Page 136 - Clinical Application of Mechanical Ventilation

102 Chapter 4

TABLE 4-5 Characteristics of the Mandatory Minute Ventilation Mode

Characteristic Description

Type of breath The ventilator increases the mandatory frequency (Note: Hamilton
Veolar increases the pressure support level).
Triggering mechanism Increase of the mandatory frequency (or the pressure support level in
the Hamilton Veolar) is triggered when the actual minute volume is
less than the preset minimal minute volume.

Cycling mechanism All mandatory breaths are volume-cycled. Patients control their own
spontaneous frequency and volume.
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volume is less than the preset minimum minute volume, the ventilator automati-
cally increases the pressure support level until the minimum minute volume is ob-
tained (Wilkins et al., 2003).
Table 4-5 summarizes the major characteristics of the mandatory minute
ventilation mode.

PRESSURE SUPPORT VENTILATION (PSV)

Pressure support ventilation (PSV) is used to lower the work of spontaneous breath-
PSV lowers the work of ing and augment a patient’s spontaneous tidal volume. When PSV is used with
spontaneous breathing and
augments the spontaneous SIMV, it significantly lowers the oxygen consumption requirement presumably due
tidal volume. to the reduced work of breathing (Kanak et al., 1985).
PSV applies a preset pressure plateau to the patient’s airway for the duration of a
spontaneous breath (Figure 4-8). Pressure-supported breaths are considered spon-
taneous because (1) they are patient-triggered, (2) the tidal volume varies with the

40
35 B C
Airway Pressure (cm H 2 O) 20 A
30
25

15
10
5
0
Inspiration Expiration

Figure 4-8 Pressure support ventilation (PSV) with PEEP of 5 cm H 2 O. (A) Inspiratory ef- © Cengage Learning 2014
fort; (B) Pressure support plateau of 30 cm H 2 O (peak inspiratory pressure of 35 cm H 2 O, PEEP
of 5 cm H 2 O); (C) Beginning expiratory phase when the inspiratory flow drops to 25% (or other
predetermined %) of its peak flow.

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