Page 140 - Clinical Application of Mechanical Ventilation

106 Chapter 4

TABLE 4-7 Characteristics of the Proportional Assist Ventilation Mode
Characteristic Description

Type of breath PAV occurs during assisted breaths only

Triggering mechanism Pressure- or flow-triggered
Cycling mechanism PAV terminates once the patient’s volume or
flow demand is met
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reduces the inspiratory effort needed to overcome systemic elastance such as restric-
tive lung defects (Navalesi et al., 1996).
PAV is achieved by a positive feedback control that amplifies airway pressure in
proportion to instantaneous inspiratory flow and volume. Unlike the traditional
modes of mechanical ventilation, PAV changes with the patient’s breathing effort.
The advantage of PAV is its ability to track changes in breathing effort over time.
By varying the pressure to augment flow and volume, a more uniform breathing
pattern becomes possible (Bigatello et al., 1997). PAV has been reported to pro-
vide mechanical ventilation while promoting patient-ventilator synchrony (Younes,
1992). However, in conditions where the elastance or airflow resistance shows sud-
In conditions where the den improvement, the pressure provided by PAV may be too high. This may lead to
elastance or airflow resistance
shows sudden improvement, overdistension, increased air trapping, and barotrauma.
the pressure provided by In terms of physiologic response, PAV improves ventilation and reduces the
PAV may be too high. This
may lead to overdistension, neuromuscular drive and work of breathing in ventilator-dependent patients with
increased air trapping, and COPD. When PAV is used with CPAP, the reduction of inspiratory muscle work
barotrauma.
reaches values close to those found in normal subjects. Exercise tolerance may be
improved with this strategy of combining PAV with CPAP (Appendini et al., 1999;
Dolmage & Goldstein, 1997). Table 4-7 summarizes the major characteristics of
the PAV mode.

VOLUME-ASSURED PRESSURE SUPPORT (VAPS)

Volume-assured pressure support (VAPS) is available in the Bird 8400 STi and
volume-assured pressure
support (VAPS): A mode of TBird ventilators. It is similar to pressure augmentation in the Bear 1000 ventilator
ventilation that assures a stable and is also known as volume-assisted pressure support.
tidal volume by incorporating
inspiratory pressure support ven- VAPS incorporates inspiratory pressure support ventilation (PSV) with conven-
tilation (PSV) with conventional tional volume-assisted cycles (VAV). This combination provides an optimal inspira-
volume-assisted cycles (VAV).
tory flow during assisted/controlled cycles, reducing the patient’s work of breathing
commonly seen during VAV. Unlike typical PSV, VAPS assures stable tidal volume
in patients with irregular breathing patterns (Amato et al., 1992).
In VAPS, the therapist must preset the desired minimum tidal volume and the
pressure support level. During VAPS, the mechanical breaths may be patient- or

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