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Diathermy and its safe use
Diathermy
 Diathermy is frequently used to coagulate a
bleeding vessel or to cut tissues.
 Unipolar diathermy is commonly used.
 Components
– 1. Diathermy active electrode.
– 2. Patient's plate.
– 3. Diathermy case (where the frequency of the current
used can be adjusted.)
An isolating capacitor is situated between the patient's
plate and earth.
Mechanism of action
• Heat is generated when a current passes through
a resistor.
• The amount of heat generated (H) is proportional
to the square of current (I2) divided by the area (A)
(H=I2/A).
• A large amount of heat is produced at the_tip of
the diathermy forceps because of its small size
(high current density).
• Whereas at the site of the patient's plate, because
of its large surface area, no heat or burning is
produced (low current density).
Mechanism of action
 A high frequency current 500 000 -1 000
000 Hz is used.
 This high frequency current behaves
differently from the standard 50 Hz current.
It passes directly across the preccordium
without causing ventricular fibrillation.
 High frequency currents have a low tissue
penetration without exciting the contractile
cells.
Mechanism of action
 The isolating capacitor has low impedance
to a high frequency current, i.e. diathermy
current.
 The capacitor has a high impedance to 50
Hz current
 Thus protecting the patient against electrical
shock.
Mechanism of action
 Earth-tree circuit diathermy can be used.
 The patient, the tip of the diathermy forceps
and the patient plate are not connected to
earth. This reduces the risk of burns to the
patient.
 This type of circuit is known as a floating
patient circuit
Mechanism of action
 Bipolar diathermy does not require a patient
plate.
 The current flows through one side of the
forceps, through the patient and then back
through the other side of the forceps.
 Usually low power can be achieved from a
bipolar diathermy.
 Bipolar diathermy is frequently used during
neurosurgery or ophthalmic surgery.
Problems in practice and safety
features
 If the area of contact between the plate and
patient is reduced, the patient is at risk of being
burned at the site of the plate.
 If the plate is completely detached, current might
flow through any point of contact between patient
and earth, for example earthed ECG electrodes or
temperature probes.
Modern diathermy machines do not function with
any of the above.
Problems in practice and safety
features
 If the area of contact between the plate and
patient is reduced, the patient is at risk of being
burned at the site of the plate.
 If the plate is completely detached, current might
flow through any point of contact between patient
and earth, for example earthed ECG electrodes or
temperature probes.
 Modern diathermy machines do not function with
any of the above.
Problems in practice and safety
features
 Electrical interference with other electrical monitoring
devices.
 The use of electrical filters can solve this problem.
 Interference with the function of cardiac pacemakers.
 Damage to the electrical circuits or changes in the
programming can occur.
 This is more of a hazard with cutting diathermy than with
coagulation diathermy.
 Modern pacemakers are protected against diathermy.
Pacemaker interference
 Electromagnetic interference (EMI)
– 0 and 109 Hz (e.g. AC power supplies and electrocautery)
– microwaves with frequencies between 109 and 1011 Hz (including
ultra high frequency radio waves and radar)
can cause device interference.
 X-rays, gamma rays and infrared and ultraviolet light do not
cause interference.
 EMI may enter the pacemaker or defibrillator by
– direct contact between the patient and the source,
– exposure to an electromagnetic field
 Device leads act as antennae.
Pacemaker interference
 Devices are generally protected by
– circuit shielding using titanium casing
– noise protection algorithms that filter out unwanted signals.
 Possible responses to external interference include
–
–
–
–
inappropriate inhibition or triggering of a paced output
asynchronous pacing
reprogramming (usually into a backup mode)
damage to device circuitry and triggering a defibrillator discharge.
 Asynchronous pacing and mode resetting are the most
common outcomes of EMI and should be considered if
pacing modes appear to change suddenly or intermittently
on ECG monitors
Pacemaker interference
 In some devices EMI may also cause a
pacing mode change.
 This is usually the ‘backup’ or ‘reset’ mode
(often the same mode as the ‘battery
depletion’ mode).
 Assessed using telemetric interrogation.
 The backup or reset mode is usually VVI or
VOO.
Pacemaker interference
 The EMI generated by electrocautery that may affect the
device is related to the distance and orientation of the
current to the patient’s device and leads.
 Radio frequency signals may be interpreted as cardiac
impulses, leading to inappropriate inhibition.
 Prolonged application of cautery can repeatedly trigger the
NSP, resulting in asynchronous pacing, with function
returning to normal when electrocautery is stopped.
 If electrocautery interference results in mode resetting, for
example from DDD to VVI or VOO, AV synchrony will be
lost and may result in haemodynamic embarrassment.
Such resetting will persist even after electrocautery is
stopped.
Pacemaker interference
 Implanted defibrillators may interpret
electrocautery interference as ventricular
fibrillation, resulting in an inappropriate shock.
 pacemaker circuitry damage resulting in output
failure,
 activation of maximum rate response pacing
 pacing lead overheating and myocardial damage
 transient threshold alteration.