Issues with Oxygenation

• Hypoxemia:
  • There are four broad physiologic causes of hypoxemia:
    • Shunting
    • VQ mismatch
    • Alveolar hypoventilation
    • Decreased partial pressure of oxygen
      • Understanding these mechanisms:
        • Allows the clinician at the bedside to quickly develop a differential diagnosis for hypoxemia and target diagnostics to assess for the precise etiology
• Shunts, or blood bypassing normal gas exchange:
  • Is one of the most common causes of hypoxemia
  • A classic example of a shunt is an intracardiac shunt:
    • In this example:
      • Much of the blood passes by the alveoli, participating in normal gas exchange:
        • However, a small amount is diverted through the heart, bypassing the lungs:
          • This deoxygenated blood mixes with the oxygenated blood:
            • Leading to hypoxemia

• When an area of the lung is perfused, but not ventilated:
  • That results in an intra-pulmonary shunt:
    • In other words:
      • The inspired oxygen cannot reach the alveoli for gas exchange
    • There are several different causes of intra-pulmonary shunts, including:
      • Atelectasis
      • Pneumonia
      • Pulmonary edema
      • Acute respiratory distress syndrome (ARDS)
      • Hemothorax
      • Pneumothorax
      • Hyperinflation or auto-PEEPing
        • All of these pathological processes:
          • Prevent effective gas exchange at the alveoli

• When an area has ventilation, but no perfusion:
  • This is dead space:
    • In other words:
      • The airways are functioning normally:
        • But there is a disease process in the vasculature
      • The best example would be a patient in cardiac arrest who is intubated and ventilated:
        • But there is an interruption of chest compressions
    • Dead space can be:
      • Anatomic and physiologic:
        • Such as oxygenation but lack of gas exchange:
          • That occurs in the upper airways, like the trachea
      • There can also be pathological causes of dead space:
        • Such as this diagram of microthrombi blocking a capillary

• Other examples of dead space include:
  • Low cardiac output and hyperinflation:
    • As occurs in obstructive lung disease:
      • In diseases such as chronic obstructive lung disease (COPD):
        • There can be a significant level of hyperinflation or auto-PEEP:
          • Which can lead to vasoconstriction of the capillaries involved in gas exchanged:
            • Thereby leading to impaired gas exchanged
• Dead space ventilation can lead to both:
  • Hypoxia and hypercapnia:
    • Due to CO2 retention

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