Page 139 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 105

0.18
0.16 WOB,res (J/s)
WOB,el (J/s)
0.14
WOB,tot (J/s)
0.12 Best Rate Point

14
0.10
WOB (J/s) 0.08

0.06

0.04

0.02

0.00 © Cengage Learning 2014
0 10 20 30 40 50
Frequency (b/min)
Figure 4-9 Work of breathing model based on the Otis Equation. Note the respiratory rate of
14 corresponds with the lowest point on the total work of breathing (WOB, tot) curve. (WOB 5 work
of breathing, res 5 resistive, el 5 elastic)

If there is no spontaneous triggering effort, the ventilator determines and provides
the mandatory frequency, tidal volume, and high pressure limit needed to deliver
the preselected tidal volume, inspiratory time, and I:E ratio. As the patient begins to
trigger the ventilator, the number of mandatory breaths decreases and the pressure
support level increases until a calculated tidal volume is able to provide adequate
alveolar volume (i.e., tidal volume 5 alveolar volume 1 2.2 mL/kg of deadspace
volume).

PROPORTIONAL ASSIST VENTILATION (PAV)

Proportional assist ventilation Proportional assist ventilation (PAV) (Evita 4, Dräger Medical) and propor-
(PAV): A mode that uses variable tional pressure support are similar modes of ventilation. With PAV, there is no
pressure to provide pressure
support. The variable pressure is in target flow, volume, or pressure during mechanical ventilation. The pressure used to
proportion to the patient’s pulmo-
nary characteristics (elastance and provide the pressure support is variable and is in proportion to the patient’s pulmo-
airflow resistance) and demand nary characteristics (elastance and airflow resistance) and demand (volume or flow)
(volume or flow).
(Appendini et al., 1999). PAV is set to overcome 80% of the elastance and airflow
resistance. For example, at 40 cm H O/L of elastance, 32 cm H O of pressure is
2
2
generated to provide 1 L of volume.
PAV may be flow assist (FA) or volume assist (VA). In FA, the applied pressure
PAV may be flow assist or is provided to meet the patient’s inspiratory flow demand. FA reduces the inspira-
volume assist and it is active
in assist breaths only. tory effort needed to overcome airflow resistance (Appendini et al., 1999). VA oc-
curs when PAV provides the pressure to meet the patient’s volume requirement. VA

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