Page 427 - Clinical Application of Mechanical Ventilation

Management of Mechanical Ventilation 393

Auto-PEEP

Auto-PEEP (intrinsic PEEP, inadvertent PEEP, occult PEEP) is the unintentional
Auto-PEEP is associ- PEEP during mechanical ventilation that is associated with excessive pressure
ated with pressure support
ventilation, significant airway support ventilation, significant airway obstruction, high frequency (.20/min),
obstruction, high frequency insufficient inspiratory flow rates, and relatively equal (about 1:1) or inversed I:E
(.20/min), insufficient in-
spiratory flow rates, relatively ratio. It is also more likely to occur when the patient has a history of air trapping
equal (about 1:1) or inversed
I:E ratio, and history of air (MacIntyre, 1986; Schuster, 1990). With auto-PEEP, the distal airway pressures in
trapping. the lungs can be as high as 15 cm H O, while the ventilator’s proximal airway pres-
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sure manometer shows zero pressure (or PEEP if PEEP is used). Auto-PEEP can be
observed on the pressure-time waveform or measured by occluding the expiratory
port just before the next inspiration (Marini, 1988). To measure it accurately, the
patient should be sedated or paralyzed.

Auto-PEEP Increases Work of Breathing. Under normal conditions, a mechanical
breath is initiated when the inspiratory negative pressure reaches the sensitivity
setting of the ventilator. For example, when the normal end-expiratory pressure is
0 cm H O and the sensitivity is set at 22 cm H O, the pressure gradient (DP) or
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work of breathing to trigger a mechanical breath is 2 cm H O (from 0 cm to 2 cm
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H O). See Figure 12-2(A).
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If auto-PEEP is present, the work of breathing is increased because the level of auto-
PEEP in the lungs at end-expiratory phase must first be overcome before additional
inspiratory negative pressure can be used to reach the sensitivity setting. For example,
when the auto-PEEP level is 6 cm H O and the sensitivity is set at 22 cm H O,
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the pressure gradient (DP) to trigger a mechanical breath becomes 8 cm H O. Fig-
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ure 12-2(B) shows the distribution of 8 cm H O of pressure (6 cm H O to bring
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auto-PEEP from 6 to 0 cm H O plus 2 cm H O to reach the preset sensitivity level).
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Strategies to Reduce Auto-PEEP. To reduce the likelihood of auto-PEEP, the tidal
Auto-PEEP may be
reduced by reducing the volume or frequency may be reduced. The frequency during pressure support ven-
tidal volume or frequency, tilation should be kept less than 20 breaths per minute if possible. Auto-PEEP may
increasing the inspiratory
flow, and eliminating airflow also be minimized or eliminated by improving ventilation or providing a longer
obstruction. expiratory time. Two methods may be useful to reduce or eliminate the auto-PEEP,
and they are (1) improving ventilation and reducing air trapping by bronchodilators
and (2) prolonging the expiratory time by increasing the flow rate or reducing the
tidal volume or frequency.
Using PEEP to Reduce Effects of Auto-PEEP. When setting changes cannot correct auto-
PEEP, therapeutic PEEP may be used to reduce the effects of auto-PEEP that is due
to air trapping in the small airways (Note: patients with fixed obstruction in the
large airways should not be managed with therapeutic PEEP). The level of thera-
peutic PEEP used to counter the effects of auto-PEEP should be kept below 85%
of the measured auto-PEEP level (Wilkins et al., 2003). For example, PEEP level
of up to 5 cm H O may be used when auto-PEEP of 6 cm H O is measured dur-
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ing mechanical ventilation. Figure 12-2(C) shows that the pressure gradient (DP)
to trigger a mechanical breath drops to 3 cm H O (1 cm H O to bring auto-PEEP
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from 6 to 5 cm H O plus 2 cm H O to reach the preset sensitivity level).
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