Philips M3921A, Service Manual

Page: 138 / 150

Theory of Operation and System Architecture
126 Chapter 10 - Training Program
Printing is accomplished on 50 mm wide thermal paper at recorder speeds
programmable up to 50 mm/s.
ECG Processing
The technique used in ECG senses the varying potential difference between
two points at the skin surface which respond to the electro-chemical actions of
the muscular activity of the heart.
Three electrodes are attached to the patient’s right arm (RA), left arm (LA)
and left leg (LL). The varying potentials at these locations are cable-
connected to the ECG circuit inputs where they are conditioned, and the
difference of potential between two selected leads is digitized before
transmitting through opto-isolators to the processor. The processor-installed
algorithms operate on the signals to develop drivers for the graphic display
and to compute the heart rate in beats per minute (BPM).
In addition to the acquisition of the QRS waveform complex, the ECG input
and subsequent signal processing computing circuitry perform a number of
other functions:
• They detect a “lead-off” condition if one of the electrode connections is
disrupted.
• They detect the presence of pacemaker signals within the QRS waveform
complex of the ECG.
• They generate a synchronization pulse for external use with
defibrillators. The Defib Sync Pulse output is available at a connector in
the rear panel.
Respiration
Processing
The patient’s respiration is detected by using two of the three leads of the
ECG electrodes and cable. A low-level excitation signal is applied to these
leads, and the variation of the thoracic impedance caused by the breathing is
sensed and processed for display and measurement.
NiBP Processing
The NiBP processing uses an oscillometric technique to provide needed
measurements at selected intervals. This technique uses an inflatable
sphygmomanometer cuff similar to those used by clinicians in routine
measurements.
A motorized pump inflates the cuff to approximately 180 mmHg initially, at
which point the pressure effectively stops the flow of blood. Then, under
monitor control, the pressure in the cuff is gradually reduced, while a pressure
transducer detects the air pressure and transmits the parameter signal to the
NiBP input circuitry.
As the pressure is reduced, blood flows in the previously occluded artery, and
changes the measurements made by the transducer. The point at which