10. Defibrillators Flashcards
What is a defib
what can it be used for
how
Defibrillators are devices
used to restore normal cardiac rhythm
by delivering a burst of electrical energy
to the heart.
This ‘burst’ depolarises all the myocytes essentially re-setting the electrical status of the heart and allowing co-ordinated myocardial depolarisation to occur again.
There are various types that can be
manual or automated,
monophasic or biphasic,
external, transvenous or implanted in the patient.
What is the difference between monophasic and biphasic waveform defibrillators?
Monophasic waveform:
This is a damped sinusoidal wave
(Lown-type waveform).
Current flows in one direction only,
from one electrode
to the other
Biphasic waveform:
This can be either a
biphasic truncated exponential waveform
or a rectilinear biphasic waveform.
Current flows in alternating directions,
completing one cycle in approximately 10 ms.
During the first phase, current flows in one direction and then reverses direction during the second phase.
This lowers the electrical threshold
for successful defibrillation,
allowing lower energy levels to be used
and reducing the risk of burns
and myocardial damage.
Biphasic defibrillation was originally developed
and used for implantable cardioverter defibrillators.
Draw defib circuit
page 175
How does a defibrillator work?
what type of current
how many volts
what is the first component
- how does this treat electricity types
Whats the next component -
whats in it
why is this used
Whats slowing the shock - how much is it
whats the energy used for each type
- > Delivers DC shock
(AC causes myocardial damage and is arrhythmogenic). - > Uses 5000 V
(this is much greater than that of the mains electricity and is produced using a step-up transformer).
- > A capacitor is used to store charge.
It consists of two conducting plates
separated by an insulating material (dielectric).
Capacitance is measured in farads (F).
The amount of charge it can store
depends on the size of the plates,
their separating gap and the
dielectric material.
- They have a low reactance to AC (i.e. passes AC) but a high resistance to DC
(i. e. blocks DC).
Charge (Q) = Capacitance (C) × Voltage (V)
Energy stored (E) = ½ CV2
- An inductor is used to prolong the duration of current discharge
It consists of coils of conducting material wound around a ferrous core (former).
A magnetic flux is induced whenever a current flows through the coils causing back EMF and prolonging the charge.
They have a high reactance to AC (i.e. block AC)
but a low resistance to DC (i.e. pass DC).
- > It produces a DC shock from 30 A , for 3 ms with 5000 V.
> The delivered (quoted) energy is less than stored charge due to some loss within the inductor.
- > Thoracic impedance is in the region of
50–150 Ω.
This is reduced after the first shock, by the use of conductive gel pads, front-to-back defibrillation and application of firm paddle pressure.
- > External biphasic defibrillators deliver an energy of 150 J
while monophasic ones deliver 360 J.
Internal cardiac defibrillators (ICD) use 20–50 J.
How do cardioversion and defibrillation differ?
> During cardioversion,
a synchronised DC shock-
prevent ‘R on T’ phenomenon,
which can trigger VF.
> During defibrillation of pulseless VT or VF, a non-synchronised shock can be administered.
> The energy delivered during cardioversion is often lower than for defibrillation
e.g.
50 J for atrial fibrillation using a biphasic defibrillator,
compared to 200 J for VF.
How can you calculate the energy that will be delivered during a shock?
This can be done using the equation E = ½ CV^2 > If C is 100 μF and V is 2000 V, then E = ½ (0.0001)(2000^2) = 200 J
> Energy stored will be 200 J.
In reality the energy delivered will be slightly less due to some loss within the system.
What are the safety considerations when using a defibrillator?
risks x5
Use of a defibrillator necessarily means discharging a large amount of energy.
If this is not done safely it can lead to:
1. > Burns.
2. > Ignition of flammable material and gases (fire and explosions).
3. > Interference with electrical components in contact with the patient such
as ICDs and pacemakers.
4. > Precipitation of VF if shock intended to cardiovert is not synchronised
correctly with patient’s cardiac rhythm.
5. > Electrocution of staff and patient.
How can risks can be minimised
- > Allowing only trained personnel to deliver shocks.
- > Ensuring all personnel are ‘standing clear’ when the shocks are delivered,
i. e. not touching the patient and trolley/bed. - > Maintaining and checking defibrillator regularly.
- > Having an audible alarm that signifies when defibrillator machine is ‘charging’ and ‘ready to shock’.
- > Having dry surroundings (patient and staff must not be in contact with fluid that can conduct the charge).
- > Placing defibrillator pads on dry skin correctly to ensure maximal contact.
If the pads are only partially in contact with the patient there will be a higher current density through the part that is in contact and this can lead to burns. - > Taking oxygen away/disconnecting it from the patient prior to delivery of shock.
- > Having regular training and simulation sessions.
