Measurement Modules
Theory of Operation
A-10 Efficia CM Series Service Guide
Mainstream CO
2
The Mainstream CO
2
sensor is used for the continuous measurement of CO
2
and
respiratory rate. The sensor measures CO
2
with the infrared absorption technique. The
infrared absorption technique is based on the principle that CO
2
molecules absorb
infrared (IR) light energy of specific wavelengths and the amount of energy absorbed is
directly related to the CO
2
concentration. When an IR beam is passed through a gas
sample containing CO
2
, the electronic signal from the photo-detector, which measures
the remaining light energy, is measured. This signal is then compared to the energy of
the IR source and adjusted to accurately reflect the CO
2
concentration in the sample.
The Mainstream CO
2
sensor's response to a known concentration of CO
2
is stored at
the factory in the sensor's memory. A reference channel accounts for optical changes
in the sensor, allowing the system to remain in calibration without user intervention.
ECG and ECG Output
The monitor measures the patient's ECG signals by sensing the varying potential
difference between several points at the skin surface which respond to the
electro-chemical actions of the muscular activity of the heart. Up to 10 electrodes are
attached to the patient’s body. The varying potentials at these locations are sent to the
ECG circuitry. These signals are amplified and filtered and then digitized before being
transmitted through opto-isolators to the microprocessor.
The microprocessor executes algorithms to:
• Display the ECG data
• Compute the heart rate
• Detect a lead-off condition if one of the electrode connections is disrupted
• Detect the presence of pacemaker signals within the QRS waveform complex of
the ECG
• Generate an ECG output signal for external use
The ECG output port is located on the back of the monitor.
Cardiac Output
The Cardiac Output (C.O.) measurement invasively measures cardiac output and other
hemodynamic parameters using the thermodilution technique. Thermodilution
determines the flow rate of a system by introducing a cold solution into the system
and measuring the resulting drop in temperature at a downstream site. The C.O.
measurement circuit measures the temperature of the blood and an injectate with
thermistors in the CO sensor set.
In the right heart thermodilution method, a fluid of known volume and temperature is
injected into the right atrium through the proximal port of a pulmonary artery (PA)
(Swan-Ganz) catheter. The injectate bolus mixes with the blood in the right ventricle
and the change in blood temperature is measured with a thermistor at the distal end of
the catheter in the pulmonary artery.
The temperature change is displayed as a curve in the C.O. window, and the monitor
calculates the C.O. value from this curve. The C.O. value is inversely proportional to the
area under the curve. As cardiac output varies continuously, a series of measurements
must be carried out to achieve a reliable C.O. average value. Always use the average of
multiple thermodilution measurements for therapy decisions.
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