Electrical safety Dr. S. Parthasarathy MD., DA., DNB, MD (Acu), Dip. Diab. DCA, Dip. Software statistics PhD (physiology) Mahatma Gandhi medical college and research institute, puducherry, India
Problems ?? Why we should know?? • Anaesthesiologist is the team leader He/She only can handle suction, OT lights, leads ECG monitor and cautery
Theatre contains equipments The operating theatre is unusual as there are numerous examples of the deliberate application of electrical equipment to the human body.
Electrical Hazards • Fires • Burns • Electrical shock • Explosions • Power failure
The basics • understand and apply electrical engineering • analyze and avoid equipment hazards, • and to apply equipment safety standards, • Clinical engineering – new field
Ohm’ s law • Voltage /Current = Resistance • Voltage / Resistance = Current • Units • Volts , ohms , amperes
Electrocution • The effects produced are dependent upon 4 factors: • (i) the amount of electricity that flows (current); • ii) where the current flows (current pathway) • (iii) the type of current (direct or alternating); • (iv) current duration.
Resistance • Dry skin = 1 mega ohm • Moist skin = 15 kiloohms • Electrode paste = 1,000 ohms • Needles and catheters = few hundred ohms
Current and effects • 1 mA --- Tingling sensation • 15 mA---- Muscle tetany, pain and asphyxia • 75 mA ----- Ventricular fibrillation • : Let go current: less than 10 mA !! • Macro shock !! – what is that ??
Macroshock and micro shock • Macroshock is the type of electrocution where electricity enters through the skin and flows through a substantial portion of the body, only a fraction actually going through the heart
Scenario is different • when monitoring catheters, pacing wires, and dye injection catheters first began to be utilized inside the heart or the coronary arteries.– two differences • Rare scenario • Micro amperes are enough • Microshock
Just that’s enough !! • 10 µA as the maximum leakage of current allowable through electrodes or catheters contacting the heart. • Micro shock !!
Grounding and un grounding • In electrical terminology, grounding is applied to two separate concepts. • The first is the grounding of electrical power, and the second is the grounding of electrical equipment. • (1) power can be grounded or ungrounded (2) power can supply electrical devices that are themselves grounded or ungrounded
Grounding and Ungrounding • Whereas electrical power is grounded in the home, it is usually ungrounded in the OR. • In the home, electrical equipment may be grounded or ungrounded, but it should always be grounded in the Operating Room.
OT is electrically dangerous • Numerous electronic devices, together with power cords and puddles of saline solutions on the floor, make the OR an electrically hazardous environment for both patients and personnel. • 40% of electrical accidents in hospitals occurred in the OR. • We don’t want macroshock ?? • Ungrounded power supplies
In the OR • We need ungrounded power supplies • Hence • Isolation transformers
We need isolation transformer- induction – no direct connection to the secondary coil induction
What is a transformer ?? • A transformer is constructed by placing two coils close to each other. • An AC source is applied to one coil, and a current is induced in the other.
From the outlet - External case is grounded but the internal circuitry of the ECG monitor is ungrounded
Internal arrangement of diathermy Isolation transformer Earthed case
Resistive and capacitative coupling • The body can act as a connection if it comes into contact with the source of electricity and the earth directly or by touching an earthed object such as drip stand. • The body can also form a connection between an electrical source and earth by acting as one plate of a capacitor • Eg. MRI scanner
Leakage currents • All AC-operated power systems and electrical devices manifest some degree of capacitance.. • Electrical power cords, wires, and electrical motors exhibit capacitive coupling to the ground wire and metal conduits and “leak” small amounts of current to ground. • This so-called leakage current partially ungrounds the isolated power system. This does not usually amount to more than a few 1 to 2 mA in an OR.
Line isolation monitor • The LIM continuously monitors the isolated power to ensure that it is indeed isolated from ground, and the device has a meter that displays a continuous indication of the integrity of the system
Line isolation monitor • The reading on the LIM meter does not mean that current is actually flowing; rather, it indicates how much current would flow in the event of a first fault. • The LIM is set to alarm at 2 or 5 mA, depending on the age and brand of the system
classification of equipment according to their means of protection. • Class 1 • Class 2 • Class 3
Class 1 Any conducting part of Class I equipment accessible to the user, such as the metal casing, is connected to earth by an earth wire. This wire becomes the third pin of the plug connecting the equipment to the mains socket. • Class I equipment should have fuses at the equipment end of the mains supply lead, in both the live and neutral conductors
Class 2 • Any accessible conducting parts of Class II equipment are protected from the live supply by either double or re-inforced insulation. • This should prevent any possibility of an accessible part becoming live and so an earth wire is not required.
Class 3 • Class III equipment provides protection against electric shock by using voltages no higher than safety extra low voltage • SELV is defined as a voltage not > 25 V AC or 60 V DC. • Battery operated
What is in store next ?? The above classes of equipment – relate to electrocution
Types for medical equipment • Type B • The equipment may be of Class I, II or III but the maximum • leakage current must not exceed 100 μA. It is therefore not suitable for direct connection to the heart. • Type BF • As for type B, but uses an isolated (or floating) circuit • Type CF • These provide the highest degree of protection, using isolated circuits and having a maximum leakage current of < 10 μA. • suitable for direct cardiac connection, • e.g. ECG leads, pressure transducers and thermodilution • computers.
Surgical diathermy • Surgical diathermy equipment uses the heating effects of high frequency (kHz–MHz) electrical current to coagulate and cut tissues. • There are two basic types – monopolar and bipolar.
Monopolar diathermy • generates electrical energy at 200 kHz to 6 MHz. • The energy is applied between two electrodes (neutral and active). • The neutral electrode has a large conductive surface area producing a low current density with no measurable heating effect • Active – resistance – heat – effect • Cutting diathermy employs a sine waveform whilst coagulation uses a modulated waveform.
Bipolar – circuit completes in forceps • Bipolar diathermy operates with a much lower power output. • The output is applied between the points of a pair of specially designed forceps producing high local current density. • No current passes throughout the rest of the body. • No neutral plate