eLFH - Electrical Safety and Diathermy Flashcards
UK mains supply current type
Alternating current (AC)
UK mains supply frequency
50 Hz
UK mains supply voltage
Oscillates between +340 V and -340 V
Root mean square definition
Generates more meaningful ‘average’ voltage for sinusoidal waves
Especially when they oscillate around zero volts as the mean will = 0 V
Therefore the waveform is squared to make the negative values positive
Root mean square voltage (rms) of UK mains supply
240 V rms
Calculation for root mean square of sinusoidal waveforms
Ohm’s law
V = IR
Voltage = Current x Resistance
Equation for Power generated by a current flowing across a resistor (or a person)
P = I^2 x R
Power = Current squared x Resistance
3 ways that electricity can cause harm to patients
Electrocution
Burns
Interference with monitoring
Electrocution definition
Occurs when current passes along an unintended path, causing either tissue or electrophysiological abnormalities
Factors which impact the effects of current flow in electrocution
How much current flows (A)
Type of current (DC vs AC)
Frequency of current
Current pathway
Current density
Duration of current flow
Effect of current flowing at 1-5 mA
Tingling sensation
Effect of current flowing at 5-10 mA
Pain
Effect of current flowing at 15 mA
No-let-go threshold
Effect of current flowing at 50 mA
Respiratory arrest
Effect of current flowing at 100 mA
Ventricular fibrillation
Classification of electrical equipment - according to means by which it provides electrical safety
Class I
Class II
Class III
Class I electrical equipment definition
Accessible conductive parts are connected to earth by and earth wire which maintains the exposed metalwork at zero potential
Provides low resistance path for current to return to local electricity substation in the event of a fault
Class I electrical equipment - process in event of a fault
Live component touches earthed casing
Casing also becomes live
Current flows via all paths to earth proportional to their relative resistances
Very low earth resistance reduces current flowing through person if they touch the casing
Total current flow also increases causing fuse to blow and breaks the circuit
Class II electrical equipment definition
Protected by double or reinforced insulation / case
Why class II electrical equipment don’t require an earth wire
Minimal chance of person coming in contact with faulty live component, so earth wire not required
Class III electrical equipment definition
Powered internally by a battery or by SELV (safety extra low voltage)
Specifications for SELV (safety extra low voltage)
Voltage not greater than 25 V AC or 60 V DC
No earth connection (usually floating circuit)
Low risk of accidental contact with higher voltage
Macroshock definition
Current flow from intact skin to skin
Microshock definition
Skin is breached and currents are delivered directly to myocardium
Higher current densities are generated near the myocardium
Currents in microshocks that can cause dangerous dysrhythmias when delivered near the myocardium
100 microA
Current threshold below which microshock is unlikely to cause harm
44 microA
Factors which increase risk of microshock induced ventricular arrhythmias
Site of stimulation - ventricles more sensitive
Increased area of stimulation
Longer duration of current passage
Equipment which predisposes patients to microshock
Saline filled (electrically conductive) CVC
Pacing wires
Oesophageal doppler probes
Classification of electrical equipment - according to degree of protection: Definition
Designated by the type and quantified by its permissible leakage under Normal Conditions (NC) and Single Fault Conditions (SFC)
Classification of electrical equipment - according to degree of protection
Type B
Type BF
Type CF
Defibrillator-safe BF
Defibrillator-safe CF
Type B electrical equipment + logo
May be class I, II or III
Not generally suitable for direct patient connection
Type B electrical equipment maximum leakage currents
Type 1 equipment:
NC < 0.1 mA
SFC < 0.5 mA
Type 2 equipment:
NC < 0.1 mA
SFC < 0.1 mA
Type BF electrical equipment + logo
May be class I, II or II
Has an isolated (floating) circuit - therefore suitable for direct patient connection
Type BF electrical equipment maximum leakage currents
Type 1 equipment:
NC < 0.1 mA
SFC < 0.5 mA
Type 2 equipment:
NC < 0.1 mA
SFC < 0.1 mA
Type CF electrical equipment + logo
May be class I, II or III
High degree of protection against shock
Suitable for direct cardiac connection
Type CF electrical equipment maximum leakage currents
All equipment:
NC < 0.01 mA
SFC < 0.05 mA
Defibrillator-safe BF electrical equipment + logo
Same specification as for Type BF but is defibrillator safe
I.e. may be left in contact with patient during defibrillation
Defibrillator-safe CF electrical equipment + logo
Same specification as Type CF but is defibrillator safe
I.e. may be left in contact with patient during defibrillation
Floating circuit definition
Earth free circuit
Further means of protection against electrical shock
Equipment separated from the earthed mains supply by an isolating transformer - transfers power from substation by magnetic field
Line isolation monitor
Should be fitted to floating circuits to ensure it has not accidentally become earthed
Why aren’t floating circuits used in all electrical equipment everywhere
Expensive
Earth leakage circuit breakers (ELCBs)
Electromechanical devices which disconnect the power supply to faulty electrical equipment when current flows down earth wire
May be voltage or current operated
Typical rating of medical infusion pumps
Type CF
Suitable for direct connection to heart because they may be connected to heart via column of electrolyte solution
Diathermy mechanism
High frequency alternating current
Current passing through any conductor dissipates power causing a heating effect
The heating effect (H) is proportional to the square of the current, and inversely proportional to the area through which it passes
Heating effect is also proportional to current density
Current density definition
Amount of current flowing per unit area
Diathermy uses high frequency currents to minimise risk of inducing dangerous dysrhythmias
Types of diathermy
Monopolar
Bipolar
Monopolar diathermy mechanism
Small active electrode at site of surgery relative to the ground electrode
Circuit formed by active electrode, ground electrode and patient’s tissue
Same current flows through both active and ground electrodes - ground plate has much lower current density due to larger area
Monopolar diathermy power used
100 - 200 Watts
Bipolar diathermy mechanism
Uses 2 electrodes to create a local circuit
High current density applied between bipolar forceps
Little effect on nearby tissue
Bipolar diathermy power used
< 100 Watts
Advantages of bipolar diathermy
Lower power used
Lower chance of current travelling via alternate pathway compared to monopolar diathermy
Microscopic effects of diathermy
Coagulation - higher temps and current density
Desiccation - lower temps and current density
Macroscopic effects of diathermy
Vaporisation - causes cutting of tissues
Tissue destruction
Diathermy waveform which achieves cutting
Continuous higher frequency (~ 400 Hz)
Lower voltage (400 - 1000V)
Higher power (> 100 Watts)
Diathermy waveform which achieves coagulation
Interrupted / modulated current
Lower frequency (250 - 400 Hz)
Higher voltages (up to 9 kV)
Lower power (< 100 Watts)
Diathermy waveform which achieves blended cutting and coagulation
Blend of both
Electrical safety mechanism with monopolar diathermy
Use of isolating capacitor
Electrical safety mechanism with bipolar diathermy
Earth free circuit used
Which type of diathermy should be used in patients with pacemakers
Bipolar
