Dräger Babylog 8000 plus Operating Instructions Manual page 138

Theory of Operation
Ventilation Monitoring
Flow and Volume Measurement
Flow is measured with a hot wire anemometer that is
either integrated into the wye or into a separate sensor
adapter between wye and ET-tube. Flow direction is
detected by using two hot wires, one of which is located
in the "shadow" of a flow obstacle from one side.
The smallest flow that can reliably be detected is
0.2 L/min. Smaller flow values are therefore ignored and
displayed as zero values.
Two different sensor types are available:
1. Wye-sensor, integrated into the wye
2. ISO sensor, connected between wye and
ET-tube connector
Both designs use the same sensor element. However,
they are not completely identical in their flow response.
Therefore, in order to optimize measurement results, the
sensor type is a parameter to be set in one of the
ventilator menus.
Reference conditions
Hot wire anemometers primarily measure gas mass, not
volumes or flows. The volume that corresponds to a
particular amount of gas depends on environmental
parameters such as atmospheric pressure, temperature,
and humidity, following the general gas law.
Babylog 8000 plus displays values measured for flow
and volume for one of two reference conditions:
1. NTPD (temperature at 20 ºC, barometric pressure
at 1013 mbar, dry air)
2. BTPS (body temperature at 37 ºC, actual barometric
pressure, water vapor saturated air)
The reference conditions desired are set in the menu.
Leak rate
With an uncuffed ET-tube, breathing gas will frequently
escape to ambient between trachea wall and tube. The
Babylog 8000 plus flow sensor is located in the wye, i.e.
before the leak. During inspiration, breathing gas will be
lost after measurement, during expiration, it will be lost
before being measured. Therefore, tidal volume meas-
ured for inspiration will be larger than true tidal volume,
tidal volume for expiration will be smaller. On average,
the difference between inspiratory and expiratory flow is
equal to the leakage flow. The gas volume that did not
flow back through the sensor must have escaped through
the leak.
138