Electrical Safety in the Operating Room

Introduction

1. Electrical Hazards - Macroshock vs. Microshock

• Sensory (tingling) threshold: 1 mA at 60 Hz
• "Let-go" threshold: ~10-20 mA
• Ventricular fibrillation: 100 mA and above
• Skin resistance normally limits current flow

• Occurs when a conductive pathway (pacing wire, saline-filled catheter, invasive monitoring line) bypasses skin resistance and delivers current directly to the myocardium
• As little as 100 µA (microamperes) can induce ventricular fibrillation when applied directly to the heart
• Maximum permissible leakage current for OR equipment: 10 µA
• Cardiac pacing wires, central venous catheters, and pulmonary artery catheters all create microshock pathways

2. Grounded vs. Isolated Power Systems

• One conductor of the supply is connected to earth ground
• A fault in equipment allows current to flow through the patient to ground, completing a circuit - this can cause electrocution
• Three-pronged plugs have a safety ground wire to divert leakage current away from the patient

• An isolation transformer isolates OR circuits from earth ground
• A single fault does NOT complete a circuit through the patient - no shock occurs
• Provides a critical extra layer of safety in the wet OR environment

3. Line Isolation Monitor (LIM)

• Continuously monitors the integrity of the isolated power system
• Detects the total hazard current (leakage) that would flow if a second fault or ground contact occurred
• Alarm activates when hazard current reaches ~2 mA (typically 5 mA threshold triggers alarm)
• An LIM alarm means the power supply has lost its isolated status - it is a warning, not an emergency
• The equipment that triggered the fault should be identified and unplugged; surgery may continue while troubleshooting

4. Electrosurgical Unit (ESU) / Surgical Diathermy

• Generates ultrahigh-frequency current (0.1-3 MHz) - this high frequency prevents ventricular fibrillation (unlike 50-60 Hz line power)
• Current flows from the active electrode (cautery tip, high current density) through the patient to the dispersal/return electrode (large surface area, low current density)
• Burns at ectopic sites occur if the return electrode is improperly placed, disconnected, or has inadequate contact - current then exits via ECG pads or metal table parts
• Precautions:
  • Proper return electrode placement (close to surgical field, away from bony prominences and prostheses)
  • Avoid all patient-to-ground contacts
  • Place return electrode close to surgical site and away from implanted cardiac devices (pacemaker, ICD) - current flow through the heart can cause device malfunction
• High ESU power levels (up to 400 W) can cause inductive coupling with monitor cables, causing electrical interference

5. Leakage Current and Equipment Safety

• All electrical equipment has leakage current due to capacitive coupling, induction, or defective insulation
• Normal leakage is imperceptible (<1 mA) and well below fibrillation threshold (100 mA externally)
• However, the same current bypassing skin and applied directly to the heart is dangerous (microshock threshold: 100 µA)
• Double insulation and safety ground wires reduce leakage
• OR circuits typically carry only 15-20 A - easily overloaded by multiple high-draw devices (fluid warmers, forced-air warmers, rapid infusers) on a single circuit

Summary Table

Key Exam Points

1. To receive a shock, the circuit must contact the person at two points with a voltage source between them
2. Microshock risk exists whenever a conductive catheter or wire terminates near the heart
3. LIM alarm = loss of isolation, NOT a call to stop surgery - identify and unplug the offending device
4. ESU burns at ectopic sites result from faulty or poorly placed return electrode
5. Isolated power + LIM together form the core OR electrical safety system