12.1 Electricity, Electrical Safety, Diathermy, Defibrillation, Implanted Devices and Fire

12.1 Electricity, Electrical Safety, Diathermy, Defibrillation, Implanted Devices and Fire

Why Electrical Safety Matters

Anaesthetised patients cannot withdraw from a painful stimulus, are connected to many devices, and may have conductive pathways to the heart such as central venous catheters or pacing wires. EDAIC questions test the effects of current, equipment classes, diathermy, defibrillation and fire.

Basic Electrical Principles

Mains supply in Europe is alternating current at about 230 V and 50 Hz. The live wire is at high potential relative to earth; the neutral is connected to earth at the substation. A person touching a live conductor while in contact with earth completes a circuit.

Effects of Electric Current on the Body

The effect depends on current magnitude, current density, frequency, duration and pathway.

• Microshock: When current enters directly through the heart (for example through a pacing wire or a saline-filled central catheter), current density at the myocardium is very high, and as little as 100-150 $\mu\text{A}$ can cause ventricular fibrillation.
• Frequency: Currents of 50-60 Hz are most likely to cause fibrillation. Above about 100 kHz, current does not depolarise excitable tissue but produces heat, which is the basis of diathermy.
• Burns depend on current density: small contact areas concentrate current.

Protection Against Electric Shock

Equipment Classes (Protection Against Mains Faults)

Types (Degree of Protection for Applied Parts)

Symbols: B is a solid figure, BF a figure in a box, CF a heart in a box; a defibrillator-proof part shows paddles beside the symbol.

Other Safety Measures

• Isolating transformers create a floating secondary circuit not referenced to earth, so touching one conductor does not complete a path to earth.
• Residual current devices compare live and neutral current and disconnect the supply within milliseconds if they differ by more than about 30 mA (or less in medical locations).
• Equipotential earthing connects all metal objects in the theatre to a common point so that no dangerous potential difference exists between them.
• Antistatic flooring and regular equipment testing reduce risk.

Surgical Diathermy

Diathermy passes high-frequency current (about 0.5-3 MHz) through tissue. Heat is generated where current density is greatest.

• Monopolar diathermy: The active electrode has a tiny contact area (high current density, cutting or coagulation); current returns through a large return (neutral) electrode with low current density. If the return electrode is partly detached, the current density rises at the remaining contact and can cause a burn; modern units monitor return electrode contact quality.
• Bipolar diathermy: Current passes only between the two tips of the forceps, so little current flows through the rest of the body. It is used for surgery on fingers, the penis and neurosurgery, and in patients with implanted cardiac devices.
• Cutting uses a continuous sine wave; coagulation uses interrupted bursts with higher peak voltages.
• Hazards: burns at the return electrode or at alternative earth contacts (ECG electrodes, metal on the table), interference with monitors and implanted devices, and ignition of flammable material or gases.
• The return electrode should be placed over well-perfused muscle, close to the operative site, and away from metal implants, with the current path not crossing an implanted cardiac device.

Defibrillation

A defibrillator stores energy in a capacitor and discharges it through the chest as a brief pulse. Modern devices deliver a biphasic waveform (current flows in one direction then reverses), which terminates fibrillation at lower energies (typically 120-200 J) than older monophasic devices (360 J).

• Transthoracic impedance (about 50-150 $\Omega$) is reduced by adhesive pads, firm contact, and delivering the shock at end-expiration.
• An inductor in older devices shaped the waveform; modern devices compensate for measured impedance.
• Synchronised cardioversion delivers the shock on the R wave to avoid the vulnerable period of the T wave, which could otherwise induce ventricular fibrillation.
• Safety: remove oxygen sources from the immediate area during shocks; ensure nobody is in contact with the patient or bed.

Cardiac Implantable Electronic Devices

Pacemakers and implantable cardioverter-defibrillators (ICDs) can be affected by electromagnetic interference, especially from monopolar diathermy above the umbilicus.

Before surgery, identify the device type, indication, pacing dependence and date of last check. Application of a magnet usually switches a pacemaker to asynchronous pacing and suspends ICD tachycardia detection without changing pacing, but responses vary by manufacturer. After surgery, the device should be checked if it was reprogrammed or exposed to significant interference.

Fires in the Operating Theatre

FIRE TRIANGLE
   OXIDISER (oxygen, nitrous oxide)  -- controlled by the anaesthetist
   FUEL (drapes, alcohol skin prep, hair, tracheal tubes, gauze, bowel gas)
   IGNITION (diathermy, laser, fibreoptic light sources, defibrillator)

Prevention

• Allow alcohol-based skin preparations to dry fully (typically at least 3 minutes) and avoid pooling.
• Use the lowest oxygen concentration that maintains adequate saturation, ideally below 30%, during head, neck and upper chest surgery with open oxygen delivery; avoid nitrous oxide, which also supports combustion.
• Use laser-resistant tracheal tubes for airway laser surgery and fill the cuff with saline (sometimes coloured with methylene blue).
• Communicate with the surgeon before diathermy near an oxygen-enriched field.

Airway Fire Response

1. Stop ventilation and remove the tracheal tube.
2. Stop all gas flow; disconnect oxygen.
3. Remove flammable material and pour saline into the airway.
4. Re-establish ventilation with air (avoid supplemental oxygen briefly if possible), then re-intubate.
5. Examine the airway by bronchoscopy for injury and debris.

Combustion of polyvinyl chloride tubes produces toxic gases, including hydrogen chloride, which injure the airway.