What is acute respiratory distress syndrome (ARDS)?

Causes and Symptoms

During acute respiratory distress syndrome
(ARDS), the lungs lose their ability to fill with air because they are filling with fluid from the capillaries (small blood vessels) instead. Damage to the vascular
endothelium can cause sepsis, while damage to the alveolar epithelium can cause the aspiration of gastric (stomach) contents. Either sepsis or aspiration of gastric contents can lead to ARDS because they both cause inflammation of the alveoli, the sacs within the lungs that normally fill with air when a person breathes. Inflammation of these alveoli causes them to fill with liquid instead. The liquid comes from the capillaries located within the walls of the alveoli that transport oxygen from the alveoli to the bloodstream. The buildup of fluid (edema) in the alveoli causes them to collapse, thus ending the transport of oxygen in the body.

The ratio of arterial partial oxygen tension, called PaO2, to FiO2, which is the fraction of inspired oxygen, is used to quantitatively describe the occurrence and severity of ARDS. The oxygen pressure of this ratio, expressed as PaO2:FiO2, indicates that the initial acute lung injury condition has occurred if its value is less than 300 millimeters of mercury (mmHg). If the oxygen pressure continues to decrease to a value of less than 200 mmHg for this ratio, then the patient is diagnosed with ARDS.

Death can result if ARDS is untreated because vital organs, such as the brain and kidneys, will stop functioning. If ARDS is diagnosed and treated in time, then death can be avoided, but permanent damage frequently occurs. Diagnosis using a chest X ray or an anteroposterior (AP) portable chest radiograph can facilitate the administration of the treatments. Additional diagnostic tests include sputum cultures and analysis, bronchoscopy, arterial blood gas testing, and blood chemistries testing.

Treatment and Therapy

Mechanical ventilation, the traditional treatment for ARDS, is a type of assisted breathing used when a patient has so much difficulty with breathing that death could otherwise result. After several days on a mechanical ventilator, however, the patient can develop pneumonia
with a 50 percent mortality rate. Therefore, alternative treatments are being developed and optimized to decrease the dependence on mechanical ventilation.

One of these alternative treatments, air pressure release ventilation (APRV), has been shown to be more effective at promoting spontaneous breathing with increased cardiac and organ responses. Because of this increased effectiveness, APRV requires a patient to spend less time on the ventilator, leading to decreased rates of pneumonia.

Positive end-expiratory pressure (PEEP) decreases the collapse of the alveoli. Ventilators plot a pressure-volume curve of the lung in order to determine the minimum PEEP to apply to a patient. This collection of pressure-volume data is crucial because the PEEP must match the surface tension of the alveoli. If the PEEP is too high, then the flow of blood to the heart will be negatively impacted.

Corticosteroids
can be a helpful treatment because they suppress the inflammation during the early phase of ARDS, when the alveoli are just beginning to fill with fluid. However, this treatment is very limited in scope. Doses of more than 2 milligrams per kilograms (mg/kg) per day are not beneficial, and these doses must be administered within the first three to five days. After that, it is too late for corticosteroids to be effective.

Nitric oxide can bind to hemoglobin and function as a selective pulmonary vasodilator. However, its effects are small and vary widely from patient to patient.

Perspective and Prospects

It was in 1994 that the American-European Consensus Conference (AECC) first defined the specific symptoms and possible causes of ARDS to facilitate the study of the pathogenesis of this syndrome and to develop treatments. Although ARDS has been recognized since World War I as a potentially fatal respiratory failure, the specific term “acute respiratory distress syndrome” was not adopted until 1967.

The results of a study by the National Institutes of Health (NIH) conducted in the 1970s indicated the incidence of ARDS to be seventy-five cases per 100,000 persons. After the formal definition was proposed in 1994 by the AECC, a study conducted from 1999 to 2000 indicated a frequency of eighty-six cases per 100,000 persons. Although ARDS can affect both males and females with equal probability at any age, there is an increase in the occurrence of ARDS with increasing age, with 306 per 100,000 for the seventy-five to eighty-four-year-old age group. In 2004, 1,736 deaths occurred in the United States as a result of 190,000 cases of ARDS.

Bibliography

"Acute Respiratory Distress Syndrome (ARDS)."  bmc.org . American Lung Association. Web. 21 May 2013.

Irwin, Richard S., and James M. Rippe, eds. Irwin and Rippe’s Intensive Care Medicine. 6th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2008.

Marino, Paul L. The ICU Book. 3d ed. Philadelphia: Lippincott Williams & Wilkins, 2007.

Wae, L., et al. “The Acute Respiratory Distress Syndrome.” New England Journal of Medicine 342 (2000): 1334–1349.