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Notes on setting the fresh gas flow
Notes on setting the fresh gas flow
The fresh gas delivered must
– cover the patient's consumption (uptake) and
– any leaks in the system.
– It must also compensate the gas sampled by the
system monitor if the measured gas return line is not
connected.
This is particularly important when setting a low fresh
gas flow.
The patient's uptake of gas depends on the anaesthesia
and primarily comprises the consumption of O
nitrous oxide. The consumption of O
approximately using Brody's equation:
BW
=
Body weight
O
uptake
=
10 x BW
2
This corresponds to an O
2
per kg body weight.
Higher O
consumption results in a lower inspiratory
2
O
concentration due to rebreathing when a low fresh
2
gas flow is set.
– The uptake of nitrous oxide varies with time and can
be approximated by the following rule of thumb:
1
√
t
Approximate steady state value: 1.5 mL/min N
per kg body weight.
– The leakage from the ventilation system depends on
the airway pressure (mean value) and can be
determined with the aid of the automatic leakage test.
168
and
2
can be calculated
2
in kg
0.75
in mL/min
uptake of roughly 3.5 mL/min
O
2
Example
for estimating the minimum fresh gas flow required in
the steady state:
Body weight
=
*
O
=
2
*
N
O
=
2
Leakage
=
Fresh gas required ≥
This example shows that the fresh gas flow (O
N
O) must be set to at least 350 mL/min. If the
2
measured gas return line is not used, the fresh gas
requirement is increased by the volume of the aspirated
measured gas.
Relationship between fresh gas flow and gas
concentration in the breathing system
The inspiratory concentration differs from the set fresh
gas concentration due to rebreathing and the O
and anaesthetic agent uptake by the patient. The lower
the fresh gas flow, the larger the concentration gradient
between fresh gas, inspiratory and expiratory gas
concentration becomes.
Since the concentration of the fresh gas flow in this flow
range bears little resemblance to the concentration at the
patient, it is important to measure the anaesthetic agent
concentration as close to the patient's airway as possible
in this mode.
The measuring system is integrated into the system
monitor.
Relationship between fresh gas flow and
time constants in the breathing system
The response time following a change of concentration of
O
, N
O or anaesthetic agent in the fresh gas depends
2
2
on the set fresh gas flow.
The inspiratory concentration in the breathing system
corresponds more and more accurately with the fresh
gas concentration as the fresh gas flow increases. At a
low fresh gas flow, a change of concentration takes
effect in the breathing system very slowly. This process
can be speeded up by increasing the fresh gas flow
abruptly.
70 [kg]:
240 [mL/min]
100 [mL/min]
+
10 [mL/min]
+
350 [mL/min]
and
2
, N
O
2
2
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