ELECTRICAL SAFETY (final).pptxBY MOHAMED ANWER RIFKY

ELECTRICAL SAFETY IN ANAESTHESIA

1. Electrical Safety MAINS SUPPLY, SKIN IMPEDANCE AND EARTH A power station>> electricity at very high voltage to a substation >>voltage is reduced >>transformer. The current >>substation>>hospital along two wires (live and the neutral), the neutral wire is connected to earth at the substation. Mains electric sockets in the hospital (live , neutral and a third >>connected to earth at the hospital). Person touching the live wire in the hospital>>electrical circuit >>body, through the earth, and back to the substation( anaesthetist touching the live wire by a damaged cable )>> current passes through him and through his antistatic footwear to the earthed antistatic floor (figure). If this is only a small current of 1 m A >> tingling sensation on touching the live parts. The United Kingdom current is at 240 V potential (in North America and some other countries it is 110 V). Current>> anaesthetist depends on the impedance( skin and tissues may be ignored ).If the impedance of the footwear and the antistatic floor is 240 a >> the current is 1 mA >> Current = Potential/Impedance >> 240/24000 X1000 mA=1mA >> tingling>> (Antistatic shoes have a strong protective effect)>> recommended that the impedance of such shoes should low enough to permit safe dissipation of electrostatic charges, but high enough to give some protection against electric shock>> between 75 and 10 M when new. An anaesthetist (figure) is touching faulty apparatus ,wearing non-standard footwear, and is standing in a pool of saline( good conductor of electricity) in contact with an earthed water pipe. As the current increases over 1 mA to 24 mA >> unable to release the handle of the electricity >> chest for only a fraction of Of a second >> ectopic beats occur>> ventricular fibrillation( footwear is negligible as an impedance),and the skin impedance is the main component 10 >>few kilohms>> Current=240/10 000 x 1000 mA >> 24 mA , But depends on many factors>> hands are wet , if the surface area of his hand is large, and if there are needles or cannulae passing through the skin. If the electric current passes through the heart during early T wave of the ECG>> risk of ventricular fibrillation is increased .Alternating current of 50 Hz is more dangerous than high frequency current of 1 kHz or greater. Finally, ventricular fibrillation >> at a lower current in patients with myocardial disease or dysrhythmias . PROTECTION AGAINST ELECTRIC SHOCK T he international Electrotechnical Commission Standard 601.>>classification of equipment according to the means of protection (shock arising) into three classes: Class I Equipment Conducting parts that are accessible to the user (metal case) , are connected to an earth wire ( third wire ) connected to mains supply socket. If a fault (the live supply and the case of the equipment)>> circuit is complete ,so that a high current flows which melts a protecting fuse (or fuses )>>disconnects the circuit ,so 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 is brown for live, blue for neutral, and yellow and green for earth. Class II Equipment Called double-insulated equipment, all accessible parts are protected by two layers of insulation, or by reinforced insulation>>no possibility of touching any conducting part with fault. An earth wire is not required for class II equipment. Internally Powered Equipment The third class of equipment ( known as internally powered equipment)and has its own power source (batteries) >> risks of mains electricity are avoided, but risk of electric shock may still be present. .

2. SHOCK RISKS WITH EARTHED EQUIPMENT I n contact with earthed equipment of class I type>> Fig.>>lamp, which is earthed ( faulty live apparatus to earth), the only protection being the size of the patient's skin impedance. If the latter is 10 >>> 24 mA >> ventricular fibrillation. Space bankets (metal coated plastic) >>electrical hazard if they come into contact with earthed metal apparatus, and they can cause burn. Patient electrodes, used for example in ECG apparatus and diathermy, may provide a route to earth and so give an increased risk of electric shock. DIATHERMY RISKS A high frequency current of about 1 MHz causes burning at the active electrode where current density is high, with no burning at the neutral plate ( current density!!). Suppose the surgeon activates the diathermy when the active electrode is not in contact with the patient, or when there is poor contact at then neutral plate>>normal circuit is broken and current may follow through a different route , such as the point of contact of the patient with the metal operating table, metal equipment or ECG electrodes>> burns at these points of contact. Even in the absence of direct contact. Diathermy current can flow by capacitance linkage >> high frequency, capacitance links offer much lower impedance >> alternative routes for current flow. A thin layer of insulating drapes may not protect the patient from the risk that diathermy, as current could flow and cause burns. The isolating capacitor >>the lead from the patient plate is earthed but the isolating capacitor has a high impedance to the low frequency 50 Hz mains current, and so protects the patient from electrocution. Nevertheless, it has a low impedance to the 1 MHz diathermy current, so that all stray currents flow through the neutral plate and do not cause burns. N.B An alternate, low resistance route is induced by a capacitor. ISOLATED PATIENT CIRCUITS A lso known as an earth free circuit (The electric circuit formed by the diathermy output, the cutting or active electrode, the patient and neutral plate have no connection to earth, still a risk of burns if the diathermy is operated incorrectly. Spacing ECG electrodes and other electrodes must be away from the area where the diathermy is being used ,but an extra lead ( reference or neutral lead was used to reduce interference, as in earlier monitoring apparatus this lead was attached to the casing, and earthed .It increase s the risk of electrocution if the patient accidentally came into contact with a source of mains electricity. Burns at the ECG electrode ( faulty diathermy earth ) >> avoid ed by using an isolated or floating patient circuit >> a transformers which ensure electrical isolation of the circuits and, all leads are insulated from earth.(an additional factor in the classification of equipment ) In class II equipment with a floating circuit, the patient connected circuit itself is isolated and insulated from the rest of the equipment. Alternating current of mains electricity can produce small currents and potentials by inductive and capacitive coupling >>can give a danger known as microshock . N.B   An isolating transformer is used with two coils insulated from each other. T he total current flow through the heart itself is only a fraction of the 24 mA, as passing from the hand to the feet. Current density determines the occurrence of ventricular fibrillation. A faulty intracardiac catheter passing from an item of monitoring MICROSHOCK equipment into the heart >>catheter touches the wall of the heart, any electric current >> a very small area of the heart, so only 150 μΑ >>the same current density in a portion of the myocardium as that produced by 24 m A flowing from a hand to the feet. (known as microshock ).Risk of microshock from a potential even lower than 1 V (figure). The anaesthetist (fig.) too may act as an earthing point though the current flowing through the anaesthetist is too small to be noticed by him. Microshock is a risk in patients who are fitted with an intracardiac pacemake r with an external lead. A t emperature-monitoring probe placed in the lower third of the oesophagus behind the left atrium may be a source of microshock . is termed type CF >>( cardiac ,floating ) circuit.>>leakage current under 50 μΑ , even if it is operating with a single fault .Other medical monitoring equipment is termed type B, or type BF >> the maximum permitted being 500 μΑ under single fault conditions. Whenever new equipment is received into a hospital >> acceptance testing must be regularly . LEAKAGE CURRENT STANDARDS E lectromedical equipment is classified according to the maximum leakage currents permissible for particulara pplications .Equipment used with electrodes which may contact the heart directly

3. CARDIAC PACEMAKERS T wo types of pacemaker are used. Firstly, for temporary use over a few days inserted through the subclavian or neck veins. X-ray control is desirable, Pulses of potential are then applied through the electrode, the potential , usually under 4 V , the pulse duration being under 1 ms .The second form of pacemaker is for longer term use. A battery powered pacemaker >>pacing lead are embedded in the patient's myocardium, but then cardiac surgery is needed Modern pacemakers are normally designed to work in a demand mode >>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. electromagnetic fields from sources outside the patient can cause interference. The electrode of the pacemaker can act as an aerial and pick up signals from Outside (misinterpret them as QRS complexes), as electric motors, microwave ovens, and the antitheft devices found in shops . also diathermy , nerve stimulators and some monitoring equipment may carry risks. Magnetic resonance imaging can also interfere with the correct function of the pacemaker. Patients may experience dizziness and syncope when interference of this sort is present. Even the potentials from adjacent chest muscles can be picked up by the pacemaker and inhibit it. Many demand mode pace makers change to a fixed rate mode when excessive interference is present. S olutions used in hospitals are good conductors of electricity.In infusion pumps >>liquid is not allowed to enter inside it. >>serious malfunction , and can be hydrolyzed into an explosive mixture of hydrogen and oxygen by electric current. N.B A capacitor is two conductive plates separated by an insulator or ‘dielectric’. CONDUCTING FLUIDS POWER FAILURE M onitoring equipment which is battery operated, and ventilators which are powered from gas cylinders or a piped supply(Failure of the mains electricity supply ). EQUIPMENT CONTAINING MICROPROCESSORS M icroprocessors ( integrated electronic circuits performing data storage, computing and control functions ). Infusion pumps>>rates are set by pressing appropriate buttons, the entered values then being stored internally in electronic memory and used to control the infusion rate of the pump. severe electrical interference may alter the stored values (infuse at a different rate) >>Good circuit design for protection against such faults. A- HOW CAN YOU CALCULATE THE ELECTRICAL CURRENT PASSING THROUGH A PERSON ?AND WHAT ARE THE METHODS USED TO AVOID ELECTRICAL SHOCK ? B- WHAT CAN HAPPEN IF A FAULTY INTRACARDIAC CATHETER PASSING FROM AN ITEM OF MONITORING EQUIPMENT C- ( G.Resons )EQUIPMENTS TERMED TYPE B,OR TYPE BF HAVE A HEIGHER PERMITTED LEAKAGE CURRENT THAN FOR THE CARDIAC FLOATING TYPE ( CF ). WHY ARE THE BIRDS NOT FRIED?

4. - 1-In cables carrying DC, one cable is designated positive and the other negative, which is not the case with AC cables. connected to earth at the ‘star point’. - 2-Most modern diathermy machines, this plate is isolated from earth as far as mains current is concerned. - 3-The current international standard regulating electromedical equipment, IEC 60601-1, lays down quite specific testing regimens for electromedical equipment before use. -4-Electrochemical effects (electrolysis)occurs, when DC can cause chemical burns. - 5-Other tissues have diverse electrical resistances, which can be grouped as follows: Low: nerve, blood, mucous membrane, muscle Intermediate: dry skin High: tendon, fat, bone - 6-Current passing ‘horizontally’ from hand to hand, whereas heart muscle damage is more often associated with a ‘vertical’ current pathway.6 - 7-Alternating current (AC) electric shock: - is about three times more dangerous than DC, at similar current flows - produces continuous muscle contractions (tetany) at 40–110 Hz - induces grip and pull as flexor muscles are much stronger than extensor muscles. If a person were to be holding onto a faulty conductor, he would be unable to let go. This prolongs the duration of the effect of the current - induces local sweating, which reduces skin resistance -8-If the number of coils in the secondary is more than the primary, then it becomes a step-up transformer. An isolation transformer is especially designed to enhance the electrical isolation between the primary and secondary coils, and between both coils and any other conductor. Isolation transformers form a very important part of modern electrical circuits, the floating circuits. - 9-High frequencies cannot be economically transmitted over long distances -10-Neutral pole is also called (‘return’) . -11-The EMF generated in the secondary coil is less than the supply EMF, and it forms a step-down transformer.