1 - Diffusion of Gasses

Question 1

Objectives: Explain diffusion of oxygen through alveolar capillary wall and Fick’s Law

Answer 1

• Oxygen will diffuse through several layers (water layer, sufactant, epithelium, interstitial space, endothelium, plasma, binds Hb
• Fick’s Law: Diffusion depends on membrane characteristics
  
  • ​V_Gas = (A x D x ΔP) / T
    
    • ​V_Gas = Rate of diffusion (direct)
    • A = Barrier surface area (direct​)
    • D = Diffusion constant (direct​)
    • ΔP = Pressure gradient (direct​)
    • T = Thickness of barrier (inverse)

Question 2

Objectives: Define perfusion limitation and diffusion limitation

Answer 2

• Perfusion Limitation: Rate limiting step is how fast you can pump blood through capillaries
  
  • If you want more O₂ delivered, pump more blood.
  • Blood is fully saturated rapidly
  • Other gasses also Perfusion Limited: N₂O, CO₂
• Diffusion Limitation: Rate limiting step is how fast gas can diffuse through membrane
  
  • Carbon Monoxide (CO) main example
  • High affinity for Hb prevents large ΔP (once bound, does not contribute to PP)

Question 3

Objectives: Explain what DL_CO is and how it’s measured

Answer 3

• Test to determined how well gas exchange is occuring; must use Fick’s Law
  
  • Direct: Area, ΔP_Gas, Diffusion Constant
  • Inverse: Thickness
• In blood, PCO = Zero, ΔP_CO = P_ACO
  
  • CO is instantly “removed” from blood by Hb
• Measurement Steps:
  
  • Breath in 0.3% CO, 10% He through spirometer
  • Hold for 10 s
  • Release through CO Meter, measure
    
    • End of exhalation = alveolar gas
    • Measure P_ACO and F_expCO
    • 0.3% of 1 L - (F_expCO x 1 L) = V’CO
  • Equation: DL_CO = V’CO / P_ACO
    *

Question 4

Objectives: How is DL_CO applied in diagnosis of lung disease?

Answer 4

• Decrease DLCO: Reduced area, increased thickness
  
  • Obstructive Disease: Alveolar simplification and loss of pulmonary capillary structures​
    
    • ​Emphysema - Destroyed lung tissues
  • Restrictive Disease: Thickening of alveolar capillar membrane, impaired diffusion
    
    • Fibrosis - Thickening of interstitial spaces
  • Pulmonary Edema - Thickening of membrane from inflammation
• Increase DLCO: Increased Area, Decreased Thickness
  
  • Alveolar Hemorrhage: Increase in RBCs (increased Hb availability)
    *

Question 5

Why do you divide DL_CO by P_ACO?

Answer 5

Normalizes the Pressure difference, eliminating it from Fick’s Equation

Becomes only dependend on Area, Thickness

Question 6

Explain the significance of Fick’s Law and it’s relation to DL_CO

Answer 6

• Fick’s Law: V’CO = (A x D x PA_CO) / T
  
  • V’CO = Update rate
  • A = Area
  • D = Diffusion Constant
  • P_ACO = Measured Alveolar Pressure of CO
  • T = Membrane Thickness
• DL_CO = V’CO / PA_CO = (A x D)/T
  
  • Normalizes to pressure
• BLUF: DL_CO depends on characteristics of the membrane barrier, its area, thickness, and composition!

Question 7

What does DL_CO measure?

Answer 7

Conductance (flux per unit driving force)

Or – efficacy of diffusion of the alveolar membrane

Changes to area available for exhange and barrier thickness will alter this value

Question 8

What is the normal rate limiting step in oxygen saturation at the alveoli?

What can this change to in disease states?

Answer 8

• Normal: Perfusion Limited
• Disease: Diffusion Limited
  
  • Blood is still flowing, however the diffusion of O₂ has been slowed and oxygen fails to fully saturate blood

Question 9

What affect will the following have on DL_CO?

Bronchitis

Neuromuscular Jx/Chest Wall Deformities

Asthma

Answer 9

• Bronchitis: Only affects upper airways, normal DLCO
• Nm JX/Chest Wall: DLCO normal
• Asthma: Variable; normal or increased

Question 10

What are some conditions which can increase DL_CO?

Answer 10

• Supine Position:
• Exercise
• Polycythemia - Increase in Hb
• Asthma - More blood reaching apices of lung; more uniform distribution of blood
• Hemorrhage - Increase in Hb
• Left-Right Cardiac Shunt - Increased pulmonary capillary volume
• Extreme Obesity - Higher pulmonary blood volume; higher DLCO