Heating of tissue by the use of the high frequency (radiofrequency) electrical current – as the electrical current passes through tissue its resistance generates heat.
Electrical circuit – generator, through active electrode and in to patients tissues, then back to generator through return electrode.
Impedance of tissue to the passage of current that causes heating.
Current density determines heat – inverse proportional to cross sectional area – i.e., tip has low cross sectional area and high current density and very hot, compared to return pad with large cross sectional area.
Hence return pad must be well contacted, over large surface area, ideally over a well vascularised muscle, on a shaved site, not folded, avoid bony prominences and scars, ensuring the skin is dry, away from prostheses and close to the operative site.
Cut – continuous waveform (sine wave) – heat produced faster
Coag – interrupted or intermittent waves – slower build-up of heat
Bipolar – return electrode is incorporated in the instrument – no return plate required.
Safer for pedicled structures and minimises tissue damage.
Hazards of diathermy:
Pedicle/channelling
- High current density may be generated in narrow pedicles between tissue being cauterised and rest of the body – this can cause ischaemia
- Prevent by laying organ against body to increase return area, reducing power, and using bipolar
Fire / ignition of flammable material
- Pooling of alcoholic prep fluids, flammable anaesthetic gases, opening obstructed bowel
Return plate burns
Poor adherence between plate and patient – the passive return electrode works by having a greater surface area – if this is lost the current density increases
- Shave skin, apply to dry skin, don’t fold the plate, well vascularised muscle mass, avoid bony prominences or scars, don’t apply over metalwork or prostheses
- Newer machines have inbuilt checks which alarm and deactivate if impedance at return electrode is high
Inadvertent activation
- Hence why plastic quivers are used – non conductive
Neuromuscular stimulation
- Particularly worrisome for TURBT
Pacemakers
- Rare reports of monopolar diathermy temporarily inhibiting pacemaker function or reprogramming PPM, or reports of damage from use right near device
- Use of a magnet prevents complications
- Use bipolar if possible, use short bursts of monopolar current, ensure the return pad is well away from the pacemaker, and ensure anaesthetists aware and have defibrillators, pacing equipment available
AICD
- Should be deactivated prior to surgery and reactivated afterwards
- Same as sacral nerve stimulators
Surgical smoke
- Contain noxious agents – viral DNA, carcinogens, mutagens and chemical irritants
Laparoscopic hazards:
Direct coupling
- Inadvertent use of diathermy on other instruments i.e., metal sucker, laparoscope
Insulation failure
- Failure of the insulation allowing current to exit not at tip and on to surrounding tissue
Capacitive coupling
- Despite insulation, there may be transmission of heat from diathermy to the metal cannula surrounding the shaft (a capacitor is two conductors separated by a non conductor)
Burns