Acute respiratory distress syndrome (ARDS)
Acute respiratory distress syndrome (ARDS) is a serious condition characterized by the sudden loss of lung function, leading to severe respiratory failure. During ARDS, the lungs become infiltrated with fluid, causing the alveoli—tiny air sacs responsible for gas exchange—to fill with liquid instead of air. This results from inflammation triggered by factors such as sepsis or aspiration of gastric contents, which damages lung tissue and disrupts the transport of oxygen into the bloodstream. A key diagnostic criterion for ARDS is the ratio of arterial oxygen tension (PaO2) to the fraction of inspired oxygen (FiO2); a PaO2/FiO2 ratio below 200 mmHg indicates ARDS. Treatment typically involves mechanical ventilation to support breathing, though alternative strategies like air pressure release ventilation (APRV) are being explored to reduce dependency on ventilators. Corticosteroids may help suppress early inflammation, but their effectiveness is limited to specific doses and timing. While ARDS can affect anyone, its incidence increases with age, and mortality rates can be significant, with variations in severity categorized by newer definitions. Timely diagnosis and intervention are crucial, as untreated ARDS can lead to organ failure and death.
Acute respiratory distress syndrome (ARDS)
Also known as: Respiratory distress syndrome (RDS), adult respiratory distress syndrome
Anatomy or system affected: Circulatory system, lungs, respiratory system
Definition: Respiratory failure caused by filling of the lungs with fluid from the capillaries, leading to a shortage of oxygen in the body that can eventually lead to death if not treated.
Causes: Lung injury, sepsis, drug overdose, blood-borne infection, transfusion, acute pancreatitis, pneumonia, trauma, aspiration, long-term illnesses
Symptoms: Difficulty breathing, low blood pressure, shock, organ failure
Duration: Symptoms develop within twenty-four to forty-eight hours; syndrome can last up to six months
Treatments: Mechanical ventilation, air pressure release ventilation (APRV), positive end-expiratory pressure (PEEP), corticosteroids, nitric oxide
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. Mortality rates for those with ARDS can range from 34 to almost 60 percent. The definition of ARDS was changed in 2012, from that of the American-European Consensus Conference to what is known as the Berlin Definition. The new definition groups diagnoses into three categories: mild, moderate, and severe (which, in turn, has four categories). The new categorizations were associated with increased rates of death.
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