12. Diathermy

Question 1

What are the basic principles of
diathermy?

What is it
/ Used for

How do the heat

WHat is the heating effect dependent on

what are the modes

Answer 1

> Diathermy devices are surgical instruments
used to cut tissues and
coagulate blood vessels.

> They use the heating effect of 
high frequency AC (0.5–1.0 M Hz) 
passing through tissues of high impedance 
to burn or vaporise tissues in contact
with the diathermy instrument.

> The heating effect of a current 
depends on the 
current density 
and
duration.

> In diathermy the current density
at the point where the instrument makes
contact with the tissue is very high.

> In ‘cutting’ mode the current flows
in an alternating sine wave pattern

while in ‘coagulation’ mode the
current flows in a pulsed sine wave pattern.

> Diathermy devices can either 
be 
monopolar 
or
bipolar.

Question 2

> In monopolar diathermy,

what are the components

where is the current density high and low

The overall power that can be delivered
is in the region of -

Answer 2

> In monopolar diathermy,

current flows through a probe
(active electrode)

at a high current density
and then returns
via a diathermy plate

(neutral plate) at low current density.

100–400 W .

Question 3

> In bipolar diathermy

Answer 3

> In bipolar diathermy,

the current is passed between

two probes within a

modified pair of forceps.

One probe delivers the current and
the second acts as the return circuit

so tissues between the probes
are exposed to the current and heated.

This system helps keep the electrical field focal.

The overall power this system
can generate is approximately

40 W .

Question 4

Why is the diathermy pad always checked at the end of surgery?

Answer 4

In bipolar diathermy the current

makes a short journey
across the tissue and

is returned at a similar density,
heating all the tissue between the electrodes.

In monopolar diathermy the current exits
at a point distant
from the site of surgery,

so it is important to ensure that the
current density is low enough
not to cause injury at this point.

Hence the neutral plate 
has a large surface area 
and 
is applied over an area of good blood supply 
so that the current density is low 

and

any heat generated can be dissipated by the blood flow.

If the plate is not applied properly the

reduced area
or
increased impedance

can lead to more heat generation and burns,

hence the reason the plate site
is checked at the end of surgery.

Question 5

What are the hazards of diathermy?

Answer 5

1
> Burns

2
> Electric shocks

3
> Pacemaker interference

4
> Monitor interference

Question 6

1

> Burns

Answer 6

• Incorrect positioning of the neutral plate
• Ignition of flammable skin preps

• Inadvertent activation of diathermy probe
(minimised by audible note when in use and use of a designated holder)

Question 7

2

> Electric shocks

Answer 7

• Disconnection of the neutral plate

may lead to current passing through

an alternative route

(e.g. ECG electrodes
or
exiting through a site

where the patient may be in contact
with a conducting surface).

This can cause electrical shocks
and burns to the patient.

Question 8

3

> Pacemaker interference

Answer 8

3
> Pacemaker interference

• Ideally, diathermy should be avoided
in patients with pacemakers and
implantable cardioverter defibrillators (ICD) as electrical interference
can cause these devices to malfunction.

• Pacemaker devices must be checked 
prior to surgery 
and 
alternative pacing techniques 
(external or transvenous) 
and 
cardiac arrest trolley should be available. 

ICDs should be disabled during surgery in which
diathermy will be used.

• Surgical and theatre team must be informed.
• Bipolar is safer than monopolar diathermy.

• If monopolar diathermy is absolutely necessary,
its use should be limited to
short bursts of less than 5 seconds.

The neutral plate must be well adhered
and
sited as far away from the pacemaker a possible.

• ECG monitoring is paramount.

Question 9

4

> Monitor interference

Answer 9

4
> Monitor interference

• Diathermy interferes with monitoring including

pulse oximetry,
ECG
and oesophageal Doppler.

Question 10

What design features are incorporated within the diathermy system to minimise risk of electrical shock?

Answer 10

1
> Use of an isolated patient circuit

2
> Use of an isolating capacitor:

Reactance of capacitors reduces
with increasing current frequency;
therefore,

they have a higher impedance to

low-frequency currents
(i.e. they block conduction of AC
and minimise the risk of macro- and microshocks)

but a lower impedance

to high frequency diathermy currents

(i.e. they allow conduction of these currents)