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Description
Ventilation Modes
Flow measurement
Regardless of whether ventilation is volume-controlled or
pressure-controlled, positive pressures are generated in
both the breathing system and patient lung during the
inspiration phase.
Depending on the ratio of lung compliance to hose
system compliance, the volume delivered by the ventilator
is distributed to the patient's lung and to the hose system
installed between the ventilator and patient.
Deviations in the measured expiration flow and derived
values, such as the minute volume and breath volume,
are low for adult patients, due to their relatively high lung
compliance in relation to the much lower compliance of
the ventilation hoses.
However, since only the volume attained and surrendered
by the lung is relevant to the efficiency of ventilation, and
since higher differences are possible during paediatric
ventilation, Evita 4 provides basic compensation for hose
compliance during ventilation.
Compensation of the effect of hose system
compliance
During the device check before ventilation, Evita 4
determines the compliance of the ventilation hoses, and
then, during ventilation, compensates for the effect of
compliance on volumetric flow measurement.
Depending on the airway pressure, Evita 4 increases the
tidal volume by the amount that remains in the ventilation
hoses.
In addition to hose system compliance, flow/volume
measurement is influenced by the environmental factors
of temperature and humidity and by leaks in the hose
system.
Evita 4 takes these factors into account and corrects the
settings and measured values accordingly.
Conversion according to ambient conditions
The volume occupied by a gas depends on the ambient
conditions of temperature, pressure and humidity.
In lung physiology, the minute volume and tidal volume
are related to the ambient conditions in the lung:
37 °C body temperature, pressure in the lung,
100 % relative humidity.
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