Biomedical Engineering Reference
In-Depth Information
of the discharge current, reduces the maximum voltage applied to the patient, and shapes
the waveform to produce a damped sinusoidal waveform. The current delivered to the
patient gradually rises to a rounded peak and drops back to zero. The discharge current
pulse duration is about 2.5
brillators.
The circuit would produce an exponential waveform if the inductor L1 is eliminated.
However, long-duration exponential-decay waveforms are unreliable for de
L
1
/C
1
, about 2.5 to 3.5 ms for most de
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brillation
because the low-amplitude long-duration currents at the end of the de
fi
brillation waveform
could re
fi
brillate the heart. If the exponential decay is truncated, however, de
fi
brillation
success is markedly increased, with an e
cacy approaching that of a damped sinusoidal
waveform de
brillator. Truncation allows larger-value capacitors to be used, which means
that the needed energy can be stored at lower voltages. This makes it possible to use solid-
state devices in the switching circuitry.
Truncated exponential waveforms are more sensitive to patient impedance changes
than to damped sinusoidal waveforms. In a typical truncated exponential waveform
de
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brillator, current drops drops from about 27 A for a patient impedance of 50
to 10 A
for a patient impedance of 150
. For damped sinusoidal waveform de
fi
brillators, the peak
current goes from about 60 A for a patient impedance of 50
to 29 A if the patient imped-
ance increases to 150
. Considering that about 30 to 40 A is commonly necessary for suc-
cessful de
brillators are suitable for low- and medium-value
patient impedances. However, damped sinusoidal waveform de
fi
brillation, both types of de
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fi
brillators are more e
ff
ec-
tive in de
brillating high-impedance patients.
A simple, practical damped sinusoidal waveform de
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fi
brillator circuit is shown in
Figure 8.32. This circuit is designed to deliver de
fi
brillation energies of up to 320 J into a
50-
load through a 5-ms Edmark (monophasic) waveform. When the power switch SW1
is on, depressing and holding the charge pushbutton SW2 energizes the primary of high-
voltage transformer T1, a of 110 V to 3 kV current-limiting transformer rated at 150 mA.
The high-voltage recti
er network formed by diodes D1-D4 charges capacitor C1
through current-limiting resistor R1. Meter M1 measures the voltage across the energy
storage capacitor. Its scale should be calibrated so that it provides an estimate of energy
(in joules) delivered to the patient, assuming a load impedance of 50
fi
.
brillation energy can be delivered to
the patient by pressing on pushbuttons SW3 and SW4 simultaneously. In commercial
de
Once C1 is charged to the desired voltage, de
fi
brillators, the insulating handles for the paddle electrodes usually house one push-
button each. This ensures that the physician administering the de
fi
brillation shock is in
control of the discharge and that the paddle electrodes do not become energized by acci-
dent. The debouncing circuit energized by SW3 and SW4 presents 12 V dc across the
coil terminals of relay K1, which is used to transfer the de
fi
brillation charge from capac-
itor C1 to the patient. Charge from capacitor C1 is delivered to the patient via pulse-
shaping inductor L1 and DPDT high-voltage relay K1. A suitable choice for this relay
is the Kilovac Products KM-14 DPDT gas-
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lled “patient” relay (about $600 in low
volumes).
A 5-k
resistor formed by R4 and R5 is connected across C1 via DPDT relay K2 dur-
ing the discharge mode. The high value of this resistor has negligible e
ect when the pulse
is being delivered to a patient. However, these resistors discharge the capacitor if the
de
ff
brillation buttons are depressed without a suitable load across the paddle electrodes.
The capacitor is also discharged if the de
fi
fi
brillator is powered down because SW1 is turned
o
, the power cord is unplugged, or because safety interlock switch SW5 opens (to pro-
tect maintenance personnel from dangerous voltages when the instrument's cabinet is
opened). The leakage current through the meter circuit slowly dissipates the stored energy
if the de
ff
brillation pushbutton switches SW3 and SW4 are not depressed soon after capac-
itor C1 is charged.
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