Page 123 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 89

Physiology of PEEP

PEEP reinflates collapsed alveoli and supports and maintains alveolar inflation
during exhalation. Once “recruitment” of these alveoli occurs and is sustained,
PEEP decreases the threshold for alveolar opening and facilitates gas diffusion and
oxygenation (Tyler, 1983).
Normally, the alveolar end-expiratory pressure equilibrates with atmospheric pres-
sure (i.e., zero pressure) and the average pleural pressure is approximately –5 cm
H O. Under these conditions, the alveolar distending pressure is 5 cm H O (alveolar-
2
2
pleural). This distending pressure is sufficient to maintain a normal end-expiratory alveo-
lar volume to overcome the elastic recoil of the alveolar wall. However, if the force of elas-
tic recoil is increased due to a decrease in compliance, the alveolar volume will decrease.
If the lung compliance continues to deteriorate, the elastic recoil forces can become great
enough to completely overcome the normal alveolar distending pressure, resulting in al-
veolar collapse and intrapulmonary shunting. PEEP increases the alveolar end-expiratory
pressure which decreases the pressure threshold for alveolar inflation. Re-expansion of
the collapsed alveoli improves ventilation and reverses intrapulmonary shunting.

PEEP
T
Decreases the pressure threshold for alveolar inflation
T
Increases FRC
T
Improves ventilation
T
(1) Increases V/Q
(2) Improves oxygenation
(3) Decreases work of breathing

Complications of PEEP

Complications and hazards associated with PEEP include (1) decreased venous
return and cardiac output, (2) barotrauma, (3) increased intracranial pressure, and
(4) alterations of renal functions and water metabolism.
Decreased Venous Return. Assuming a normal intravascular volume, venous return to
the right atrium is influenced by the difference in the central venous pressure and the
negative pleural pressure that surrounds the heart. During PEEP, the pleural pressure
becomes less negative and the pressure gradient between the central venous drainage
and the right atrium will decrease resulting in a decreased venous return. This in turn
results in a decreased cardiac output and hypotension (Qvist et al., 1975).
Since PEEP increases both Experience has shown that significant increases in the mean airway pressure are more
peak inspiratory pressures
and mean airway pressures, it likely to increase pleural pressures sufficiently to decrease venous return. Since PEEP
has the potential to decrease increases both peak inspiratory pressures and mean airway pressures, it has the potential
venous return and cardiac
output. to decrease venous return and cardiac output. It is vital to closely monitor the patient re-
ceiving PEEP therapy for any drop in blood pressure, especially when PEEP is either first

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