3-Safety_ch14.pdf

Medical Instrument Electrical Safety Significance of safety 10s of thousands device related patient injuries in U.S every year. Even a single harmful event can lead to significant damage in terms of reputation and legal action. Different level of protection...

Medical Instrument Electrical Safety Significance of safety 10s of thousands device related patient injuries in U.S every year. Even a single harmful event can lead to significant damage in terms of reputation and legal action. Different level of protection required as compared to household equipment. Minimum performance standards introduced in 1980s –relatively new practice. Physiological Effects of Electricity Physical effect vs. current level Experiments from 160lb human with 60Hz current 911! ouch woah current level ECE 445: Biomedical Instrumentation Ch14 Safety. p. 2 ECE 445: Biomedical Instrumentation Ch14 Safety. p. 1  Susceptibility Parameters Mean “threshold of perception” 1.1mA for men 0.7mA for women Minimum threshold of perception 500 µA 80 µA with gel electrodes (reduces skin impedance)  Mean “let-go current”  let-go current = max current where you can still release your grip  16.5 mA for men 10.5 mA for women Let-go current vs. frequency Minimal let-go current occurs at commercialpower-line frequencies of 50-60 Hz ECE 445: Biomedical Instrumentation Ch14 Safety. p. 3  Macro vs. Micro Shock Macroshock externally applied current spreads through the body so less concentrated Microshock applied current is concentrated at an invasive point accepted safety limit is only 10 µA generally only dangerous if current flows through the heart Macroshock Hazards Most probable cause of death due to macroshock ventricular fibrillation Factors skin/body resistance design of electrical equipment Skin and body resistance dry skin has high resistance (~15k-1M ohm) limits current through body wet/broken skin has low resistance (~1% that of dry skin) internal body resistance ~ 200 ohm for each limb ~100 ohm for trunk of body resistance between two limbs = ~500 ohm procedures that bypass skin resistance can be dangerous example: gel electrodes, surgery, oral/rectal thermometers ECE 445: Biomedical Instrumentation Ch14 Safety. p. 5  Microshock Hazards Main causes leakage currents in line-operated equipment undesired currents through insolated conductors at different potentials differences in voltage between grounded conductive surfaces Conductive Paths Direct connection to an internal organ (during measurement or surgery) makes patients susceptible to mircoshock External electrodes of temporary car diac pacema kers Electrodes for intracardiac measuring devices Liquid filled catheters placed in the heart liquid filled catheters have much greater resistance than electrodes Worst !danger! currents flowing through the heart Electrode current densit y experiments suggest smaller electrode are more dangerous ECE 445: Biomedical Instrumentation Ch14 Safety. p. 8  Leakage currents if ground is broken → current flows through patient  if low resistance ground is available → no problem  Power Distribution Electrical power system in Healthcare Facility must control available power (fuse/breaker to set max current) must provide good ground Patient’s Electrical Environment -Grounding NEC code: max potential between two surfaces general care areas: 500mV under normal operation  critical care areas: 40mV under normal operation Isolated Power Systems Ground fault short circuit between hot conductor and ground injects large current into grounding system can create hazardous potentials on grounded surfaces Isolation transformer isolates conductors against ground faults  may include ground fault monitor/alarm ECE 445: Biomedical Instrumentation Ch14 Safety. p. 9  Electrical Isolation Isolation amplifiers  devices that break ohmic continuity of electric signals between input and output of the amplifier different supply voltage sources and different grounds on each side of the barrier Barrier isolation transformer, optical or capacitive isolation no current across barrier Implants  p pro er insulation required to prevent microshocks ECE 445: Biomedical Instrumentation Ch14 Safety. p. 11