32 Pathophysiology of Air Space Filling Diseases

Question 1

Airspace filling diseases

• Characterized by…
• Most commonly…

Answer 1

• Characterized by…
  
  • The replacement of alveolar gas with some type of “foreign” material
• Most commonly…
  
  • Edema fluid (pulmonary edema)
  • Inflammatory cells (pneumonia)
  • Blood (alveolar hemorrhage)

Question 2

Airspace filling diseases:
Alterations in respiratory mechanics

Answer 2

• Lung volumes are reduced in these diseases simply because there are fewer “empty” alveoli to receive the inspired gas
• In a sense, then, airspace filling diseases produce small lungs by markedly reducing the number of available alveoli

Question 3

Airspace filling diseases:
Alterations in gas exchange

• These diseases interfere with…
• What occurs since perfusion remains relatively intact
• Depending on disease severity, a variable proportion of the lungs receives…
• For example, in patients with severe ARDS,…

Answer 3

• These diseases interfere with ventilation to affected alveoli
• Since perfusion remains relatively intact (even with hypoxic vasoconstriction), a marked increase in V/Q mismatching occurs, with low V-Q ratios predominating
• Depending on disease severity, a variable proportion of the lungs receives no ventilation, and this produces a right to left shunt
• For example, in patients with severe ARDS, as much as 50% of the cardiac output may pass through unventilated alveoli

Question 4

Airspace filling diseases:
Alterations in gas exchange

• The presence of V/Q mismatching and shunt causes…
• As always, V/Q mismatching also produces…
• The reduction in the number of gas-filled alveoli cause…
• This leads to…

Answer 4

• The presence of V/Q mismatching and shunt causes PaO2 to fall and PA-aO2 to increase
• As always, V/Q mismatching also produces high V/Q regions, which increase alveolar and physiologic dead space
• The reduction in the number of gas-filled alveoli cause the tidal volume to fall
• This leads to an increase in VD/VT and to a further increase in dead space ventilation
• Patients are usually able to compensate for these changes by increasing minute ventilation
• In those with diffuse alveolar filling (e.g. ARDS, pulmonary edema) such an increase may not be possible due to a marked increase in dead space ventilation, and these patients may develop acute hypercapnia

Question 5

Airspace filling diseases:
Alterations in gas exchange

• Patients are usually able to…
• In those with diffuse alveolar filling (e.g. ARDS, pulmonary edema),…

Answer 5

• Patients are usually able to compensate for these changes by increasing minute ventilation
• In those with diffuse alveolar filling (e.g. ARDS, pulmonary edema), such an increase may not be possible due to a marked increase in dead space ventilation, and these patients may develop acute hypercapnia

Question 6

Airspace filling diseases:
Pathophysiology of dyspnea and exercise intolerance

Answer 6

• In the airspace filling diseases, these symptoms result primarily from alveolar filling, which reduces tidal volume and forces the respiratory rate to increase
• Other factors include increased physiologic dead space and arterial hypoxemia