Overview
This lecture explains the five basic mechanisms of hypoxemia, how to distinguish between them, and the main causes and diagnostic strategies for low blood oxygen.
Mechanisms of Hypoxemia
- Hypoxemia is defined as low partial pressure of oxygen in arterial blood (PaO2).
- Five mechanisms: low inspired oxygen (high altitude), hypoventilation (high PaCO2), impaired diffusion, ventilation-perfusion (VQ) mismatch, and shunt.
- Only right-to-left intracardiac shunts are linked to a single mechanism; most causes involve multiple mechanisms.
Key Equations and Concepts
- The alveolar-arterial (A-a) gradient measures difficulty in oxygen exchange; a high gradient suggests pathology.
- Alveolar gas equation estimates alveolar oxygen pressure (PAO2).
- Fixed law describes gas diffusion: rate = diffusion coefficient × surface area × pressure gradient / membrane thickness.
Impaired Diffusion
- Usually caused by increased alveolar-capillary membrane thickness (e.g., pulmonary fibrosis).
- Exacerbated by high altitude or high cardiac output (less diffusion time).
- CO2 diffusion is less affected due to its higher solubility.
Ventilation-Perfusion (VQ) Mismatch
- Optimal gas exchange requires matched alveolar ventilation and blood perfusion; normal lung VQ ≈ 0.8.
- Causes include pulmonary edema, airway obstruction (atelectasis), or pulmonary embolism (dead space).
- Adaptive hypoxic vasoconstriction diverts blood from poorly ventilated areas.
- Supplemental oxygen often corrects hypoxemia unless the underlying disease is severe.
Shunt
- Right-to-left shunt: blood bypasses lungs (e.g., tetralogy of Fallot), leading to hypoxemia unresponsive to oxygen.
- Shunt fraction ≥30%: little improvement in PaO2 with supplemental oxygen.
Diagnostic Approach
- Step 1: Check A-a gradient; normal suggests hypoventilation or low inspired O2.
- Step 2: Assess correction with 100% oxygen; incomplete correction indicates shunt.
- Step 3: Combine history, physical exam, chest X-ray, and possibly echocardiogram with bubble study to localize shunt.
Radiographic Patterns in Hypoxemia
- Normal X-ray: COPD/asthma, PE, hypoventilation, intracardiac shunt, AVM.
- Single lobe abnormal: pneumonia, infarct, collapse.
- Unilateral diffuse: large effusion, collapse, severe pneumonia, re-expansion edema, selective intubation.
- Bilateral diffuse: pulmonary edema, ARDS, interstitial disease, viral/fungal pneumonia.
Key Terms & Definitions
- A-a Gradient — difference between alveolar and arterial O2 pressures, indicates oxygenation efficiency.
- VQ Mismatch — regions with uneven ventilation and perfusion, impairing gas exchange.
- Shunt — blood bypasses alveolar gas exchange, resistant to supplemental oxygen.
- Hypoventilation — inadequate ventilation, leads to increased PaCO2 and hypoxemia.
- Fixed Law — law describing diffusion rate of gases across membranes.
- Dead Space — ventilated lung regions not participating in gas exchange.
Action Items / Next Steps
- Review lectures 16 and 17 for background on ABGs if needed.
- Read up on Fixed law and alveolar gas equation for full understanding.
- Next lecture: study carbon monoxide poisoning and methemoglobinemia.