What is the difference between hypoxia and hypoxemia, and how is hypoxemia measured?

Hypoxemia vs Hypoxia: Definitions and Measurement

Hypoxemia refers specifically to low oxygen levels in the blood (low PaO₂ or SaO₂), while hypoxia refers to insufficient oxygen supply at the tissue level, which can occur due to multiple mechanisms including hypoxemia. 1

Definitions and Key Differences

Hypoxemia

• Definition: Abnormally low partial pressure of oxygen (PaO₂) in arterial blood or low arterial oxygen saturation (SaO₂)
• Clinical threshold: Generally defined as SaO₂ <90% or PaO₂ <60 mmHg (8 kPa) 1, 2
• Direct cause: Inadequate oxygenation of blood in the lungs
• Primary focus: Blood oxygen content

Hypoxia

• Definition: Insufficient oxygen supply to tissues to meet metabolic demands
• Clinical focus: Tissue oxygen delivery and utilization
• Broader concept: Can occur even with normal blood oxygen levels
• Types: Can be classified into four main categories 1, 2:
  • Hypoxemic hypoxia: Due to low PaO₂ (reduced oxygen in blood)
  • Anemic hypoxia: Due to reduced oxygen-carrying capacity (e.g., low hemoglobin)
  • Stagnant hypoxia: Due to inadequate blood flow to tissues
  • Histotoxic hypoxia: Due to inability of tissues to utilize oxygen

Measurement of Hypoxemia

Direct Measurement

• Arterial Blood Gas (ABG): Gold standard for measuring hypoxemia
  • Measures PaO₂ directly
  • Normal PaO₂: 80-100 mmHg (10.6-13.3 kPa)
  • Hypoxemia threshold: PaO₂ <60 mmHg (8 kPa) 1
  • Also provides information on pH and PaCO₂

Indirect Measurement

• Pulse Oximetry (SpO₂):
  • Non-invasive method
  • Measures peripheral oxygen saturation
  • Normal SpO₂: 95-100%
  • Clinical hypoxemia threshold: SpO₂ <90% 1
  • Limitations:
    • Less accurate at saturations <80%
    • Can be affected by poor perfusion, anemia, carbon monoxide poisoning
    • May overestimate actual SaO₂ in dark-skinned patients (risk of occult hypoxemia) 3

Mechanisms of Hypoxemia

Five primary mechanisms can lead to hypoxemia 2, 4:

1. Ventilation-perfusion (V/Q) mismatch:

   • Most common cause of hypoxemia
   • Areas of lung are ventilated but not perfused, or perfused but not ventilated
   • Responds relatively well to oxygen therapy

2. Right-to-left shunt:

   • Blood passes from right to left heart without oxygenation
   • Relatively refractory to oxygen therapy
   • Seen in pneumonia, ARDS, and certain congenital heart defects

3. Alveolar hypoventilation:

   • Reduced air volume reaching alveoli
   • Causes both hypoxemia and hypercapnia (elevated PaCO₂)
   • Responds well to oxygen therapy
   • Common in respiratory depression, neuromuscular disorders

4. Diffusion impairment:

   • Oxygen transfer across alveolar-capillary membrane is impaired
   • Seen in pulmonary fibrosis, pulmonary edema
   • Usually not a major cause of hypoxemia at rest

5. Low inspired oxygen (FiO₂):

   • Occurs at high altitude or in confined spaces
   • Responds well to supplemental oxygen

Clinical Relevance and Pitfalls

• Prevalence: Hypoxemia is common in acute illness, with pooled prevalence of 24.5% in neonates, 12.1% in children, and 10.8% in adults admitted to hospitals in low and middle-income countries 1

• Mortality risk: Hypoxemia is associated with 4.84 times higher odds of death compared to no hypoxemia 1

• Measurement pitfalls:

  • SpO₂ measurements in sleeping patients should be interpreted with caution due to normal nocturnal dips 1
  • SpO₂-SaO₂ discrepancies occur in approximately 23.6% of ICU patients, with occult hypoxemia (normal SpO₂ despite low SaO₂) in 4.7% 3
  • Black patients and those with higher BMI are at increased risk of SpO₂-SaO₂ discrepancies 3

• Treatment considerations:

  • Target saturation should be 88-92% in patients at risk of hypercapnia (e.g., COPD) 2
  • Oxygen therapy only corrects hypoxia due to hypoxemia; other types of hypoxia require different interventions 1

By understanding the difference between hypoxemia and hypoxia, and the appropriate methods for measuring hypoxemia, clinicians can better diagnose and manage respiratory compromise in their patients.