GE HEALTHCARE B20, Руководство пользователя

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Airway gas
17-19
ET balance gas
You can obtain a calculated value for balance gas, EtBal. End-tidal balance gas is the
percentage of gas concentration not measured by the gas sensors. It is displayed in digit field
with the MAC value.
An increased balance gas value may indicate the amount of nitrogen flushed out from the
patient into the circuit. The increase may be due to an accumulation of nitrogen during low
flow anesthesia.
The monitor calculates end-tidal balance gas when all gases (CO
2
, O
2
, AA, N
2
O) measurements
are active. If more than one of gases measurements off or the status are invalid, the monitor
displays the balance gas value as invalid.
Basics of CO
2
measurement
Normal CO
2
waveform
The CO
2
waveform is referred to as capnogram and it reflects the different stages in breathing.
The capnogram of a healthy patient under controlled ventilation has a normal shape. Changes
in the CO
2
waveform may indicate compromised patient respiratory and/or circulatory
function or improper mechanical ventilator functionality.
The origin of the CO
2
waveform
The following illustration shows a normal capnogram. In this illustration, the letters indicate the
following:
•
A: The gas first exhaled is from the anatomical and apparatus dead-space. It contains no
CO
2
because it has not been in the alveoli and no gas exchange has taken place.
•
B: Briefly, the exhaled gas is a mixture of gas from the anatomical dead-space and gas
from the alveoli.
•
C: A plateau is reached when the gas exhaled is entirely from the alveoli. The end-tidal
CO
2
(EtCO
2
) concentration is measured at the end of this plateau.
•
D: When the next inspiration starts the capnogram rapidly falls towards the baseline. The
minimum level of CO
2
measured during the inspiratory phase is called the inspired CO
2
concentration (normally 0.0%).
•
E: With a scale, the height of the capnogram tells you the end-tidal CO
2
concentration.
The monitor automatically calculates and display the EtCO
2
in numbers. EtCO
2
approximates the alveolar CO
2
concentration because it is measured when the patient
exhales virtually pure alveolar gas.