30-06-2012, 04:53 PM
DIATHERMY
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Definition:
Diathermy involves the passage of high frequency alternating current through body
tissue. Where the current is locally concentrated heat is produced (1000 deg. C).
Diathermy involves current frequencies between 400KHz to 10MHz, thus allowing
currents of 500mA to safely pass through patient (100mA at a household frequency of
50Hz will cause electrocution & ventricular fibrillation).
Monopolar diathermy
· Electrical plate is placed on patient and acts as indifferent electrode
· Current passes between instrument and indifferent electrode
· As surface area of instrument is an order of magnitude less than that of
the plate
· Localised heating is produced at tip of instrument
· Minimal heating effect produced at indifferent electrode
Bipolar diathermy
· Two electrodes are combined in the instrument (e.g. forceps)
· Current passes between tips and not through patient
Effects of diathermy
· The effects of diathermy depends on the current intensity and waveform
used
· Coagulation
Produced by interrupted pulses of current (50-100 per second) Square wave-form
dessication & sealing of blood vessels,
· Cutting
Produced by continuous current
Sinus wave-form
cell water is vaporized
· Blend
combination of cutting & coagulation waveforms to increase the
degree of haemostasis during cutting.
Risk and complications
1. May interact with pacemakers -> arrythmias
2. Full thickness burns if use spirit based skin preparation
3. Diathermy burns under indifferent electrode if plate improperly applied
4. Channeling effects if used on appendages
5. Arcing can occur with metal instruments and implants (not likely - see below)
Safety
· Responsibility
It is the surgeon's responsibility
· Alarms
plate continuity alarm - sounds if plate is not connected to the machine.
connect patient to plate, then plate to machine.
· Patient Plate
1. site the plate close to the operation site
2. ensure current is moving away from the ECG electrodes
3. skin area under plate should have a good blood supply
4. avoid bony prominences & scars
5. shave hairy skin
6. ensure plate is not kinked or bent
7. do not let skin fluids seep under plate (esp. spirit based)
· Surgical points
only the surgeon wielding the active electrode should activate the machine
always replace the electrode in an insulated quiver
do not use monopolar diathermy on appendages (fingers)
· Pacemakers
contact a cardiologist pre-operatively
use bipolar wherever possible
ensure the current flows awa from the pacemaker system
use short bursts only
stop all diathermy if arrythmia occurs.
Diathermy over Metallic Prostheses (Michael Mayor)
· An established metallic implant will conduct the radio-frequency electrical
energy of the cautery safely, because of the relatively large surface area of
metal in contact with the patient's tissues.
· Cautery produces its effect due to the high density of current flowing from a
very small conductor (the tip of the electrocautery pencil).
· The larger the surface over which the current is diffused, the lower the
resistance to flow, and the lower the resulting heat.
· Even a single bone screw has such a large area that you cannot achieve any
significant heating of the screw by applying cautery to the screw.
· If the cord is looped extravagantly, you can induce an inductive impedance in
the circuit, lowering the effectiveness of your tool.
· Draping the cord over a long surface of steel (even under the sterile covers)
can induce a capacitive impedance, also draining power.