Pediatric Acute Respiratory Distress Syndrome Workup

Updated: Nov 23, 2016
  • Author: Prashant Purohit, MD; Chief Editor: Timothy E Corden, MD  more...
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Workup

Approach Considerations

The diagnosis of ARDS is established based on the definitions described above, Berlin definition for adults and The Pediatric Acute Lung Injury Consensus Conference Group definition of PARDS for children. Chest imaging including a radiograph and/or a computed tomogram are part of both the definitions. Rest of the work up is usually geared towards identification of underlying disease, assessment of ARDS progress, prevention and treatment of co-morbidities and the other aspects of ongoing management as discussed below.

Chest radiography may be useful beyond its use as part of the diagnostic criteria, as discussed below in radiography section. Computed tomography (CT) of the chest, although not routinely performed, may be helpful in differentiating between atelectasis and consolidation. Ultrasonography is an easy method of further assessing pleural effusions and differentiating between transudative and exudative fluid. Echocardiography may help exclude cardiogenic edema and would provide information regarding cardiac contractility, intraventricular volume, pulmonary hypertension, and other potential anatomic abnormalities. There appears at present to be no particular indication for magnetic resonance imaging (MRI). Other laboratory work up including hematology and blood chemistry would help identify involvement of the other organ system and further management accordingly. Persistent regional areas of atelectasis may suggest the use of bronchoscopy including bronchoalveolar lavage (BAL) for diagnostic and therapeutic consideration. Routine use of BAL is not recommended in ARDS. Indications, risks and yield of the procedure should be determined by the treating physician. Similarly, there are no specific guidelines for other laboratory work up. Utility of the other tests should be determined by treating physician.

Go to Acute Respiratory Distress Syndrome and Barotrauma and Mechanical Ventilation for complete information on these topics.

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Other Tests

The only role for chest ultrasonography in patients with ARDS is to define the presence of pleural effusions and to determine whether loculation of the pleural fluid is present if drainage of the effusion is being considered.

To the authors’ knowledge, no data are available concerning the role of MRI in patients with ARDS.

The primary role of echocardiography in ARDS is to detect congenital or acquired heart disease as a cause of respiratory distress and pulmonary edema. Echocardiography may provide evidence of pulmonary hypertension; however, the practical implications of this finding are unclear, because little evidence supports the clinical benefit of pulmonary vasodilators in ARDS.

Many authorities debate the use of determining pulmonary mechanics as a means of defining optimal ventilatory strategies. As of yet, no clear consensus on their use has been established.

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Histologic Findings

Three classic histopathologic phases of ARDS are described. These correspond to the time course of the disease.

Typical histologic appearances of the exudative phase include diffuse hemorrhage, edema, leukocyte infiltration, and cellular necrosis or apoptosis. Evidence of the initiating illness may also be apparent, such as pneumonia or aspiration.

The main features of the proliferative phase include fibroblast proliferation, hyperplasia of type II pneumocytes, and ongoing evidence of inflammation.

The fibrotic phase is characterized by the presence of fibrosis, honeycombing, and bronchiectasis.

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Laboratory Tests

Suggested laboratory tests include arterial blood gas (ABG) measurements, a complete blood count (CBC) with differential, and an electrolyte panel with blood urea nitrogen (BUN) and creatinine.

Arterial blood gas measurements

Measurement of P/F ratio is an essential part of Berlin definition. However, the Pediatric Acute Lung Injury Consensus Conference group has recommended utilization of oxygen saturation and OSI. This would eliminate the burden of invasive tests in pediatric population. In case of presence of arterial catheter, the group also recommended utilizing OI ratio. All these parameters (P/F ratio, OI and OSI) will also help identify the severity of ARDS.

The onset of capillary congestion and changes in the alveolar epithelium during the initial exudative stage leads to significant ventilation/perfusion (V/Q) mismatching and intrapulmonary shunting. During this stage of ARDS, oxygen diffusion is impeded to a much greater extent than carbon dioxide diffusion. Respiratory alkalosis reflecting a relative hyperventilation and hypocarbia is an early sign of ALI/ARDS. This difference is attributable to the much greater solubility of carbon dioxide. Hypercarbia develops with worsening disease, reflecting an increasing shunt fraction and an increased dead space.

Complete blood count

The CBC may indicate an infectious etiology. Leukocytosis may be evident, reflecting either the initiating stimulus or a nonspecific inflammatory response. The CBC may also uncover significant anemia, which will further compromise oxygen-carrying capacity. Anemia may secondary to acute illness, underlying chronic disease, acute blood loss, or hemodilution from massive fluid resuscitation. Thrombocytopenia may be present.

Electrolyte panel

An electrolyte panel may also screen intravascular volume status, anion gap acidosis, and other potential comorbidities. Additional laboratory tests would be indicated pending specific concerns toward individual patients.

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Radiography

Chest radiography is essential for diagnosing ARDS or ALI. The radiologic findings in ARDS are nonspecific (see the images below). Radiographic findings immediately after the inciting event may be entirely normal or may show only the primary disease process. Early changes reflect increased pulmonary alveolar and endothelial permeability. Studies of pediatric and adult patients reveal low levels of interobserver agreement for radiographs obtained early in the course.

Eight-year-old girl with diagnosis of pneumonia. C Eight-year-old girl with diagnosis of pneumonia. Chest radiograph on day of admission.
Eight-year-old girl with pneumonia and impending r Eight-year-old girl with pneumonia and impending respiratory failure. Chest radiograph on day 2.

Subsequently, progressive bilateral interstitial and alveolar infiltrates develop without cardiomegaly.

As the disease progresses, the lung fields become diffusely and homogeneously opaque. However, this homogeneous appearance is misleading, as chest CT scanning demonstrates. Although the radiographic appearance may initially be indistinguishable from that observed in cardiac failure, numerous characteristic differences are present.

ARDS-related edema and edema secondary to heart failure may be difficult to distinguish on radiographs. Cardiomegaly is not a feature of ARDS; it is usually present with marked cardiac failure. Kerley B lines, which indicate interstitial edema or lymphatic swelling, are rarely observed in ARDS.

Other radiologic differential diagnoses of the infiltrates observed in ARDS include aspiration, hemorrhage, pneumonia, and atelectasis. Distinguishing these entities on the basis of chest radiographic appearances is often difficult. As opacification of the lung fields increases, air bronchograms may become apparent.

Air-leak syndromes are commonly observed on plain chest radiographs of patients with ARDS. These include pneumothorax (see the image below), pneumomediastinum, pneumopericardium, subcutaneous emphysema (see the image below), pneumoperitoneum, and pneumoretroperitoneum (free air in the retroperitoneal space).

Subcutaneous emphysema and pneumothorax. Subcutaneous emphysema and pneumothorax.

In intubated patients, free air rises to the high caudal areas overlying the diaphragm because of their supine position. Early and subtle signs suggestive of free air include the deep sulcus sign, which is increased radiolucency in the costophrenic angle of the affected side and increased acuteness of the costophrenic angle on the same side.

The double-diaphragm sign is also reported in association with air leaks; subpulmonic air produces the impression of a second diaphragm formed by the basal border of the lower lobe. Air below the diaphragm, which does not cross the midline, suggests pneumoretroperitoneum.

Characteristic radiologic changes of late ARDS corresponding to histopathologic changes are well described. After a variable period (ie, usually days to weeks), patchy areas of increased lucency appear. Associated with clinical resolution of illness, radiologic improvement follows slowly.

Although radiologic changes completely resolve in most children, chronic changes are apparent in a small subset. Whether the persisting changes (often ascribed to fibrosis) are the result of the primary illness or ventilator-induced lung injury (VILI) is often unclear. Iatrogenic features visible on a chest radiograph in a patient with ARDS may include an endotracheal tube (see the first image below), central venous lines, and chest tubes (see the second image below).

Chest radiograph in 3-year-old girl who developed Chest radiograph in 3-year-old girl who developed acute respiratory distress syndrome due to overwhelming gram-negative sepsis. Salient features include endotracheal tube; diffuse, bilateral infiltrates; air bronchograms on left side; and central venous catheter. Ratio of arterial oxygen tension to fraction of inspired oxygen at time of chest radiography was 100.
Chest radiograph demonstrates complication of acut Chest radiograph demonstrates complication of acute respiratory distress syndrome. Patient presented with respiratory failure after near-drowning episode. Peak inspiratory pressures were 40 cm water. Patient had sudden desaturation and decreased bilateral air entry, as well as cool peripheries and decreased blood pressure. Needle evacuation of both pleural spaces confirmed pleural air. Chest tubes were placed, with immediate improvement in clinical status. Pulmonary status continued to deteriorate; high-frequency oscillatory ventilation was given. Patient subsequently required second chest tube on left side.
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Computed Tomography

Since CT scanning of the chest was first reported, its usefulness for understanding the pathophysiologic mechanisms underlying ARDS and the response of the ARDS lung to ventilator maneuvers has been described many times.

Gattinoni et al have been at the forefront of this research. [48]  Before the introduction of CT imaging, clinicians assumed that ARDS was a homogeneous lung process. The use of chest CT scanning demonstrated that although pulmonary involvement in ARDS was diffuse, it also was heterogeneous. In 1994, Gattinoni et al reported that, in adults with ARDS, areas of normal lung were interspersed with poorly aerated lung parenchyma. [48]  

Researchers have shown a marked spatial distribution of parenchymal collapse in the lungs of ARDS patients. In patients ventilated in a supine position, collapse was most pronounced in the more dorsal regions. A combination of edematous lung, the weight of the chest wall and mediastinal structures (specifically, the heart), and supine positioning are postulated to play a part in the development of dorsal atelectasis (see the image below). These findings provide an intellectual basis for the role of prone positioning in severe ARDS (see Treatment).

Chest CT in 6-month-old male infant with newly dia Chest CT in 6-month-old male infant with newly diagnosed cystic fibrosis. Patient was intubated for respiratory failure and subsequently developed acute respiratory distress syndrome. Image demonstrates numerous cystic and bronchiectatic areas. Note dorsal distribution of atelectasis, particularly on right side.

CT findings support the "baby lung" hypothesis. Simply stated, the lungs of patients with ARDS are functionally smaller than normal lungs. Indeed, some authors suggest that the volume reduction may be approximately 75% of total lung volume. Hence, ventilation with normal physiologic tidal volume may lead to iatrogenic lung damage. The data showing improved outcomes in patients with ARDS ventilated with small tidal volumes lend credence to this theory.

Gattinoni proposed 2 types of ARDS: ARDS due to primary pulmonary disease (eg, aspiration, pneumonia) and ARDS arising secondary to extrapulmonary disease (eg, sepsis, trauma). [48]

In support of this hypothesis, Goodman et al described CT findings in adults with ARDS due to pulmonary and extrapulmonary disease and noted marked differences between populations. [49] The group with pulmonary-related ARDS had ground-glass opacification or consolidation, which tended to be asymmetric. The group with extrapulmonary ARDS generally had symmetric ground-glass opacification.

In both groups, pleural effusions and air bronchograms were common, whereas Kerley B lines and pneumatoceles were rare. Mortality tended to increase in the group with extensive consolidation versus those with extensive ground-glass opacification; this difference was not statistically significant.

In the present clinical setting, the main use of chest CT scanning is for determining the presence of coexisting illness—specifically, thoracic abscess formation, barotrauma undefined on plain radiography, or other unsuspected pathology. CT is not routinely required for diagnosis or management of ARDS.

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Bronchoalveolar Lavage

BAL is not required for diagnosis of ARDS. It may be useful in determining the underlying etiology in patients with primary pulmonary ARDS in whom pneumonia or an infective pneumonitis is thought to be the cause. This is especially true for immunocompromised patients. It is also used in cases of persistent regional areas of atelectasis for diagnostic and therapeutic consideration. Routine use of BAL is not recommended in ARDS. Indications, risks and yield of the procedure should be determined by the treating physician. Many investigators are interested in the use of BAL as a research tool. 

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