Diagnosis and Management of Acute Respiratory Failure - Pathofysiology of Acute Respiratory Failure - Hypoxemia

6 important questions on Diagnosis and Management of Acute Respiratory Failure - Pathofysiology of Acute Respiratory Failure - Hypoxemia

What is the most common underlying physiologic abnormality in hypoxemic respiratory failure?

A mismatch of alveolar ventilation (V) and a pulmonary perfusion (Q).

Causes of low V/Q mismatch. (3)

  • Obstruction of distal airways
  • Alveolar filling/ flooding (eg. ARDS)
  • Atelectasis

(eg, pneumonia, aspiration, pulmonary edema).

What is a compensatory mechanism in low V/Q mismatch?

Decrease blood flow to abnormal lung units though hypoxic pulmonary vasoconstriction. However this is not effective enough to fully compensate the decrease in oxygen availability. A greater proportion of deoxygenated blood returns to the left atrium.
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What is an extreme V/Q mismatch?

A shunt due to ARDS (alveolar flooding) -> perfusion of the unventilated lung -> refractory hypoxemia.

Treatment:
  • Oxygen therapy/ mechanical ventilation
  • Treating the underlying cause (infection, obstruction)
  • Reopening atelectatic lung zones (recruiting)

Causes of decreased diffusion of oxygen across the alveolocapillary membrane? (3)

  • Interstitial edema
  • Inflammation
  • Fibrosis

What is the range of a normal P/F ratio?

300 - 500

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