CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 437

3. Every 1 s (every fourth 250-ms wake-up) this module performs the following actions:
a. If a charge command has been issued, checks that once the desired charge has
been reached, charge is not maintained for more than 30 s, and if past, initiates
charge dump.
b. Tests the lead impedance and sets the “check lead impedance” LED appropriately.
c. Checks if ac power is available and turns peripheral power on constantly (if ac
power is not available, peripherals such as the A/Ds, D/A, and impedance meas-
urement circuit are powered only when needed).
4. Every minute, this module increments the reform counter, which keeps track of the
time elapsed since last capacitor reform.

Lead Impedance Verification This module initiates lead impedance measurement and
runs the A/D that reads the impedance sensor output. This function is initiated either man-
ually (through a GUI request for lead impedance) or before de

brillation is delivered to
the load. A submodule drives the impedance-measurement A/D and takes eight readings
(2.048 ms apart) and averages the measurements prior to using them to enable or disable
de

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fi brillation as well as to communicate the impedance measurement to the GUI.

Charge Control This software module enables the high-voltage power supply, estab-
lishes DAC output proportional to the desired stored energy level, and veri fi es successful

completion of the charge cycle.

Deliver This software module runs a sequencer to turn the H-bridge switches on and off ff

according to the de brillation waveform desired. This module also initiates a lead imped-

ance check prior to delivery.

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Assessment of Charge Delivered This module measures the remaining voltage on the
capacitor bank after shock delivery and communicates the measured value to the control
computer. Estimation of charge delivery is done by the control computer. A submodule
takes care of controlling the A/D that reads the voltage across the capacitor bank and aver-
ages eight readings.

Reform This module keep tabs on the time elapsed since the last full-charge MANUAL
charge (through front-panel control) or reform. If the time elapsed runs over 30 days, this
module enables a full charge of the capacitor bank in order to reform. The module then
dumps the charge into the dummy load after the reform operation.

Verification of Correct Clock Operation es correct operation of the

timing mechanisms of the microcontroller. If a fault is found, the module deactivates auto-
matically.

This module veri fi

Read Manual Switches This module takes care of charging the isolated capacitor used
to power the optocouplers that respond to the manual charge and manual de fi brillation

pushbutton commands.

CARDIAC FIBRILLATOR

No, it's not a typo—a cardiac fibrillator is an instrument for electrical arrest of the action

of the heart. Deliberate cardiac arrest is frequently required during cardiac surgery after

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