Acute Respiratory Distress Syndrome

Samuel E. Greenberg, M.D.

When a patient suffers from a severe infection, blunt chest trauma of significant magnitude, or inhalation of toxic smoke or fluid, it is incumbent for the treating physician to suspect ARDS. This Syndrome or constellation of symptoms and signs results in death in greater than 50% of those who develop it.

Acute Respiratory Distress Syndrome is a diffuse injury to the lung tissue, presenting clinically as a patient, who is short of breath, often pale and moist, and initially agitated. Later, the patient often becomes lethargic and may progress to a comatose state. Because of diminished oxygenation, bluish-gray (cyanosis) of the extremities are present.

The tissue injury involves the membrane between the air sacs (alveoli) and the blood vessels. This injury is manifested by increased permeability of this alveolar/vascular membrane allowing fluid to seep into the air sacs and the development of an inflammatory reaction. Later, if the inflammation does not subside, scar tissue (fibrosis) develops causing permanent lung damage.

This fluid in the air sacs, clinically, is similar to that of Congestive Heart Failure (CHF). The physical findings of crackles and wet breath sounds (rales) are present in both conditions. But, in the instance of CHF, there is an elevation of the Pulmonary Artery pressure and a weakening of the left ventricle. This differentiation can be confirmed by the insertion of a Pulmonary Artery catheter to measure the pressure.

Predispositions:

1.) Trauma to Chest-Suspect the possibility of ARDS if there exists

A.) Multiple Rib Fractures
B.) Contusion of the lungs

2.) Toxic Inhalation
Near drowning

3.) Sepsis-Pneumonia, Pancreatitis, etc.

4.) Head injuries

5.) Overdose or drug inter-reaction, etc.

6.) Multiple Blood Transfusions

Clinical Findings:

1.) The condition generally announces itself shortly after admission to the Emergency Room or Hospital, within the first 24 to 48 hrs.

2.) The Chest X-ray shows bilateral fluffy infiltrates and may progress to a "whiteout". This is indistinguishable from CHF.

3.) The PaO2 is in the hypoxic range, below 50 and the PaO2/FIO2 ratio is less than 200. If it is less than 300, a milder form of ARDS, called ALI (Acute Lung Injury) is present.

4.) Normal Pulmonary Artery Wedge Pressure.

Laboratory Tests:

1.) There is no specific test for ARDS

2.) Arterial Blood gases-if it is below 50 mm.Hg.

3.) Check the FIO2 (Fractional concentration of Oxygen in the inspired air. The severity of Hypoxemia needed to make the diagnosis of ARDS is defined by the ratio of Arterial Oxygen Partial Pressure (PaO2) to the FIO2. If the PaO2/FIO2 is 200 or below then ARDS is diagnosed. If the PaO2/FIO2 is 300 or below, then ALI (Acute Lung Injury), a less severe form of ARDS is diagnosed.

4.) Sputum cultures, either from endotracheal tube suction or by bronchoscopy to identify cause of infection.

5.) Blood Culture for infection to implicate Sepsis.

6.) Complete blood count, urinalysis-to search for infection.

7.) Chest X-ray

8.)Swan-Ganz catheter to measure Pulmonary Artery Oxygen Pressures.

9.) Blood for FIO2.

10.) Echocardiogram to exclude CHF

Treatment
The most important aspect of treatment is to properly Oxygenate the patient, generally by Mechanical Ventilation. Increasing the pressure (PEEP) can assist in pushing the fluid out of the air sacs so more air and oxygen can be offered to the tissues. Since the most common cause of ARDS is Sepsis, the institution of antibiotics is justified. Maintenance of vital signs by cardiovascular mediation.

SUMMARY
Acute Respiratory Distress Syndrome is a serious consequence of tissue damage to the membrane between the air sacs and the arteries in the lungs. It is a result of the changes which allow this membrane to become permeable to the fluids in the blood and inflammation occurs, ultimately culminating in fibrous scar tissue if healing does not occur. The fluid in the air sacs and the attendant inflammation causes decreased oxygen in the blood stream and the patient manifests symptoms and signs of this oxygen deprivation. Shortness of breath, peripheral cyanosis, agitation, confusion and somnolence associated with fluid noises heard in the lungs are present.

The patient must be placed on Mechanical Ventilation and treated with Antibiotics and cardiovascular support. The mortality rate is greater than 50%, unless the physician anticipates and diagnoses ARDS's presence in a timely fashion.