Anesthesiology APPLIED Exam › Pharmacology, equipment and physics

Electrical safety, electrocautery and lasers

Pharmacology, equipment and physics·~5 min read· Physician-reviewed✓ Sourced · 4

Stem

A 67-year-old man, 84 kg, is having an open sigmoid colectomy under general anesthesia in a room with isolated power. A right internal jugular triple-lumen catheter was placed after induction, and the surgeon is using monopolar electrocautery with a dispersive pad on the left thigh. Forty minutes in, the circulating nurse plugs in a second forced-air warmer, and a few seconds later the line isolation monitor alarms, reading 6 mA.

Question

What does this alarm mean, and what will you do about it?

Ideal answer

HeadlineThe isolated power system has lost its isolation: a first fault, not a shock in progress. I unplug the warmer that just went in.

Unplug the new warmer now. The alarm sounded right after it went in; the threshold is 5 mA on newer monitors and 2 mA on older ones.¹
If the alarm clears, the warmer leaves service. If plugging it back in sets the alarm off again, it is tagged and goes to biomedical engineering;¹ the first warmer and warm fluids keep him warm.
If not, unplug one at a time, in reverse order. Nonessential first; the ventilator and monitors keep running.
Respect the central line. Current straight to the heart causes ventricular fibrillation at 50 to 100 microamps, far below what the monitor detects,¹ so hubs stay capped and dry and I never touch catheter and device together.

Breaking down the answer

Isolated power and the monitor — an isolation transformer makes the supply ungrounded, and the monitor continuously shows the current that could flow to ground.² One fault makes the room behave like grounded power, so a second fault could macroshock the patient or a staff member.
Macroshock numbers — 1 mA is perceived, 10–20 mA is the let-go threshold, and 50–100 mA through intact skin causes ventricular fibrillation.¹ That is why a 5 mA alarm is a warning, not an emergency.
Microshock is the real danger with a central line — the maximum allowed leakage current in the OR is 10 µA, and isolated power was never designed to prevent microshock.¹ ² Saline-filled catheters and pacing wires are the low-resistance path to the myocardium.
Why not a ground fault interrupter — a GFCI cuts power at a 5 mA difference,¹ safe for a hair dryer, unacceptable for a ventilator; the monitor warns without shutting anything off. ORs have counted as wet locations since 2012 unless a risk assessment says otherwise.¹
Monopolar cautery is a circuit through the patient — current leaves the active tip and returns through the dispersive pad, placed so the path avoids any implanted generator and its leads.³ A pad that lifts concentrates current in a small area and burns.
Lasers are ignition sources — in the airway, a tube resistant to that wavelength, a cuff filled with saline tinted with methylene blue, oxygen at the minimum that avoids hypoxia, and a few minutes' wait before firing.⁴
Eyes in a laser room — everyone wears eye protection matched to the laser, and the patient's eyes are covered.

Follow-up question

You unplug the new warmer and the alarm stops. Ten minutes later the surgeon complains the cautery "isn't cutting" and asks the nurse to turn the power up, and the nurse notices the edge of the dispersive pad lifting off the thigh. What do you do?

Follow-up answer

HeadlineA failing return electrode. I tell the room, "Hold the cautery; the pad is lifting, and turning it up will burn him under the edge."

A request for more power is the classic warning that current is not returning through the full pad area. Activation stops and the setting stays down. The nurse places a new pad on clean, dry skin over a well-vascularized muscle mass close to the field, away from bony prominences and metal, with the current path away from any implanted device.³ Cautery resumes only once the generator's contact alarm clears. Afterward I inspect the old pad site with the nurse, document, and file an event report so biomedical engineering checks the unit.

Pitfalls that cost points

Calling the alarm an electrocution in progress and stopping the operation.
Saying the monitor protects against microshock.

Rapid-fire

Q: What is the maximum leakage current allowed in the OR? A: 10 µA, well below the thresholds of the line isolation monitor and a GFCI.¹

Q: How close must cautery be to a pacemaker to cause trouble? A: Interference within 6 inches of the generator can inhibit pacing or trigger inappropriate tachycardia therapy.³

Q: How do you fill a laser tube cuff? A: With saline tinted with an indicator dye such as methylene blue, so a punctured cuff shows itself.⁴

Q: Why does radiofrequency cautery not stimulate muscle or the heart? A: At radiofrequency the current alternates too fast to depolarize nerve and muscle, so it heats tissue instead.

Q: What should the surgeon do before activating the laser? A: Give the anesthesiologist adequate notice, so the oxygen concentration can be reduced first.⁴

Numbers to know

Item	Value
LIM alarm	5 mA (newer) or 2 mA (older)¹
Macroshock perception / let-go	1 mA / 10–20 mA¹
Macroshock VF	50–100 mA¹
Microshock VF	50–100 µA¹
Maximum OR leakage	10 µA¹
GFCI trip	5 mA difference¹
CIED interference zone	Within 6 inches³

Sources

1.Electrical safety in the operating room (keyword) · OpenAnesthesia · openanesthesia.org
2.Current Questions in Patient Safety: Isolated Power Systems in the OR · APSF · apsf.orgLIM threshold up to 5 mA; isolated power never protected against microshock (as small as 10 µA).
3.Change of Pace: An Update on the Perioperative Management of Cardiovascular Implantable Electronic Devices (CIEDs) · APSF · apsf.orgDispersive electrode placement; EMI within 6 inches of the generator.
4.Practice Advisory for the Prevention and Management of Operating Room Fires · ASA, 2013 · asahq.orgAnesthesiology 2013;118(2); laser tube and oxidizer recommendations.

Facts on this page were checked against these sources on Oct 16, 2026. Guidelines change; confirm doses against current institutional protocols.