Biomedical Engineering Reference
In-Depth Information
region. With a more unipolar system with one electrode
under the shoulder the current path is more optimal, and
this is used if the deflbrillation is planned (electro-
conversion).
Earlier conductive paste was used on the skin, today
contact pads are used because they are quick to apply.
They also make it possible to avoid usual contact paste
that is easily smeared out on the skin surface and causes
stray currents (either short-circuiting the shock energy,
or representing a hazard for the personnel involved). The
current density is so high that reddening of the skin often
occurs, especially at the electrode edge. The large elec-
trode and the large current cause an extremely low-
ohmic system. Fifty ohm is the standardized resistance of
the complete system with two electrodes and the tissue
in between. The resistance is falling for each shock given,
and this is attributed to tissue damage.
Wemust assume that it is the local current density in the
heart that is the determining parameter for a successful
conversion. As this is unknown, it is usual practice to
characterize the shock in energy (wattseconds
¼
joules).
This refers to the capacitor used to store the energy
(
Fig. 4.1-16
). Stored energy is
CV
2
/2, so a chock dose is
simply chosen by choosing charging voltage. Maximum
stored energy is usually 400 Ws. Not all the energy will be
dissipated in the patient system, a part will be dissipated in
the internal resistance
R
i
of the coil used to shape the
waveform of the discharge current pulse. External shock is
given transcutaneously, so the voltage must be high enough
to break down the skin even at the lowest dose. For in-
ternal, direct epicardial application (internal shock), sterile
electrode cups are used directly on the heart without any
paste or pad. The necessary dose is usually less than 50Ws.
There is a certain range of accepted current duration.
Figure 4.1-17
shows some current discharge current
waveforms. Note that some marks use biphasic wave-
forms, some use truly monophasic.
Defibrillators are also made as implanted types, using
intracardial catheter electrodes. In order to reduce
50
40
30
20
10
0
0
5
10
15
t
(ms)
Figure 4.1-17 Some classical current discharge waveforms.
energy consumption new waveforms have been taken
into use: the exponential truncated waveform. It may be
monophasic or biphasic. The idea of the biphasic wave-
form is that the second pulse shall cancel the net charge
caused by the first pulse and thereby reduce the chance
of refibrillation.
Tissue impedance measurements with the defibrillator
electrodes are used both in some external and internal
defibrillator models. Measuring current and voltage
during a shock gives a high current level, minimum value,
non-linear region, peak voltage to peak current ratio. Be-
tween shocks the small signal, linear impedance is also
monitored. The measured impedance value is used to
customize both waveform and energy level for each shock
given.
4.1.12.2 Electroshock (brain
electroconvulsion)
Electroshock therapy is a somatic method in psychiatry,
for the treatment of depressions. The traditional current
waveform is a quarter-period power line 50 Hz sine wave,
starting at the waveform maximum. Pulse duration is
therefore 5 ms, followed by a pause of 15 ms. Automatic
amplitude increase, or pulse grouping, is used. It is now
often replaced by another waveform, a train of pulses of
1 ms duration with a total energy around 20 J. It is be-
lieved that with this waveform the memory problems are
less. The corresponding voltage and current are several
hundred volts and milliamps. Large bipolar electrodes are
used on the temples. The positioning is usually bilateral,
but ipsilateral positioning is also used.
ECT is a much discussed procedure, partly because it
has been perceived as a brutal medical treatment. It is
performed under anesthesia, and because of the heavy
DEFIBRILLATOR
L
R
i
C
R
pas
Charger
Figure 4.1-16 Classical defibrillator circuit. Typical values are
C ¼ 20 mF, L ¼ 100 mH, R
i
¼ 15 U, R
pas
¼ 50 U.
