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Datex-Ohmeda S/5 Light Monitor
12
Document No. 8000060-1
Noise and pacemaker are detected by a slew rate detector.
Lower frequency is determined by a high pass (HP) filter 0.5 Hz (monitor bandwidth).
ECG filtering
The Light Monitor has two ECG filtering modes:
MONITORING
0.5 - 30 Hz (-3 dB, with 50 Hz reject filter)
0.5 - 40 Hz (-3 dB, with 60 Hz reject filter)
The purpose of filtering is to reduce high frequency noise and low frequency (e.g. respiratory)
movement artifacts.
The high-pass filter 0.5 Hz is done with hardware. The monitor sends a command to the LM-ECG
board determining the corner frequency 0.5 Hz.
The 50 Hz and 60 Hz reject filters are both low-pass filters with zero at 50 Hz or 60 Hz
correspondingly and they are done with software. They are for the mains supply filtering. When
these filters are used, -3 dB value for low-pass filter is 30 Hz or 40 Hz.
Respiration section
ECG electrodes are used to measure impedance change across the thorax. Impedance change is
caused by movements and changing air volume in the lungs.
The analog switches control the current supply source of the impedance respiration measurement,
and the lead selection for the 3-lead cable can be seen from the following table:
Lead
selection
Signal fed between
electrodes
Signal measured
between electrodes
I
L and R
F* and ground
II
F* and R
L and ground
III
F* and L
R and ground
*) may be neutral (N) in some older cables
When the 5-lead cable is used, the current source is between L-F and the signal is measured from
the N, independently on the lead selection.
The respiration amplifier consists of the operational amplifiers, and the components around them.
There is an analog switch for controlling the gain of the first stage of the preamplifier.
Synchronous rectifier consists of the analog switches, which are used for detecting the respiration
signal from 31 kHz amplitude modulated raw signal.
The amplifier stage consists of the differential amplifier and the last amplifier. The differential
amplifier consists of the operational amplifiers and the components around them. This stage is AC-
coupled on both sides for minimising the offset voltages.
The last amplifier is used for amplifying the signal derived from differential amplifier stage.
The respiration signal is zeroed at the beginning of the measurement. Zeroing is also used for fast
recovering the measurement after the motion artefact. This is done in amplifier section.
NOTE:
The respiration measurement is switched “OFF” automaticly for 20 seconds when
defibrillation is detected at the defibrillation detector.
Summary of Contents for F-LM1
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