This means that only part of the stroke volume is
effectively used to ventilate the lungs, the rest remaining
in the compressible hose volume. This compressible
volume must be known for ventilation to be effective.
Example:
A patient with a lung compliance of 3 mL/mbar
requires a tidal volume of VT = 60 mL.
Disregarding the compressible hose volume, this
would yield an effective airway pressure of
VT 60 mL
P = = = 20 mbar.
C1 3 mL / mbar
The actual compressible hose volume (hose system)
equals 3 L, however, which corresponds to a
compliance of C2 = 3 mL/mbar.
The actual airway pressure is therefore:
VT 60 mL
P = = = 10 mbar
C1 + C2 (3 + 3) mL / mbar
In other words, 30 mL stroke volume ventilate the
lungs, the other 30 mL remaining in the hoses.
The compliance of the breathing system (breathing gas
block, soda lime container, hoses, etc.) is determined
during the leakage test and saved by the Cato.
This calculated compliance value is used to calculate the
volume stored in the breathing system and hoses for
each ventilation pressure. In order to correct it, the Cato
starts with the set tidal volume and reaches the correct
volume after three to six breaths. The corrected volume is
constantly verified automatically.
The measured value must be limited to plausible ranges
for safety reasons. This limit is set at 3.9 mL/mbar when
using adult hoses (tidal volumes greater than 200 mL)
and at 0.8 mL/mbar when using infant hoses (tidal
volumes less than 200 mL). The maximum length of the
breathing hoses should therefore not be exceeded (see
table below).
Automatic leakage test IPPV
This leakage test identifies any leaks of relevance for
automatic ventilation in subsystems 1 and 2 of the Cato
ventilation system. It also encompasses the breathing
hoses up to the Y-piece, as well as the measured gas
sampling and return lines if installed. The overall system
compliance is determined at the same time.
The IPPV leakage test and the leakage test started in
»standby« mode is carried out by building up a constant
pressure of 30 mbar. The piston movement necessary to
compensate the gas escaping through leaks is measured
and calculated and indicated as a volume per unit time.
The effective leakage over the complete ventilation cycle
is lower than the value indicated, since the effective mean
pressure Pmean in IPPV ventilation mode is considerably
lower than the test pressure.
The relationship depends on the rate of pressure
increase, the plateau time and the ratio TI:TE. The
effective leakage value varies with the value measured in
the leakage test as Pmean : Ptest.
Example:
Test leckage = 30 mL/min
Ptest = 30 mbar
Pmean = 6 mbar
Effective leakage = Test leakage x Pmean / Ptest
Effective leakage = 6 mL/min
Automatic MAN leakage test
This leakage test is also part of the self-test and locates
any leaks of relevance for manual ventilation in subsystem
3 of the Cato ventilation system. The breathing bag, fresh
gas hose, Vapor and internal connections up to the bank
of measuring tubes are tested for leaks.
The test is normally carried out at a pressure of 30 mbar.
If the leakage value remains below 300 mL/min, this is
not indicated and the self-test continues. This subsystem
contributes only marginally to the overall leakage, since
the mean pressure is normally below 5 mbar.
Automatic compliance correction
The stroke volume applied by a ventilator not only
ventilates the patient's lungs, but also the hose system
connecting the patient to the ventilator.
127
Beschreibung
Automatic leakage test IPPV / MAN
Automatic compliance correction
Table of maximum hose lengths:
Hose Maximum length
with filter without filter
Adults; black 3.0 m 3.5 m
Adults; blue 6.5 m 7.0 m
Infants; black 2.2 m 4.4 m
Infants; blue 2.2 m 4.4 m
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