 50 W patient Lown Waveform 335 J to 405 J • Monophasic • Biphasic • Escalating high levels of energy delivered in one direction • Delivers energy in both directions • More preferred A defibrillator provides a 5 ms pulse of 20 A to a 50 W load. Thus the energy delivered is E = Pt = I2Rt = (20 A)2(50 W)(0.005 s) = 100 J. • Output • Watt-seconds or Joules as a measure of electrical energy stored in a capacitor • Range: 0-400 J • External: • Metal discs about 3-5 cm in diameter attached to highly insulated handles • Circular, concave shaped with sharp rims and insulated-back side • Should be able to deliver large currents • Contain safety switches • Capacitor is discharged only when electrode makes good firm contact with patient’s skin • Internal: • Large spoon shaped electrodes (a) A spoon-shaped internal electrode that is applied directly to the heart. (b) A paddle-type electrode that is applied against the anterior chest wall. • Some electrodes are provided with spring contacts. • Burns can be prevented • Pre-gelled and self-adhesive electrodes are used now-a-days • Provides better conformity to body shape • Chest wall impedance • Size of electrode, paddles, energy of discharge, no. and time-interval of previous shocks and interface material used between paddle electrode and chest wall • Mainly used when there is atrial fibrillation, ventricular tachycardia, and other arrhythmias • Synchronization is required – to avoid ventricular fibrillation • The period ~20 to 30 ms after QRS peak is most preferred – least risk of ventricular fibrillation • Called Cardio-version • Shock-wave during vulnerable T-wave is avoided • Synchronizer consists of ECG amplifier that detects QRS complex and uses this to trigger a time delay circuit • After the time delay defibrillator capacitor is discharged across the chest through the electrodes • Generates a marker pulse on ECG monitors to show when the counter shock occured • Output of defibrillating circuit is kept isolated or floating • Total energy is always contained between the two electrodes • There is no loss of energy to grounds – high efficiency is maintained • No direct path to ground – electrical safety • Portable Defibrillators operate on batteries • Have DC to Dc converter to step-up the voltage required to charge capacitor • Patient impedance is very critical for defibrillator effectiveness • Knowledge of peak current, patient impedance changes, actual delivered energy is very important for operator to choose right parameters • Can be determined if stored energy and defibrillator circuit parameters (C, L, R) are known • Operator selects desired energy to be delivered to 50 W load • Microprocessor determines corresponding value of stored energy • internal resistance and patient impedance are taken for setting this value • Storage capacitor voltage (V) is determined using: • E stored = 0.5 CV 2 • Discharge current passes through a current-sensing transformer • It also provides ground isolation for patient circuit • Provides voltage that is peak-detected and recorded by µP Peak discharge current, Stored energy Patient impedance Poor paddle contact alarm when patient impedance > 100W • Also called Advisory External Defibrillators (AED) • Accurately analyses ECG and makes reliable shock decisions • Detects ventricular fibrillation with sensitivity and specificity comparable to paramedics • Then delivers automatic or recommended high energy defibrillating shock • Requirement • Self adhesive electrodes • More accurate, fast and safer • Four key indicators • Heart rate, conduction (width of R wave), stability and amplitudes • Automatic self-test features are included: - internal discharge and recalibration • Maintenance free