Diathermy
Used for coagulation and cutting.
Heat generated as current passes through a resistance in the probe.
Heat ∝ power
Power ∝ current/area
Sine waveform for cutting 0.5 MHz
Damped waveform for coagulation 1-1.5 MHz
Heating at all frequencies but high frequencies used for safety – high frequency has minimal tissue penetration and doesn’t induce VF.
Unipolar
2 connections to patient – indifferent and active.
Current through both but current density v. high at active and v. low at indifferent.
Circuits have isolating capacitors or are floating for safety.
Bipolar
Current passes from one blade of forceps to other.
Low power so limited efficacy.
Safety
If placed over metal prosthesis → current flow through metal → burn.
Alcoholic skin prep can ignite.
Avoid in structures supplied by fine end arteries.
Indifferent electrode
- Inadequate contact of indifferent electrode.
- High current density → burn.
- Current may flow to earth via another route if any metal contact → burn.
- Apply well, adhesive conductive gels used, well perfused area will dissipate heat quickly.
- Pacemakers
- Use bipolar if possible.
- If unipolar unavoidable place plate as far as possible from pacemaker and use only in short bursts. Cutting worse than coag.
- Monitoring – minimised by filters.