Remember that the objective of using PEEP is
to 1) restore functional residual capacity, & 2) to increase mean airway
pressure and improve oxygenation by reducing ventilation-perfusion
mismatch. The required PEEP depends on:
1) The extent of lung injury:
determined by the alveolar-arterial oxygen gradient (or the PaO2/FiO2
ratio).
2) The patient's chest wall compliance.
As stated previously (click here), additional weight to the chest reduces
FRC, by reducing the tendency of the chest wall to spring outwards; the
chest wall compliance. This is examplified in obese patients, and explains
their chronic respiratory failure. Low chest wall compliance can be
acquired in critical illness due to circumferential chest dressings,
extensive edema, and, in particular, raised abdominal pressure. Patients
who have had large volume fluid resuscitation develop extensive tissue
edema, bowel distension, ascites and abdominal hypertension. The
diaphragmatic excursion is limited, and dependent atelectasis results.
Moreover, the heart increases in size and weight and compresses the left
lower lobe.
The result of this is that patients with low chest wall compliance, such
as surgical critically ill patients, require higher trans-alveolar
pressure to achieve the same tidal volumes, which means higher PEEP to
restore FRC. Elderly patients, particularly those with COPD, may have very
high chest wall compliance, and require relatively low levels of pressure
to generate target tidal volumes.
