Surgical Diathermy in the OT
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Direct answer
Radiofrequency alternating current at roughly 0.3–3 megahertz heats tissue to cut or coagulate, at frequencies far above the threshold where muscle and nerve are stimulated. Monopolar diathermy pushes current from an active electrode through the patient's body to a large return pad; bipolar confines the current between the two tips of a forceps and needs no pad at all. Cutting uses a continuous, lower-voltage waveform for a clean blade of vaporisation; coagulation uses an interrupted, higher-peak-voltage waveform that desiccates and seals — and blend settings mix them. The technician's real responsibility is the return electrode: generous pad area over well-vascularised muscle, full contact, and vigilance, because nearly every serious diathermy burn is a return-pathway failure.
What you must remember
- Frequencies: around 0.3–3 MHz; below roughly 100 kHz current stimulates nerve and muscle, which is why electrosurgery is high-frequency by design.
- Cutting versus coagulation: continuous low-voltage sine wave vaporises (cut); intermittent high-voltage spikes desiccate (coag); blend combines the two for haemostatic cutting.
- Monopolar: current traverses the whole body between pencil and return pad — pad placement, contact area (about 70 cm² or more) and adhesion are critical checks.
- Bipolar: current crosses only the grasped tissue between forceps tips; no return pad, mandatory near pacemakers, implantable devices, delicate neural and vascular structures.
- Return electrode rules: dry, hair-clipped, intact skin over a fleshy, well-perfused area (thigh, upper arm or flank); avoid bony prominences, scar tissue, implants and pressure points.
- Burn mechanisms: pad edge-lift with partial contact concentrates current density; spirit or alcohol pooling under drapes ignites; stray contact with grounded metal creates alternate return paths.
- Pacemaker patients: bipolar preferred, short bursts, pad placed so current does not cross the device, and pacing function monitored — with the cardiology team aware.
- Alarm habits: a persistently sounding return-electrode alarm means the circuit is genuinely compromised — investigate, never silence and continue.
Setting up and running a monopolar case
Before the patient enters, the return pad is applied to a suitable site and the cable plugged in; the generator self-tests and confirms pad continuity. The active pencil is checked for insulation cracks — a frayed shaft near the surgeon's hand delivers burns to the glove or the patient tissue touching it. Power is set to the surgeon's preference (commonly in the range of 30–60 watts for coagulation and cutting in general surgery, higher settings justified by tissue type rather than habit), footswitch or finger switch tested, and the holster in which the pencil rests between uses positioned so an accidental activation cannot burn drapes or the patient.
Mid-case vigilance matters more than the setup. Patient repositioning can peel a pad edge; pooled prep spirit under drapes can flash when the pencil arcs; a wet glove or towel contacting a grounded table edge can create an unintended return path. After the case, the pad site is inspected and documented in the recovery notes — a pink rectangle is normal, a blister or barcode-pattern burn is a reportable incident. Plume from electrosurgery is evacuated at the field like laser smoke; the hazards of chronic inhalation are reason enough to keep the suction near the tip.
Where students slip
The exam trap is "why does the patient not get burned under the return pad?" — because current density there is low: the same total current spread over a large area produces negligible heating, while the millimetre tip of the pencil concentrates it intensely. Invert the area — pad half-detached — and density at the remaining edge rises to burning levels, which is why Return Electrode Monitoring pads and alarms exist. The second slip is bipolar confusion: candidates attach a return pad for bipolar forceps when none is needed, or fail to say why neurosurgeons and pacemaker patients prefer bipolar. Third, waveform definitions: candidates describe coagulation as "higher power" when the discriminating facts are the interrupted delivery and higher peak voltage of the coag waveform at lower average current.
Frequently asked questions
Why does diathermy use very high-frequency current?
Frequencies around and above 300 kHz pass through tissue without stimulating nerve or muscle, allowing thermal effect without convulsion or pain response.
Where should the return electrode be placed?
Over clean, dry, well-vascularised muscle such as the thigh or upper arm — avoiding bone, scars, implants and any position where it could peel during repositioning.
Does bipolar diathermy require a return pad?
No; current passes only between the two forceps tips through the grasped tissue, which is why it is preferred near pacemakers and delicate structures.
How does the cutting waveform differ from coagulation?
Cutting is a continuous lower-voltage waveform that vaporises tissue; coagulation is an interrupted, higher peak voltage waveform that desiccates and seals vessels.
What is the commonest cause of a diathermy burn?
Return-pad failure — edge lift or poor contact concentrating current density at a small skin area; hence full-contact application and functioning pad alarms.