The concept of microshock derives from the fact that, if there is an intracorporeal contact on or close to the heart, currents as low as 10 A may provoke ventricular fibrillation. Intracorporeal electrodes include monitoring devices such as central venous catheters and pulmonary artery catheters, particularly if they are filled with a conductive solution like saline. There is also a risk of microshock in patients with external pacing wires and from temperature monitoring probes placed in the lower third of the esophagus, behind the left atrium.

Protection against electrocution

Any protective cut-out system built into electrical equipment for use in intensive care needs to be very sensitive to prevent electrocution. However, it must not be responsible for unnecessary cut-outs. The use of earth leakage circuit breakers, a common feature in domestic and industrial electrical equipment, is inappropriate in the ICU because low but variable levels of current may be permitted to pass through the patient. Instead, equipment design has concentrated on isolating the patient circuit from the mains supply.

International design standards are published jointly by the International Standards Organization (ISO) and the International Electrotechnical Commission (IEC).

IEC 601 Part 1 classifies equipment according to the means of protection it provides against electric shock. There are three classes for electrical equipment for medical use.

1. Class I equipment has any conducting parts accessible to the user, such as the metal casing, connected to earth. If a fault occurs in the equipment in which an inadvertent connection is made between the live supply and the casing, the circuit is completed to earth. A high current then flows, melting protective fuses and disconnecting the circuit. This system requires that the correct fuses are installed and the earth wire is properly connected.

2. Class II equipment is double insulated. All accessible parts are protected by two layers of insulation, or by reinforced insulation, so that there is no possibility of a person touching any conducting part that may become live through a fault. Therefore an earth wire is not required.

3. Class III equipment has no potentials exceeding 24 V a.c. or 50 V d.c. Normally, it is not possible for these potentials to produce an electric shock in healthy humans. However, there may still be a risk of microshock.

A further classification of equipment is based on the permissible leakage current. This relates to the use for which the equipment is designed. These type classifications include the following.

1. Type B may be Class I, II, or III, or battery powered, but the maximum leakage current to the patient must not exceed 100 A. Type B equipment is not suitable for direct connection to the heart even via a catheter.

2. In type BF equipment, the parts in contact with the patient are electrically isolated.

3. Type CF has a very high degree of isolation with a maximum leakage current of less than 10 A and therefore is suitable for direct connection to the heart or connection via a saline-filled catheter.

All ECG leads, pressure transducers, and cardiac output computers should be rated as type CF.

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