Dräger Babylog 8000 plus Operating Instructions Manual page 140

Theory of Operation
Ventilation Monitoring
Measurement of Lung Mechanics Parameters
For each cycle of inspiration and expiration, Babylog
8000 plus calculates the following parameters from the
waveforms for pressure, flow, and volume:
- C dynamic compliance of the respiratory system
using linear regression
- R airway and ET-tube resistance using
- r coefficient of correlation
- TC time constant of the respiratory system
(TC = R . C)
- C /C lung overdistension index according to [1]
20
Babylog 8000 plus first saves all measured values for
P (t), Flow(t), and V(t) for a breath up to a length of 5 s.
aw
120 data points per second are sampled. The data is
evaluated and the results are displayed. Subsequently,
another cycle is sampled, and so on. Hence, the numeric
results on display do not belong to a breath in progress
but are normally a few seconds old.
The calculation is based on the equation:
= R . Flow +V/C
P
aw
which holds true for the single compartment model of the
respiratory system at each point in time during a breath.
Resistance and compliance are assumed to be constant.
The method of linear regression finds the best matching
values of R and C for the pressure flow and volume data
set. The correlation coefficient r, a number between 0
and 1, is a measure for this match. The closer r is to 1,
the better the match.
Since Babylog 8000 plus only has information about air-
way pressure, but not about pleural or esophagal pres-
sure, spontaneous breathing cannot be evaluated.
Considerable spontaneous breathing superimposed on
ventilation will therefore alter the results.
The same is true for a substantial leak. Resistance and
compliance displayed will be greater than true values.
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