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

#Arrangoiz #Surgeon #Teacher

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s