Electricity, diathermy & theatre safety

Overview & definition

The electrical safety in the operating theatre is the management of the risks from the electrical equipment and the surgical diathermy (the electrosurgery). The anaesthetist works surrounded by the mains-powered equipment (the monitors, the anaesthetic machine, the warming devices, the diathermy) in an environment that is wet (the fluids, the cleaning), conductive (the metal table, the metal instruments), and rich in the flammable agents and the high oxygen concentrations. The understanding of the electrical physics and the diathermy is an exam-critical topic and the foundation of the theatre safety. [1]

The electric shock — the macroshock and the microshock

The electric shock is the physiological effect of the current passing through the body. Two distinct risks are recognised:[3]

• The macroshock — the whole-body current received through the skin, from the contact with the live mains. The effect depends on the current magnitude: 1 milliampere is the perception threshold (a tingle); 5 mA is the maximum harmless current; 10 to 20 mA is the "let-go" threshold (the tetanic muscle contraction that prevents the release); 50 to 100 mA through the chest causes the ventricular fibrillation; and the higher currents cause the burns. The skin resistance (high when dry, low when wet) determines the current for a given voltage.
• The microshock — a very small current (as little as 10 to 100 microamperes) delivered DIRECTLY to the myocardium via a low-resistance pathway (a central venous catheter, a saline-filled monitoring line, a pacing wire). The microshock induces the ventricular fibrillation at currents far below the macroshock threshold because the current is concentrated on the heart with no skin resistance. The microshock risk is specific to the intensive care and the cardiac theatre.[3]

The protective measures — earthing, the LIM, the RCD

Several protective measures reduce the electric-shock risk:[3]

• The earthing — the metal casing of the equipment is connected to the earth; a fault that makes the casing live diverts the current to the earth rather than through the operator, blowing the fuse. The modern equipment uses the double-insulated (Class II) casings that need no earth.
• The line isolation monitor (the LIM) — the operating theatre is supplied through an isolated supply (an isolation transformer) that is not referenced to the earth. A single fault to the earth does not complete a circuit (no current flows through the patient), so the first fault is non-lethal; the LIM alarms, alerting the staff to fix the fault before a second fault creates a live circuit. This is why the theatre power is isolated.
• The residual current device (the RCD) — detects the imbalance between the live and the neutral currents (the leakage to the earth) and trips the circuit within milliseconds. The RCD protects against the macroshock but does not prevent the microshock (the leakage current is too small to trip it).
• The equipotential earthing — in the cardiac areas, all the conductive surfaces are bonded to a common earth point so that no potential difference can drive a current through the patient. [1]

The diathermy — the electrosurgical unit

The surgical diathermy (the electrosurgery) uses the high-frequency (about 300 kilohertz to 3 megahertz) alternating current to cut and to coagulate the tissue. The high frequency is chosen because the nerve and the muscle stimulation (and the shock) occur only below about 100 kilohertz; at the megahertz frequencies, the current passes through the tissue without the tetany or the cardiac stimulation, generating the heat by the resistive (Joule) heating of the tissue.[1][6]

The diathermy has two modalities:[1]

• The monopolar diathermy — the current passes from the active electrode (the surgical pen) through the patient's body to the large return plate (the dispersive electrode, the patient plate) on the thigh. The current density is high at the active electrode (the cutting or the coagulation at the surgical site) and low at the large plate (no burn). The current traverses the whole body between the two electrodes.
• The bipolar diathermy — the current passes between the two tips of the forceps (the active and the return in one instrument), through the small amount of the tissue grasped between them. The current does not traverse the body. The bipolar diathermy is safer for the patient with a pacemaker and for the surgery near the nerves and the delicate structures. [1]

The diathermy waveforms — the cutting and the coagulation

The electrosurgical generator produces different waveforms for the different effects:[1]

• The cut — a continuous, high-frequency, low-voltage sine wave. The rapid, sustained heating vaporises the tissue and explodes the cells, producing the clean cutting with the minimal coagulation. The current density is very high at the fine electrode.
• The coagulation — an intermittent (damped), higher-voltage pulse waveform. The pulses heat the tissue more slowly, desiccating and sealing the blood vessels (the coagulation) rather than vaporising. The coagulation cuts poorly but seals well.
• The blend — a mixture of the two (the bursts of the cut waveform), giving the combined cutting and coagulation. [1]

The waveform and the power setting are chosen for the tissue effect; the principle is that the current density (not the total power) determines the local effect.[1]

The patient-return plate and the burn risk

The safety of the monopolar diathermy depends entirely on the patient-return plate:[2]

• The plate must be large and well-applied to a well-perfused, hairless, clean area (the thigh, the buttock) over a good contact area. The large area keeps the current density low so no burn occurs at the plate.
• A poorly applied plate (the small contact, the detached edge, the bony prominence, the metal prosthetic implant underneath) concentrates the current at the remaining contact, causing a severe plate-site burn. The modern plates have the split-surface with the contact-quality monitor (the return electrode monitor) that alarms if the plate is detached.
• The metal implants, the body piercings, and the tattoos (some contain metal) can concentrate the current and burn. The plate should be placed on the same side of the body as the surgical site to keep the current path away from the pacemaker and the heart.[2][6]

The pacemaker and the implanted-device interaction

The diathermy can interfere with the cardiac pacemakers and the implantable cardioverter-defibrillators (the ICDs):[5]

• The monopolar current can be sensed by the device as the cardiac electrical activity (the electromagnetic interference), causing the inappropriate inhibition of the pacing, the false tachyarrhythmia detection, or the spurious shock.
• The current can traverse the device or the lead, causing the lead heating, the capture threshold change, or the direct induction of the ventricular fibrillation. The case reports document the diathermy-induced ventricular fibrillation.[5]
• The precautions: prefer the bipolar diathermy; if the monopolar is unavoidable, keep the current path (the active electrode and the plate) as far from the device as possible, use the short, intermittent bursts at the lowest effective power, and have the magnet and the external defibrillator available. The device should be checked and reprogrammed perioperatively by the device team.[5]

The surgical fire — the fire triangle

The surgical fire requires the three elements of the fire triangle:[4]

• The fuel — the alcohol-based skin preparation (the chlorhexidine, the iodine in the alcohol), the drapes, the gowns, the intestinal gas, the hair, the ointments, the patient's hair and the body.
• The oxidiser — the oxygen (the high inspired oxygen, the open delivery, the supplemental oxygen), the nitrous oxide (which supports the combustion), and the room air.
• The ignition source — the diathermy, the laser, the fiboptic light source, the high-speed burr, the defibrillator. [1]

The fire risk is highest around the head, the neck, and the airway (the "fire triangle" zone — the oxygen-enriched atmosphere, the alcohol prep, and the diathermy near the face). The prevention is the avoidance of the three elements together: let the alcohol prep dry completely before the draping and the diathermy; use the lowest oxygen compatible with the patient; use the air or the FiO2 below 30 per cent where the surgery allows; and keep the ignition sources away from the oxygen-rich areas.[4]

The airway fire

The airway fire is the most catastrophic form of the surgical fire, occurring during the surgery of the airway (the tracheostomy, the laser laryngeal surgery) where the oxygen-rich tracheal gas, the endotracheal tube (the fuel), and the laser or the diathermy (the ignition) coexist. The prevention: use the lowest FiO2 (ideally below 30 per cent and no nitrous oxide), use the laser-resistant tubes for the laser surgery, fill the cuff with the saline (not air), and wet the pledgets. If the airway fire occurs, the immediate management is to stop the gas flow (disconnect the oxygen), remove the burning tube, douse the fire with the saline, mask-ventilate with the room air, then re-intubate and assess the airway for the damage (the bronchoscopy).[4]

The surgical smoke and the plume

The diathermy and the laser vaporise the tissue and generate the surgical smoke (the plume), which contains the particulate matter, the toxic gases (the benzene, the hydrogen cyanide, the formaldehyde), and potentially the viable cellular material and the viruses (the HPV, the HIV). The chronic exposure of the theatre staff is an occupational hazard (the respiratory irritation, the possible carcinogenicity). The prevention is the local-exhaust evacuation (the smoke extractor at the source) and the filtration masks (the high-efficiency masks) for the high-plume procedures. The coronavirus pandemic heightened the awareness of the plume risk.[7][8]

The laser safety

The surgical laser (the CO2, the Nd:YAG, the argon) delivers the intense, coherent light that is used for the cutting, the vaporisation, and the photocoagulation. The laser is itself an ignition source (the laser airway fire) and a direct hazard to the eye (the retinal damage from the direct or the reflected beam) and the skin. The laser safety requires the controlled access (the warning signs, the closed doors), the eye protection (the wavelength-specific goggles for everyone in the room), the matte instruments (to avoid the reflection), and the wet drapes and the fire precautions. The CO2 laser is absorbed by the water and is precise on the surface; the Nd:YAG penetrates the deeper tissue.[4]

The electrical interference with the monitoring

The diathermy generates the electromagnetic interference that can disrupt the monitoring: the ECG artefact (the spurious waves that mimic the arrhythmia or the pacemaker spikes), the pulse oximeter interference, and the artefactual asystole on the monitor. The recognition of the diathermy artefact (it coincides with the activation) prevents the misdiagnosis. The modern monitors filter the interference, but the awareness is essential — never act on a monitor reading that changes with the diathermy activation without confirming it independently.[8]

The electrical burns — the differential

The electrical burns in the theatre have several mechanisms: the return-plate burn (the poor contact), the alternate-site burn (the current returning through an alternate pathway — the ECG electrode, the touch of the metal table — when the plate is poorly applied), the capacitive coupling (in the laparoscopic surgery, the current coupling to the metal cannula and burning the adjacent bowel), and the direct insulation failure (the broken active-electrode insulation burning the unintended tissue). The bipolar diathermy and the active-electrode monitoring reduce these risks. The careful plate application and the avoidance of the alternate return paths are the key preventions.[2][6]

[1] [1]

Red flags

[1] [1] [1] [1] [1]

References

1. [1]Vilos GA, et al. Electrosurgical generators and monopolar and bipolar electrosurgery J Minim Invasive Gynecol, 2013.PMID 23659748
2. [2]Link T. Guidelines in Practice: Electrosurgical Safety AORN J, 2021.PMID 34181252
3. [3]Burgess RC. Electrical safety Handb Clin Neurol, 2019.PMID 31277877
4. [4]Mortada H, et al. Preventing and Managing Operating Room Fires in Plastic Surgery: A Review of Incidence, Risk Factors, and Recommendations With Case Experiences J Burn Care Res, 2024.PMID 38158891
5. [5]Chacko S, et al. Diathermy-induced Ventricular Fibrillation J Innov Card Rhythm Manag, 2021.PMID 33654572
6. [6]Al Baalharith MM, et al. Understanding the safe application of electrosurgery: A cross sectional study of surgeons in KSA J Taibah Univ Med Sci, 2023.PMID 36818175
7. [7]Zhang S, et al. Prevalence of operating room occupational health cluster: a systematic review and meta-analysis Front Public Health, 2026.PMID 42273630
8. [8]Karuppal R. It is time for a more cautious approach to surgical diathermy, especially in COVID-19 outbreak: A schematic review J Orthop, 2020.PMID 32425415