Diathermy: monopolar vs bipolar

Core concepts (what diathermy is doing)

Electrosurgery uses high-frequency alternating current to generate heat in tissue (Joule heating) to cut/coagulate; it is not “cautery” (which uses an externally heated instrument).
- Heat generated ∝ I² × R × t (current squared, tissue resistance, time).
- High frequency (typically ~300 kHz–3 MHz) reduces neuromuscular stimulation compared with mains frequency.
Tissue effect depends on current density at the active electrode and waveform (continuous vs pulsed) → cutting vs coagulation.
- Smaller contact area → higher current density → more heating at that point.

Monopolar vs bipolar (clinical mental model)

Monopolar: current flows from active electrode → patient → return electrode (dispersive plate) → generator.
- Large return pad area keeps current density low at pad to avoid burns.
Bipolar: active and return are both on the instrument tips (e.g., forceps) → current confined to tissue between tips.
- No separate patient return pad required (usually).

Immediate safety behaviours in theatre

Before activation: confirm correct mode/power, dry field where possible, avoid pooling prep solutions, and ensure correct placement/contact of return pad (monopolar).
- Avoid pad over scar tissue, bony prominences, metal implants close to pad edge, or poorly perfused areas.
During use: use lowest effective power, short bursts, avoid activation in air, keep cables intact/uncoiled where possible, and keep active electrode in holster when not in use.
- Be alert to unexpected patient movement (inadequate anaesthesia/analgesia) vs true electrical stimulation (rare at HF).
High-risk contexts: pacemakers/ICDs, oxygen-enriched fields (airway surgery), laparoscopic surgery (capacitive coupling/insulation failure), and patients with poor skin integrity.
- Prefer bipolar where feasible; if monopolar required, keep current path away from device and use short bursts.

Monopolar diathermy: circuit, advantages, limitations

Circuit: generator → active electrode → patient tissues → dispersive return pad → generator.
Advantages: versatile (cut + coag), effective for large fields and deep dissection; common in open and laparoscopic surgery.
Limitations: requires return pad; greater risk of unintended burns (alternate site burns, pad burns, coupling phenomena), and more electromagnetic interference with implanted devices.

Bipolar diathermy: circuit, advantages, limitations

Circuit: generator → one tip → tissue between tips → other tip → generator (localized current path).
Advantages: reduced risk of remote burns; less EMI to pacemakers/ICDs; useful in neurosurgery, ENT, ophthalmic and delicate work; safer near flammable gases/oxygen-enriched fields (still not risk-free).
Limitations: generally less effective for rapid cutting; can cause tissue sticking/charring; still possible thermal injury from hot tips and lateral thermal spread.

Waveforms and tissue effects (cut vs coag)

Cut: typically more continuous waveform → rapid heating → cell vaporisation and tissue division; less haemostasis.
Coag: intermittent/pulsed waveform with higher peak voltage → slower heating, protein denaturation, vessel sealing; more charring if excessive.
Blend modes vary duty cycle/peak voltage to balance cutting and haemostasis.

Return electrode (monopolar): physics and safe placement

Purpose: provide low-impedance path back to generator with low current density at skin by using large area contact.
Pad burn mechanisms: poor contact (edge lift, hair, dried prep, wrinkles), small effective area, high power/prolonged activation, placement over scar/bone, or pad partially detached.
Modern systems may use return electrode monitoring (REM) to detect rising impedance/poor contact and alarm/disable output.

Unintended burns: mechanisms you must be able to explain

Alternate-site burn: return current finds an unintended pathway to earth/return (e.g., ECG electrode, metal table contact) due to poor pad contact or high impedance at pad.
Direct coupling: active electrode unintentionally contacts another metal instrument (e.g., laparoscopic grasper) transferring current to unintended tissue.
Capacitive coupling: current transfers through intact insulation via capacitance (common in laparoscopy with long insulated instruments, metal trocars, high voltage coag mode).
- Risk increased by high peak voltage, long activation times, and small capacitance barriers; can cause unseen internal burns.
Insulation failure: microcracks in laparoscopic instrument insulation allow current leakage to adjacent tissue.
Thermal injury: hot instrument tips or residual heat causes contact burns even without current flow.

Interactions with implanted cardiac devices (pacemakers/ICDs)

Risks: oversensing → inhibition of pacing; inappropriate ICD shock; device reset; lead heating (rare).
Risk reduction: prefer bipolar; if monopolar needed use short bursts, lowest effective power, place return pad so current path does not cross device/leads (e.g., pad on thigh for upper body surgery), keep diathermy cables away from device.
Perioperative planning: identify device type/dependence, consider magnet/reprogramming per local policy, ensure external pacing/defib available, continuous ECG and pulse monitoring (pleth).

Fire and explosion risk (airway and oxygen-enriched fields)

Fire triangle: ignition source (diathermy/laser), oxidiser (O2/N2O), fuel (drapes, alcohol prep, ETT).
High-risk: airway surgery, head/neck with open O2, bowel with flammable gases (rarely relevant clinically), alcohol-based skin prep not dried.
Mitigation: minimise FiO2, avoid N2O, allow prep to dry, use cuffed tube, consider wet swabs around field, communicate before activation.

Electrical safety and monitoring artefact

Diathermy causes ECG/SpO2 artefact; rely on pleth/pulse palpation/arterial line waveform during activation.
Ensure correct placement of ECG electrodes away from operative field and not in direct path between active site and return pad (monopolar).

Explain the difference between monopolar and bipolar diathermy in terms of circuit and current pathway.

Focus on where current flows and why burns occur at specific sites.

Monopolar: generator → active electrode → patient → dispersive return pad → generator; current traverses a large part of the patient.
Bipolar: generator → one forceps tip → tissue between tips → other tip → generator; current confined locally, no separate return pad.
Clinical implication: monopolar has higher risk of remote/alternate-site burns and EMI; bipolar reduces these but still has thermal injury risk at tips.

Why does diathermy use high-frequency current? What would happen at low frequency?

Link frequency to neuromuscular stimulation and arrhythmia risk.

High frequency (hundreds of kHz to MHz) reduces stimulation of nerves and muscle compared with 50 Hz mains.
At low frequency, current can cause painful muscle contraction, interference with cardiac conduction, and potentially VF at sufficient current.
Despite HF, burns still occur because heating depends on I²R and current density, not on frequency alone.

A patient sustains a burn under the return electrode. Explain the mechanism and how you would prevent it.

This is a common viva: show you understand current density and contact area.

Mechanism: poor pad contact reduces effective area → increased current density at pad → heating and skin burn; factors include edge lift, hair, wet/dried prep, wrinkles, placement over scar/bone.
Prevention: place pad on well-perfused, muscular area; ensure full contact on clean, dry skin; avoid bony prominences/scars; use lowest effective power and short bursts; use REM-equipped systems where available.

Describe alternate-site burns with monopolar diathermy and give examples.

Emphasise unintended return pathways.

If the intended return pad pathway has high impedance (poor contact), current may return via another contact point with lower impedance, producing a small-area high current density burn.
Examples: burn at ECG electrode, metal cannula hub touching skin, patient touching metal table/arm board, wet drapes creating conductive pathway.
Prevention: ensure good pad contact; isolate patient from metal; keep monitoring electrodes away from current path; avoid pooled fluids.

In laparoscopy, explain capacitive coupling and insulation failure. Why are injuries sometimes not seen?

A frequent FRCA theme: unseen internal burns.

Capacitive coupling: HF current transfers through intact insulation via capacitance to adjacent conductive material (e.g., metal trocar) then to tissue, especially with high-voltage coag modes.
Insulation failure: cracks/defects in instrument insulation allow direct leakage of current to tissue outside the visual field.
Injuries may be unseen because they occur outside the camera view or on hidden surfaces; may present post-op as perforation/bleeding.

Compare cutting vs coagulation settings: what changes electrically and what changes in tissue?

Talk waveform/duty cycle/peak voltage and tissue effect.

Cut: more continuous waveform (higher duty cycle) → rapid heating → vaporisation → tissue division; typically lower peak voltage than coag.
Coag: intermittent/pulsed waveform → slower heating, protein denaturation and haemostasis; higher peak voltage increases risk of arcing/coupling.
Blend: intermediate duty cycle/voltage to balance cut and haemostasis.

How does bipolar diathermy reduce risk in patients with pacemakers/ICDs? What precautions remain necessary?

Show you understand EMI and current path.

Bipolar confines current to tissue between tips → less stray current and less electromagnetic interference reaching device/leads.
Precautions: still use lowest effective power/short bursts; keep cables away from device; ensure appropriate device plan (magnet/reprogramming) and external pacing/defib available; monitor ECG and pulse/pleth.

Where would you place the return electrode for surgery above the umbilicus in a patient with a left pectoral pacemaker, and why?

Answer should describe avoiding current path across the generator/leads.

Place return pad on a lower limb (e.g., right thigh) so the current pathway is kept away from the left pectoral device and leads.
Aim for the shortest practical path between active site and pad that does not traverse the device; avoid placing pad on torso near pacemaker.

Explain the difference between electrocautery and diathermy (electrosurgery).

This is commonly asked and often muddled.

Electrosurgery/diathermy: electrical current passes through tissue → heat generated within tissue (I²R).
Electrocautery: instrument tip is heated electrically but no current passes through the patient (e.g., battery cautery).

List the main causes of operating theatre fires related to diathermy and how you would reduce risk during airway surgery.

Expect mention of oxidiser control and alcohol prep.

Causes: diathermy ignition + oxygen-enriched atmosphere (high FiO2/N2O) + fuel (drapes, alcohol prep, ETT).
Risk reduction: minimise FiO2, avoid N2O, allow alcohol prep to dry fully, use cuffed tube, communicate before activation, consider wet swabs around field and suction to scavenge O2.

A surgeon complains the diathermy is ‘not working’. Give an anaesthetist’s systematic check (monopolar).

Practical viva: show safe troubleshooting without delaying critical care.

Check patient safety first: no burns/smoke/fire risk; oxygen-enriched field considerations.
Confirm settings: correct mode (cut/coag), power, foot pedal/hand switch functioning, correct socket selection.
Check circuit: return pad attached, good contact, cable intact, REM alarm status; active electrode tip clean and correctly connected.
Check environment: excessive fluid pooling, instrument insulation damage, poor tissue contact, or surgeon using in air.