624074/07 C-25
C.3.6 Dynamic adjustment of optimal breath pattern
After calculated, the optimal breath pattern is revised with
each breath according to the measurements of RCexp. Apply
Otis’ equation and a new target breathing pattern is calcu-
lated. The targets do not change under steady-state condi-
tions. However, if the patient‘s respiratory system mechanics
change, the target values also change.
In this example: the bronchi of our normal 70 kg sample
patient (being ventilated at 15 b/min and with a Vt of 467 ml)
constrict due to asthma, and the expiratory resistance increases
to values higher than 5 cmH
2
O/l/s. For this reason, more time is
needed during exhalation for the lungs to reach the end-expi-
ratory equilibrium position. In technical terms, the RCexp has
increased and this increase requires a longer expiratory time.
For a given minute ventilation, this calls for an increase in Vt
and a decrease in rate (longer expiratory time). Otis’ equation
yields new targets:
f = 11 b/min and Vt = 636 ml.
Figure C-12 shows the change. Notice also that the increase in
resistance results in a decrease in the volume/pressure ratio
(V/P). The changes in RCexp and dynamic compliance affect
the safety limits accordingly and with each breath (see previous
subsection).
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