ARDS 

What is the definition of ARDS:

• ARDS (acute respiratory distress syndrome) refers to the severe end of the spectrum of “acute lung injury”. 

• Acute lung injury is characterized by three clinical features: 

  • Widespread, bilateral radiographic infiltrates 

  • A ratio of partial pressure of arterial oxygen to the fraction of inspired oxygen (PaO2/FiO2) less than or equal to 300 mm Hg. 

  • No clinical evidence for an elevated left atrial pressure.  Pulmonary capillary wedge pressure of 18 mm Hg or less.  

• For a diagnosis of ARDS to be made, the second criteria above requires a PaO2 to FiO2

  • ratio of 200 mm Hg or less. 

What is the etiology: for ARDS?

•  (to date, more than 60 causes of ARDS have been identified.)

• Sepsis (most common)

• Aspiration of gastric contents

• Infectious pneumonia

• Trauma

• Near-drowning

• Massive/multiple blood transfusions

• Drugs

• Lung and bone marrow transplantation

• Head injury

• Lung contusion

• Multiple other causes as well 

What is the pathophysiology: of ARDS

• Clinical signs and symptoms of ARDS occurs primarily as a result of inflammatory injury to the alveoli causing diffuse alveolar damage.  

• Pro-inflammatory cytokines and neutrophils accumulate in the lungs in response to a variety of precipitants.  

• Eventually the capillary endothelium and alveolar epithelium are damaged, resulting in an accumulation of bloody, proteinaceous edematous fluid in the alveoli. 

• This causes impaired gas exchange, impaired lung compliance, and pulmonary hypertension. 

Classic Clinical Presentation:

• Initially, the patient will present with clinical features that reflect the precipitant of ARDS. 

• For example, if sepsis is the cause of ARDS, the patient may appear febrile and hypotensive.  

• As the patient’s disease progresses to include ARDS, usually this is marked by severe hypoxia, dyspnea, tachypnea, stiff lungs, and diffuse radiographic infiltrates. 

• Pulmonary dysfunction typically develops within 24 to 48 hours.  

• Patients can also complain of chest pain and a dry cough. 

• Mechanical ventilation is almost always required. 

• Physical exam may reveal tachycardia, trachypnea, diffuse rales, rhonchi, and wheezes. 

• ABG usually show an acute respiratory alkalosis, an elevated alveolar-arterial oxygen gradient, and severe hypoxemia reflecting a right to left shunt. 

Radiographic findings: 

• The chest X-ray usually shows diffuse, fluffy alveolar infiltrates in all lung zones with prominent air bronchograms.  

• Although this finding is sometimes seen in patients without ARDS.  

• For instance, patients with CHF may have similar chest X-ray findings.  

• ARDS is favored over CHF when there is an absence of radiographic findings that are characteristic of CHF, such as cardiomegally, pulmonary venous congestion, and Kerley B lines.  

• To be sure that the patient’s symptoms are from ARDS and not CHF, one can check the pressure of the left side of the heart by measuring the pulmonary wedge pressure. 

• A left ventricular pressure of less than 18 would suggest that the pulmonary edema is not do to a back-up of blood from the left heart causing orthostatic pulmonary edema.  

• By the same token, an elevated wedge pressure does not exclude the possibility of ARDS, because it is estimated that 20% of patients with ARDS have concomitant left ventricular dysfunction. 

Complications:

• Complications of ARDS are usually from mechanical ventilation and include:

  • pneumothorax

  • subcutaneous emphysema

  • pneumomediastinum

  • interstitial emphysema

  • air embolism.   

Prognosis:

• Mortality 35-40%, with most patients dying of from the underlying cause of ARDS.

• Long term survivors of ARDS often show only mild abnormalities in pulmonary function and are usually asymptomatic.