1) The document discusses electrical safety and risks related to mains electricity supply, skin impedance, and earth connections. 2) It describes how small leakage currents can cause microshock and ventricular fibrillation, particularly if a cardiac catheter is in contact with heart tissue. 3) Standards classify electromeical equipment according to maximum permissible leakage currents to prevent microshock, especially for equipment used near the heart.

1. Electrical Safety MOHAMED ANWER RIFKY
MAINS SUPPLY, SKIN IMPEDANCE AND EARTH CLASS ONE
1-High voltage to a substation >> voltage reduced by a transformer. 2-The current from substation to the hospital along two wires ;live and the neutral which is
connected to earth at the substation. 3-Mains electric sockets in the hospital provide connections to the live and neutral conductors which is connected to earth at
the hospital. 4-If a person touches the live wire (an anaesthetist) in the hospital, an electric circuit can be completed through his body, through the earth, and back
to the substation ( through his antistatic footwear to the earthed antistatic floor).If this is only a small current of 1 m A>>tingling sensation on touching the live parts
with no serious effects. 5-The mains electricity supply in the United Kingdom is at 240 V and 110 V in North America. The impedance of the skin and tissues may be
ignored as this is small compared to that of the antistatic footwear and floor. If the impedance of the footwear and the antistatic floor is 240 k ohm and the volt is
240 V >> Current = (Potential) 240/240 000x 1000 mA= 1mA (Antistatic shoes have a strong protective effect )>>It is recommended that the impedance of such
shoes should be between 75 k ohm and 10 M ohm>> (low enough to permit safe dissipation of electrostatic charges, but high enough to give some protection
against electric shock). 6-If the anaesthetist is touching faulty apparatus ,and not wearing non-standard footwear, and is standing in a pool of saline with contact
with an earthed water pipe and if the current increases is over 1 mA>>sensation of tingling and pain , stimulateing muscular contraction. But if it is 24 mA >>the
anaesthetist would be unable to release the handle of the electrically faulty apparatus. The current goes >> his body to earth through the footwear, the pool of
saline and the water pipe (through the chest for only a fraction of a second) before ectopie beats occur, and even ventricular fibrillation). 7-If impedance=10 K
ohm>> Current = (Potential) 240/(Impedance)10 000x 1000 mA= 24 mA. Skin impedance is not constant, however, but depends on many factors. If the
anaesthetist's hands are wet, and if the surface area of his hand touching the live equipment is large, the impedance may be much less than 10 k ohm.And even
lowered further if there are needles or cannulae passing through the skin. 8-The risk of VF increases if the electric current passes through the heart during the
repolarization of the muscle cells.Alternating current of 50 Hz is more dangerous than high frequency current of 1 kHz or greater. Finally, ventricular
fibrillation from electric shock can occur at a lower current in patients with myocardial disease or dysrhythmias.
PROTECTION AGAINST ELECTRIC SHOCK
Must meet the requirements of British Standard 5724: Safety of Medical Electrical Equipment, and the corresponding international standard
is International Electrotechnical Commission Standard 601.the classification of equipment according to the means of protection (three classes
for electromedical equipment).
Class I Equipment: Where the metal case of an instrument, are connected to an earth wire via the plug to the mains socket .If a fault occurs in
the equipment >>connection>>inadvertently between the live supply and the case of the equipment a circuit is completed >>a high current
flows which melts a protecting fuse >>disconnects the circuit .For this protective system to operate the earth wire must be connected correctly
and fuses must be present in the live and neutral wires. In the UK a fuse is also incorporated in the mains plug. The colour code in the cable
connecting the mains plug to the equipment is brown for live, blue for neutral, and yellow and green for earth. Class II Equipment:
Also, called double-insulated equipment, all accessible parts are protected by two layers of insulation, or by reinforcement !!.An earth wire is not
required for class II equipment. Internally Powered Equipment: The third class of equipment is known as internally powered equipment, such as
batteries. Although the risk of electric shock may still be present, the particular risks associated with the use of mains electricity are avoided.
SHOCK RISKS WITH EARTHED EQUIPMENT : The lamp (earthed) >>route through the patient from a faulty live apparatus (the only
protection being the size of the patient's skin impedance),but If (10 k ohm) >>ventricular fibrillation.Space blankets (metal coated plastic) used
to reduce hypothermia >>contact with earthed metal apparatus >>low resistance path to earth, so diathermy may >> burn.Patient electrodes
>>with diathermy>>provide a route to earth>> electric shock.. Because diathermy current has a high frequency, capacitance links offer much
lower impedance to its flow and such capacitances between the patient and earth or between the patient and the diathermy leads give
alternative routes for current flow. Earthed equipment Isolated or floating patient circuit

2. MICROSHOCK
1-Ventricular fibrillation>> the total current flow through the heart itself is only a fraction of the 24 mA .The fraction of the current
passing through the myocardium (current per unit area) determines whether ventricular fibrillation will occur.
2-If there is a faulty intracardiac catheter passing from an item of monitoring equipment into the heart itself >>catheter touches the
wall of the heart >>current flowing along the catheter >>very small area of the heart, thus a current of only 150 μΑ (same current
density in a portion of the myocardium as that produced by 24 m A >> the phenomenon of microshock.
3-The risk is greatest at low frequencies such as mains frequency and with direct current.
4-A small leakage current can flow through the catheter and through the heart in the earthed patient (even lower than 1 V). Feet
being in contact with a metal lamp (figure). 5-The anaesthetist too may act as an earthing point for the patient if he is in contact
simultaneously with the earthed casing of apparatus and with the patient >>faulty equipment travels via the cardiac catheter to give a risk of
microshock in the patient (anaesthetist recieves a too small current to be noticed by him). 6-Microshock is a risk in patients who are
fitted with an intracardiac pacemaker with an external lead. A temperature-monitoring probe placed in the lower third of the
oesophagus >> electricity to pass in close proximity to the heart.
LEAKAGE CURRENT STANDARDS PACE MAKER
1-Electromedical equipment is classified according to the maximum leakage currents permissible for particular applications.
2-Equipment used with electrodes which may contact the heart directly is termed type CF (cardiac use) and has a floating circuit. In this case,
the leakage current through its intracardiac connection must be under 50 μΑ.
3-Other medical monitoring equipment is termed type B, or type BF if it has a floating circuit. For these types of equipment, the maximum
permitted patient leakage current is higher than for CF equipment, being 500μΑ.
4-Equipment which is received into a hospital should !! (our duty ).
CARDIAC PACEMAKERS
1-Two types of pacemaker are used. Firstly, for temporary use over a few days, a special
pacemaker catheter can be inserted through the subclavian or neck veins
into the right ventricle (X-ray control ).Pulses of potential are then applied through the electrode, the potential being adjusted to the minimum needed, usually
(under 4 V, duration being under 1 ms).
2-The second form of pacemaker is for longer term use. A battery powered pacemaker and its pacing lead are embedded in the patient's tissues. Instead of an
endocardial pacing lead, electrodes may be embeded in the myocardium (cardiac surgery is needed) .
3-Modern pacemakers are normally designed to work in a demand mode. This means that the stimulating electrode has a second function; it senses
the electrical complexes from the atria and ventricles. The pacemaker then paces the heart only if the patient's heart rate is outside an acceptable
range. 4-A problem with demand mode pacemakers is that electromagnetic fields from sources outside the patient can cause interference
>>electrode of the pacemaker can act as an aerial and pick up signals, then misinterpret them as QRS complexes. Electric motors, microwave
,ovens, and the antitheft devices !! .
5-The diathermy, nerve stimulators and some monitoring equipment may carry risks. Magnetic resonance imaging equipment and physiotherapy equipment can also interfere with
the correct function of the pacemaker. In class II equipment with a floating circuit, not only is the power supply circuit insulated from all accessible parts but the patient connected circuit itself is
isolated and insulated from the rest of the equipment.The electric circuit is divided into two parts (An isolated part seperated from the mains by an electerical barrier) should a fault develop in the
mains part.