Page 368 - Clinical Application of Mechanical Ventilation

334 Chapter 11

100
V (L/min) 50 50% End Flow

5 L/min End Flow

1 2 3 4 5 6 7

P (cm H 2 O) 2100 Increased End Flow

40 Increased End Flow Increased End P TA
Increased End P
TA

P ALV P ALV

22 1 2 3 4 5 6 7 © Cengage Learning 2014

Time (sec)
Figure 11-16 Effects of end flow on the pressure waveforms during descending ramp flow
ventilation. Figure shows two end flows: 5 L/min (left) and 50% of peak inspiratory flow (right).
Increase of end flow raises the transairway pressure (P TA ), but not the alveolar pressure (P ALV ).

Effects of End-Flow on End-Transairway Pressure

Figure 11-16 demonstrates the changes in P patterns that could be expected
TA
(Figure 11-16) During when end-flows for DRFWs are increased. In the first example, the end-flow is
descending ramp flow ventila-
tion, a higher end-flow raises 5 L/min (PB840). The pressure pattern shows a slight increase in the P above
TA
the P TA at end-inspiratory, but the P at the end of inspiration as a result of the slight resistance to end-flow
not the alveolar pressure (P ALV ). ALV
being above baseline. The second set of waveforms shows a 50% DRFW where
end-flow is 50% of the initial peak flow set. The initial flow set is very high
(100 L/min), which causes an exponential rise in initial P (40 cm H O) because
2
TA
of turbulent flow and circuit/airway resistance. The end-P is also relatively high
TA
from resistance to flow above the peak P ALV compared to the first example since
end-flow is 50 L/min (50% of 100 L/min) compared to 5 L/min. These flow and
pressure patterns do not represent ventilation of the same patient. Peak P ALV is
at the same level, but V delivered is much greater for the second flow waveform
T
example compared to the first.

Distribution of Delivered Tidal Volume

In Figure 11-17, volume-time waveforms have been added for comparison with
DRFW and pressure-time waveforms. Since flow rate is highest in the beginning
of inspiration for a DRFW, the slope in rise to peak volume, like P ALV , is initially

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