1. Dr. Ali Saad, BMT department1 Dr. Ali Saad, Biomedical Engineering Dept. College of applied medical sciences King Saud University BMT414 Biomedical Engineering Defibrillators/Cardioversion

2. Dr. Ali Saad, BMT department2 Defibrillation used to treat ventricular fibrillation (cardiac arrest) loss of coordinated contraction of muscle fibers in ventricles, death occurs in minutes if left untreated. 50,000 cardiac arrest cases occur annually in US. defibrillation involves the application of a strong electrical shock designed to depolarize most of the heart cells simultaneously, which often reestablishes coordinated contractions and a normal sinus rhythm. exact physical mechanism leading to ventricular fibrillation is not known. Is thought to originate from myocardial ischemia due to a complication of atheroschlerosis.

3. Dr. Ali Saad, BMT department3 Defibrillation Strength Duration Curve energy (joules) charge (coulombs) current (amps) pulse duration defibrillation occurs no defibrillation

4. Dr. Ali Saad, BMT department4 Defibrillation Strength Duration Curve (cont.) minimum defibrillation energy occurs for pulse durations of 3 - 10 ms (for most pulse shapes). 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.)

5. Dr. Ali Saad, BMT department5 External Defibrillators For each minute elapsing between onset of ventricular fibrillation and first defibrillation, survival decreases by 10%. defibrillators should be portable, battery operated, small size. energy in defibrillators usually stored in large capacitors. total energy stored in capacitor: V c = capacitor voltage

6. Dr. Ali Saad, BMT department6 External Defibrillator power supply energy storage patient ECG monitor timing circuitry gate charge discharge standby switch is under operator control applies shock about 20 ms after QRS complex, avoids T-wave

7. Dr. Ali Saad, BMT department7 Capacitive Discharge Defibrillator power supply C L R lead R chest 10 ms t 2kV C: 10  F - 50  F, takes about 10s to charge V c : 4,000 - 9,000 V up to 40% of energy in C can be dissipated in L and R i response is slightly underdamped (depends on chest R) VcVc + _ VcVc

8. Dr. Ali Saad, BMT department8 Example power supply C R chest VcVc + _ R chest = 95  R lead = 5  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? R lead V C (0) = initial capacitor voltage after charging V chest + _

9. Dr. Ali Saad, BMT department9 Example (cont.) energy stored in capacitor at t = 0: (1) (2)

10. Dr. Ali Saad, BMT department10 Example (cont.) energy delivered to R chest : substituting (1): using (2):

11. Dr. Ali Saad, BMT department11 Example (cont.) C = 54.3  F initial voltage across capacitor: V c (0)=3,322.90V solving for C:

12. Dr. Ali Saad, BMT department12 Square Wave Defibrillators power supply C R chest VcVc + _ V chest + _ R lead timing circuit SCR1 SCR2 n during charging, SCR1 and SCR2 both open n to defibrillate, SCR2 closes, current flows to chest n after a fixed interval, SCR1 closes, shorts out C V chest

13. Dr. Ali Saad, BMT department13 Defibrillation Electrodes n metal, 70-100 cm 2 surface area n must be coupled to skin using conductive material (otherwise can burn patient) n two types: n hand-held: conductive gel must be manually applied, reusable. n adhesive: adhesive conducting material holds electrode to skin, disposable

14. Dr. Ali Saad, BMT department14 Electrode Placement anterior wall placement front-to-back placement

15. Dr. Ali Saad, BMT department15 Automatic External Defibrillators (AEDs) 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. n Less operator training needed

16. Dr. Ali Saad, BMT department16 (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 R L.

17. Dr. Ali Saad, BMT department17 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.

18. Dr. Ali Saad, BMT department18 Hand-held transthoracic electrode (From Tacker Jr. 1980).

19. Dr. Ali Saad, BMT department19 Some examples of the pre-applied adhesive electrodes (From Tyco/Healthcare Kendall  LTP 2001)

20. Dr. Ali Saad, BMT department20 LIFEPAK 500 Automatic External Defibrillator (AED) weighs 3 kg and is portable (Medtronic inc. 2001).

21. Dr. Ali Saad, BMT department21 Cardioversion Cardioversion is also a shock supplied to treat other types of arrhythmias such as: atrial fibrillation ventricular tachycardia (rapid heart rate) these types of arrhythmias are not life threatening but do result in reduced cardiac output it is important that the cardioversion pulse not coincide with a T-wave otherwise ventricular fibrillation may result.

22. Dr. Ali Saad, BMT department22 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.

23. Dr. Ali Saad, BMT department23 Implantable cardioverter defibrillator (ICD) (Medtronic inc., 2001).

24. Dr. Ali Saad, BMT department24 (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)

25. Dr. Ali Saad, BMT department25 A defibrillator provides a 5 ms pulse of 20 A to a 50  load. Thus the energy delivered is E = P = I 2 Rt = (20 A) 2 (50  )(0.005 s) = 100 J.

26. Dr. Ali Saad, BMT department26 Defibrillator Safety n Shock for treating atrial flutter or atrial fibrillation should not be applied during T-wave. n Operator should be careful not to touch electrodes during defibrillation. n Other personnel should remain clear of patient and any metal objects contacting patient during defibrillation.

27. Dr. Ali Saad, BMT department27 Defibrillator Case Study CardioServe

28. Dr. Ali Saad, BMT department28 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

29. Dr. Ali Saad, BMT department29 Case Study (cont.) Defibrillation electrodes Hard paddles: Hard paddles (80 cm 2 ) 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

30. Dr. Ali Saad, BMT department30 Implantable Defibrillators n Treat v. fibrillation, tachycardia n Usually incorporated as part of an implanted cardiac pacemaker (typically VVI). n Defibrillation threshold: J (n = 102), biphasic pulse n Defibrillation electrodes also transvenous. n Must detect arrhythmia prior to defibrillation: n HR n PR, RR interval stability n more sophisticated electrogram analysis (EE 5345)

31. Dr. Ali Saad, BMT department31 References for Defibrillation n Willis A Tacker, “External Defibrillators,” in The Biomedical Engineering Handbook, J. Bronzino (ed) CRC Press, 1995. n M. Neuman, in Webster (ed), Medical Instrumentation: Application and Design, Houghton Mifflin, 1992. n