Page 525 - Clinical Application of Mechanical Ventilation

Critical Care Issues in Mechanical Ventilation 491

normal compliant lung units are mingled with collapsed noncompliant lung
acute respiratory distress
syndrome (ARDS): A condition units. When these lungs are ventilated by positive pressure, the noncompliant
of sudden onset, characterized units are recruited intermittently while the compliant units suffer from overdis-
by non-cardiogenic pulmonary
edema on chest radiograph and a tention. As early as 1970, a study showed that when a peak transalveolar pressure
PaO 2 /F I O 2 of # 200 mm Hg.
of 30 cm H O was applied to the nonhomogenous lung units, the stress on the
2
wall between normal and collapsed units reached 140 cm H O (Mead, 1970). This
2
pressure harms the compliant lung unit since they cannot sustain extreme high
pressures.
When non-homogenous
lungs are ventilated by
positive pressure, the non-
compliant units are recruited Definitions of ALI and ARDS
intermittently while the
compliant units suffer from
over-distention. Acute respiratory distress syndrome (ARDS) is a more severe form of acute lung
injury (ALI). The mortality rate of patients with ARDS ranges from 10% to 90%.
This wide range may be attributed to the nonuniformity of the disease processes,
definition for ARDS, therapy modality, and patient population. In 1992, the
American-European Consensus Conference on ARDS recommended four evalu-
ation parameters for the classification of ALI and ARDS. These parameters are
timing of onset, oxygenation, chest radiograph, and pulmonary capillary wedge
pressure (PCWP) (Bernard et al., 1994). Table 15-1 lists the criteria and thresholds
for the classification of ALI and ARDS.

TABLE 15-1 Criteria and Thresholds to Classify ALI and ARDS

Evaluation Parameter ALI ARDS Notes

1. Timing Acute onset Acute onset
2. Oxygenation PaO /F O # 300 mm PaO /F O # 200 mm Hg Effects of PEEP are
I
2
2
I
2
2
Hg (regardless of (regardless of PEEP time-dependent
PEEP level) level)
3. Chest Radiograph Bilateral infiltrates on frontal chest radiograph Review pulmonary
capillary wedge
pressure (PCWP)
to evaluate cause
of pulmonary
edema

4. Pulmonary Capillary #18 mm Hg when measured or no clinical To rule out cardio-
Wedge Pressure evidence of left atrial hypertension genic pulmonary
(PCWP) edema

(Bernard et al., 1994.)
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