5. Defibrillation

Question 1

Capacitance:

Answer 1

two plates that are separated by an insulator and which will store electrons after the application of a potential difference

ability to hold electric charge

Question 2

Defibrillator circuit:

Answer 2

4,000 and 6,000 V is applied across the capacitor to produce a store of electrons

When the defibrillator is activated, this stored charge is
released as a pulse of current across the patient’s heart

inductance coil (inductor) in the output circuit decelerates
the rapid discharge of the capacitor to give a shock that is slowed to 4–10 ms

This duration gives the optimal chance of synchronous myocardial depolarization

Question 3

Impedance

Answer 3

The efficiency of the applied shock is greater

if transthoracic impedance is minimized by the use of conductive gels,
firm paddle pressure, and by defibrillation

from front-to-back rather than from sternum to apex. (Impedance is the sum of all
forces impeding electron flow in an AC circuit.)

Question 4

Waveform

Answer 4

In a defibrillator
that uses a biphasic waveform, the current is reversed halfway through the
discharge to move both in a positive and a negative direction. (There are also two
biphasic waveforms: truncated exponential decay and rectilinear

not only more effective than monophasic, but they also cause less myocardial
injury. Their use is now becoming widespread

Question 5

Supplementary Information and Clinical Considerations

Answer 5

Atrial and ventricular fibrillation:
in health, the sinus impulse is conducted evenly
and concentrically to all parts of the atria and thence to the ventricles.

fibrillation (AF) supervenes, the excitation and recovery of different parts of the atria
becomes uncoordinated,

with various areas at different stages of excitation and
recovery. It is similar with ventricular fibrillation (VF).

changing amplitude of the ECG reflects electrical activity,
but depolarization is chaotic and unable therefore
to generate any cardiac output.

Question 6

Effects Fibrilliation

Answer 6

: in AF, there is loss of the atrial contribution to ventricular filling, which is
usually around 20%. In addition, the risk of thrombus formation is substantially
increased. A fibrillating ventricle produces no cardiac output

Question 7

Common causes of AF

Answer 7

Common causes of AF include
ischaemic heart disease
and acute critical illness,
particularly sepsis
( mitral stenosis, Thyrotoxicosis)
Alcohol

Question 8

VF is caused

Answer 8

by myocardial disease,

both ischaemic and myopathic,

by hypoxia, by profound hypothermia,

by electrolyte imbalance, by some drugs and by electrocution

Question 9

The Electrical Management of Fibrillation

refractory AF

Answer 9

refractory AF is treated by the application of a DC shock,

which is synchronized to the peak deflection of the ‘R’

wave of ventricular depolarization on the ECG.

The risk of inducing VF is very high during repolarization (as shown by the ‘T’ wave on the ECG).

This is why the ‘R-on-T’ rhythm is particularly
dangerous.

Question 10

Ventricular fibrillation (VF):

Answer 10

this can be treated either by mechanical defibrillation
or electrical defibrillation. The application of mechanical energy in the form of a
praecordial thump (sometimes known as ‘thumpversion’) may convert VF to a viable
rhythm only if it is applied very early. It normally achieves only around 5–10 joules
of mechanical energy. In electrical defibrillation, a defibrillator delivers a charge
across the chest which causes simultaneous depolarization of myocardial cells. If
the procedure is successful, there is a short refractory period after which there is
resumption of normal pacemaker activity with myocardial contraction and a stable
rhythm.