Positive End-Expiratory Pressure (PEEP)
Manipulation of inspiration by means of the phase variables and modes
just discussed is one of the two main processes involved in mechanical
ventilation. The other is manipulation of end-expiratory pressure, which
may be kept equal to that of the atmosphere or deliberately raised to
produce positive end-expiratory pressure (PEEP). The application of PEEP
has two primary purposes:
- To increase lung volume in patients who have acute lung restriction
that produces hypoxemia; and,
- To reduce the effort required for patients to trigger the ventilator
or breathe spontaneously in the presence of dynamic hyperinflation and
auto-PEEP (see below).
When PEEP is applied to the breathing circuit connected to the closed
respiratory system of an intubated patient, all breaths start and end at a
pressure above ambient. Continuous pressurization of the system from which
a patient breathes spontaneously is referred to as CPAP, a term applicable
only during spontaneous breathing. Whenever positive pressure above the
end-expiratory level is applied during inspiration, the term PEEP is used.
Thus, a patient can be on A/C + PEEP, or on SIMV + CPAP, or on SIMV + PSV
+ PEEP.
As end-expiratory, end-inspiratory, and mean airway pressures are all
increased in the presence of PEEP, the potential exists for a fall in
cardiac output because of diminished venous return to the right side of
the heart. Regional or generalized lung overdistention can also stretch
pulmonary vessels, which reduces their caliber and increases pulmonary
vascular resistance. In the presence of a reduced cardiac output secondary
to either or both of these mechanisms, any gain in arterial oxygenation
may be offset, and tissue oxygen delivery may actually fall. In addition,
the application of PEEP may increase end-inspiratory lung volume to the
point at which individual lung units become overdistended and rupture
alveolar membranes, which leads to clinical barotrauma.
Unless PEEP is being adjusted according to the ARDS Network PEEP-FIO2
ladder according to institutional protocol, whenever feasible a
systematic, incremental ‘PEEP trial’ should be performed in as controlled
a manner as possible (Table 6
[32k PDF*]). Ideally, only one variable—the amount of PEEP—is
altered during the trial, with Vt, fraction of inspired oxygen (FIO2
), body position, and other factors that might affect oxygenation
unchanged. An assessment for both favorable and adverse PEEP effects is
made at each level as PEEP increased. As the condition of the patient may
change over time, to determine the effects of PEEP most clearly the
intervals at each level must be kept short.
As PEEP is increased, the occurrence cardiac impairment becomes
progressively more likely. Direct measurement of cardiac output during the
trial should be considered if any of the following circumstances exist:
- Levels of PEEP are to be used that are likely to impair cardiac
function (e.g. 15 cm H2O or more);
- Unexplained tachycardia or other manifestations of possible
hypovolemia are present; or
- The patient has underlying cardiac disease.
Changes in cardiac output and lung compliance are likely to occur
rapidly following an increase in PEEP, and should be sought within the
first 3-5 minutes at each level.
As PEEP is increased, PaCO2 is measured sequentially
as the primary index of a favorable response. If a substantial increase in
PaO2 occurs with no evidence of either cardiac impairment or alveolar
overdistention (as assessed using static compliance), that PEEP level can
be maintained and the FIO2 titrated downward to maintain
the target PaCO2. Improvement in PaCO2 tends
to occur more slowly than changes in cardiac function or compliance as
PEEP is increased, and arterial blood gas specimens should be drawn 10 to
20 minutes after each change.
If PEEP is reduced prematurely, some alveoli may remain sufficiently
unstable to collapse, which worsens oxygenation. If this happens, PEEP
higher than the previous baseline level may be required to reopen the
collapsed alveoli and, conceivably, the patient’s requirement for
mechanical ventilation may be unnecessarily prolonged. It is thus
important to be able to predict when patients are ‘ready’ for PEEP
weaning. The protocol given in Table 7
[22k PDF*] is designed to facilitate PEEP withdrawal while
protecting the patient from possible worsening of hypoxemia.
For patients whose arterial oxygenation has previously been shown to be
‘PEEP responsive’, the following criteria should be met before PEEP is
reduced:
- Hemodynamic stability with no changes in PEEP during the previous
6–12 hours;
- No signs of sepsis present; and
- Adequate arterial oxygenation, as shown by a PaCO2
/FIO2 ratio of 200 mm Hg or more.
Using the protocol in Table 7
[22k PDF*], maximum protection from unintended hypoxemia caused by
premature PEEP weaning can be achieved if a 3-minute trial of PEEP
reduction is carried out.
Advances in understanding of ventilator-induced lung injury, along with
results from major clinical trials, have led to a new overall approach to
the ventilator management of acute lung injury (ALI) and the acute
respiratory distress syndrome (ARDS), as discussed below. The
lung-protective ventilation protocol summarized in
Table 8 [43k PDF*] and the "FIO2
-PEEP Ladder" shown in Table 9 [20k
PDF*] provide overall guidance for application of both PEEP trials
and PEEP weaning as presented here.
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