Microshock vs macroshock

Clinical relevance in anaesthesia

• Key concept: the risk depends on whether current passes through the myocardium and how it gets there.
  • Macroshock: current enters via intact skin/body surface and may traverse the thorax.
  • Microshock: tiny currents delivered directly to the heart via an intravascular/intracardiac conductor (e.g., pacing wire, PA catheter).
• Where you see microshock risk: patients with invasive conductive pathways to the heart in ICU/theatre.
  • Temporary pacing wires, intracardiac EP catheters, central venous/PA catheters (especially if conductive path to myocardium), arterial lines connected to transducers, ECMO/CRRT circuits (multiple devices/earth references).
• Where you see macroshock risk: faulty mains-powered equipment, damaged insulation, wet skin, poor earthing, patient contacting live conductor.
  • Operating table/diathermy return plate issues, fluid spills, extension leads, non-medical devices brought into theatre/ICU.

Immediate practical prevention (what you actually do)

• Assume microshock risk if any intracardiac/intravascular conductor is present; minimise leakage currents and earth loops.
  • Use medical-grade equipment with intact insulation; avoid daisy-chaining extension leads; keep connectors dry; ensure correct equipment checks.
  • Prefer isolated patient connections where applicable; ensure all equipment connected to the same equipotential earth in critical care areas.
• If a shock is suspected: stop the source safely, call for help, treat arrhythmia/ALS, and quarantine equipment for investigation.
  • Do not touch patient and suspected live equipment simultaneously; isolate power first if safe (switch off/unplug).

Definitions

• Macroshock: electric current applied across intact body surface producing systemic effects; hazard is mainly ventricular fibrillation (VF) when current traverses the heart.
• Microshock: very small currents applied directly to the myocardium via an invasive conductor; can cause VF at currents far below perception thresholds.

Why microshock is more dangerous (physics/physiology)

• Skin is a major resistor; with macroshock much of the applied voltage is dropped across skin, limiting current reaching the heart.
  • Dry intact skin resistance can be high (kΩ range); wet/broken skin markedly reduces resistance and increases current for a given voltage.
• Microshock bypasses skin resistance: the conductor delivers current to blood/myocardium (low resistance), so tiny voltages can drive sufficient current to trigger VF.
• VF risk relates to current density through myocardium and timing within the cardiac cycle (vulnerable period around the T wave).

Typical current thresholds (order-of-magnitude figures used in FRCA)

• Macroshock (50–60 Hz AC, hand-to-hand/hand-to-foot): perception ~1 mA; pain ~5 mA; ‘let-go’ ~10–20 mA; respiratory tetany/paresis ~20–30 mA; VF risk typically ≥50–100 mA depending on path/duration.
  • Lower thresholds if current path includes heart (hand-to-hand), prolonged contact, wet skin, or reduced body resistance.
• Microshock: VF can occur with currents in the tens of microamps delivered directly to myocardium (≈10–100 µA; commonly quoted ~50 µA).
  • This is ~1000× smaller than macroshock VF currents (mA vs µA).
• DC vs AC: 50 Hz AC is more arrhythmogenic than DC at comparable currents; DC tends to cause a single muscle contraction and may ‘throw’ the victim clear, but can still be lethal at high energy.

Leakage current and equipment safety concepts

• Leakage current: unintended current that flows from mains parts to accessible conductive parts/earth due to capacitance, insulation imperfections, filters, or faults.
• Microshock protection aims to limit patient-applied leakage currents to very low levels (µA range) for applied parts connected to the heart (Type CF).
  • Type BF: body floating (not intended for direct cardiac connection); Type CF: cardiac floating (highest protection).
• Equipotential earthing reduces potential differences between simultaneously touchable metalwork, reducing macroshock risk and reducing earth-loop currents.
• Isolation transformers (isolated power systems) reduce risk of macroshock from a single fault by removing a direct reference to earth; they do not eliminate shock risk (a second fault can be dangerous).

Clinical scenarios to mention in viva

• ICU patient with PA catheter + multiple infusion pumps + monitor + warming blanket: risk of earth loops and cumulative leakage currents.
• Temporary pacing wires post-cardiac surgery: treat as high microshock risk; ensure only appropriately rated equipment connections and intact insulation.
• Wet patient/diathermy: macroshock risk increases with reduced skin resistance; also increased burn risk at contact points.