V/Q Mismatch and Shunting
V/Q ratio affects oxygenation. Dead space (high V/Q) = ventilation without perfusion. Shunt (low V/Q) = perfusion without ventilation.
Detailed Explanation
**Normal V/Q Ratio:** ~0.8 (ventilation slightly less than perfusion)
V/Q Mismatch Types:
1. **Dead Space (V/Q → ∞):**
- Ventilation without perfusion
- Wasted ventilation
- Examples: Pulmonary embolism, decreased cardiac output
- Physiologic dead space = anatomic + alveolar
2. **Shunt (V/Q → 0):**
- Perfusion without ventilation
- Blood bypasses gas exchange
- Does NOT respond to 100% O2
- Examples: Pneumonia, ARDS, pulmonary edema
A-a Gradient:
Key Equation:
A-a gradient = PAO2 - PaO2
PAO2 = (FiO2 × 713) - (PaCO2/0.8)
Clinical Correlation
A patient with pneumonia has a shunt - giving 100% oxygen will not significantly improve their hypoxemia because blood is passing through non-ventilated lung. ARDS is an extreme example of shunting.
Memory Trick
"Shunt is STUCK - oxygen doesn't help. Dead space is DEAD perfusion - like talking to a wall, your ventilation goes nowhere."