29 Pathophysiology of Restrictive Disorders

Question 1

Restrictive disorder

• A large and diverse group of disorders accompanied by…
• The mechanism by which this occurs…
• Three major pathophysiologic categories

Answer 1

• A large and diverse group of disorders accompanied by a decrease in TLC and VC
• The mechanism by which this occurs varies with the underlying disease process
• Three major pathophysiologic categories
  
  • Disorders causing increased lung elastic recoil
  • Disorders limiting the expansion of the chest wall
  • Disorders causing impaired respiratory muscle function

Question 2

Alterations in respiratory mechanics:
Disorders increasing lung elastic recoil

• Disorders that increase the elastic recoil (stiffness) of the lungs lead to…
• The most common disorders causing increased lung elastic recoil
  
  • Diseases
  • Characterized histologically by…

Answer 2

• Disorders that increase the elastic recoil (stiffness) of the lungs lead to…
  
  • A decrease in VT, FRC, VC, TLC, and RV
  • Downward displacement of the pressure-volume curve of the respiratory system
    
    • More pressure must be generated by the respiratory muscles to inflate the lungs to a given volume
    • Conversely, at a given volume, more pressure (the elastic recoil pressure) is available to drive expiratory flow
• The most common disorders causing increased lung elastic recoil
  
  • Interstitial lung diseases
  • Characterized histologically by interstitial inflammation and fibrosis, which lead to reduced lung distensibility

Question 3

Alterations in respiratory mechanics:
Disorders limiting expansion of the chest wall

• This heterogeneous group of disorders includes…
• All…

Answer 3

• This heterogeneous group of disorders includes…
  
  • Kyphoscoliosis
  • Morbid obesity
  • Massive ascites
• All…
  
  • Decrease the equilibrium volume of the chest wall
  • Impede its expansion
  • This, in turn, causes VC, TLC, and all other lung volumes to be abnormally low

Question 4

Alterations in respiratory mechanics:
Disorders causing impaired respiratory muscle function

• TLC and RV are largely determined by…
• Disorders leading to impaired respiratory muscle function will, therefore, cause…
  
  • TLC
  • RV
  • Vital capacity
  • FRC
• The extent to which TLC and RV change will depend on…
• Respiratory muscle dysfunction may result from diseases affecting…

Answer 4

• TLC and RV are largely determined by…
  
  • The balance between the elastic recoil of the respiratory system
  • The strength of the respiratory muscles
• Disorders leading to impaired respiratory muscle function will, therefore, cause…
  
  • TLC to fall
  • RV to increase
  • Vital capacity to fall, since it is the difference between these two volumes
  • FRC to remain normal, since it is determined only by the elastic recoil of the lungs and chest wall
• The extent to which TLC and RV change will depend on…
  
  • The severity of the disorder
  • Whether inspiratory or expiratory muscles are affected to a greater degree
• Respiratory muscle dysfunction may result from diseases affecting…
  
  • The central nervous system
  • The peripheral nerves
  • The neuromuscular junction
  • The muscles themselves

Question 5

Alterations in gas exchange

• Interstitial inflammation and fibrosis cause…
• This leads to…
• This produces…

Answer 5

• Interstitial inflammation and fibrosis cause…
  
  • Occlusion and destruction of the pulmonary capillary bed
• This leads to…
  
  • Under-perfusion of the most diseased regions of the lungs
  • Over-perfusion of less-involved areas
  • Relatively little change in alveolar ventilation
• This produces…
  
  • Significant ventilation-perfusion mismatching, which leads to a drop in the PaO2 and a rise in the PA-aO2

Question 6

Alterations in gas exchange

• Interstitial lung disease can also be accompanied by…
• Equilibration requires…
• Alveolar gas and capillary blood are normally in contact for an average…
• As capillary destruction occurs,…
  
  • Cardiac output
  • Flow rate
  • Time allowed for equilibration
  • Diffusion

Answer 6

• Interstitial lung disease can also be accompanied by…
  
  • Incomplete equilibration between the PO2 of alveolar gas and end-capillary blood
• Equilibration requires…
  
  • Approximately 0.25 second
• Alveolar gas and capillary blood are normally in contact for an average…
  
  • Of 0.75 second
• As capillary destruction occurs,…
  
  • Cardiac output is forced through a smaller and smaller number of capillaries
  • Fow rate of the blood increases
  • Time allowed for equilibration progressively falls
  • When the increased diffusion distance due to interstitial thickening is combined with decreased blood transit time, it is evident that diffusion impairment may occur, especially when heart rate and cardiac output increase

Question 7

Alterations in gas exchange

• Exchange of CO2 by the lungs is also affected by…
• Abnormal V/Q relationships…
• This, coupled with a fall in VT, leads to…
• Most patients with interstitial lung disease…
• CO2 retention can, however, be seen in…

Answer 7

• Exchange of CO2 by the lungs is also affected by…
  
  • Interstitial lung disease
• Abnormal V/Q relationships…
  
  • Cause an increase in alveolar and physiologic dead space
  • Impair CO2 excretion by the lungs
• This, coupled with a fall in VT, leads to…
  
  • An increase in VD/VT
• Most patients with interstitial lung disease…
  
  • Increase respiratory rate and minute ventilation to compensate
• CO2 retention can, however, be seen in…
  
  • End-stage disease when abnormal mechanics prevent further increases in minute ventilation

Question 8

Alterations in gas exchange

• Other disorders that may impair gas exchange
• Low VT often leads to…
• PaCO2 will increase if patients…

Answer 8

• Other disorders that may impair gas exchange
  
  • Disorders affecting the chest wall and the respiratory muscles
• Low VT often leads to…
  
  • Atelectasis (focal collapse) of the lung parenchyma
  • Leads to V/Q mismatching, shunt, and arterial hypoxemia
• PaCO2 will increase if patients…
  
  • Are unable to maintain adequate VA either because of excessive load on or impaired function of the respiratory muscles

Question 9

Pulmonary function tests

• In all types of restrictive disorders
  
  • TLC and VC
  • FEV1
  • The amount exhaled in one second
  • FEV1/FVC
• Restrictive disorders affecting the lung parenchyma and chest wall cause…
  
  • RV to be…
  • FRC to be…
• Respiratory muscle dysfunction typically causes…
  
  • RV to be…
  • FRC to be…
• Diffusing capacity in interstitial lung diseases

Answer 9

• In all types of restrictive disorders
  
  • TLC and VC
    
    • Decreased
  • FEV1
    
    • Also reduced, not because expiratory flow is decreased, but because of the low VC
  • The amount exhaled in one second
    
    • Since there is relatively little air in the lungs at TLC, the amount exhaled in one second will be abnormally low
  • FEV1/FVC
    
    • Because there is no obstruction to expiratory flow, the reduced VC can be exhaled quickly, and FEV1/FVC is normal or elevated
• Restrictive disorders affecting the lung parenchyma and chest wall cause…
  
  • RV to be abnormally low
  • FRC to be abnormally low
• Respiratory muscle dysfunction typically causes…
  
  • RV to be abnormally high
  • FRC to be maintained as normal
• Diffusing capacity in interstitial lung diseases
  
  • Diffusing capacity is usually reduced in the interstitial lung diseases because of capillary destruction and increased diffusion distance

Question 10

Pulmonary function tests:
Interstitial lung disease:
Chest wall disorders

• FVC
• FEV1
• FEV1/FVC
• PEFR
• FEF25-75
• FRC
• RV
• TLC
• DLCO

Answer 10

• FVC: decreased
• FEV1: decreased
• FEV1/FVC: normal to increased
• PEFR: normal to increased
• FEF25-75: decreased
• FRC: decreased
• RV: decreased
• TLC: decreased
• DLCO: normal to decreased

Question 11

Pulmonary function tests:
Respiratory muscle dysfunction

• FVC
• FEV1
• FEV1/FVC
• PEFR
• FEF25-75
• FRC
• RV
• TLC
• DLCO

Answer 11

• FVC: decreased
• FEV1: decreased
• FEV1/FVC: normal to increased
• PEFR: decreased
• FEF25-75: decreased
• FRC: normal
• RV: increased
• TLC: decreased
• DLCO: normal

Question 12

Pulmonary function tests:
Flow-volume curve

Answer 12

• The abnormally low FVC causes the base to narrow as the restrictive process worsens
• The volume-time curve (not shown) rapidly reaches a volume plateau
• Image: flow-volume curves in patients with mild (A) and severe (B) restrictive disease

Question 13

Pathophysiology of dyspnea and exercise limitation

• Disorders that cause a progressive fall in VC and VT
• As VT falls, respiratory rate…
• In patients with interstitial lung disease, dyspnea and exercise intolerance are exacerbated by…

Answer 13

• Disorders that cause a progressive fall in VC and VT
  
  • Disorders that increase the elastic recoil of the lungs
  • Disorders that impede expansion of the chest wall
  • Disorders that lead to respiratory muscle dysfunction
• As VT falls, respiratory rate…
  
  • Must increase to maintain a normal PaCO2
  • Any activity will, of course, further increase the required respiratory rate
• In patients with interstitial lung disease, dyspnea and exercise intolerance are exacerbated by…
  
  • Arterial hypoxemia
  • Increases in dead space ventilation