Definition of Hypoxia
Hypoxia occurs when oxygen supplies are insufficient to meet oxygen demands in a particular compartment (such as alveolar or tissue hypoxia). 1
Core Distinction: Hypoxia vs. Hypoxemia
It is critical to distinguish hypoxia from hypoxemia:
- Hypoxemia refers specifically to low partial pressure of oxygen (PaO2) in the blood, typically defined as PaO2 <8 kPa (60 mm Hg) or oxygen saturation (SaO2) <90% 1
- Hypoxia is the broader condition where tissues receive inadequate oxygen to meet metabolic demands, regardless of blood oxygen levels 1
Four Main Types of Tissue Hypoxia
Hypoxia can result from four distinct mechanisms, each requiring different therapeutic approaches 1:
1. Hypoxaemic Hypoxia (Hypoxic Hypoxia)
- Occurs when blood oxygen content is low due to reduced partial pressure of oxygen
- Causes include: altitude exposure, right-to-left cardiac shunts, ventilation-perfusion (V/Q) mismatch, alveolar hypoventilation, or diffusion impairment
- This is the only type correctable with supplemental oxygen therapy 1
2. Anaemic Hypoxia
- Results from reduced hemoglobin available for oxygen transport
- Despite normal PaO2 and SpO2, reduced oxygen-carrying capacity leads to tissue hypoxia
- Examples: anemia, carbon monoxide poisoning (which impairs hemoglobin's oxygen-binding ability) 1
3. Stagnant Hypoxia
- Low tissue oxygen due to inadequate blood flow (globally or regionally)
- Occurs in: low cardiac output states, severe peripheral vascular disease, cold exposure to extremities
- Can cause gangrene in severely ischemic tissue 1
4. Histotoxic Hypoxia
- Tissues cannot utilize oxygen due to disrupted cellular metabolism
- Classic example: cyanide poisoning (impairs cytochrome function)
- Emerging concept: mitochondrial dysfunction in sepsis despite adequate oxygen delivery, termed "cytopathic dysoxia" 1
Clinical Significance
The threshold for tissue injury risk is generally SaO2 <90% (PaO2 <8 kPa or 60 mm Hg), with most experts emphasizing maintaining SaO2 above 90% for acutely ill patients 1. Sudden exposure to SaO2 below 80% can cause altered consciousness even in healthy individuals 1.
Important Caveats
- Oxygen therapy only addresses hypoxaemic hypoxia; other types require alternative interventions (transfusion, improving cardiac output, treating toxins) 1
- Normal oxygen saturation does not exclude tissue hypoxia if the underlying mechanism is anaemic, stagnant, or histotoxic 1
- In children, hypoxemia definitions vary by age, with chronicity defined as lasting ≥2 weeks 2