Pulmonary Diffusing Capacity

Question 1

What are the two types of repsiratory insufficiency?

Answer 1

1. Adequate Ventilation
2. Inadequate Ventilation (Hypoventilation)

Question 2

• What two processes can lead to inadequate ventilation (hypoventilation)?
• What can these processes lead to?

Answer 2

1. Obstructive
   
   • anatomic
   • allergic
2. Restrictive
   
   • ↓ lung wall compliance
     
     • elastic tissue
     • ↑ surface tension
   • ↓ chest wall compliance

Question 3

What factors influence the rate of diffusion across the respiratory membrane?

Answer 3

• D = Diffusion rate
• P = Pressure
• S = Solubility
• d = distance
• MW = Molec. wt.
• A = Area

Question 4

Which factors might contribute to diffusion impairment in the lung?

Answer 4

• Surface area
• Distance (thickness)
• Pressure gradient

Question 5

1. What is the surface area (A) of the lung?
2. What is the thickness (d) of the lung membrane?

Answer 5

1. A = ~ 80 meters²
2. d = ~ 0.6 µ

Question 6

How is the pressure gradient of O₂ different based upon the presence of hemoglobin (Hb)?

Answer 6

• Without Hb
  
  • PO₂: 40 ⇒ 100 mmHg
  • Vol O₂: 0.18 ml/100 ml
• With Hb
  
  • PO₂: 40 ⇒ 100 mmHg
  • Vol O₂: 5 ml/100 ml
  • Hb is an oxygen sink
    
    • restores the pressure gradient

Question 7

Anything that affects Hb will also affect the …

Answer 7

rate of diffusion

Question 8

Why is O₂ more susceptible to diffusion impairment than CO₂?

Answer 8

CO₂ is 20x more soluble in blood than O₂ (Henry’s Law)

• SO₂ = ~0.03 mL/L/mm Hg
• SCO₂ = ~0.6 mL/L/mm Hg

Question 9

1. On average, how many ml of O₂ are removed from 100 ml of blood passing through systemic capillaries?
2. What is the normal value for cardiac output (CO)?
3. What is the normal oxygen requirement in a normal adult at rest (ml O₂/min)?

Answer 9

1. O₂ removed per 100 ml = 5 mL
2. CO = 5000 ml
3. normal oxygen requirement = 250 ml O₂/min

Question 10

What is diffusion capacity (D_L)?

Answer 10

• D_L = Diffusing capacity
• D = Diffusion rate
• P_A = Alveolar partial pressure
• P_c = Mean capillary partial pressure

Question 11

1. Why can’t the D_L for oxygen be measured directly?
2. Why is carbon monoxide (CO) used to calculate D_L?

Answer 11

1. We cannot measure the mean capillary pressure for oxygen
2. CO is used instead:
   
   • CO is immediately taken up by Hb (200x greater affinity than O₂)
   • HbCO is not dissolved in the blood (i.e. P_c = 0)

Question 12

What is the Steady-State method for measuring D_L and what are its advantages?

Answer 12

• Breathe 0.1 - 0.2% CO for 5 to 6 minutes
• Collect expired gas over the last 2 minutes

Advantages:

1. more “natural” (physiological)
2. easier for persons with lung disease
3. can be used during exercise
4. sometimes more invasive

Question 13

What is the Single Breath method for measuring D_L and what are its advantages?

Answer 13

• Full VC inspiration of trace CO and reference gas
• 10 second breath-hold, then maximal expiration

Advantages:

1. systems for rapid testing and evaluation
2. test is more difficult to perform
3. values are higher than Steady-state DL

Question 14

How can the D_L for oxygen be derived from the D_L for CO?

Answer 14

D_LO2 = D_LCOx 1.23

• Example: 17 x 1.23 = 21 ml/min/mmHg

Question 15

Measuring Volume:

1. ∆P across mesh screen ⇒
2. Flow integrated over time ⇒

Answer 15

1. ∆P across mesh screen ⇒ Flow rate
2. Flow integrated over time ⇒ Volume

Question 16

How is D_L correlated with age, height and gender?

Answer 16

• Males:
  
  • D_LCO = [0.250 x (ht)] – [0.177 x (age)] + 19.93
• Females:
  
  • D_LCO = [0.284 x (ht)] – [0.177 x (age)] + 7.72

where ht is inches and age is in years

Question 17

What is the normal value of D_L for O₂ at rest (obtained from the D_L for CO)?

Answer 17

Normal D_LO2 = 21 ml/min/mmHg

Question 18

How can you measure the oxygen exchange efficiency based on this graph?

Answer 18

Take the points on the graph to get the average capillary pressure:

40 + 80 + 100 + 100 + 100 + 100 + 100 ⇒ 620/7 = 88

Question 19

How does exercise affect D_L for O₂?

Answer 19

Increases it by 3

• Take the average between two points (almost linear increase):

25 + 100 ⇒ 125/2 = 63

Question 20

How does abnormal oxygen exchange affect the D_L for O₂?

Answer 20

Decreases D_LO2

Take the average of the two endpoints (linear increase):

40 + 100 ⇒ 140/2 = 70

Question 21

How does the decrease in D_L for O₂ in the abnormal lung emphasize the physiological reserve of the lung?

Answer 21

The patient is not hypoxemic, however, there is no reserve for excersise

Question 22

Calculate the D_L values for:

1. Normal oxygen exchange:
   
   • P_c = 88
2. Exercise oxygen exchange (increase x10 for D):
   
   • P_c = 63
3. Abnormal oxygen exchange:
   
   • P_c = 70

Answer 22

1. Normal oxygen exchange:
   
   • D_L = 250/(100 - 88) = 21 ml/min/mmHG
2. Exercise oxygen exchange:
   
   • DL = 2500/(100 - 63) = 68 ml/min/mmHG
3. Abnormal oxygen exchange:
   
   • DL = 250/(100 - 70) = 8.3 ml/min/mmHG

Question 23

In diffusion impairment, would you expect a reduced P_aO₂ to be accompanied by an increase in P_aCO₂?

Why?

Answer 23

CO₂ Advantages:

• More soluble
• More carefully controlled
• High VA/Qc compensate for low VA/Qc
  
  • CO₂ does not saturate in the blood like O₂

Question 24

How would anemia or hemorrhage affect D_L?

Answer 24

• D_M = membrane diffusing capacity
• Q_c = capillary volume
• θ = blood O₂ affinity

Question 25

How would D_L be affected in a subject with an **obstructive or restrictive ventilatory impairment**?

Answer 25

1. **Restrictive:** * ↓DL and ↓VA ⇒ **normal DL/VA** 2. **Obstructive:** * ↓DL and VA normal or ↑ ⇒ **low DL/VA**

Question 26

How can you have **oxygen delivery impairment** if the there is **adequate ventilation**?

Answer 26

1. **Lung** * Diffusion Impaired * V_A/Q_c Mismatch 2. **Blood** * Abnormal Hb * Reduced blood volume

Question 27

What can lead to **diffusion impairment**?

Answer 27

1. **Thickening of the respiratory membrane ⇒ **Pneumonia * fluid and blood cells in alveoli * Alveolar walls thickened by edema 2. **Loss of surface area ⇒ **Ephysema * Loss of elastic recoil * Leads to airway collapse during expiration because of forcible compression * Confluent alveoli

Question 28

What can lead to **V_A/Q_c Mismatch**?

Answer 28

1. **Shunt** * V/Q = 0 2. **Dead space** * V/Q = ∞ * Normal V/Q = 1

Question 29

What problems in the **blood** can lead to problems in O₂ delivery? * **Q_c:** * **θ:**

Answer 29

1. **Qc:** * hemorrhage 2. **θ:** * carbon monoxide