Theory Of Operation; Control; Ecg And Respiration; Blood Pressure - Fluke MPS450 Service Manual

MPS450 SERVICE MANUAL FUNCTIONAL DESCRIPTION

THEORY OF OPERATION

The MPS450 generates physiological waveforms simulating actual conditions in a patient.
CON TROL
The MPS450 is controlled by a microcomputer (MCU). The MCU runs its own embedded program. It
obtains user input from the keyboard or serial port, writes to the display, and controls the circuits that
generate patient simulations.
E C G a n d R E S P IR A T IO N
The MPS450 ECG circuits simulate signals generated by a patient's heart. They are low-level signals
in the millivolt range. The MCU writes the ECG waveforms to an eight-channel digital-to-analog
converter (DAC). The signals from the DAC are attenuated to the proper low levels by resistor
networks.
Patient monitors measure respiration by measuring a change in impedance across the patient's chest as
the patient breathes. Breathing stretches the chest and, therefore, causes the impedance to change. A
monitor uses the ECG electrodes to measure this impedance at the same time that the monitor senses
the ECG. The MPS450 switches in a digital variable resistor to simulate respiration. The MCU writes
the changes in impedance to the variable resistor to generate the respiration waveform.

BLOOD PRESSURE

Patient monitors measure instantaneous blood pressure internal to the patient's heart and blood vessels
by connecting to pressure transducers that are inserted physically into the area being monitored. A
blood pressure transducer is a 300-ohm bridge device. The monitor supplies an excitation voltage to
the bridge. The transducer then returns a signal proportional to the excitation voltage and the measured
pressure. The MPS450 circuits simulate blood pressure transducers. They contain multiplying DACs
with the excitation voltage driving the DACs' reference inputs.
TEMPERATU RE
Patient monitors measure temperature by connecting to a thermistor. The MPS450 contains precision
resistors that have the same resistance as thermistors at specified temperatures.
CARD IAC OU TPUT
Patient monitors can measure the quantity of blood pumped out of the heart. In the cardiac-output
procedure, water (either "iced" or room temperature) is injected into the heart. Then, the temperature
of the blood coming out of the heart is measured. The monitor uses the temperature change to
determine how much blood has been pumped. The MPS450 uses a digital variable resistor to simulate
a thermistor measuring the blood temperature.
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