C-18 624074/07
CASV
®
(Adaptive Support Ventilation)
A: High tidal volume limit
WARNING
Check Vt high setting to make sure the target min-
ute ventilation can be reached in passive patients.
The tidal volume applied by ASV is limited (see A in Figure C-8)
by three operator settings: high Pressure alarm limit, Vt high
alarm limit, and Patient height.
The operator must set the high Pressure limit before connect-
ing a patient to the ventilator. It was recommended by a group
of physicians (Slutsky 1994) that the plateau pressure not
exceed 35 cmH
2
O. To achieve this with ASV, the high Pressure
limit must be set to 45 cmH
2
O. The maximum pressure to be
applied in the ASV mode is 10 cmH
2
O below the high Pressure
limit.
For example, a normal 70 kg normal (post-operative) patient
would have a compliance of about 50 ml/cmH
2
O. A high Pres-
sure limit of 45 cmH
2
O will result in a maximum applied pres-
sure of 35 cmH
2
O. With a PEEP level of 5 cmH
2
O, the effective
pressure swing will be 30 cmH
2
O. This in turn leads to an effec-
tive Vt of equal to, or less than 1500 ml. If the patient‘s lungs
stiffen, to a compliance of 30 ml/cmH
2
O, the maximum tidal
volume becomes 900 ml.
If the operator sets the Pressure limit to a very high pressure,
say 60 cmH
2
O, the target volume is limited by the second crite-
rion: 22 x IBW. For the 70 kg sample patient, a maximum tar-
get volume of 1540 ml results.
Additionally the target volume is limited to 1.5 * VT high limit,
and pressure support actually is limited in a way that the
inspired volume does not exceed Vt high limit in mechanical
breaths for more than a few breaths.
B: Low tidal volume limit
To determine the minimum target Vt in ASV (see B in
Figure C-8) use the IBW calculated from the Patient height,
which corresponds to 4.4 ml/kg. In this example for a 70 kg
patient, the minimum target Vt is 308 ml.
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