Chapter 14: Electrical Safety

microshock

usually results from leakage currents in equipment or differences in voltage between the grounded conductive surfaces due to large currents in the grounding systems

microschocks

very small currents

240 V

voltages greater than ______ can puncture the skin

ground

when a fault occurs between the hot conductor and the chassis, the current flows safely to ______ on the green conductor

only a small fraction of the total current flows through the heart

when current is applied at two points on the surface of the body

sustained myocardial contraction

when the current is high enough, the entire heart muscle contracts

the subject feels a tingling sensation

when the local current density is large enough to excite nerve endings in the skin

10 mA

widely accepted safety limit to prevent microshocks is

leakage current

microshocks generally result from

threshold of perception

minimal current that an individual can detect varies considerably among individuals and with measurement conditions

50 - 60 Hz

minimal let-go currents occur for commercial power-line frequencies of

6mA

minimum threshold for let-go current

brief high-current pulse defibrillator

normal rhythmic activity returns only if a _______ from a _______ is applied to depolarize all the cells of the heart muscle simultaneously

microshock

all the current applied through an intracardiac catheter flows through the heart

isolation amplifiers

devices that break the ohmic continuity of electric signals between the input and output of the amplifier

ventricular fibrillation

does not stop when the current that triggered it is removed

18-22 mA

during let go experiments, respiratory arrest has been observed at ____

current enters the body

electric stimulation of excitable tissue (nerve and muscle) resistive heating of tissue electrochemical burns and tissue damage for direct current and very high voltages

biopotential amplifiers

electrical isolation of patient leads to

sustained myocardial contraction

entire heart muscle contracts

to pull the muscle attachment away from the bone

excessive currents may stimulate muscular contractions that are strong enough to _____

macroshock

externally applied current spreads throughout the body

failures of insulation, shorted components, or mechanical failures that cause shorts

faults inside electric devices

macroshock hazards

faults inside electric devices power cords fluids wet conditions

electric power and the grounds in the patients' environment

first step in providing electrical safety is to control the availability of

1 s

for 60 hz current applied to the extremities, the fibrillation threshold increases sharply for shocks that last less than about _____

let go current

for higher levels of current, nerves, and muscles are vigorously stimulated, and pain and fatigue eventually result involuntary, withdrawals

grounded casing

help reduce macroshock

wet conditions

hemodialysis and physical therapy areas

underground cables

high voltage (4800 V) enters the building usually via

ventricular fibrillation

rapid, disorganized cardiac rhythm is called

ventricular fibrillation

rapid, disorganized heart rhythm caused by disruption of normal propagation of electrical activities in the heart

let go current

max current at which the subject can withdraw voluntarily

40 mV

maximal potentials permitted critical care areas, _____ under normal operation

500 mV

maximal potentials permitted general care areas, _____ under normal operation

cardiac repolarization T wave

a single electric stimulus pulse can induce ventricular fibrillation if it is delivered during the vulnerable period of ____ that corresponds to the ____ on the ECG

fluids

blood, urine, intravenous solutions, and even baby formulas - can conduct enough electricity to cause temporary short circuits if they are accidentally spilled into normally safe equipment

epicardial/endocardial electrodes of externalized temporary cardiac pacemakers electrodes for intracardiac electrogram liquid-filled catheters placed into the heart

clinical devices that make patients susceptible to microshock

microshock

current can flow either into or out of the electric connection to the heart

brain and other nervous tissue

lose all functional excitability when high currents pass through them

microshocks induce ventricular fibrillation

if a device provides a conductive path to the heart that is insulated except at the heart

300 beats per minute

if the cardiac electric activity is sufficiently disrupted, the heart rate can rise to _______

ventricular fibrillation

if the magnitude of the current is sufficient to excite only part of the heart muscle, then the normal propagation of electric activity in the heart muscle is disrupted

small

if two points are both on the same extremity, the risk of fibrillation is ____ even for high currents

grounded at a single patient-grounding point

in critical care areas, all exposed conductive surfaces in the vicinity of the patient must be

performance standards

international electrotechnical commision (IEC) 601-1 standard UL Standard ANSI/AAMI ES1-1993 standards

respiratory paralysis

involuntary contraction of respiratory muscle to bring about asphyxiation

ground fault

is a short circuit between the hot conductor and ground that injects large currents into the grounding system

ventricular fibrillation

is the major cause of death due to electric shock

unity-grain isolation stage

isolation amplifiers usually consist of an instrumentation amplifier at the input followed by a

macroshocks

large, externally applied currents

microshock hazards

line-operated equipment, differences in voltage between grounded conductive surfaces due large currents in the grounding system

threshold of perception

local current density is strong enough to excite nerve endings in the skin so that the subject feels a tingling sensation

burns

resistive heating causes ____ usually on the skin at the entry points, because skin resistance is high

let-go current

subject can voluntarily withdraw, before involuntary contractions of the muscles can prevent one from doing so

power cords

susceptible to strain and physical abuse, as are plugs and receptacles

supply-voltage sources and grounds on each side of the isolation barrier

the isolation amplifiers include different

1 - 6 A

the minimal currents for complete myocardial contraction range from ____

240 V

the secondary of a stepdown transformer develops

75-400 mA

the threshold for ventricular fibrillation for an average-sized human varies from _____