Microshock
The passage of even small amounts of electric current through
the body can interfere with normal functioning of muscles and nerves. Microshock
refers to the direct application of very small voltages or currents to the heart,
often intentionally, by means of external or internal cardiac pacemaker electrodes.
Inadvertent microshock, however, is dangerous because it can produce ventricular
fibrillation. Deaths from accidental microshock were caused by a faulty endoscope
and by faulty dialysis equipment.[9]
[10]
Before 1980, arterial and central venous pressure transducers were a potential source
of microshock because they brought electrodes very close to heparinized saline that
is in contact with the intravascular compartment.[35]
[36]
However, in 1982, miniature disposable pressure
transducers appeared that could be powered and operated with low-voltage telephone
cables. They are now used exclusively, and their safety with respect to microshock
has been demonstrated by years of frequent use without reported complications. Such
systems, however, are vulnerable to more subtle dangers, as was reported when a faulty
cable connection permitted precision pressure tracings but caused a substantially
lower pressure reading.[37]
Electric current is the most appropriate variable to address when
setting safety limits. Because investigators have reported that electric current
passing through the human heart needs to be at least 50 µA before ventricular
fibrillation occurs,[33]
[38]
[39]
the American National Standards Institute (ANSI),
on December 2, 1993, set 10 µA as the maximum leakage of current allowable
through electrodes or catheters contacting the heart. This amount of current is
considerably smaller than the peak electric current generated by an implanted pacemaker
during a single pulse (0.1 to 10 mA). Because 2 mA is the LIM warning level, the
LIM does not provide protection against microshock hazards.
ECG monitoring electrodes are electrically isolated from power
circuits in the monitor by an isolation transformer in the main unit, a second level
of isolation. However, an anesthesiologist who wishes to perform intravenous or
intracardiac ECG monitoring, as occurs during p-wave guided brachial placement of
a central venous pressure catheter, should have qualified personnel check the ECG
monitor for current leaks. In some ECG and electroencephalographic (EEG) systems,
electrode signals from the patient are translated into optical signals by a battery-powered
amplifier having a photodiode output that is
optically coupled to the main ECG or EEG unit, an even more thorough second level
of isolation.
Endovascular procedures performed by interventional neuroradiologists
include the placement and activation of electrical coagulation coils inside certain
types of intracranial aneurysms. Electricity in such coils causes coagulation more
slowly than occurs in conventional electrosurgery. The currents in these unipolar
devices have not caused macroshock or microshock. Despite the good safety record
for such equipment, anesthesiologists monitoring the care of patients in an interventional
neuroradiology suite should be aware of potential electrical hazards that come with
new techniques.[40]
