At Glance
Acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS)

Acute respiratory distress syndrome (ARDS) is a medical condition occurring in critically ill patients characterized by widespread inflammation in the lungs. ARDS is not a particular disease, rather it is a clinical phenotype which may be triggered by various pathologies such as trauma, pneumonia and sepsis. The hallmark of ARDS is diffuse injury to cells which form the alveolar barrier, surfactant dysfunction, activation of the innate immune response, and abnormal coagulation. In effect, ARDS results in impaired gas exchange within the lungs at the level of the microscopic alveoli. The syndrome is associated with a high mortality rate between 20 and 50%. The mortality rate with ARDS varies widely based on severity, the patient's age, and the presence of other underlying medical conditions. Although the terminology of "adult respiratory distress syndrome" has at times been used to differentiate ARDS from "infant respiratory distress syndrome" in neonates, international consensus is that "acute respiratory distress syndrome" is the best moniker because ARDS can affect those of all ages.

Signs and symptoms

The signs and symptoms of ARDS often begin within two hours of an inciting event, but can occur after 1–3 days. Signs and symptoms may include shortness of breath, fast breathing, and a low oxygen level in the blood due to abnormal ventilation.

Causes

Diffuse compromise of the pulmonary system resulting in ARDS generally occurs in the setting of critical illness. ARDS may be seen in the setting of severe pulmonary (pneumonia) or systemic infection (sepsis), following trauma, multiple blood transfusions, severe burns, severe pancreatitis, near-drowning, drug reactions, or inhalation injuries. Some cases of ARDS are linked to large volumes of fluid used during post-trauma resuscitation.

Diagnosis

Diagnostic criteria for ARDS have evolved over time, in step with an increasing understanding of both the pathophysiology and the limits of diagnostic capability in general practice. The international consensus criteria for ARDS were most recently updated in 2012 and are known as the "Berlin definition". In addition to generally broadening the diagnostic thresholds, other notable changes from the prior 1994 consensus criteria include discouraging the term "acute lung injury," and defining grades of ARDS severity according to degree of hypoxemia (see history). With these 1994 criteria, arterial blood gas analysis and chest X-ray were required for formal diagnosis. Although severe hypoxemia is generally included, the appropriate threshold defining abnormal PaO2 has never been systematically studied. A severe oxygenation defect is not synonymous with ventilatory support. Any PaO2 below 100 (generally saturation less than 100%) on a supplemental oxygen fraction of 50% meets criteria for ARDS. This can easily be achieved by high flow oxygen supplementation without ventilatory support. Medical imaging is key to diagnosis. While a chest x-ray is often ordered as a first-line test, CT scanning showing a bilateral infiltrative process may also be helpful. Lung ultrasound, performed at the point of care, is becoming increasingly adopted in critical care medicine and aids in the diagnosis of ARDS as well, accurately distinguishing among the various causes of acute respiratory failure. Ultrasound findings suggestive of ARDS, as opposed to cardiogenic pulmonary edema.

Pathophysiology

ARDS is a form of noncardiogenic pulmonary edema provoked by an acute injury to the lungs that results in flooding of the lungs' microscopic air sacs responsible for the exchange of gases such as oxygen and carbon dioxide with capillaries in the lungs. Additional common findings in ARDS include partial collapse of the lungs (atelectasis) and low levels of oxygen in the blood (hypoxemia). However, in ARDS, these changes are not due to heart failure. The clinical syndrome is associated with pathological findings including pneumonia, eosinophilic pneumonia, cryptogenic organizing pneumonia, acute fibrinous organizing pneumonia, and diffuse alveolar damage (DAD). Of these, the pathology most commonly associated with ARDS is DAD, which is characterized by a diffuse inflammation of lung tissue. The triggering insult to the tissue usually results in an initial release of chemical signals and other inflammatory mediators secreted by local epithelial and endothelial cells. Neutrophils and some T-lymphocytes quickly migrate into the inflamed lung tissue and contribute in the amplification of the phenomenon. Typical histological presentation involves diffuse alveolar damage and hyaline membrane formation in alveolar walls. Although the triggering mechanisms are not completely understood, recent research has examined the role of inflammation and mechanical stress.

Inflammation

Inflammation, such as that caused by sepsis, causes endothelial dysfunction, fluid leakage from the capillaries and impaired drainage of fluid from the lungs. Elevated inspired oxygen concentration often becomes necessary at this stage, and may facilitate a 'respiratory burst' in immune cells. In a secondary phase, endothelial dysfunction causes cells and inflammatory exudate to enter the alveoli. This pulmonary edema increases the thickness of the alveolocapillary space, increasing the distance the oxygen must diffuse to reach the blood, which impairs gas exchange leading to hypoxia, increases the work of breathing and eventually induces fibrosis of the airspace. Edema and decreased surfactant production by type II pneumocytes may cause whole alveoli to collapse or to completely flood. This loss of aeration contributes further to the right-to-left shunt in ARDS. Similar to a traditional right-to-left shunt which refers to blood passing from the right side of the heart to the left side, thus bypassing oxygenation, lung right-to-left shunting occurs within the lungs. As the alveoli contain progressively less gas, the blood flowing through the alveolar capillaries is progressively less oxygenated, resulting in massive intrapulmonary shunting. The collapse of alveoli and small airways interferes with the process of normal gas exchange. It is common to see patients with a PaO2 of 60 mmHg (8.0 kPa) despite mechanical ventilation with 100% inspired oxygen. The loss of aeration may follow different patterns depending upon the nature of the underlying disease and other factors. These are usually distributed to the lower lobes of the lungs, in their posterior segments, and they roughly correspond to the initial infected area. In sepsis or trauma-induced ARDS, infiltrates are usually more patchy and diffuse. The posterior and basal segments are always more affected, but the distribution is even less homogeneous. Loss of aeration also causes important changes in lung mechanical properties that are fundamental in the process of inflammation amplification and progression to ARDS in mechanically ventilated patients.

Mechanical stress

Mechanical ventilation is an essential part of the treatment of ARDS. As the loss of aeration and the underlying disease progress, the end tidal volume grows to a level incompatible with life. Thus, mechanical ventilation is initiated to relieve respiratory muscles of their work and to protect the usually obtunded patient's airways. However, mechanical ventilation may constitute a risk factor for the development—or the worsening—of ARDS. Aside from the infectious complications arising from invasive ventilation with tracheal intubation, positive-pressure ventilation directly alters lung mechanics during ARDS. When these techniques are used the result is higher mortality through barotrauma. In 1998, Amato et al. published a paper showing substantial improvement in the outcome of patients ventilated with lower tidal volumes (Vt) (6 mL•kg−1). This result was confirmed in a 2000 study sponsored by the NIH. Both studies were widely criticized for several reasons, and the authors were not the first to experiment with lower-volume ventilation, but they increased the understanding of the relationship between mechanical ventilation and ARDS. This form of stress is thought to be applied by the transpulmonary pressure (gradient) (Pl) generated by the ventilator or, better, its cyclical variations. The better outcome obtained in patients ventilated with lower Vt may be interpreted as a beneficial effect of the lower Pl. The way Pl is applied on alveolar surface determines the shear stress to which lung units are exposed. ARDS is characterized by a usually inhomogeneous reduction of the airspace, and thus by a tendency towards higher Pl at the same Vt, and towards higher stress on less diseased units. The inhomogeneity of alveoli at different stages of disease is further increased by the gravitational gradient to which they are exposed and the different perfusion pressures at which blood flows through them. The different mechanical properties of alveoli in ARDS may be interpreted as having varying time constants—the product of alveolar compliance × resistance.Slow alveoli are said to be "kept open" using PEEP, a feature of modern ventilators which maintains a positive airway pressure throughout the whole respiratory cycle. A higher mean pressure cycle-wide slows the collapse of diseased units, but it has to be weighed against the corresponding elevation in Pl/plateau pressure. Newer ventilatory approaches attempt to maximize mean airway pressure for its ability to "recruit" collapsed lung units while minimizing the shear stress caused by frequent openings and closings of aerated units.

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