Page 96 - Clinical Application of Mechanical Ventilation

62 Chapter 3

A B
Returned = Delivered Returned < Delivered
Flow Flow Flow Flow

© Cengage Learning 2014

Patient Patient
Figure 3-10 Flow-trigger mechanism. (A) Before an inspiratory effort, the delivered flow
equals the returned flow. Flow trigger is not activated because there is no drop in returned flow.
(B) At beginning inspiration, some of the delivered flow goes to the patient and this leads to a
lower returned flow. Flow trigger is activated when the ventilator senses an inspiratory effort (the
returned flow is lower than the delivered flow).

between the start of inspiratory flow and the beginning of expiratory flow. If one or
more variables (pressure, flow, or volume) is not allowed to rise above a preset value
during the inspiratory time, it is termed a limit variable. In this definition, inspira-
tion does not end when the variable reaches its preset value. The breath delivery
continues, but the variable is held at the fixed, preset value. (Note that the pressure
limit variable discussed above is not the same as the high pressure limit one would
set on the ventilator to prevent excessive pressure during inspiration.) Figure 3-11
provides a useful algorithm for determining the limit variable (pressure-limited,
volume-limited, or flow-limited) during the inspiratory phase.

Cycle Variable

Inspiration ends when a specific cycle variable (pressure-cycled, volume-cycled,
cycle variable: A measurement
that causes the breath to end. flow-cycled, or time-cycled) is reached (Figure 3-11). This variable must be mea-
sured by the ventilator and used as a feedback signal to end inspiratory flow deliv-
ery, which then allows exhalation to begin.
Again, it is easy to make false assumptions regarding many ventilators by classify-
ing them as volume-cycled. Most newer ventilators measure flow and are flow con-
trollers. Since flow is measured and used as a feedback signal for gas delivery, volume
becomes a function of flow and time [Volume (L) 5 Flow (L/sec) 3 Inspiratory
Time (sec)]. Therefore, these ventilators are really time-cycled, rather than “volume-
cycled.” Inspiration ends because a preset time interval has passed, and volume has
not been directly measured.

Baseline Variable

Expiratory time is defined as the interval between the start of expiratory flow
and the beginning of inspiratory flow of the next breath. This is also termed the

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.