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MT414 al Engineering rs/Cardioversion Dr. Ali Saad, Biomedical Engineering Dept. College of applied medical sciences King Saud University Dr. Ali Saad, BMT department 1 efibrillation • • • • used to treat ventricular fibrillation (cardiac arrest) loss 50,000 cardiac arrest cases occur annually in US. defibrillation involves the application of a strong electr exact physical mechanism leading to ventricular fibrilla Dr. Ali Saad, BMT department 2 ength Duration Curve defibrillation occurs current (amps) no defibrillation pulse duration Dr. Ali Saad, BMT department 3 ngth Duration Curve (cont.) • minimum defibrillation energy occurs for pulse durations of 3 - 10 m • pulse amplitude in tens of amperes (few thousand volts). • operator selects energy delivered: 50-360 joules, depends on: – intrinsic characteristics of patient – patient’s disease – duration of arrhythmia – patient’s age – type of arrhythmia (more energy required for v. fib.) Dr. Ali Saad, BMT department 4 l Defibrillators • • • • For each minute elapsing between onset of ventricular fibrillation an defibrillators should be portable, battery operated, small size. energy in defibrillators usually stored in large capacitors. total energy stored in capacitor: 1 WC CVC2 2 Vc = capacitor voltage Dr. Ali Saad, BMT department 5 External Defibrillator standby power supply charge discharge gate patient switch is under operator control energy storage timing circuitry ECG monitor applies shock about 20 ms after QRS complex, avoids T-wave Dr. Ali Saad, BMT department 6 Capacitive Discharge Defibrillator L power supply 2kV Vc + Vc _ Rlead Rchest C C: 10 mF - 50 mF, takes about 10s to charge Vc: 4,000 - 9,000 V up to 40% of energy in C can be dissipated in L and Ri response is slightly underdamped (depends on chest R) t 10 ms Dr. Ali Saad, BMT department 7 Example power supply Rlead + Vc _ C + Vchest _ Rchest Rchest = 95W, Rlead = 5W, total energy stored in C is W = 300 J, want to deliver 90% of W to heart in 8 ms. What value of C should be used? RL Rlead Rchest VC t VC 0e t / RL C , t 0 VC(0) = initial capacitor voltage after charging Dr. Ali Saad, BMT department 8 Example (cont.) Rchest Vchest t VC t Rchest Rlead VC 0e t / RL C Rchest VC 0e t / RLC Rchest Rlead energy stored in capacitor at t = 0: 0 0 (1) dVC W VCiC dt VC C dt dt 0 1 2 C VC dVC CVC 0 (2) 2 Dr. Ali Saad, BMT department 9 Example (cont.) energy delivered to Rchest: tV2 chest Wchest t 0 substituting (1): 0 using (2): Rchest tV2 2 C Wchest t t dt 2t / RLC 0 e dt Rchest 0.008 V 2 0e 2 t / R L C 1 0.9 CVC2 0 2 C dt 2 Rchest 0 Dr. Ali Saad, BMT department 10 Example (cont.) 0.9 / e 2( 0.008)/ R L C 1 0.0526 e 2( 0.008)/ R LC solving for C: C = 54.3 mF 1 300J CVC2 0 2 V (0)=3,322.90V initial voltage across capacitor: c Dr. Ali Saad, BMT department 11 Square Wave Defibrillators power supply n n n + Vc _ SCR2 Rlead + SCR1 C timing circuit Vchest _ Rchest during charging, SCR1 and SCR2 both open to defibrillate, SCR2 closes, current flows to chest after a fixed interval, SCR1 closes, shorts out C Vchest Dr. Ali Saad, BMT department 12 Defibrillation Electrodes n n n metal, 70-100 cm2 surface area must be coupled to skin using conductive material (otherwise can burn patient) two types: n hand-held: conductive gel must be manually applied, reusable. n adhesive: adhesive conducting material holds electrode to skin, disposable Dr. Ali Saad, BMT department 13 Electrode Placement anterior wall placement front-to-back placement Dr. Ali Saad, BMT department 14 Automatic External Defibrillators (AEDs) n n Two modes of operation: n Automatic: AED recognizes specific arrhythmias via signal processing algorithms, applies shock as needed. No manual control. n Semi-automatic: operator must confirm shock advisory from AED to deliver the shock. Less operator training needed Dr. Ali Saad, BMT department 15 (a) Basic circuit diagram for a capacitive–discharge type of cardiac defibrillator. (b) A typical waveform of the discharge pulse. The actual waveshape is strongly dependent on the values of L, C, and the torso resistance RL. Dr. Ali Saad, BMT department 16 Electrodes used in cardiac defibrillation (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. Dr. Ali Saad, BMT department 17 Hand-held transthoracic electrode (From Tacker Jr. 1980). Button Dr. Ali Saad, BMT department 18 Some examples of the pre-applied adhesive electrodes (From Tyco/Healthcare Kendall LTP 2001) Dr. Ali Saad, BMT department 19 LIFEPAK 500 Automatic External Defibrillator (AED) weighs 3 kg and is portable (Medtronic inc. 2001). Dr. Ali Saad, BMT department 20 dioversion • Cardioversion is also a shock supplied to treat other types of arrhyt • atrial fibrillation • ventricular tachycardia (rapid heart rate) • these types of arrhythmias are not life threatening but do result in re • it is important that the cardioversion pulse not coincide with a T-wa Dr. Ali Saad, BMT department 21 A cardioverter The defibrillation pulse in this case must be synchronized with the R wave of the ECG so that it is applied to a patient shortly after the occurrence of the R wave. Dr. Ali Saad, BMT department 22 Implantable cardioverter defibrillator (ICD) (Medtronic inc., 2001). Dr. Ali Saad, BMT department 23 Lead wire Electrode Electrode Electrode Lead wire (a) (b) (c) Electrodes for the automatic implantable cardioverter defibrillator (ICD). (a and b:modified from Tacker Jr. 1994, c: modified from Owens et al., 1990) Dr. Ali Saad, BMT department 24 A defibrillator provides a 5 ms pulse of 20 A to a 50 W load. Thus the energy delivered is E = P = I2Rt = (20 A)2(50 W)(0.005 s) = 100 J. Dr. Ali Saad, BMT department 25 Defibrillator Safety n n n Shock for treating atrial flutter or atrial fibrillation should not be applied during T-wave. Operator should be careful not to touch electrodes during defibrillation. Other personnel should remain clear of patient and any metal objects contacting patient during defibrillation. Dr. Ali Saad, BMT department 26 Defibrillator Case Study CardioServe Dr. Ali Saad, BMT department 27 Case Study (cont.) Instrument type: Portable defibrillator with monitor Operation modes: Non-synchronized, synchronized, HR monitoring Energy levels External defibrillation: 2, 5, 7, 10, 20, 30, 50, 70, 100, 150, 200, 300, 360 J Capacitor charging time: To 360J from power line or fully charged battery typically 8 seconds. To 200J typically 4 seconds. Waveform: Damped sinusoidal halfwave (Edmark) Synchronized delay: From R-wave trigger to pulse discharge approximately 40 ms Pulse output: Isolated Safety discharge: Capacitor discharge via internal load resistance Dr. Ali Saad, BMT department 28 Case Study (cont.) Defibrillation electrodes Hard paddles: Hard paddles (80 cm2) with charge/shock and printer buttons. Pediatric adapter (17 cm2). Pads: Adapter cable for adult and pediatric pads. Buttons for charge/shock and printer on the defibrillator. Synchronization Signal: With ECG signal of either polarity ECG acquisition: Via hard paddles/pads or ECG patient cable. Via ECG patient cable up to 7 leads are selectable. ECG Monitor Input: ECG via 3 or 5 leadwire cable or defibrillation hard paddles or pads Frequency response: 0.5 to 100 Hz (ECG patient cable) Isolation: Class CF according to IEC. Input protected against high voltage defibrillator pulses. Common mode rejection: >110 dB RL referred to ground Dr. Ali Saad, BMT department 29 Implantable Defibrillators n n n n n Treat v. fibrillation, tachycardia Usually incorporated as part of an implanted cardiac pacemaker (typically VVI). Defibrillation threshold: 9.8 6.6 J (n = 102), biphasic pulse Defibrillation electrodes also transvenous. Must detect arrhythmia prior to defibrillation: n HR n PR, RR interval stability n more sophisticated electrogram analysis (EE 5345) Dr. Ali Saad, BMT department 30 References for Defibrillation n n Willis A Tacker, “External Defibrillators,” in The Biomedical Engineering Handbook, J. Bronzino (ed) CRC Press, 1995. M. Neuman, in Webster (ed), Medical Instrumentation: Application and Design, Houghton Mifflin, 1992. n Dr. Ali Saad, BMT department 31