Acute Respiratory Distress Syndrome
ARDS is a syndrome of symptoms of respiratory failure like Dyspnea, hypoxia, cyanosis.
ARDS is a lung condition in which alveoli that are normally air filled balloons when filled with fluid result in respiratory distress by diffused alveolar damage. The peripheral organs as a fatal consequence do not get the required amount of oxygen and ultimately results in multi-organ failure.
ARDS is one of the diseases having a high mortality rate and devastating consequences.
What are the causes of ARDS?
There are two major causes of ARDS- direct lung injury and indirect lung injury
Direct Injury:
- Pneumonia (streptococcus pneumonia or staphylococcus aureus)
- SARS Covid-2
- Aspiration of gastric content (acids, bacteria and other nasty molecules)
- Lung Contusions by any trauma
- Drowning (near drowning)
- Toxic smoke inhalation
Indirect Injury:
Now these are not the elements that directly cause an impact. There is something fishy going on systemically that affects the lungs.
- Sepsis – It can be an inflammatory immune reaction damaging the alveolar epithelium.
- Pancreatitis
- Long bone fractures caused by a trauma
How does ARDS happen?
Whenever lung damage is caused by any kind of injury, it tends to damage either Type 1 pneumocytes or Type 2 pneumocytes in the alveoli. These cells release cytokines that stimulate macrophages to let them know that something is wrong.
In case of direct injury macrophages can also be directly triggered by pathogens.
Macrophages once stimulated start releasing certain chemicals or cytokines like Interleukin-1, IL-6, IL-8, etc. that leak into pulmonary capillary blood and affect endothelial cells there causing oedema and inflammation.
These cytokines enjoy increasing the capillary permeability that result in fluid leaking into the interstitial spaces. On the other hand IL-8 stimulates neutrophils to enter the alveoli that secrete some weird molecules like proteases and reactive oxygen species, which cause additional damage to the type-1 pneumocytes causing large scale damage.
Now the alveolar membrane consisting of these type 1 cells has lost its wall barriers which allows-in the fluid that was leaked into the interstitial spaces previously. Many other components like proteins, WBCs, RBCs, etc. also enter the alveoli making a thick wall. This wall then acts as a barrier between the oxygen and pulmonary capillary that affects gas exchange resulting in hypoxemia, dyspnea and cyanosis.
Type 2 pneumocytes are responsible for releasing surfactant that decreases the surface tension. When these cells are affected it leads to increase in surface tension and collapsing pressure. Increased fluid in alveoli result in washing of surfactant and alveoli starts collapsing.
Now this situation leads to hypoxemia that further stimulates the respiratory center to increase respiratory rate.
Signs and Symptoms of ARDS
- Crackling sound while auscultating
- Dyspnea
- Tachypnea
- Tachycardia
- Hypoxemia
Diagnosis
- If there are some signs of sepsis, test like sputum culture, blood culture, ESR and urine analysis, etc. are done.
- Imaging test to see if they have bilateral opacities in their lungs. They include:
- Chest X-ray
- CT Scan
- Ultrasound
Treatment
- Non invasive positive pressure ventilation is given in patients with mild ARDS.
- In moderate to severe ARDS, Endotracheal intubation is done for Lung protective ventilation together with other aids like NMBA, dexmathasone, tocilizumab, etc,
- Proning is considered as one of the most efficient treatment for ARDS.
- Extracorporeal membrane oxygenation is ultimately done in the most severe cases where machinery takes over the role of lungs.