CARDIAC DEFIBRILLATORS

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

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

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.

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)

Contd:

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)

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

Contd: Mono phasic or biphasic

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)

Contd:

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

Contd:

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

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

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

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

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

Block diagram of the automatic defibrillator:

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

Contd:

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

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