Dräger Babylog 8000 plus Instructions For Use Manual page 165

Measurements
Airway pressure measurement
The device measures airway pressure indirectly in
order to avoid the need for a hose for pressure
measurement on the Y-piece. Within the device,
two piezoresistive pressure sensors record the
pressures P at the inspiratory connection and P
I
at the expiratory connection. These two measured
values are used to calculate the airway pressure
P .
Y
Due to operation with continuous flow, the
inspiratory pressure P
I
point. The flow causes a pressure drop P in
insp
the inspiratory side of the breathing circuit, which
is dependent on the resistance R
hose and :
insp
P = x R
insp
I
P is only minimally dependent on the ventilation
pattern. The pressure at the Y-piece is
approximately equal to the pressure P
P = P – P = P –
insp
Y I I
Thus, the airway pressure at the Y-piece can be
calculated if the resistance R
The total resistance of the breathing circuit can be
measured during ventilation using the two sensors
for P and P ; however, the inspiratory fraction
I E
cannot be measured.
Comprehensive laboratory tests have
demonstrated that the inspiratory branch has
approx. 70 % of the total resistance in all typical
breathing circuits. The device uses this estimated
value to calculate the following:
P = P – 0.7 x < P – P
Y I I
< P – P >: Represents the pressure drop
I E
throughout the entire breathing circuit averaged
over time. If a breathing circuit has a resistance
distribution of other than 30 % to 70 %, there is a
measurement error for the airway pressure. In the
breathing circuits typically used, this error is less
than 1 mbar (1 cmH O). In systems with very high
2
Instructions for use Babylog 8000 plus SW 5.n
serves as the reference
in the inspiratory
I
less P:
I
x R
I
is known.
I
>
E
resistance that also have high continuous flow, it
may be greater. For this reason, only use
breathing circuits with inner diameters of at least
10 mm (0.4 in).
Flow and volume measurement
E
The measuring principle used for flow
measurement is based on hotwire anemometry.
Hot-wire anemometry is a thermal measuring
procedure where the measuring wires of the flow
sensor are maintained at a constantly high
temperature. The higher the flow, the more power
is needed to maintain a constantly high
temperature. The flow rate can then be calculated
from the amount of power consumed.
To ensure correct functioning check the flow
sensor for visible damage, soiling, and particles
before inserting it. Replace damaged, soiled, or
non-particle-free flow sensors.
If the measuring wires of the flow sensor glow
continuously during operation, this is a sign of
soiling. Immediately reprocess or replace the flow
sensor.
The lowest flow at which detection functions
reliably is 0.2 L/min. Lower flow values are
therefore suppressed and displayed as zero.
Principles of operation
165