Page 366 - Clinical Application of Mechanical Ventilation

332 Chapter 11

TABLE 11-7 Volume Delivered in the First Half of Inspiration for the True DRFW Compared to the CFW
(see Figure 11-14)

True DRFW Volume in the first half of inspiration: % volume in the first half of inspiration:
Volume 5 Average Flow 3 T I

Volume 5 ½ (PF L/sec 1 *EF L/sec) 3 ½ T (sec) 80 L/min 3 min/60 sec 5 1.33 L/sec
I
Volume 5 ½ (1.33 L/sec 1 0.67 L/sec) 3 ½ (1.0 sec) 40 L/min 3 min/60 sec 5 0.67 L/sec
Volume 5 ½ (2 L/sec) 3 0.5 sec V 5 0.67 L for DRFW
T
Volume 5 0.5 L 0.5 L/0.67 L 3 100% 5 75%
CFW Volume in the first half of inspiration: % volume in the first half of inspiration:

Volume 5 Average Flow 3 ½ T I V 5 0.67 L for CFW
T
Volume 5 0.67 L/sec 3 ½ (1.0 sec) 0.33 L/0.67 L 3 100 5 50%
Volume 5 0.67 L/sec 3 0.5 sec

Volume 5 0.33 L

*End-flow for the DRFW in the first half of inspiration 5 40 L/min or 0.67 L/sec
© Cengage Learning 2014

Given that T is held constant in the first example of descending ramp flow com-
I
pared to constant flow ventilation, greater volume (75% vs. 50%) is delivered by the
DRFW, and therefore, there is a faster rise in P ALV in the first half of inspiration in
the first pressure waveform for the DRFW compared to the rise in P ALV for the CFW.
In the second half of inspiration, the slope of the rise in P ALV for the DRFW is lower
than for the CFW because only 25% of the V is delivered for the DRFW compared
T
to 50% of the V delivered for the CFW. There may be an improvement in oxygen-
T
ation using the DRFW compared to the CFW, according to research, because the
alveoli are more distended throughout inspiration. Given the same T and V , mean
T
I
Given the same T I and P is higher during descending ramp flow compared to constant flow ventilation.
V T , mean P ALV is higher dur- ALV
ing descending ramp flow The same concepts explored above for the DRFW can be applied to the P ALV
compared to constant flow pattern presented in the second pressure waveform in Figure 11-14, but the
ventilation.
comparison of pressures for the waveforms is a little more complicated. As stated,
the ventilator holds peak flow constant for the adjustment in flow waveforms. In
this second example, however, the P ALV slope for the CFW is higher throughout
inspiration compared to the P ALV slope of the DRFW’s pressure waveform. Since
T is extended for descending ramp flow ventilation to take place, the entire V
T
I
is delivered by the CFW within the time period for which only 75% of the V is
T
delivered by the DRFW. Thus, given this condition, pressure-rise is steeper for the
CFW. But also note that in the second example, as in the first, because there is a
proportionate decrease in volume delivered per unit time during descending ramp
flow ventilation, the P ALV generated per unit time steadily decays during inspiration
compared to the linear rise in pressure over time for the CFW. Thus, Figure 11-14

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