6.0 Acute respiratory distress syndrome

Acute respiratory distress syndrome

a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs.

Acute respiratory distress syndrome (ARDS) is a life-threatening lung injury that develops following injury to the capillary endothelium of the alveolar space. This injury results in diffuse alveolar damage, where extracellular fluid seeps into the alveolar space thereby restricting the ability of air to reach the alveolar membrane and for gaseous exchange to occur.

ARDS usually occurs in people who are already critically ill or have major injuries, many of which will already be inpatients in hospital with related disease. The severity of ARDS means that most patients will not survive, with an increased mortality rate observed in the elderly. Of the people who survive ARDS, some fully recover, but others have chromic resulting lung damage.

ARDS has many risk factors which induce the inflammation to trigger the cascade that induces the syndrome. People are especially at risk if they have an pulmonary infection, such as pneumonia or pulmonary aspiration.

In addition to pulmonary infection or aspiration, extra-pulmonary sources include:

• Sepsis. The most common cause of ARDS is sepsis, a serious and widespread infection of the bloodstream.

• Coronavirus disease 2019 (COVID-19). People who have severe COVID-19 may get ARDS. Because COVID-19 mainly affects the respiratory system, it can cause lung injury and swelling that can lead to COVID-19-related ARDS.

• Head, chest or other major injury. Accidents, such as falls or car crashes, can damage the lungs or the portion of the brain that controls breathing.

• Breathing in harmful substances. Breathing in a lot of smoke or chemical fumes can lead to ARDS, as can breathing in vomit. Breathing in water in cases of near-drownings also can cause ARDS.

• Other conditions and treatments. Swelling of the pancreas (pancreatitis), massive blood transfusions and severe burns can lead to ARDS.

• Advanced age, smoking, alcohol use and being female.

• Drugs. Including radiation, chemotherapeutic agents and amiodarone

These extra-thoracic illnesses/injuries trigger an inflammatory cascade, culminating in pulmonary injury

The seriousness of ARDS symptoms can vary depending on the underlying aetiology and whether there is underlying heart or lung disease. Symptoms include:

• Severe shortness of breath.

• Laboured and rapid breathing that is not usual.

• Cough.

• Chest discomfort.

• Fast heart rate.

• Confusion and extreme tiredness.

ARDS can cause other medical problems while in the hospital, including:

• Blood clots. Lying still in the hospital while you're on a ventilator can make it more likely that you'll get blood clots, particularly in the deep veins in your legs. If a clot forms in your leg, a portion of it can break off and travel to one or both of your lungs, where it can block blood flow. This is called a pulmonary embolism.

• Collapsed lung, also called pneumothorax. In most people with ARDS, a breathing machine called a ventilator brings more oxygen into the body and forces fluid out of the lungs. But the pressure and air volume of the ventilator can force gas to go through a small hole in the very outside of a lung and cause that lung to collapse.

• Infections. A ventilator attaches to a tube inserted in your windpipe. This makes it much easier for germs to infect and injure your lungs.

• Scarred and damaged lungs, known as pulmonary fibrosis. Scarring and thickening of the tissue between the air sacs in the lungs can occur within a few weeks of the start of ARDS. This makes your lungs stiffer, and it's even harder for oxygen to flow from the air sacs into your bloodstream.

• Stress ulcers. Extra acid that your stomach makes because of serious illness or injury can irritate the stomach lining and lead to ulcers.

The chief treatment strategy is supportive care, focusing on:

1. Reducing shunt fraction,

2. Increasing oxygen delivery, 

3. Decreasing oxygen consumption, and 

4. Avoiding further injury. 

Patients are mechanically ventilated and given nutritional support until improvement is observed. Interestingly, the mode in which a patient is ventilated affects lung recovery. Evidence suggests that some ventilatory strategies can exacerbate alveolar damage and perpetuate lung injury in the context of ARDS. Care is placed on preventing volutrauma (exposure to large tidal volumes), barotrauma (exposure to high plateau pressures), and atelectrauma (exposure to atelectasis).

Non-ventilatory strategies include:

• Prone positioning and conservative fluid management once resuscitation has been achieved.

• Extracorporeal membrane oxygenation (ECMO). This has recently been advocated as salvage therapy in refractory hypoxemic ARDS, however two major trials that compared venovenous ECMO to standard care showed no difference in mortality between the two groups.

• Nutritional support via enteral feeding. A high-fat, low-carbohydrate diet containing gamma-linolenic acid and eicosapentaenoic acid has been shown in some studies to improve oxygenation. 

Diuretics - A patient can be guarded against fluid overload with diuretics which are prescribed as per guidelines listed in the BNF.

Glucocorticoids - these can be administered in patients in whom ARDS has been precipitated by a steroid-responsive process (eg, acute eosinophilic pneumonia) and to those with refractory sepsis or community-acquired pneumonia. Most patients who have persistent or refractory moderate to severe ARDS are relatively early in the disease course (within 14 days of onset with a PaO2/FiO2 ratio <200 mm Hg) despite initial management with standard therapies, including low tidal volume ventilation, can also be managed with glucocorticoids.[19] However, glucocorticoids are generally avoided in patients with less severe ARDS or those with persistent ARDS beyond 14 days. Moreover, their use is associated with worse outcomes in patients with certain viral infections, including influenza. 

The Surviving Sepsis Campaign guidelines recommend the use of intravenous hydrocortisone, 200 mg per day, in patients with sepsis who are hemodynamically unstable despite fluid administration and vasopressor therapy. Given the evolving literature, it is reasonable to consult with an intensivist about the use of corticosteroids when caring for a patient with ARDS.