EvitaXL

Measurements
Flow measurement
Measurements
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
larger differences are possible during paediatric ventilation,
EvitaXL provides basic compensation for hose compliance during
ventilation.
Compensation of the effect of hose system compliance
During the device check before ventilation, EvitaXL determines
the compliance of the ventilation hoses and then compensates
for the effect of compliance on volumetric flow measurement
during ventilation.
Depending on the airway pressure, EvitaXL increases the tidal
volume by the amount that remains in the ventilation hoses.
In addition to hose system compliance, flow/volume measurement
is also influenced by the ambient factors of temperature
and humidity, as well as by leaks in the hose system. These factors
are taken into account by EvitaXL and the settings and
measured values are corrected 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 oC body temperature, pressure
in the lung, 100 % relative humidity.
The flow and volume values measured under these conditions
are marked with BTPS*. Medical gases from cylinders or from
the central supply are dry (approx. 0 % rel. humidity) and are
delivered at 20 o C by the ventilator. The flow and volume
values measured under these conditions are marked with
NTPD**.
* BTPS = Body Temperature, Pressure, Saturated.
** NTPD = Normal Temperature Pressure Dry.
218 Instructions for Use EvitaXL