624074/07 C-19
The operator must use caution with low tidal volumes to avoid
insufficient alveolar ventilation. The determining parameter for
alveolar ventilation is dead space (VDaw). Tidal volume value
must always be greater than the VDaw value. It is widely
accepted that a first approximation of dead space can be
obtained by the following simple equation (Radford 1954):
The lower limit for tidal volume is based on this equation and
calculated to be at least twice the dead space. Or, the mini-
mum Vt is 4.4 x IBW.
VDaw = 2.2 * IBW (1)
C: High rate limit
You derive the maximum rate (see C in Figure C-8) from the
operator-set %MinVol and the calculated IBW, which is calcu-
lated from the operator-set Patient height. The equation used
to calculate the maximum rate is:
fmax = target MinVol / minimum Vt (2)
For example, the 70 kg patient described above will have
a maximum rate of 22 b/min, when %MinVol is set to 100%.
However, as an example, if you choose an excessively high
%MinVol of 350%, the maximum rate becomes 77 b/min.
To protect the patient against such high rates, ASV employs
a further safety mechanism, which takes into account the
patient’s ability to exhale.
A measure of the ability to exhale is the expiratory time con-
stant (RCexp) (Marini 1989, Brunner 1995). To achieve a nearly
complete exhalation to the equilibrium point of the respiratory
system (90% of the maximum potential volume change), an
expiratory time of at least 2 x RCexp is theoretically required.
For this reason, ASV calculates the maximum rate based on
the principle of giving a minimum inspiratory time equal to
1 x RCexp and a minimum expiratory time equal to 2 x RCexp,
which results in these equations:
fmax = 60 / (3 x RCexp) = 20 / RCexp
fmax
≤
60 b/min (3)
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