Page 419 - Clinical Application of Mechanical Ventilation

Management of Mechanical Ventilation 385

used with considerable success as an oxygenation strategy for infants with severe
refractory hypoxemia. In adult patients, however, ECMO has not been shown to
provide better oxygenation over conventional mechanical ventilation with PEEP
(Zapol et al., 1979).

Initiate High Frequency Oscillatory Ventilation
(HFOV) for Adults

High frequency oscillatory ventilation (HFOV) is traditionally used in neonates
when conventional ventilation fails to provide adequate ventilation or oxygen-
ation. In recent years, HFOV has been used successfully for the treatment of
acute respiratory failure in adult patients based on clinical trials (Viasys Health-
care, 2005).
Unlike conventional mechanical ventilation, the PaCO is controlled by the power
2
In HFOV, hypoventilation (amplitude) and frequency of oscillation. In HFOV, hypoventilation is managed by
is managed by using a higher
amplitude or a lower frequency. using a higher amplitude or a lower frequency, and hyperventilation is managed by
using a lower amplitude or a higher frequency.
Procedure. The following procedure is the suggested clinical guideline based on the
3100B ventilator (Viasys Healthcare, Yorba Linda, CA). The actual application of
HFOV must be determined by the physician and based on the patient’s condition
and requirement (Viasys Healthcare, 2005).
Since the mean airway pressure (mPaw) is affected by the power setting (see next
paragraph), the initial mPaw should start at 5 cm H O above the mPaw obtained
2
during conventional mechanical ventilation. In patients with severe hypoxia, a
mPaw of 40 cm H O may be applied for 40 to 60 sec. The mPaw may be increased
2
in 3- to 5-cm H O increments every 30 min until the maximum setting. When this
2
strategy is used, oxygenation may worsen in the first 30 min. A chest radiograph
should be done within 4 hours to evaluate changes in lung volume.
The power setting determines the amplitude of oscillation and thus the tidal vol-
ume. For adult patients, the power is set at 4 and rapidly increased to achieve chest
wiggle. Chest wiggle is defined as visible vibration from shoulder to midthigh area. If
the PaCO rises (with a pH .7.2), the power setting is increased to achieve a change
2
of amplitude in 10 cm H O increments every 30 min until it reaches the highest
2
setting.
The initial frequency is set at 5 to 6 Hz and may be decreased if unable to control
Unlike conventional the elevated PaCO with amplitude. It is important to note that a lower Hertz set-
2
mechanical ventilation, a lower
frequency in HFOV provides a ting yields a larger tidal volume. The hertz setting is decreased by 1 Hz increment
larger tidal volume. every 30 min until 3 Hz.
The initial inspiratory time is set at 33% and may be increased up to 50% if
unable to ventilate adequately (i.e., by increasing the amplitude or decreasing the
frequency). The F O is initially set at 100%. The initial settings for ECMO are
2
I
summarized in Table 12-6.
Weaning from HFOV in Adults. As oxygenation improves, the F O is weaned to 40%.
2
I
Once it reaches 40%, the mPaw is reduced in 2- to 3-cm H O increments every
2

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