1 - Alveolar Ventilation

Question 1

Objectives: Explain effect of gravity on ventilation of the lung

What is the result on distribution of inspired air?

Answer 1

• Gravity alters pleural pressure due to pull on the lung tissue and organs
• Top: More Negative
  
  • ​Lung suspended from top of chest wall; pleural pressure = more negative
• Bottom: Less Negative
  
  • ​Gravity pushes lung down; pleual pressure = less negative
• Air is not distributed evenly due to:
  
  • ​Gravitational gradient (negative top, less negative bottom)
  • Non-linear compliance curve in lungs

Question 2

Objectives: Explain changes in partial pressure of gasses during respiration

(Series of Calculations)

Answer 2

• During gas exhange, Oxygen consumption is 250 ml/min and CO₂ production is 200 ml/min
• You must subtract O₂ and add CO₂ to the totals in the alveoli!
  
  • Inspiration Total = 3600* - 250 + 200 = 3550 total (*This is total inspiration, this value can change)
  • From previous calculations: Room Air Values
    
    • O2 = 756 ml
    • N2 = 2844 ml
    • CO2 = 200 ml
  • Alveoli Values:
    
    • O2 = 756 - 250 = 506 ml
    • N2 = 2844 (no exchange)
    • CO2 = 0 + 200 = 200 ml
• Partial Pressures have now changed; you need to recalculate with the P_Total that factors in Water Vapor (713 mmHg)
  
  • ​Calculate Fractions First: F_Gas = V_Gas / V_Total
    
    • _​FO₂ = 506 / 3550 = 0.14
    • FN₂ = 2844 / 3550 = 0.80
    • FCO₂ = 200 / 3550 = 0.06
  • Calculate Partial Pressures: P_Gas = F_Gas x P_Total
    
    • _​PO2 = 0.14 x 713 = 102
    • PCO2 = 0.06 x 713 = 40

Question 3

Objectives: Define minute ventilation and alveolar ventilation (Rate of Alveolar Ventilation)

What is the Respiratory Exchange Ratio?

How does alveolar ventilation compare to total ventilation?

Answer 3

• Minute Ventilation: V_E = Freq x V_T
• Alveolar Ventilation Per Minute (V_A): Volume of air entering alveoli per minute
  
  • V_A = R_r x (V_T - V_D)
    
    • R_r = Respiration Rate
    • V_T = Tidal Volume
    • V_D = Dead Space Volume
• Respiratory Exchange Ratio (R): CO₂ Output / O₂ Uptake
  
  • R = 200 / 250 = 0.8
• Alveolar ventilation is always LESS because it subtracts volume of deadspace in the lungs

Question 4

Objectives: What is dead space and how do you measure it?

Answer 4

• Amount of air perminut which does not partake in gas exchang–two components:
  
  • Anatomical Dead Space - Large airways (no gas exchange)
  • Alveolar Dead Space - Alveoli ventilated, but not perfused (any alveoli where gas exchange is not happening)
• V_DTotal = V_DAnatomical + V_DAlveolar
• Measurement: Bohr’s Method - Measure Expired CO_2​
  
  • Equation: V_D / V_T = (P_ArtCO₂ - P_ExpCO₂) / P_ArtCO₂
    
    • _​P_ArtCO₂ = Partial pressure arterial CO2
    • P_ExpCO₂ = Partial pressure expired CO2

Question 5

Objectives: How does alveolar ventilation change with changes in CO₂ levels?

Answer 5

• Rate of Alveolar Ventilation (V_A) is directly proportional to Rate of CO2 Exhalation (V_CO2) and inversely proportional to Partial Pressure Arterial CO2 (Pa_CO2)
• Requires Constant (K) factor
• V_A = K ( V_CO2 / Pa_CO2 )

Question 6

Objectives: How does alveolar ventilation change with body temperature? (metabolism)

How would you adjust a ventilator for various conditions?

Answer 6

• V_A α Metabolic Rate (temperature)
• High Body Temp requires higher ventilation settings
• Lower Body Temp requires lower ventilation settings

Question 7

Objectives: Explain the alveolar gas equation

Answer 7

• P_AO₂ = F_insO₂ x (P_ATM - P_H2O) - (P_aCO₂ / 0.8)
  
  • P_AO₂ = Partial Pressure Alveolar O₂
  • F_insO₂ = Fraction O₂ in inspired gas
    
    • Usually 20% (0.20)
  • P_ATM - P_H2O ; At sea level = 740-47
  • P_aCO₂ = Partial Pressure arterial CO₂
  • 0.8 = Respiratory Constant
• Sea Level Short Hand: 0.20(740-47)=150
  
  • ​P_AO₂ = 150 - (P_aCO₂ / 0.80

Question 8

Due to _____ and _____ , how are alveoli sizes impacted during ventilation in the lung?

Where will alveoli be on the compliance curve?

Answer 8

• Due to two forces, alveoli are inflated to different sizes in the lungs
  
  • Gravitational forces (negative at top, less negative at bottom)
  • Non-linear compliance curve
• Alveoli are inflated to different sizes
  
  • Top = Largest (most negative pleural pressure)
    
    • Further along compliance curve
  • Bottom = Smalles (least negative pleural pressure)
    
    • Not as far on compliance curve (they have hypothetical room to expand)

Question 9

Explain the distribution of ventilation within the lungs (alveoli in different regions)

Answer 9

• Alveoli at the top are further along the non-linear compliance curve
  
  • They have the smallest ΔV during inspiration; they are already the most inflated due to lowest negative pressures
• Alveoli at the bottom have more room to expand
  
  • They have the largest ΔV during inspiration; they are the least inflated due to less negative pressures
• Bottom line: Change in volume (ΔV) is greatest at the bottom of the lungs

Question 10

Where is the greatest ventilation in the lungs?

How does lung position change this?

Answer 10

Ventilation is greatest at the bottom

This is also greatest change in volume (ΔV)

Changes due to Gravitational Forces and non-linear Compliance Curve

Bottom is a relative term to the orientation of the lungs and gravity (mg) direction on the lungs; e.g. bottom is relative to the position of the body

Question 11

How do diseases impact regional ventilation?

Answer 11

• Areas with different compliances are ventilated differently
• Low compliance will be ventilated poorly
  
  • Fibrotic Disease
  • Restrictive Disease
• Blood passing through these regions undergoes poor gas exchange; they are Low V/Q units

Question 12

Explain Dalton’s Law and Partial Pressures

Answer 12

• Dalton’s Law: PTotal = PA + PB
  
  • ​As each new gas (PC , PD, etc) is added, the influence (P) of each gas decreases, as PTotal does not change.
• Gas Fractions: PA = FA x PTotal
  
  • F = Fraction of A

Question 13

What is the pressure influence of Oxygen and Nitrogen at ATM?

Answer 13

• O₂ = 21% of Barometric Pressure (760 mm Hg)
  
  • 760x(0.21) = 160 mm Hg O₂
  • Equals PO₂
• N₂ = 79% of Barometric Pressure (760 mm Hg)
  
  • 760x(0.79) = 600 mm Hg N₂
  • Equals PN₂

Question 14

How does the introduction of water vapor alter partial pressures in the airways?

Answer 14

• During gas exchange, O₂ and CO₂ are exchanged in the lungs, so their partial pressures will change
• Water Vapor (47 mm Hg) reduces their partial pressures (Dalton’s Law, adding additional PGas to PTotal)
  
  • ​P_Barometric = 760 = PO2 + PN2 + P_{Water_Vapor}
  • PO₂ + PN₂ = (760 - 47) = 713
• In airways, you MUST factor in Water Vapor
  
  • ​PO₂ = 0.21 x 713 = 150 mm Hg
  • PN₂ = 0.79 x 713 = 563 mm Hg

Question 15

In what cases can physiological deadspace be strongly elevated?

Answer 15

Pulmonary embolus (lyng units with high V/Q ratio)

Artificial increases to deadspace (ventilators, snorkels)

Question 16

What are the clinical implications pf the rate of alveolar ventolation and its relationship to rate of CO2 exhalation and the partial pressure of arterial CO2?

Why does this case not happen every time we exercise?

Answer 16

Remember: V_A = K (V_CO2 / P_A-CO2)

• Hypoventilation: Reduce Rate of Alveolar Ventilation (V_A) ; CO₂ production constant ; P_A-CO2 will increase at inverse rate
  
  • ​Lowers blood pH; Respiratory Acidosis
  • High P_ACO2 = Acidic
• Hyperventilation: Increase Rate of Alveolar Ventilation (V_A) ; CO₂ production constant ; P_A-CO2 will decrease at inverse rate
  
  • ​Increase blood pH; Respiratory Alkalosis
  • Low P_ACO2 = Basic (Alkaline)
• During exercise:
  
  • Rate of Ventilation increases
  • Rate of CO2 production increases direct to rate
  • P_aCO2 is unchanged

Question 17

In hyper and hypventilation, how are ΔP_aCO₂ and ΔP_aO₂ related to eachother?

How do you solve these types of problems?

What equations do you need?

What known values do you need?

Answer 17

They are inversely related–they will be roughly the same end values after changes.

To Solve:

1. VA*=K x V*CO2/PaCO2 ; need for changes based on changes in rate; PaCO2 will increase or decrease
2. PAO2=FIO2(PB-PH2O)-(PaCO2/0.8) or 150-PaCO2/0.8 ; Solve for PAO2 with change in PaCO2 from #1
3. Solve ΔPaO₂ and ΔPaCO₂ ; these will be roughly equal to eachother

Question 18

What does venous blood gas tell you about respiratory function?

Answer 18

Nothing, it tells you nothing!