Explain the cardiac conduction pathways with the electrical impulse originating in the SA node (right atrium) which travels to the AV node (division of the right and left ventricles), down along the right and left bundle branches to the Purkinje fibers.Intrinsic heart rates at the SA node are 80 bpm, the AV node is 60 bpm, and the ventricular rate (Purkinje fibers) is 40 bpm.
1. Define defibrillation.
2. Describe Need and history of defibrillation.
3. Describe the principle and mechanism of
4. Types and classes of defibrillator
5. Describe the Automated external defibrillator
6. Identify the precautions and risk
7. Troubleshooting of defibrillator
Defibrillation is a process in which an electronic device
sends an electric shock to the heart to stop an extremely
rapid, irregular heartbeat, and restore the normal heart
Defibrillation is a common treatment for life
threatening cardiac dysrhythmias, ventricular
fibrillation, and pulse less ventricular tachycardia.
Ventricular fibrillation is a serious cardiac emergency
resulting from asynchronous contraction of the heart
Due to ventricular fibrillation, there is an irregular
rapid heart rhythm.
Fig. Ventricular fibrillation
Fig. Normal heart beat
Ventricular fibrillation can be converted into a more
efficient rhythm by applying a high energy shock
to the heart.
This sudden surge across the heart causes all muscle
fibres to contract simultaneously.
Possibly, the fibres may then respond to normal
physiological pace making pulses.
The instrument for administering the shock is called
Defibrillation is performed to correct lifethreatening fibrillations of the heart, which could
result in cardiac arrest. It should be performed
immediately after identifying that the patient is
experiencing a cardiac emergency, has no pulse,
and is unresponsive.
Defibrillation was invented in
by Prevost and
Batelli, two Italian physiologists. They discovered
that electric shocks could convert ventricular
fibrillation to sinus rhythm in dogs.The first case of a
human life saved by defibrillation was reported by
Beck in 1947 .
Energy storage capacitor is charged at relatively
slow rate from AC line.
Energy stored in capacitor is then delivered at a
relatively rapid rate to chest of the patient.
Simple arrangement involve the discharge of
capacitor energy through the patient‟s own
The discharge resistance which the patient
represents as purely ohmic resistance of 50 to 100Ω
approximately for a typical electrode size of 80cm2.
This particular waveform Fig is called „ Lown‟
The pulse width of this waveform is generally 10 ms.
• minimum defibrillation energy occurs for pulse
durations of 3 - 10 ms (for most pulse shapes).
• pulse amplitude in tens of amperes (few
• operator selects energy delivered: 50-360
joules, depends on:
intrinsic characteristics of patient
duration of arrhythmia
type of arrhythmia (more energy required for v.
Fibrillations cause the heart to stop pumping
blood, leading to brain damage.
Defibrillators deliver a brief electric shock to the
heart, which enables the heart's natural pacemaker
to regain control and establish a normal heart
Higher voltages are required for external defibrillation
than for internal defibrillation.
A corrective shock of 750-800 volts is applied within a
tenth of a second.
That is the same voltage as 500-533 no of AA batteries!
Types of Defibrillator electrodes:a) Spoon shaped electrode
• Applied directly to the heart.
b) Paddle type electrode
• Applied against the chest wall
c) Pad type electrode
• Applied directly on chest wall
fig: Electrodes used in defibrillator (a) a spoon shaped internal
electrode that is applied directly to the heart. (b) a paddle type
electrode applied against the anterior chest wall.
Anterior electrode pad
Apex electrode pad
Fig: anterior –apex scheme of electrode placement
Monophasic pulse or waveform
Bi-phasic pulse or waveform
There are two general classes of waveforms:
a) mono-phasic waveform
Energy delivered in one direction through the
a) Biphasic waveform
Energy delivered in both direction through the
The biphasic waveform is preferred over
monophasic waveform to defibrillate. Why?????
• A monophasic type, give a high-energy shock,
up to 360 to 400 joules due to which increased
cardiac injury and in burns the chest around the
shock pad sites.
• A biphasic type, give two sequential lowerenergy shocks of 120 - 200 joules, with each
shock moving in an opposite polarity between
Electrodes placed directly to the heart
b) External defibrillator
Electrodes placed directly on the heart
• For each minute elapsing between onset of
ventricular fibrillation and first defibrillation,
survival decreases by 10%.
• defibrillators should be portable, battery operated,
• energy in defibrillators usually stored in large
• total energy stored in capacitor:
Vc = capacitor voltage
switch is under
applies shock about 20 ms after
QRS complex, avoids T-wave
AED is a portable electronic device that automatically diagnoses the ventricular fibrillation
in a patient.
Automatic refers to the ability to autonomously
analyse the patient's condition.
AED is a type of external defibrillation process.
AEDs require self-adhesive electrodes instead of hand
The AED uses voice prompts, lights and text
tell the rescuer what steps have to take next.
Turned on or opened AED.
AED will instruct the user to:-
• Connect the electrodes (pads) to the patient.
• Avoid touching the patient to avoid false
readings by the unit.
• The AED examine the electrical output from
the heart and determine the patient is in a
shock able rhythm or not
When device determined that shock is warranted, it
will charge its internal capacitor in preparation to
deliver the shock.
When charged, the device instructs the user to ensure
no one is touching the victim and then to press a red
button to deliver the shock.
Many AED units have an 'event memory' which
store the ECG of the patient along with details of the
time the unit was activated and the number and
strength of any shocks delivered.
The paddles used in the procedure should not be
placed:• on a woman's breasts
• over an internal pacemaker patients.
Before the paddle is used, a gel must be applied to
the patient's skin
• Skin burns from the defibrillator paddles are the
most common complication of defibrillation.
• Other risks include injury to the heart muscle,
abnormal heart rhythms, and blood clots.
• Attach the external and internal paddles if the monitor
reads, "No paddles."
• Check to ensure that the leads are securely attached if
the monitor reads, "No leads.“
• Connect the unit to AC power if the message
reads, "Low battery."
• Verify that the Energy Select control settings are correct
if the defibrillator does not charge.
• Change the electrodes and make sure that the electrodes
adapter cable is properly connected if you receive a
message of "PACER FAILURE." Restart the pacer.
• Close the recorder door and the paper roll if the monitor
message reads, "Check recorder”.
Willis A Tacker, “External Defibrillators,” in The
Biomedical Engineering Handbook, J. Bronzino (ed)
CRC Press, 1995.