1. CARDIAC DEFIBRILLATORS

2. NEED: Ventricular fibrillation leads to serious cardiac emergency
Asynchronous contraction or uncoordinated movement of the ventricular walls
Ventricles quiver instead of pumping
Steep fall of cardiac output

3. Remedy:
Can be converted into more rhythm by applying a high energy shock to the heart
Instrument is defibrillator

4. Types: Requires more voltage
External: placed on the chest of the patient
Requires more voltage
Internal: the electrodes may be held directly against the heart when the chest is opened.

5. DC Defibrillator: step up transformer Capacitor : 16 micro farad
Charges and discharges (provides shock to the subject)
L and C decides the shape of the output (nearly trapezoidal)

6. Contd:

7. Contd: Enormous votage( approx. 4000 V) Duration : 20 micro seconds
Human skin is used as a resistance to discharge
Calibrated in terms of watt-seconds (0 – 400 Ws)

8. Defibrillator electrodes:
Metal discs (3-5 cm in diameter)
Highly insulated handles
Spoon shaped electrodes
Impedance depends on :
Size of the electrode, paddles
Energy discharged, time interval between two shocks
Interface material

9. Contd:
Mono phasic or biphasic

10. DC defibrillator with synchronizer:
To permit placement of discharge at the right point on the patient’s ECG.
Vulnerable point of time is avoided
After 20 to 30 ms after the peak of the R wave of the patient’s ECG (capacitor is discharged)
Rechargeable batteries are used (400 Ws)

11. Contd:

12. Automatic or Advisory External Defibrillators (AED) :
Accurately analysis the ECG and give reliable shocks
Detect ventricular fibrillation with high sensitivity
Less noise
Higher quality

13. Contd:

14. Contd: Four indicators Heart rate
Conduction (electrical wave flow – R wave)
Stability (repeatability of QRS complexes)
Amplitude(magnitude)
Lithium battery( 75 discharges )

15. Implantable defibrillators:
For who have high risk of ventricular fibrillation
Reduce the device size
Biphasic waveforms are more efficient and probably safer than mono phasic waveform
Additional calculations like rhythm calculations and storage of heart signal

16. Contd: Three main systems: Defibrillator (AID) Lead system 6cm* 9 cm
Programmer recorder / monitor(PRM)
Provides a bidirectional communications link to AID (telemetry link)

17. Contd: Battery : lithium silver Vanadium oxide (LiSVO) 3 V
High voltage capacitors for timed delivery

18. Pacer-Cardioverter-Defibrillator:
Cardioverter: an energy-storage capacitor-discharge type of condenser which is discharged with an inductance; it delivers a direct-current shock which restores normal rhythm of the heart.
For who suffer from tachyarrhythmias

19. Block diagram of the automatic defibrillator:

20. Contd: Sensing circuit High voltage converter Switching circuit
Five battery powered units:
Sensing circuit
High voltage converter
Switching circuit
Defibrillation control circuit
Pacing control circuit

21. Contd:

22. Defibrillator analysers:
To measure the energy content in the discharge pulse
Where
E= energy in watt-seconds
e(t)=voltage as a function of time
i(t)=current as a function of time
T=time duration of the pulse

23. Contd: Energy dissipated in the resistance is given as:
Input squaring integrator output