41: Acute Respiratory Distress Syndrome (ARDS)

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CHAPTER 41 Acute Respiratory Distress Syndrome (ARDS)

James B. Haenel, RRT, Jeffrey L. Johnson, MD

1 What is the difference between acute lung injury and acute respiratory distress syndrome?

In 1992 the North American-European Consensus Committee (NAECC) on acute respiratory distress syndrome (ARDS) convened with the goal of providing a clearer and more uniform definition of acute lung injury (ALI)/ARDS. Two years later ALI was defined as an acute inflammatory condition resulting in increased lung permeability. The clinical presentation of ALI and ARDS in terms of timing, etiologies, chest film appearance, and lack of primary left atrial hypertension is comparable. ALI/ARDS is the same syndrome, but the degree of hypoxemia is worse in ARDS. Of importance, the degree of the hypoxemia is reflective of disease severity only; it does not predict mortality.

2 How would you define acute respiratory distress syndrome?

Three clinical definitions are commonly used: the NAECC definition, the Murrary Lung Injury Score, and the Delphi definition (Table 41-1). The NAECC definition of ALI/ARDS offers two advantages; first, it provides a method of classifying patients who have less severe or a milder form of lung injury; and second, the definition is simple enough for those with even limited experience to readily apply. Unlike other definitions the NAECC has no reference to the set positive end-expiratory pressure (PEEP) level. This absence of a minimum level of PEEP and other ventilator settings maintains the simplicity of the NAECC definition but makes it too nonspecific. For example, a patient on 5 cm H2O of PEEP could have a PaO2/FiO2 ratio of <200; but, after a recruitment maneuver (RM) using a higher level of PEEP, 5 minutes later that same patient may have a PaO2/FiO2 level of >300.

TABLE 41-1 Definitions of Acute Lung Injury and Acute Respiratory Distress Syndrome (American-European Consensus Conference)

Acute Lung Injury Criteria
Timing: acute
Oxygenation: PaO2/FiO2 ≤300 mm Hg (regardless of PEEP)
Chest radiograph bilateral infiltrates on anteroposterior film
Pulmonary artery occlusion pressure: <8 mm Hg or no clinical evidence of left arterial hypertension
ARDS criteria
Same as acute lung injury except
Oxygenation: PaO2/FiO2 ≤200 mg Hg regardless of PEEP)
Murray
Lung Injury Score
Chest Radiograph Score
No alveolar consolidation 0
Alveolar consolidation: 1 quadrant 1
Alveolar consolidation: 2 quadrants 2
Alveolar consolidation: 3 quadrants 3
Alveolar consolidation: 4 quadrants 4
Hypoxemia Score
PaO2/FiO2 ≥300 0
PaO2/FiO2 225–299 1
PaO2/FiO2 175–224 2
PaO2/FiO2 100–174 3
PaO2/FiO2 100 4
PEEP Score (when ventilated)
PEEP ≥ 5 cm H2O 0
PEEP 6–8 cm H2O 1
PEEP 9–11 cm H2O 2
PEEP 12–14 cm H2O 3
PEEP ≥15 cm H2O 4
Respiratory System Compliance Score
Compliance ≥ 80 ml/cm H2O 0
Compliance 60–79 ml/cm H2O 1
Compliance 40–59 ml/cm H2O 2
Compliance 20–39 ml/cm H2O 3
Compliance ≤19 ml/cm H2O 4
The final value is obtained by dividing the aggregate sum by the number of components that were used: no lung injury, 0; mild-to-moderate injury, 1–2.5; severe lung injury (ARDS), 2.5.
DELPHI Definition of ARDS
Timing: acute onset
Oxygenation: PaO2/FiO2 ≤200 With PEEP > 10
Chest radiograph: bilateral infiltrates
Absence of congestive heart failure or presence of recognized risk factors for ARDS

ARDS, Acute respiratory distress syndrome; PEEP, positive end-expiratory pressure.

3 What are the risk factors for acute respiratory distress syndrome?

Historically ARDS has been described as being a homogeneous and overwhelming inflammatory reaction of the pulmonary parenchyma to a variety of insults. Currently classification and identification of risk factors for ARDS are based on whether the inciting event is either a direct or indirect insult to the lung parenchyma (Table 41-2).

TABLE 41-2 Classification of Inciting Events Associated with ALI/ARDS

Direct Lung Injury Indirect Lung Injury
Aspiration of gastric contents Sepsis
Pulmonary contusion Multisystem trauma associated with shock
Diffuse pulmonary infections

image Bacterial
image Fungal
image Viral
image Pneumocystis

Massive transfusion > 6 units PRBCs in ≤12 hours
Drug overdose
Acute pancreatitis
Cardiopulmonary bypass (rare)
Fat emboli syndrome

Inhalational injury Near drowning

ALI, Acute lung injury; ARDS, acute respiratory distress syndrome; PRBC, packed red blood cells.

Stratification of risk factors based on whether ARDS is caused by a pulmonary or extrapulmonary etiology has important implications beyond establishing its epidemiology and incidence. There are identified important mechanical differences in the lungs and chest wall compliance of patients based on whether there is a pulmonary or extrapulmonary cause for the ARDS. ARDS resulting from direct pulmonary disease is associated predominantly with lung tissue consolidation; therefore the response of a stiff lung to an RM with PEEP may be modest at best and carry the risk of overdistention of normal alveoli. In contrast, the lung suffering from an indirect insult demonstrates increased interstitial edema and diffuse alveolar collapse. Application of an RM with PEEP in this situation often results in a salient improvement in lung compliance and oxygenation.

4 What is the most common cause and mortality rate for acute respiratory distress syndrome?

In the past sepsis syndrome was the most common risk factor for the development of ARDS. In the 1980s three prospective studies evaluated the risk factors for ARDS, and sepsis was identified as a risk in 43% of the cases. The most common diagnosis associated with ARDS, reported in the National Institutes of Health (NIH) Acute Respiratory Distress Syndrome Network (ARDS NET) study, was pneumonia (incidence = approximately 35%); sepsis as an etiology had an incidence of 27%.

The difficulty with interpreting mortality data is that the definition for this syndrome has changed at least three times. Yet three large randomized trials involving over 2000 patients provided consistent mortality data of between 33.9% and 36.3%.

5 Describe the pathogenesis of acute respiratory distress syndrome

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