3. Sally has been very sick with HINI virus. She has \"problems breathing\" so s

Question

3. Sally has been very sick with HINI virus. She has "problems breathing" so she goes to her doctor. The doctor diagnoses ARDS (acute respiratory distress syndrome. Sally's increase in respiratory rate and volume is due to the fact that her alveolar sacs have become filled with fluid. The doctor decides to hospitalize Sally and she prescribes oxygen therapy. During oxygen therapy Sally will breath pure oxygen instead of air. Explain why this treatment will return Sally's respiratory rate& volume to normal and aid in her recovery.

Explanation / Answer

In acute respiratory distress syndrome, there is acute onset, hypoxia, diffuse infiltrates on chest X-ray, with no cardiac failure, or pulmonary edema. In ARDS, there is diffused alveolar damage along with increased permeability of the alveolar–capillary membrane. The edema fluid and plasma proteins enter the alveolar spaces. Macrophages and neutrophils are present in increased amounts in the interstitium. The lungs have pro-inflammatory cytokines with hyaline membranes forming in the alveoli.

ARDS causes hypoxemia, which is low arterial oxygen tension below the normal expected value (85-100 mmHg). Hypoxemia results in hypercapnia (high carbon dioxide) and reduced blood pH. In ARDS, the alveolar–capillary barrier fails. This causes flooding of the alveolar space leading to a severe impairment of gas exchange, mostly as hypoxemia reduces the sodium transporters in the lung cell plasma membrane.

By administering pure oxygen, the arterial oxygenation can increase to nearly 88-95% along with maintaining carbon dioxide and pH. Higher concentration of oxygen is required to correct the hypoxemia. When pure oxygen is administered to an ARDS patient, the amount of deoxygenated haemoglobin decreases in blood as more oxygen is carried to the tissues. As a result, carbon dioxide levels falls due to reduced capacity of deoxyHb to carry CO2. This will increase the respiration rate by improving gas exchange in the lungs.

Fluid can be reabsorbed from the alveolar spaces due to active vectorial flux of sodium from the airspaces into the lung interstitium and the pulmonary circulation. Sodium is pumped out of the alveolar epithelial cells by the Na,K-ATPase pump in place of K+. Entry of Na+ into the cells occurs apically through the amiloride-sensitive and -insensitive epithelial Na+ channel (ENaC). This osmotic gradient created by vectorial sodium transport process clears fluid from the alveolar space. When pure oxygen is administered, the sodium transporters will be increased, leading to effective clearing of the alveolar fluids. As a result, respiration rate and volume are restored to normal.

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