04: Respiratory System Mechanics & Pulmonary Function Tests

Question 1

Airway caliber

Answer 1

• Intraluminal: altered by secretions, foreign bodies, tumors
• Intramural: altered by inflammation, fibrosis, smooth muscle tone, edema
• Extraluminal: altered by tumor/node/blood vessel compression, loss of radial traction

Question 2

FEV1/FVC

Answer 2

Normal = 80%

Airflow obstruction if < 70% or lower limit of normal

Question 3

Airflow obstruction

Answer 3

• AKA obstructive ventilatory defect
• Conditions characerized by OVD:
  
  • Asthma (episodic, reversed w/ bronchodilator meds)
  • COPD/emphysema
  • Bronchiectasis (chronic airway infx)
  • Focal airway obstruction (tumor, stenosis, foreign body)
  • Small airway disease (bronchiolitis, chronic lung allograft rejection)

Question 4

Compliance

Answer 4

Change in volume/change in pressure

1/Elastance

Determined by properties of lung tissue (parenchyma) and surface tension in alveoli.

Question 5

Hysteresis

Answer 5

• Compliance curves different for inspiration vs. expiration
• At given transpulmonary pressure, lung volume greater during expiration than during inspiration

Question 6

Pulmonary surfactant

Answer 6

• Protein-rich lipid layer at alveolar surface
  
  • Lipids: DPPC
  • Proteins: Surfactant proteins A, B, C, D
• Produced by type II pneumocytes

Question 7

Newborn respiratory distress syndrome (NRDS)

Answer 7

• Premature newborns lack surfactant
• Lungs stiff and collapsed
• Tx: intratracheal surfactant

Question 8

“Wash in”

Answer 8

• Gas equilibriation to determine FRC
• Breathe in known mass and known concentration of He
• Allow system to equilibriate and measure final concentration of He
• Lung volume = He mass/final He concentration

Question 9

“Wash out”

Answer 9

• Gas equilibriation to determine FRC
• Ambient air and lungs contain 80% nitrogen (known concentration)
• While breathing 100% FiO2, measure how much nitrogen (mass) is washed out of lungs in exhaled gas.
• Lung volume = measured N2 mass/N2 concentration

Question 10

Restrictive ventilatory defect

Answer 10

• Reduced total lung capacity
• TLC < 75-80% predicted volume or < lower limit normal
• Causes
  
  1. ↓lung compliance (abn. parenchyma)
  2. ↓chest wall compliance (pleural/chest wall dz)
  3. Lung removal, destruction or collapse
  4. ↓respiratory muscle force generation (does not have symmetric reduction in all lung volumes)
• Spirometry
  
  • ↓FVC, ↓FEV1, normal FEV1/FVC ratio
    
    • NOT diagnostic (can be gas trapping or poor muscle effort/weakness)
  • Diagnose by ↓TLC

Question 11

Restrictive pattern in neuromuscluar weakness

Answer 11

• Smaller volumes with each breath
• FRC normal
• Inspiratory capcity (IC) mainly decreased
• TLC reduced

Question 12

Assessing respiratory system mechanics via volume control ventilation

Answer 12

• Flow and tidal volume programmed
• Airway resistance and lung/chest wall compliance depend on patient
• Thus, changes in airway pressure on a ventilator reflect changes in respiratory system mechanics
  
  • Peak pressure: resistive and elastic components
  • Plateau pressure: measured during “inspiratory breath hold” (zero flow); determined by two factors:
    
    • Respiratory system compliance
    • Tidal volume
• Peak inspiratory pressure - plateau pressure = **Delta P **(airway resistance)
• Plateau pressure - expiratory pressure = Compliance
• In the setting of elevated peak pressure:
  
  • High plateau, normal delta P: compliance problem
  • Normal plateau, high delta P: resistance problem

Question 13

Static compliance

Answer 13

= tidal volume / (plateau pressure - expiratory pressure)

Follow in patients with a “low compliance” problem.

Normal value on MV = 60-80 mL/cmH20

Decreased is < 60

Severely low is < 10-30

Question 14

Dynamic compliance

Answer 14

= tidal volume / (peak pressure - expiratory pressure)