Respiratory Failure

Respiratory failure may be divided into two broad categories: hypoxemic (type 1) and hypercapnic (type 2). Hypoxemic respiratory failure is defined as a partial pressure of oxygen in arterial blood (PaO2) of less than 55 mm Hg when the fraction of inspired oxygen (FIO2) is 0.60 or more. Hypercapnic respiratory failure is defined as a partial pressure of carbon dioxide in arterial blood (PaCO2) of more than 45 mm Hg. Disorders that initially cause hypoxemia may be complicated by respiratory pump failure and hypercapnia . Conversely, diseases that produce respiratory pump failure are frequently complicated by hypoxemia resulting from secondary pulmonary parenchymal processes (e.g., pneumonia) or vascular disorders (e.g., pulmonary embolism).

Hypoxemia may be broadly divided into four major categories.
  1. Hypoventilation and low FIO2
  2. Diffusion limitation
  3. Ventilation/Perfusion (V/Qÿ) mismatch
  4. Shunt
Common Causes of Hypoxemia and Hypercapnia
1. Hypoxemia
  • Pneumonia
  • Acute respiratory distress syndrome (ARDS)
  • Pulmonary embolism
  • Congestive heart failure (CHF)
2. Hypercapnia
  • Muscle weakness
  • Factors that increase CO2 production (e.g., fever, sepsis, trauma)
  • Airway obstruction


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