Download Defibrillator - galileo.harvard.edu

• 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