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PHYSICS

© 2005 The Medicine Publishing Company Ltd411ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:12

A defibrillator is a device that delivers direct electrical current

across the myocardium to cause synchronous depolarization of 

the cardiac muscle, with the aim of converting a dysrhythmia into

normal sinus rhythm. It is used to treat ventricular fibrillation

(VF), which is one of the main causes of sudden death. Although

it was discovered in 1899 that a higher strength electrical signal

applied to the heart could convert VF into normal sinus rhythm,

the defibrillator was not invented until 1932.

A typical defibrillator includes a power supply, capacitor, induc-tor, variable transformer and rectifier. Figure 1 shows the defibril-

lator circuit. The power source comes from the mains supply or

a battery.

Power supply 

The mains power supply voltage of 240 V is converted to higher

voltage (usually 5000 V) with the help of a step-up transformer. A

rectifier then converts AC voltage to DC voltage. DC energy, rather

than AC energy, is used because it is more effective, causes less

myocardial damage and is less arrhythmogenic.

Capacitor The capacitor is the most vital part of the defibrillator. It stores

electrical energy in the form of electrical charge. A capacitor is

formed by a pair of conductors (metal plates) separated by an

insulator (a layer of air).

When switch A is activated, the current from the power supply

flows through the capacitor. The flow of current starts to build up

an electrical charge on the metal plates of the capacitor. The stored

charge (C) on the plates creates a potential difference across the

plates, which oppose the flow of current (Q). Initially the potential

difference (V) is smaller allowing more current to flow across the

capacitor plates. As the voltage of stored charge on the plates pro-

gressively rises, current flowing through the capacitor decreases.

Once the voltage of the stored charge in the capacitor equals thatof the source, current flow stops. Thus, capacitor charging is an

exponential process (Figure 2).

In the capacitor, the quantity of electrical charge stored for a

given charge potential is determined by the surface area of the

capacitor plates, the thickness of the insulating layer and the ability

of the capacitor to store charge (permitivity).

Capacitance is measured in farads (F) and is the ability of the

capacitor to hold the electrical charge.

When paddles are applied to a patient’s chest and switch B

is activated, the capacitor discharge circuit is completed. As the

current flows through the discharge circuit and between the two

paddles, electrical energy is delivered across the patient’s heart.

The quantity of current flow across the chest is partly limited bythoracic impedance. To reduce thoracic impedance and to achieve

Physical principles of thedefibrillator Maheshwar Chaudhari

P M Baker

Maheshwar Chaudhari is Specialist Registrar in Anaesthesia in the West 

Midland Deanery Rotation Scheme. He qualified from the University 

of Poona, India. He trained in India and Peterborough, UK. His special

interest is in the physics of anaesthesia and regional anaesthesia.

P M Baker is Consultant in Anaesthesia to Peterborough and Stamford 

Hospitals NHS Foundation Trusts, UK. He qualified from the University of Newcastle-upon-Tyne and trained in Newcastle, Sheffield and Leeds, UK.

The defibrillator circuit

Capacitor

Inductor

Paddles

Switch A Switch B

L

N

Power

source

1

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PHYSICS

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higher peak current flow, gel pads should be applied to the chest,

paddles should be applied firmly and defibrillation should be car-

ried out in the expiratory phase of ventilation (as impedance is less

when lung volume is reduced). Capacitor discharge is associated

with a progressive fall in the potential difference (V) across the

plates. At the beginning of discharge, the potential difference ismaximum, allowing more energy to be delivered to the heart, but

as V progressively falls, this delivered energy also declines in an

exponential way.

During the discharge of a capacitor, the delivered energy falls

exponentially and some of the energy available is lost in circuit

resistance, inductor and paddles. About half of the stored energy

is delivered at the patient’s chest. Energy delivered is calculated

as:

Energy (J) = ½ × stored charge (C) × potential difference (V)

Modern defibrillators are calibrated in terms of delivered energy,

not stored energy.

Inductor 

An inductor is a coil of wire. The defibrillator has an inductor in

its output circuit. During discharge of a defibrillator, current flows

through an inductor. The flow of current generates a magnetic field

around the inductor that further slows the flow of current through

it. This minimizes the rapid decay of current flow (delivered

energy) and allows it to flow for optimum duration. Effective defib-

rillation depends on sustained release of energy at the heart.

Monophasic versus biphasic energy 

In a monophasic defibrillator, during discharge of the defibrilla-

tor, current flows in one direction between the two plates of the

capacitor.

In a biphasic defibrillator (newer type) the current flows in

one direction for a specified duration before reversing to the other

direction for the remainder of the electrical discharge.

The biphasic waveform of energy reduces the effective energyrequired for defibrillation. Thus, smaller capacitors with less bat-

tery power are required. A longer refractory period follows biphasic

shock, which helps to block recurring fibrillating waveforms.

 Automated implantable cardioverter-defibrillator (AICD)

AICD is a very small defibrillator (the size of a pager) containing

its own power source in the form of a battery. It is implanted in

the body in the same way as a conventional implantable pace-

maker.

The output of the AICD may vary between 0.1 and 30 J. A

transvenously placed lead has sensing electrodes as well as shock-

ing electrodes. The sensing electrodes sense the heart rate and

analyse the morphology of complexes. The AICD can be control-led externally with a magnet. The history of AICD activity can be

analysed by telemetry.

FURTHER READING

Hutton P, Cooper G M, James III F M, Butterworth J F. Fundamental

 principles and practice of anaesthesia. London: Martin Dunitz, 2002.

Moyle J T B, Davey A, eds. Ward’s anaesthetic equipment . London: WB

Saunders, 1998.

Sykes M K, Vickers M D, Hull C J. Principles of measurement and 

monitoring in anaesthesia and intensive care. Oxford: Blackwell

Scientific Publications, 1991.

Charging a capacitor 

    P   o   t   e   n   t    i   a    l    d    i    f    f   e   r   e   n   c   e    (

    V    )

Current flow (Q)

V

Q 

Max Min

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