ARDS, SARS, and Sepsis
I Definition of Acute Respiratory Distress Syndrome (ARDS)
A ARDS: A diffuse, heterogenous inflammatory response of the lungs, resulting in hypoxemia, consolidation, and decreased compliance.
B The American-European Consensus Conference has provided the most precise definition of this syndrome (Box 23-1):
1. Sudden and acute onset of disease.
2. The presence of bilateral pulmonary infiltrates in all lung regions seen on frontal chest radiograph.
3. A pulmonary artery wedge pressure ≤18 mm Hg or a lack of clinical evidence of left atrial hypertension.
4. Severe hypoxemia: A Pao2/FIO2 ≤200 mm Hg regardless of positive end-expiratory pressure (PEEP) or FIO2 level.
5. A less severe form of ARDS, referred to as acute lung injury (ALI), is defined as a Pao2/FIO2 ratio ≤300 mm Hg.
C Although the aforementioned definition has become the most accepted definition of ALI/ARDS, it has been shown that alterations in FIO2 and PEEP can markedly affect the Pao2:FIO2 ratio, moving patients into and out of the classification of ALI or ARDS.
D Others have used varying assessment mechanisms to define ARDS. The most commonly reported is the Murray lung injury score.
1. This score is based on four areas:
| a. | Chest radiograph | Score | |
| (1) | No consolidation | 0 | |
| (2) | Consolidation confined to one quadrant | 1 | |
| (3) | Consolidation confined to two quadrants | 2 | |
| (4) | Consolidation confined to three quadrants | 3 | |
| (5) | Consolidation confined to four quadrants | 4 | |
| b. | Hypoxemia | Score | |
| (1) | Pao2:FIO2 ≥300 | 0 | |
| (2) | Pao2:FIO2 225 to 299 | 1 | |
| (3) | Pao2:FIO2 175 to 224 | 2 | |
| (4) | Pao2:FIO2 100 to 174 | 3 | |
| (5) | Pao2:FIO2 <100 | 4 | |
| c. | PEEP (if mechanically ventilated) | Score | |
| (1) | ≤5 cm H2O | 0 | |
| (2) | 6 to 8 cm H2O | 1 | |
| (3) | 9 to 11 cm H2O | 2 | |
| (4) | 12 to 14 cm H2O | 3 | |
| (5) | ≥15 cm H2O | 4 | |
| d. | Respiratory system compliance (when ventilated) | Score | |
| (1) | ≥80 ml/cm H2O | 0 | |
| (2) | 60 to 79 ml/cm H2O | 1 | |
| (3) | 40 to 59 ml/cm H2O | 2 | |
| (4) | 20 to 39 ml/cm H2O | 3 | |
| (5) | ≤19 ml/cm H2O | 4 | |
2. A score of 0 to 4 is given for each of the above available, and then scores are averaged.
3. ARDS is defined as a score >2.5; a mild to moderate injury is scored 0.1 to 2.5; and 0.0 indicates no lung injury.
E It is important to remember that there is no test or measurement that can precisely define or identify ARDS. Diagnosis is always based on the signs and symptoms described previously.
F Until a test is identified that can definitively diagnose ARDS there will continue to be controversy whether a patient truly has ARDS.
G Many believe there is a genetic predisposition of ARDS and that one day an “ARDS gene” will be identified.
II Incidence and Mortality of ARDS
A Because no precise definition for ARDS exists, it is difficult to precisely identify its occurrence in the general population.
B However, the epidemiologic data currently available indicate that approximately 3 to 19 cases of ARDS occur in every 100,000 individuals per year.
C Similarly the reported mortality of ARDS varies widely. Early reports indicate a mortality of 80% to 90%.
D Randomized clinical trials evaluating select populations of ALI and ARDS patients have reported mortalities as low as 25% to 30%.
E Epidemiologic data from widely distributed intensive care units indicate that the mortality for all patients with ARDS is still approximately 50% to 60%.
A By 1 year after hospital discharge most patients have regained the majority of pulmonary function lost during the acute illness.
B However, most have a decreased diffusing capacity, resulting in desaturation with exertion.
C At 1 year after discharge, feelings of anxiety, depression, and posttraumatic stress also are common.
D Quality-of-life surveys also indicate decrements in general and respiratory-associated parameters at 1 year after discharge.
A Two general categories of causative factors have been defined: Direct or primary pulmonary lung injury, and indirect or secondary nonpulmonary cause of lung injury.
A ARDS is characterized by diffuse alveolar damage and microvascular injury.
B Three distinct phases of ARDS/ALI from a pathophysiologic perspective have been defined: Exudative phase, fibroproliferative phase, and resolution phase.
C ARDS does not necessarily progress to the fibroproliferative phase; many patients rapidly move from the exudative phase to the resolution phase.
D Exudative (acute) phase (Figure 23-1)
1. On histologic examination of the lung the following are observed:
2. The adhesion and activation of neutrophils lead to the secretion of proinflammatory mediators, potentially leading to more injury (see Figure 23-1; Table 23-1).
TABLE 23-1
Proinflammatory Mediators Associated with the Development of ARDS/ALI
| Mediator Category | Mediators |
| Tumor necrosis factors | TNF-α, TNF-β |
| Interleukins | IL-1β, IL-2, IL-6, IL-10, IL-12 |
| Chemokines | IL-8, MIP-1, MCP-1, growth-regulated peptides |
| Colony-stimulating factors | G-CSF, GM-CSF |
| Interferon | IFN-β |
ARDS, Acute respiratory distress syndrome; ALI, acute lung injury.
From Wiedemann H: Systemic Pharmacolic Therapy of ARDS Resp Care Clin North Am 3:732, 1998.
3. The composition of pulmonary surfactant and its quantity are altered, increasing surface tension.
4. As the disease progresses deadspace ventilation increases.
5. Early in this exudative phase the major gas exchange issue is oxygenation. As this phase transitions into the fibroproliferative phase, ventilation generally becomes more of a problem.
6. The exudative phase generally lasts for approximately 3 to 7 days.
