C-20 624074/07
CASV
®
(Adaptive Support Ventilation)
For example, the 70 kg patient with a respiratory system com-
pliance of 50 ml/cmH
2
O (equal to 0.05 l/cmH
2
O), an airway
resistance including endotracheal tube of 5 cmH
2
O/l/s, and
a resistance of the expiratory hose and valve of another
5cmH
2
O/l/s, would have an RCexp of
0.05 l/cmH
2
O x (5+5) cmH
2
O/l/s = 0.5 s
and thus a maximum rate of 40 b/min. Since this value is
higher than the one calculated above, the lower of the two
values is in effect, that is, 22 b/min.
This limit applies to the respiratory rate of the ventilator only,
not to the respiratory rate of the patient.
D. Low rate limit
The lowest target rate (see D in Figure C-8) is fixed at 5 b/min.
This low rate in turn limits the maximum tidal volume to
1400 ml in the example of the 70 kg patient above, when
%MinVol is set to 100%.
C.3.4 Optimal breath pattern
Although the lung-protective rules strategy limits possible com-
binations of Vt and f, ASV prescribes an explicit target combi-
nation. Using the example in Figure C-8, this shows
considerable room for selection within the dotted rectangle.
The selection process is an exclusive feature of ASV. The device
works on the assumption the optimal breath pattern is identi-
cal to the one a totally unsupported patient will choose natu-
rally (assuming the patient is capable of maintaining the
pattern).
It is common knowledge that the choice of breathing pattern
is governed by either work of breathing, or the force needed
to maintain a pattern. ASV uses the original equation by Otis
(Otis 1950) and calculates the optimal rate based on operator
entries of %MinVol and the IBW (based on the Patient height
setting) as well as on the measurement of RCexp (see
Section C.4).
For example, with the 70 kg patient, a setting of 100 %Min-
Vol, and a measured RCexp of 0.5 s, the optimal rate is
15 b/min according to Otis’ equation.
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