ELECTRIC SHOCK
Macroshock
Macroshock refers to disturbances
of neural or muscular function, or both, caused by application of large voltages
or currents. Problems can arise if macroshocks occur near the heart, as in a fatal
case when a patient's ECG leads were accidentally attached to a power cord.[11]
However, macroshock can also cause injury if contact is made at locations remote
from the heart.
If someone's hand or arm accidentally touches a voltage terminal,
the response to electric current flowing through the arm will depend on the amplitude
and the frequency of the current. Sensation of a 60-cycle current occurs at approximately
300 µA, and pain occurs at approximately 1 mA. If the current exceeds a certain
value, called the let-go current, the person is unable
to release objects.[33]
The value of the let-go
current varies with frequency and among individuals. Figure
87-5
shows the range of average values as a function of AC frequency.
The 50- and 60-cycles/sec currents have the lowest values and are therefore the most
dangerous. The response of cardiac muscle to electroshock also depends on frequency.
Electric current is more important than voltage when issues of safety arise. Table
87-1
indicates physiologic responses to increasing total body currents.
Continuous, total-body currents that reach 0.1 to 2.5 A (approximately
10 times the let-go value) can cause ventricular fibrillation. The electric current
generated by one pulse of an implanted cardiac pacemaker ranges from 0.1 to 10 mA.
Only a small part of the total-body current needs to pass through the heart to affect
cardiac function. An electrosurgical unit generates so much power that it can induce
microshock currents when it is used near implanted pacemaker wires. However, thanks
to compliance with the safe grounding practices described earlier, the last convincing
report of ventricular fibrillation induced by electrosurgery occurred in 1968.[34]
Defibrillation is often based on the same principle—that
the skin is a good electrical insulator requiring large electrical fields at its
surface for the production of small electrical fields internally. A jolt of 400
watt-seconds
Figure 87-5
The response to electric current flowing through an arm
that accidentally touches a voltage terminal depends on the amplitude and the frequency
of the current. Sensation of a 60-cycle current occurs at approximately 300 µA,
and pain occurs at approximately 1 mA. If the current exceeds a certain value, called
the let-go current, the person is unable to release
objects. The value of the let-go current varies with frequency and among individuals.
RMS, root-mean-square average.
TABLE 87-1 -- Average effects on humans of 60-Hz currents applied at body surface and passing
through the trunk
|
Current (mA) for 1-Second Contact |
Effect |
|
1 |
Threshold of perception |
|
5 |
Accepted as maximum harmless current intensity |
|
10–20 |
"Let-go" value is exceeded; sustained muscle contraction is next. |
|
50–100 |
Pain; possible fainting, exhaustion, mechanical injury; heart
and respiratory function continue |
|
100–2500 |
Ventricular fibrillation; respiratory center intact |
|
≥6000 |
Sustained myocardial contraction followed by normal rhythm; temporary
respiratory paralysis; burns if the current density is high (usually if current >
100 mA/cm2
) |
|
Adapted from Bruner JMR: Hazards of electrical apparatus.
Anesthesiology 28:396, 1967. |
(or joules) during closed-chest cardiopulmonary resuscitation delivers the same amount
of energy to the body as that produced by a large-caliber handgun. (A 0.45-caliber
bullet weighs 250 grains and has a muzzle velocity of 860 ft/sec when fired. This
corresponds to an energy of approximately 540 watt-seconds.) Only a very small portion
of a defibrillator's energy dose is needed to depolarize the heart.
