BT_SQ 1.7 Describe microshock and macroshock and the mechanisms for preventing these, with particular reference to ensuring the compatibility of medical procedure, treatment area, and medical equipment used.


The risk of microshock is now very low. In the past people used to connect central lines to ECG monitor to aid in positioning. I am aware of cases in which this led to shock.

The incident reported in Acta Anaesthesiol Scand 2006 May. 50:641-2, whilst it is macroshock rather than microshock, shows how we still can electrocute patients with invasive instruments.


Before you answer the first question, see if you can pick what is wrong with this photo.


The chapter in Miller I alluded to in the previous post states that no equipment should be present in the operating theatre if it doesn’t have an earth wire. How does this fit with your understanding of classes of electrical equipment?

  1. What is Class I, II and III equipment?
  2. How do manufacturers reduce the risk of shock from electrical devices such as an ECG?

The isolated front-end coupling, which you would have read about in answering the previous question, seems to have made microshock very rare. Some would argue that isolated power supplies are unnecessary. Personally I think they are of limited utility, but you will work in hospitals which have them, so you need to know their characteristics.

  1. What is an isolated power supply?
  2. How do you monitor the integrity of an isolated power supply?
  3. What is the purpose of the equipotential earthing system?

If you confuse a Line Isolation Monitor with an RCD in your viva, you will not score well.

One final point:

Machines are labelled “body protected” or “cardiac protected” based on the amount of leakage current that can occur under normal or fault conditions. A single fault condition would include something like a broken earth wire. You don’t need to know the exact values for the leakage currents, but you should know that this is the basis for the classification.




BT_SQ 1.7 Describe microshock and macroshock and the mechanisms for preventing these, with particular reference to ensuring the compatibility of medical procedure, treatment area, and medical equipment used.


Earth Wire 3

Electrical safety is a subject which many candidates seem to have trouble with.

The examiners consider it an important topic and ask it regularly.

Because of the environment you are practising in, you may underestimate the importance of electrical safety. In a hospital I worked at some time ago, there were several staff members who received electrical shocks before RCDs were introduced.

I gave a talk on the topic when I was a registrar, and dug up all sorts of reports of electrocutions. Anesthesiology 1973 Feb. 38:181-3 has an example of this kind of incident.

Rather than give True/False statements in this post, I will give a series of questions, which cover the important areas in the topic.

  1. Why is the domestic power supply “earthed”?
  2. What risks occur because of the “earthing” of the power supply?
  3. What is the earth wire connected to?
  4. If you touch a live wire, how is a circuit formed?
  5. What is an RCD and how does it protect against shock?
  6. What is the difference between an RCD and a circuit breaker?
  7. Why does shock from domestic power induce VF, whilst shock from a defibrillator terminates it?

The answer to the first few questions can be found in Magee & Tooley.

RCDs are explained well in Russell if you can find a copy.

The 2015 Miller Cap. 109 covers electrical safety, but I find the explanations somewhat unclear. It has a lot on isolated power supplies, and does have a diagram which you can extrapolate to answer question 4. They call RCDs Ground Fault Current Interrupters.

The previous edition of Miller has electrical safety in chapter 100. Both editions have a couple of dubious statements on earthing. More on that in a later post…

BT_SQ 1.19 Describe the principles of surgical lasers, their safe use and the potential hazards

TRUE/FALSE  CO2 lasers may cause retinal damage if protective eyewear is not worn

TRUE/FALSE  Nd-YAG lasers cause injury confined to the cornea if eye protection is not worn

TRUE/FALSE  A laser may ignite material under a drape without igniting the drape

TRUE/FALSE  “Laser” stands for light amplification by stimulated electron radiation

TRUE/FALSE  Laser hazards include atmospheric contamination


BT_SQ 1.18 Discuss the principles of surgical diathermy, its safe use and the potential hazards

T/F Diathermy units operate at frequencies of approximately 100Hz to minimise the risk of VF

T/F Unipolar diathermy is contra-indicated in patients with implanted pacemakers

T/F Bipolar diathermy cannot be used for cutting tissue

T/F It is ok to leave metal jewellery on a patient as long as a capacitative coupling ground electrode is used

T/F The neutral wire of the diathermy is earthed to reduce the risk of electric shock


Bonus Question:What is this device, and how does it work?Megadyne