Microshock
If an aberrant current were to occur in a faulty device connected to an intra-cardiac catheter, e.g. CVP line, external pacemaker, there is a risk that the entire current could enter the myocardium through a small area, thus generating a high current density (Fig 1).
Under these circumstances, a current as low as 0.1 mA may be sufficient to trigger VF. This effect, called microshock, may be evident even at low voltages.
A major cause of microshock is a leakage current, i.e. a small current passing between circuits or to earth because of incomplete insulation and which would normally be harmless via surface contact. Such currents may flow in the equipment casing and in earth and patient connections. They are mainly due to capacitance in the supply lead and mains transformer causing a conductor to be raised to a potential above that of earth. In this situation, the earth connection becomes a hazard by providing a return pathway for leakage currents.
Any electrical device to be connected to an intra-cardiac catheter has to meet the highest British Standards classification (Table 1), i.e. Type CF, for electrical isolation, incorporating a floating circuit.
British Standards classification for isolation
All medical equipment is subjected to stringent tests to assess leakage currents before acceptance for use, and is given a British Standards classification. There are three classifications for allowable leakage currents in medical equipment:
| Type CF |
The most stringent standard for equipment with direct cardiac connections. It has a floating circuit. |
|---|---|
| Type BF |
For equipment with no direct cardiac connection, e.g. ECG leads. It also has a floating circuit. |
| Type B |
The least stringent classification. It is used for devices with non-conducting connections to the patient. It is earthed and has no floating circuit. |
Floating circuit
A floating circuit, or isolated patient circuit, is usually achieved using a transformer (Fig 2) which generates an identical electrical source using electromagnetic induction, but breaks the continuous circuit between the mains supply and the patient.
Fig 2 A transformer