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Respiratory > Pulmonary Diffusing Capacity > Flashcards

Pulmonary Diffusing Capacity Flashcards

1
Q

What are the two types of repsiratory insufficiency?

A
  1. Adequate Ventilation
  2. Inadequate Ventilation (Hypoventilation)
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2
Q
  • What two processes can lead to inadequate ventilation (hypoventilation)?
  • What can these processes lead to?
A
  1. Obstructive
    • anatomic
    • allergic
  2. Restrictive
    • ↓ lung wall compliance
      • elastic tissue
      • ↑ surface tension
    • ↓ chest wall compliance
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3
Q

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

A
  • D = Diffusion rate
  • P = Pressure
  • S = Solubility
  • d = distance
  • MW = Molec. wt.
  • A = Area
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4
Q

Which factors might contribute to diffusion impairment in the lung?

A
  • Surface area
  • Distance (thickness)
  • Pressure gradient
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5
Q
  1. What is the surface area (A) of the lung?
  2. What is the thickness (d) of the lung membrane?
A
  1. A = ~ 80 meters2
  2. d = ~ 0.6 µ
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6
Q

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

A
  • 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
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7
Q

Anything that affects Hb will also affect the …

A

rate of diffusion

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8
Q

Why is O2 more susceptible to diffusion impairment than CO2?

A

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
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9
Q
  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)?
A
  1. O2 removed per 100 ml = 5 mL
  2. CO = 5000 ml
  3. normal oxygen requirement = 250 ml O2/min
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10
Q

What is diffusion capacity (DL)?

A
  • DL = Diffusing capacity
  • D = Diffusion rate
  • PA = Alveolar partial pressure
  • Pc = Mean capillary partial pressure
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11
Q
  1. Why can’t the DL for oxygen be measured directly?
  2. Why is carbon monoxide (CO) used to calculate DL?
A
  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. Pc = 0)
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12
Q

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

A
  • 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
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13
Q

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

A
  • 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
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14
Q

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

A

DLO2 = DLCOx 1.23

  • Example: 17 x 1.23 = 21 ml/min/mmHg
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15
Q

Measuring Volume:

  1. ∆P across mesh screen ⇒
  2. Flow integrated over time ⇒
A
  1. ∆P across mesh screen ⇒ Flow rate
  2. Flow integrated over time ⇒ Volume
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16
Q

How is DL correlated with age, height and gender?

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

where ht is inches and age is in years

17
Q

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

A

Normal DLO2 = 21 ml/min/mmHg

18
Q

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

A

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

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

19
Q

How does exercise affect DL for O2?

A

Increases it by 3

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

25 + 100 ⇒ 125/2 = 63

20
Q

How does abnormal oxygen exchange affect the DL for O2?

A

Decreases DLO2

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

40 + 100 ⇒ 140/2 = 70

21
Q

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

A

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

22
Q

Calculate the DL values for:

  1. Normal oxygen exchange:
    • Pc = 88
  2. Exercise oxygen exchange (increase x10 for D):
    • Pc = 63
  3. Abnormal oxygen exchange:
    • Pc = 70
A
  1. Normal oxygen exchange:
    • DL = 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
23
Q

In diffusion impairment, would you expect a reduced PaO2 to be accompanied by an increase in PaCO2?

Why?

A

CO2 Advantages:

  • More soluble
  • More carefully controlled
  • High VA/Qc compensate for low VA/Qc
    • CO2 does not saturate in the blood like O2
24
Q

How would anemia or hemorrhage affect DL?

A
  • DM = membrane diffusing capacity
  • Qc = capillary volume
  • θ = blood O2 affinity
25
Q

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

A
  1. Restrictive:
    • ↓DL and ↓VA ⇒ normal DL/VA
  2. Obstructive:
    • ↓DL and VA normal or ↑ ⇒ low DL/VA
26
Q

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

A
  1. Lung
    • Diffusion Impaired
    • VA/Qc Mismatch
  2. Blood
    • Abnormal Hb
    • Reduced blood volume
27
Q

What can lead to diffusion impairment?

A
  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
28
Q

What can lead to VA/Qc Mismatch?

A
  1. Shunt
    • V/Q = 0
  2. Dead space
    • V/Q = ∞
  • Normal V/Q = 1
29
Q

What problems in the blood can lead to problems in O2 delivery?

  • Qc:
  • θ:
A
  1. Qc:
    • hemorrhage
  2. θ:
    • carbon monoxide

Respiratory (31 decks)

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