Question 22

With reference to electrical safety in the ICU:

a) What is meant by the term: "Cardiac protected electrical area"? (35% marks)

b) What is meant by the term: "Microshock"? (35% marks)

c) What patient related factors in a critically ill patient theoretically increase susceptibility to microshock? (30% marks)

[Click here to toggle visibility of the answers]

College Answer


Cardiac protected electrical area:                                                                     

Power reticulation and devices are designed and constructed to minimise unequal electrical potentials between different devices, so that potential current flow between a device and a patient is limited to a defined level.  Class 1c (cardiac protection) ensures leakage currents do not exceed 50 microamps)



"Micro-shock" is a sub milliamp current applied directly or in very close proximity to the heart muscle of sufficient strength, frequency, and duration to cause disruption of normal cardiac function. There are no incontrovertibly demonstrated fatal cases, but proving causality is difficult.


Microshock Susceptibility:                                                                                                  

  • Invasive devices, (CVC, PAC, pacing wires) 
  • Altered fibrillation thresholds:
    • Electrolyte abnormality
    • Underlying heart disease


Electrical safety in the ICU is discussed at length elsewhere.


A "cardiac protected area" is an area where where microshock is likely, eg. areas where patients have intravascular devices such as central lines or IV infusions. These areas have the following features:

  • Equipotential earthing which ensures all equipment is earthed at the same low potential; usually identified by a sign in the room
  • Residual current devices  which detect small current leaks and break the circuit if the leak is detected
  • Line isolation monitors which monitor escess current and alarm when excess current is detected.

These areas contrast with "body protected electrical areas", where microshock is unlikely (eg. outpatient clinics, anywhere there is ECG monitoring)


A microshock is a small current delivered via electrodes directly into the body, bypassing the resistance of the poorly conductive skin. In the definitive-sounding words of Lester AH Critchley (Oh's Manual, Chapter 83, p. 844- Electrical Safety and Injuries):

"Microshock occurs when there is a direct current path to the heart muscle that bypasses the protective electrical resistance of the skin surface. Such a pathway may be provided by saline-filled arterial or venous-pressure-monitoring catheters or transvenous pacemaker wires. The current required to produce ventricular fibrillation in microshock settings is extremely small, in the order of 60 µA."

The key issue is that a lethal magnitude of current (1-2mA) is not cutaneously detectable by a normal person with dry skin, and yet it can kill an ICU patient if it happens to be conducted to their fluid giving set, for example.


Patient-related factors which increase susceptibility to microshock? As per Walter Olson (1978), "The following clinical devices make patients susceptible to microshock":

  1. Epicardial or endocardial electrodes of externalized temporary cardiac pacemakers
  2. Electrodes for intracardiac electrogram (EGM) measuring and stimulation devices
  3. Liquid-filled catheters placed in the heart to
    1. Measure blood pressure (eg. PA catheter)
    2. Withdraw blood samples
    3. Inject substances such as dye or drugs into the heart

To these, in the modern era we may add:

  1. Diathermy
  2. Continuous vacuum wound management systems
  3. MRI-induced current in implanted conductors

The college also mention altered fibrillation thresholds, which certainly make it more likely that one responds with fibrillation to any irritant, be it microshock, dobutamine infusion, low PICC line tip or a sudden loud noise.


O'HARA Jr, JEROME F., and THOMAS L. HIGGINS. "Total electrical power failure in a cardiothoracic intensive care unit." Critical care medicine 20.6 (1992): 840-845.


Olson, Walter H. "Electrical safety." Medical instrumentation. Boston: Houghton Mifflin Co (1978): 667-707.

Χριστοδούλου, Χριστόφορος. Recommendations and standards for building and testing an Intensive Care Unit (ICU) electrical installation. Diss. 2011.

Oh's Manual, Chapter 83   (pp. 844)  Electrical  safety  and  injuries by Lester  AH  Critchley.