Acute Respiratory Distress Syndrome (ARDS)

ARDS is a clinical devastating syndrome that affects both medical and surgical patients. Despite great advances in understanding the pathogenesis of the disease, the mortality rate is still high. Even survivors of ARDS usually experience long ICU stays, hospital stays, and several co-morbidities.  Survivors require prolonged rehabilitation time till full recovery.

ARDS is an acute condition characterized by bilateral pulmonary infiltrates and severe hypoxemia in the absence of evidence for cardiogenic pulmonary edema.

ARDS Causes

The probable causes of ARDS may include −

Direct precipitating causes −

• Pneumonia

• Aspiration

• Pulmonary embolism

• Pulmonary contusion

• Inhalation injury

• Reperfusion injury

• Chest trauma

• Near drowning

Indirect systemic precipitating causes −

• Sepsis

• Blood transfusions with transfusion-related acute lung injury

• Trauma  with multiple fractures and fat embolic syndrome

• Burns

• Acute pancreatitis

• Post cardio Pulmonary bypass

• Toxic injections, e.g., aspirin tricyclic antidepressants

The four most frequent causes include −

• Sepsis − Most common cause

• Aspiration

• Pneumonia

• Severe trauma

ARDS Symptoms

The major symptoms include −

• Acute dyspnea

• Hypoxemia

• Tachypnoea

• Tachycardia

• Fever

• Hypothermia

• Hypotension

• Cold extremities

• Peripheral vasoconstriction

• Bilateral rales

Cardiogenic pulmonary edema must be distinguished from ARDS. Carefully look for signs of congestive heart failure or volume overloads like cardiac murmurs, hepatomegaly, and edema.

ARDS Risk Factors

The major risk factors include −

• Chronic alcohol abuse

• Hypoproteinemia

• Advanced age

• Increase severity and extent of the injury is measured by injury severity score or Apache score

• Hyper transfusion of blood products

• Cigarette smoking

The three stages of the syndrome include

• Exudative stage

• Proliferative stage

• Fibrotic stage

ARDS Diagnosis

The diagnosis of ARDS can be done in the following ways −

Chest X-ray shows diffuse bilateral alveolar infiltrates consistent with pulmonary edema. Infiltrates in the early stage may be mild or dense, interstitial or alveolar, patchy or confluent. Infiltrates can progress to diffuse bilateral with ground glass changes or frank alveolar infiltrates

Arterial blood gas analysis −

• ARDS severity − P A O2 / FiO2

• Mild − 200 to 300

• Moderate − 100 to 200

• Severe − Less than 100

In addition to hypoxemia arterial blood gases initially show respiratory alkalosis.

A metabolic acidosis with or without respiratory compensation may be present as the condition progresses and the work of breathing increases the partial pressure of carbon dioxide begins to rise and respiratory alkalosis give way to respiratory acidosis.

Computerized tomography (CT) − A CT scan combines X-ray images taken from many different directions into cross-sectional views of internal organs. CT scans can provide detailed information about the structures within the heart and lungs.

Heart Tests − Because the signs and symptoms of ARDS are similar to those of certain heart problems, your doctor may recommend heart tests such as

• Electrocardiogram This painless test tracks the electrical activity in your heart. It involves attaching several wired sensors to your body.

• EchocardiogramA sonogram of the heart, this test can reveal problems with the structures and the function of your heart.

Hematological Tests −

• Leukopenia or leucocytosis

• DIC

• RFT

• LFT

• Von Willebrand factor may be elevated

• Cytokines  IL1  IL 6  IL 8  are elevated

Invasive hemodynamic monitoring − Pulmonary artery wedge pressure

ARDS Treatment

The main modes of treatment include −

• Treatment of respiratory system abnormalities

• Diagnose and treat the precipitating cause of ARDS

• Maintain oxygenation

• Prevent ventilator-induced lung injury

• Keep PH in the normal range

Enhance patient-ventilator synchrony and patient comfort by use of sedation with amnesia, opioid analgesia, and pharmacological paralysis if necessary.

Liberate or wean from mechanical ventilation when the patient can breathe without assisted ventilation.

Treatment of Non-Respiratory System Abnormalities

• Support or treat other organ system dysfunction or failure

• General Critical Care

• Adequate early nutritional support

• Prophylaxis against deep veins thrombosis and Gastrointestinal bleeding

Maintain Adequate Oxygenation

• Positive end-expiratory pressure is employed

• When utilized in sufficient amounts PEEP allows Fi O 2 to be loaded from highly potentially toxic concentrations.

• Lung protective mechanical ventilation

• Mechanical ventilation using Limited tidal volume

The goal is to avoid injury due to over-expansion of alveoli during inspiration and injury to repeated opening and closing of lungs during inspiration and expiration.

• Permissive hypercapnia

• Fluid management

• Hemodynamic management

• Prone Positioning

• Inhaled nitric oxide

• Inhaled prostacyclins

• Tracheal gas insufflation

Corticosteroids

Corticosteroids are a group of natural and synthetic analogues of hormones secreted by the hypothalamic-pituitary-adrenocortical axis. These drugs are potent anti-inflammatory, antifibrotic, and immunomodulator agents which exert inhibitory effects at several stages of the inflammatory cascade.

These drugs exhibit their anti-inflammatory activity through the following three independent mechanisms: the induction and activation of annexin-I, induction of mitogen-activated protein kinase phosphatase-1, and repression of transcription of cyclooxygenase-2 resulting in blockade of transcription of various cytokines, chemokines, cell adhesion molecules, and complement factors responsible for the development of ARDS.

High-frequency Oscillatory Ventilation

Extracorporeal CO2 removal (ECMO) is a very complicated treatment that takes blood outside of your body and pumps it through a membrane that adds oxygen, removes carbon dioxide, and then returns the blood to your body. This is a high-risk therapy with many potential complications. It is not suitable for every ARDS patient.

Recovering from ARDS

ARDS patients may require ventilation for long periods of time. On average this is seven to 14 days. Beyond this time, doctors may suggest a tube be placed directly into the windpipe through the neck (tracheostomy) by a surgeon. Usually, the doctor believes it may take weeks more to recover from ventilator support. This tube can easily be removed once the patient is free of the need for a ventilator.

It is important to note that most people survive ARDS. They will not require oxygen on a long-term basis and will regain most of their lung function. Others will struggle with muscle weakness and may require re-hospitalization or pulmonary rehabilitation to regain their strength.

ARDS: Prevention

There’s no way to prevent ARDS completely. However, you may be able to lower your risk of ARDS by doing the following

• Seek prompt medical assistance for any trauma, infection, or illness.

• If you smoke, consider stopping smoking cigarettes.

• Try to stay away from secondhand smoke.

• Avoid alcohol. Chronic alcohol use may increase your mortality risk and prevent proper lung function.

• Get your flu vaccine annually and pneumonia vaccine every 5 years. This decreases your risk of lung infections.

Conclusion

ARDS is a multi-system syndrome. It is characterized by the accumulation of excessive fluid in the lungs with resulting hypoxemia and ultimately some degree of fibrotic changes.

The most frequent causes of ARDS include sepsis, pneumonia, and severe trauma. Treatment is primarily supportive and can include non-traditional types of ventilation and oxygenation strategies.

The best-proven strategy to improve survival is low tidal volume ventilation. ARDS patients may require ventilation for long periods of time. On average this is seven to 14 days. Beyond this time, doctors may suggest a tube be placed directly into the windpipe through the neck (tracheostomy) by a surgeon.