Acute respiratory distress syndrome

What is Acute respiratory distress syndrome?

Acute Respiratory Distress Syndrome (ARDS) is a disease of the lungs. The lungs essentially provide the interface between air and blood. The lungs consist of a series of folded membranes (the alveoli), which are located at the ends of very fine branching air passages (bronchioles). Blood which arrives into the lungs from the pulmonary artery gets into smaller and smaller blood vessels until it ends up in the capillaries located within the walls of the alveoli. In this moist environment, oxygen diffuses from within the alveoli into the blood stream, while carbon dioxide moves out of the blood stream into the alveoli and is expelled out of the air passages.

Acute Respiratory Distress Syndrome (ARDS) is caused by a number of medical and surgical conditions that cause damage to the alveolar walls and pulmonary capillaries surrounding them. There is pulmonary oedema arising from increased leakiness of blood vessels which can cause severe respiratory failure due to impaired gas exchange. ARDS is often accompanied by failure of other organs in the body, particularly in the case of sepsis. Alternative names for ARDS include adult respiratory distress syndrome/failure, diffuse alveolar damage, acute alveolar injury and traumatic wet lungs.

Statistics on Acute respiratory distress syndrome?

Acute respiratory distress syndrome (ARDS) is a relatively common disease in Australia particularly in very ill patients admitted to the intensive care unit (ICU). There are about 30 cases for every 100,000 of the population (aged greater than 15 years) each year. Approximately 1 in 9 patients admitted to Australian ICUs will develop acute lung injury. These patients can have a number of illnesses, not just lung disease.

Risk Factors for Acute respiratory distress syndrome

ARDS is really a non-specific reaction of the lungs and airways to a number of irritants. These can be directly affecting the lung or indirectly lead to lung damage. Some predisposing factors for ARDS include the following:

Pulmonary conditions

Severe pneumonia- Infection of the lungs.
Aspiration (or breathing in stomach contents).
Near drowning.
Inhalation of toxins and other irritants such as smoke.
Lung injury and bruising.
Oxygen toxicity.
Fat embolism- Where bubbles of fat travel through the bloodstream and block off airways.

Systemic conditions

Shock- Including septic shock and shock due to trauma.
Septicaemia- Caused by bacteria spreading hrough the bloodstream.
Burns.
Pancreatitis.
Diabetic Ketoacidosis (acidic pH level in blood due to the body breaking down ketones instead of sugar).
Hypersensitivity reactions.
Drugs reactions due to aspirin, heroin or paraquat.
Multiple blood transfusions.
Malaria.
Acute liver failure.
Obstetric complications- Problems during pregnancy or delivery such as preeclampsia.
Cardiac surgery and other complicated surgeries.

Progression of Acute respiratory distress syndrome

ARDS can be caused by an acute lung injury or severe systemic illness. The lung is damaged and a cycle of acute inflammation begins. There is damage to the alveolar wall and capillary lining, which increases leakage into the alveoli. Alveolar "surfactant" (needed to keep air spaces open) is lost or degraded, causing reduced compliance and lung collapse. The end result is an acute pulmonary oedema and multi-organ failure. Other complications include pneumothorax (due to ventilation and reduced lung compliance) and secondary pneumonia. In most cases ARDS will follow a rapid course from its development, progressing rapidly thought the various phases of oedema, proliferation and later fibrosis or scarring of the airways.

Symptoms of Acute respiratory distress syndrome

ARDS is a rapidly progressive and acute illness that normally initially presents with unexplained breathlessness and rapid breathing. You may also notice your lips and tongue turning blue (central cyanosis). Your doctor will take a brief history to determine if you have any of the predisposing conditions and find out what symptoms you may be having. Sometimes patients with ARDS are too ill to give a history so the doctor will make the diagnosis based on witnesses and medical records.

Clinical Examination of Acute respiratory distress syndrome

To diagnose ARDS your doctor will look for a number of clinical signs that have an acute onset. Your doctor will examine your general appearance, carefully listen to your chest and check your oxygen saturations with and oxygen probe on the finger. Some signs of ARDS that the doctor will be looking for include:

Cyanosis- bluish discolouration.
Tachypnoea- rapid breathing.
Tachycardia- increased heart rate.
Increasing hypoxaemia (low oxygen concentrations in the blood) .
Peripheral vasodilatation (opening up of the blood vessels in the peripheries of your body).
Bilateral crackles in the chest- When the doctor listens to your chest with a stethoscope they can hear grating or crackling sounds which indicate your air spaces are not working properly.

How is Acute respiratory distress syndrome Diagnosed?

If you are suspected to have ARDS your doctor will perform a number of different tests. Full blood count, kidney function, liver function tests (LFT), clotting studies, and blood cultures are all important. These can help identify whether you have a predisposing infection, the likely bugs and give the doctor an idea of how well your other organs are functioning. A Chest x-ray is an extremely important tool in diagnosis and monitoring progression of disease. In this investigation, your doctor can visualise fluid build up in the lungs. You will also have regular measurements of the oxygen concentration in your blood. You may also have other more specialised investigations such as insertion of special catheters (thin tubes) to measure certain blood pressures. This is to exclude other causes for fluid build up in the lungs such as congestive heart failure.

Prognosis of Acute respiratory distress syndrome

The overall prognosis of ARDS is poor and many patients will not recover from the condition. ARDS is a hard condition to treat as some treatments (such as high concentrations of oxygen) can further exacerbate the condition. Between 50-75% of patients with ARDS will die from the disease, although there is evidence that this figure may be improving. Prognosis in ARDS depends on your age, previous health and the underlying cause for your condition. In general if ARDS is caused by sepsis and associated with damage to multiple organs, you have a lower likelihood of survival.

How is Acute respiratory distress syndrome Treated?

ARDS is a serious and life-threatening condition that requires management in an intensive care unit where appropriate respiratory and circulatory support is available. Management then focuses on:

Identifying and treating the underlying cause- If the suspected underlying cause is sepsis (pus-forming bacteria or their toxins in the blood or tissues) broad spectrum antibiotics should be started. Other conditions such as trauma will be treated appropriately.
Avoiding complications of treatments- Careful ventilation methods should avoid the development of ventilation-induced pneumonia and other lung injuries.
Supportive care- Most patients will require oxygen and ventilation to allow enough oxygen to reach the lungs and to keep the airways open. Circulatory support includes careful fluid management (reducing fluid intake to remove the excess fluid of the lungs), blood transfusions and medications to help vessels dilate and the heat to pump better. These all make sure the heart maintains adequate output. Arterial lines will be inserted to monitor your haemodynamic state. Nutritional support is essential in critically ill patients and you may require insertion of a tube down your nose for feeding. Dialysis may be required in renal failure.
Other- A number of other agents have been trialed in the management of ARDS including nitrous oxide (which dilates blood vessels in the lungs), surfactant (a fluid which hold the airways open) and steroid treatment. However, most of these have failed to show improvements in survival. Steroids may however be used in the later stages of disease to speed recovery by reducing inflammation.

Acute respiratory distress syndrome References

Bersten A. Optimal mechanical ventilation for ARDS, Critical Care and Resuscitation 2003; 5: 7-13. Available [online] at URL: http://www.jficm.anzca.edu.au/pdfdocs/Journal/Journal2003/J2003 (a) March/Editorials.pdf
Cotran R, Kumar V, Collins T. Robbins Pathological Basis of Disease Sixth Ed. WB Saunders Company 1999.
Kumar, Clark. Clinical Medicine. 5th Edition. Saunders. 2002.
Levy B, Shapiro S. 'Acute Respiratory Distress Syndrome' in Kasper et al. Harrison's Principle of Internal Medicine, 16th Edition (Chapter 251), McGraw-Hill, 2006.
Longmore, Wilkinson, Rajagopalan. Oxford Handbook of Clinical Medicine. 6th Edition. Oxford University Press. 2004.
Talley NJ, O'Connor S. Clinical examination- A Systematic Guide to Physical Diagnosis, 4th Edition. MacClennan & Petty, Sydney 2001.