624074/07 C-27
Figure C-13. Three different relationships between rate
and WOB are plotted for a hypothetical lung: (+) purely
resistive load causes WOB to rise with rate, (x) purely elastic
load creates highest load at low rates, (o) the total lung shows
a clear minimum which can be calculated according to the
equation below.
The following equation was found to represent the rate where
WOB is minimum:
f = (1 + 2a*RCe*(MinVol-f*Vd)/(Vd))
-0.5
-1/a*RCe
where a is a factor that depends on the flow waveform. For
sinusoidal flows, a is 2
π
2
/60.
The corresponding tidal volume is calculated as:
Vt = MinVol/f
Example: A 70 kg male patient with normal lungs (Rtotal =
5cmH
2
O/l/s, expiratory resistance hose and valve =
5 cmH
2
O/l/s, Crs = 50 ml/cmH
2
O) may have a measured RCexp
of 0.5 s, an estimated VDaw of 154 ml, and an operator-set
%MinVol of 100%. With these values, the target MinVol
becomes
MinVol = 100% x 70 kg x 0.1 l/min/kg = 7 l/min
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