Pediatric Acute Respiratory Distress Syndrome Workup

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.

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

The first of the 3 phases, the exudative phase, occurs during days 1-7 of the initial injury. Typical histologic appearances 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 proliferative phase begins at about day 7 of the illness. The main features of this period include fibroblast proliferation, hyperplasia of type II pneumocytes, and ongoing evidence of inflammation.

The fibrotic phase begins approximately 3 weeks after the onset of illness; its main features are fibrosis, honeycombing, and bronchiectasis.

No definitive laboratory tests establish the diagnosis of acute respiratory distress syndrome (ARDS). However, because ARDS often develops concomitantly with severe acute illness, major derangement of laboratory indices may be present, including thrombocytopenia and abnormal liver function, renal function, electrolyte levels, blood glucose concentrations, lactate values, and coagulation parameters. Hypoproteinemia is predictive of ARDS, weight gain, and death in patients with severe sepsis.^[20]

Chest radiography may be useful beyond its use as part of the diagnostic criteria. Persistent regional areas of atelectasis may suggest the use of bronchoscopy including bronchoalveolar lavage (BAL) for diagnostic and therapeutic consideration. Computed tomography (CT) of the chest, although not routinely ordered, 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 give 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).

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

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

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.

Because of the uncertainty imposed by the measurement of the partial pressure of oxygen in arterial blood (PaO₂) and the necessity of standardizing the definition of ARDS, the ratio of PaO₂ to the fraction of inspired oxygen (FiO₂) is used as a measure of disease severity. Without an arterial catheter in place, however, arterial blood gas samples are often deferred. Oxygenation is typically assessed by pulse oximetry and ventilation by capillary or venous blood gas sampling. Because of instances when arterial blood gas sampling is deferred, methodology has been developed to correlate the PaO₂/FiO₂ ratio using pulse oximetry as described above.^{[5, 6]}

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.

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.^[21]

Subsequently, progressive bilateral interstitial and alveolar infiltrates develop without cardiomegaly (see the images below).

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).

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).

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.^[22] 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.^[22]

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).

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).^[22]

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.^[23] 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.

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.

Many investigators are interested in the use of BAL as a research tool. Cytokine levels in BAL fluid have been determined in ARDS patients. Much has been learned regarding the complex interplay of the inflammatory response in ARDS. In a small series of patients, elevated levels of IL-8 in BAL fluid was predictive of ARDS in at-risk patients and predictive of mortality in patients with ARDS.^[24]