V/Q Mismatch

• Both V and Q ↓ as you move from base to apex.
  • ↓ total number of alveoli → ↓ diffusive area → ↓ V
  • ↑ compression of intra-alveolar vessels → ↑ west zone 1 → ↓ Q
  • V ↓’s slower than Q
• The intersection of V/Q is at rib 3, here V/Q = 1
• V/Q = 3 at the apex
• V/Q = 0.67 at bases

• Shunt
• P_ACO₂ and P_AO₂ = mixed venous blood
  • P_ACO₂ = 46mmHg
  • P_AO₂ = 40mmHg

• Matched ventilation and perfusion
• P_ACO₂ and P_AO₂ = normal blood gas for healthy patient
  • P_ACO₂ = 40mmHg
  • P_AO₂ = 100mmHg

• Dead space
• Alveolar pressures equal that of inspired air
  • P_ACO₂ = 0mmHg 
  • P_AO₂ = 150mmHg

• Increased V/Q inequality causes:
  • Mild decrease in P_aO₂
  • At Apex
    • decrease in dissolved O2
    • Minimal decrease in Hb saturation due to flat part of OHDC
    • Thus overall decrease in C_aO₂
  • Base
    • P_AO₂ = P_vO₂ = 40
• Addition of a small quantity of poorly oxygenated shunted blood flow (PO₂ < PcO2) added to a large volume well-oxygenated PECB →  small fall in C_aO₂ BUT a LARGER fall in P_aO₂
• Because carrying capacity of blood to apex is decreased there is a decrease in P_aO₂

Remembering that the relationship for the Alveolar gasses:

• Increased V/Q inequality causes:
  • Mild increase in P_aCO₂  which is rapidly compensated for due to Chemoreceptor reflexes
  • Apex to base
    • ↓P_ACO₂ towards the apices of the lungs
    • ↓gas transfer → ↑P_aCO₂ 
    • Sensed by chemoreceptors (sensitive to ±3mmHg) 
    • ↑RR and T_v as per the CO₂ dissociation curve
• Normal or slightly lower (after compensation) CO₂