Acute Respiratory Distress Syndrome is a secondary illness of lungs which is due to underlying primary Lungs injury. ARDS is an emergency medical condition which required immediate mechanical ventilation. Prognosis of ARDS is very poor nearly about 40%, even after recovery also the quality of life decreases.
Cause/Etiology of ARDS
ARDS is caused by any direct or indirect injury to lungs
- TRALI (Transfusion induced acute lung injury)
- Diffuse injury
Pathophysiology of ARDS
In responding to lung injury inflammation process started in the body which leaks fluid into the small sac of the lung called alveoli. Fluid-filled alveoli affect oxygen and carbon dioxide exchange, which may lead to tissue hypoxia and death.
Diagnosis of ARDS
Chest Radiography, Chest CT, ABG, and PaO2 /FiO2 ratio
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PaO2/FiO2 ratio for diagnosis of ARDS
PaO2 is Partial Pressure of oxygen and FiO2 is Fraction of inspired oxygen. These 2 have greater role in the diagnosis of ARDS.
|Severity of ARDS||PaO2 / FiO2||Mortality|
|Mild ARDS||201 to 300||Approx 27%|
|Moderate ARDS||101 to 200||Approx 32%|
|Severe ARDS||<100||Approx 45%|
Example 1 – Patient is on vent support, his FiO2 is set on 80% and his PaO2 in ABG is 60mmHg.
Then PaO2 / FiO2 is 60/0.8 = 75, I.epatient having Severe ARDS
Example 2 – A patient is on Oxygen mask 8 litter/min and his PaO2 in ABG shows 65mmHg.
Then PaO2 / FiO2 is 65/0.53 = 122, I.e the patient is having Moderate ARDS
Tip – In Oxygen mask 1 litter of oxygen is equal to 4% and always remember air has 21% of oxygen, so in the above equation, the patient is on 8lt of oxygen that means 8*4 = 32% and air having 21% so 32+21 = 53%.
Ventilator Settings in ARDS
Mechanical ventilation is necessary for moderate and severe Acute Respiratory Distress Syndrome. The rule is High PEEP(Positive end-expiratory pressure) and Low tidal volume(4 to 6ml per kg body weight). High peep is helping to keep open alveoli and prevent from collapse after expiration and in pause state; It makes the exchange of carbon dioxide and oxygen going on `without disruption even during expiration and pause state.
Low tidal volume is very necessary to prevent lungs from secondary trauma because in Acute Respiratory Distress Syndrome lungs capacity decreases due to fluid accumulation.
If saturation is still not improving then ECMO(Extracorporeal membrane oxygenation) is the only option.
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