- • Acute onset of respiratory failure.
- • Bilateral infiltrates consistent with pulmonary
edema on chest radiograph.
- • Hypoxemia.
- • No evidence of left atrial hypertension.
The acute respiratory distress syndrome (ARDS) was first formally
described by Dr. Ashbaugh and colleagues in 1967. They reported
a total of 12 cases of patients with acute respiratory failure with
an overall mortality rate of 57%. The syndrome was defined
in that initial description as the acute onset of severe hypoxemia
that responded to initiation of mechanical ventilation with positive
end-expiratory pressure (PEEP), diffuse bilateral infiltrates on
chest radiograph, and decreased pulmonary compliance. Since that
original description the definition of ARDS has been debated and
refined. Definitions have evolved to better incorporate the clinical
spectrum of disease that was recognized to occur and to try to better
correlate the diagnostic definition with the pathophysiology of
the syndrome. In 1994 a group of experts convened and developed
the American–European Consensus Conference Committee definition
that is currently utilized by most practitioners and investigators
to identify ARDS. This definition has four components:
- 1. Acute onset of respiratory
- 2. Pao2/Fio2:<300 mm Hg = acute
lung injury (ALI) <200 mm Hg = acute respiratory distress
- 3. Chest radiograph: bilateral
alveolar infiltrates consistent with pulmonary edema.
- 4. No evidence of left atrial
hypertension. Pulmonary capillary wedge pressure <18 mm Hg if
In addition, the consensus document suggests that patients should
have an identified risk factor for the development of ALI and should
not have significant chronic lung disease. This definition incorporates
the concept that there is a spectrum of disease. All patients with
an arterial oxygen tension–pressure/fraction of
inspired oxygen (Pao2/Fio2) ratio < 300 mm Hg have
acute lung injury; only that subset with more severe hypoxemia,
as reflected in a Pao2/Fio2 < 200 mm Hg, are now
categorized as having ARDS.
The definition still has some elements that are being evaluated.
The meaning of acute onset has been discussed. Studies of patients
at risk for the development of ARDS have shown that the majority
of patients who develop ARDS do so within 3–5 days so that
is what most investigators use for the timeframe defined by acute.
The chest radiograph description has also been identified as an
area that needs to be more refined. Rubenfeld and colleagues had
a panel of experts review a set of randomly selected radiographs
from patients with Pao2/ Fio2 < 300. They found that
there was significant interobserver variability in the interpretation
of these films. For example, the percentage of radiographs identified
as being consistent with ALI varied from 36 to 71%. Accordingly,
there is now a taskforce working to establish more rigid criteria
for the interpretation of radiographs for this syndrome.
The diagnosis of left atrial hypertension may also be difficult.
Recent studies suggest that as many as 25% of patients
identified clinically as having ALI with no evidence of left atrial
hypertension actually had pulmonary artery wedge pressures greater
than 18 mm Hg when the measurements were made.
A number of clinical syndromes have been identified as risk factors
for ALI (Table 15–1). The most common predisposing
condition for ALI is sepsis. The incidence of ALI from sepsis ranges from
approximately 30 to 50%, depending on the definition used
for sepsis. Other common predisposing conditions include aspiration
of gastric contents and severe trauma. Pneumonia is identified as ...