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 _____
