Page 124 - Clinical Application of Mechanical Ventilation

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applied or increased to high levels. If PEEP decreases the blood pressure, first be sure that
the patient is not hypovolemic (Shapiro et al., 1991). If the blood volume is adequate,
then the PEEP should be decreased until an adequate blood pressure is reestablished.
A given amount of PEEP does not impede venous return to the same degree in
The detrimental effects different patients. If a patient has extremely low lung compliance, the airway pres-
of PEEP are dependent on the
compliance characteristics of sure is less readily transmitted into the pleural space. In effect, the low lung compli-
the patient. ance shields the pleural space from the full effects of the increased alveolar pressure.
Patients with adult respiratory distress syndrome (ARDS) usually have a very low
lung compliance and often require very high PEEP levels. However, despite high
PEEP levels, hemodynamic instability is seldom a problem unless the patient has
preexisting cardiovascular disease (Shapiro et al., 1991).
In contrast, if a patient has a normal or elevated lung compliance, the increased
alveolar pressure due to the PEEP will more readily be transmitted into the pleural
space. In other words, PEEP therapy in patients with normal or high lung compli-
ance will more likely produce an elevated pleural pressure and therefore a decreased
venous return (Shapiro et al., 1991).

Barotrauma. Barotrauma is lung injury that results from the hyperinflation of
PEEP greater than 10 cm alveoli past the rupture point. Although each patient is different, a PEEP greater
H 2 O (or mean airway pressure
.30 cm H 2 O, peak inspira- than 10 cm H O (or mean airway pressure .30 cm H O, or a peak inspiratory
2
2
tory pressure .50 cm H 2 O) is pressure .50 cm H O) is associated with an increased incidence of alveolar rup-
associated with an increased 2
incidence of barotrauma. ture or barotrauma (Bezzant et al., 1994; Slutsky, 1994). Alveolar rupture can
produce pneumothorax, tension pneumothorax, pneumomediastinum, pneu-
mopericardium, and pneumoperitoneum. Subcutaneous emphysema or crepitus
of unknown cause should always be interpreted as a sign that barotrauma has
occurred.
Since PEEP increases alveolar pressures and alveolar volumes, it has the poten-
tial to produce barotrauma (Petersen et al., 1983), especially when combined with
volume-controlled ventilation. Therefore plateau pressures should be closely moni-
tored and the therapist should be vigilant for signs of barotrauma.
In patients with normal Increased Intracranial Pressure. In patients with normal lung compliance, PEEP may
lung compliance, PEEP may
increase the intracranial raise the intracranial pressure (ICP) (normal 8 to 12 cm H O) due to an imped-
2
pressure due to impedance of ance of venous return from cerebral perfusion. However, in patients with ARDS or
venous return from the head.
noncompliant lungs, transmission of the excessive pressure generated by PEEP is
minimal and it does not cause as much adverse effect on a patient’s ICP.
Alterations of Renal Functions and Water Metabolism. Kidneys play an important role
Positive pressure ventila- in eliminating wastes, clearance of certain drugs, and regulating fluid, electrolyte,
tion can reduce the blood flow
to the kidneys and affect their and acid-base balance. They are highly vascular and at any one time receive about
normal functions.
25% of the body’s circulating blood volume (Brundage, 1992). Because of these
characteristics, the kidneys are highly vulnerable to a decrease in blood flow, as
would occur during positive pressure ventilation.
When perfusion to the glomeruli of the kidneys is decreased, filtration becomes
less effective (Baer et al., 1992). Subsequently, the urine output is decreased, as the
kidneys try to correct the perceived hypovolemic condition by retaining fluid. If
hypoperfusion of the kidneys persists or worsens, renal failure may result.

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