Page 367 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 333

demonstrates that the slope for the P ALV curve generated during descending ramp
flow ventilation proportionately decreases per unit time as the summative P ALV in-
creases. The result is that a convex (progressive decay) pressure pattern is always
created during inspiration for descending ramp flow ventilation, which is consistent
with the pattern of volume delivery.

Peak Flow and Tidal Volume Relationship
in Time-Limited Ventilation

Figure 11-15 demonstrates descending ramp flow ventilation where T is main-
I
tained but flow is reduced. The first example (on left) shows that the P is high for
TA
the set flow rate (80 L/min). Given the same lung and circuit characteristics for each
set of waveforms, the second example (on right) shows the results to the pressure
pattern if T is maintained (time-limited ventilation), but flow is reduced. Given
I
the same T and a reduction in flow, the V is reduced (V 5 Average Flow 3 T ).
(Figure 11-15) During I T T I
time-limited ventilation The consequence of the reduction in flow is that V is reduced, as is the P and
T
TA
(constant T I ), a decrease in P , causing a change in the pattern. With the reduction in the initial P , the
flow causes a lower V T , P TA , ALV TA
and P ALV . pressure pattern is relatively constant and PIP is no longer at the beginning of
inspiration.

80 Peak Flow Reduced
Volume Reduced
V (L/min)

1 2 3 4 5 6 7

P (cm H 2 O) 280 P TA

40
P Reduced
TA
P ALV Reduced
P ALV P ALV

22 © Cengage Learning 2014
1 2 3 4 5 6 7
Time (sec)
Figure 11-15 Characteristic changes of P TA and P ALV waveforms created by changes in peak
descending ramp flow during time-limited ventilation. Decrease of peak descending ramp flow
reduces the volume delivered, P TA , and P ALV .

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