VENTILATION/PERFUSION

1
VENTILATION/PERFUSION GAS EXCHANGE

• AIMS
• What factors are important in gas exchange?
• What factors affect gas transfer?
• What is the ventilation/perfusion ratio?

2
GAS EXCHANGE

• ADEQUATE ALVEOLAR VENTILATION(VA)
• TRANSFER OF GAS ACROSS ALVEOLAR-CAPILLARY
  MEMBRANE
• ADEQUATE PERFUSION
• MATCHING OF VENTILATION AND PERFUSION

3
TRANSFER OF GASES

• FICKS LAW
• GAS FLOW?PAS/ TMw
• Where, ?P pressure gradient
  
• A surface area
• S solubility
• T thickness
• Mw molecular
  weight
• Diffusion- THIN MEMBRANE(0.2-1?m) LARGE
  SURFACE AREA (50-70 m2)
• Measurement of gas transfer- transfer factor,
  use CO (0.3)- DCO (KCO)

4
VENTILATION/PERFUSION

• VENTILATION- VT f PERFUSION- cardiac output
• RATIO OF VENTILATION TO PERFUSION
• efficiency of gas transfer by alveoli
• ratios 0.5-3.0, mean 0.8
• extreme situations
• diffusion limitation
• perfusion limitation
• limitations in both
• Posture (gravity) - affects ratios
• P h?g - pressure 1cmH2O /cm of lung
• standing BF? from base to apex, VA?from base to
  apex, ratios? from base to apex
• top of lung not very important

5
CONCLUSIONS

• 1. Factors important for gas exchange - VA, gas
  transfer, perfusion and matching.
• 2. Gas transfer is directly proportional to
  pressure difference, surface area, inversely to
  thickness- Ficks law.
• 3. Efficiency of gas exchange is increased if
  ventilation and perfusion are matched.