6. Physiology of Pulmonary Function Tests - Richard

• Type 1 pneumocyte

  • cover most of internal surface of each alveolus
  • thin and squamous - ideal for gas exchange
• Type 1 pneumocyte function

  • share a basement membrane with pulmonary capillary endothelium
  • form air-blood barrier where gas exchange occurs
  • form tight junction to prevent fluid into alveoli
• Brush cells - serve as receptor that monitor air quality
• Type II pneumocyte

  • progenitor for type I cells
  • express ACE2
  • within cytoplasm - are lamellar bodies containing surfactant
  • decrease surface tension of alveoli
• Alveolar macrophages - mononuclear phagocytes

  • they contain microtubules to changes shape during chemotaxis
• Where is surface tension the greatest in the lung?

  smaller alveoli
• surface tension is a contractile force

  • due to strong attraction of water molecules at the air-liquid interface
  • can cause collapsing pressure
• Surface tension effect does what to alveoli?

  it requires a lot of distending pressure to open alveoli
• If 2 different size alveoli are connected by a common airway, smaller alveoli will empty into the larger alveolus

  • Therefore, smaller alveolus would collapse when the surface tension is high
• Surfactant is made by?

  type II alveolar epithelial cells
• Surfactant production start at 24 weeks and usually present by week 25

  • premature baby - less surfactant
• What can be used to indicates mature surfactant production?

  DPPC:sphingomyelin ratio > 2:1
• Surfactant work by?

  • Interferes with hydrogen-bonding between H2O molecules: reduces alveolar surface tension
  • Stabilizes pressure gradients between connected alveoli of differing sizes
• Surfactant deficiency - may cause

  • atelectasis - complete or partial collapse of the entire lung or area (lobe) of the lung 
• Immature lungs with surfactant production deficiency resulting in newborn respiratory distress syndrome (NRDS)

  • Infant born preterm (particularly <28 weeks gestation age)
• Positive transpulmonary pressure throughout passive expiration keeps airway open
• • During passive expiration
  •  intrapleural pressure remain negative
  • Transpulmonary pressure remain positive
  • Therefore
  • AP > IP
  • Airway remain open with minimal resistance to airflow down the pressure gradient form alveoli to atmosphere
• Passive expiration is due to elasticity
• Vital capacity (VC) is the maximum volume of air that can be moved quickly in a single breath
• Vital capacity (assessment) is to expire as quickly & forcefully as possible producing a forced vital capacity or FVC

  • used to diagnosed pulmonary dysfunction
• • What creates/causes expiratory flow?
  • Muscle contraction
  • Very low resistance causes activation of expiratory muscles = generating peak flow
  • Effort dependent phase
• • What happens to lung elasticity during forceful expiration?
  • Lung elasticity - decrease during expiration
  • Low lung volume --> increases compliance
• • Why does airflow rate drop dramatically?
  • Increase in resistance
  • Decrease elasticity

  
• Intrapleural pressure higher than airway pressure (above EPP) --> airway compressed and may collapse

  • Cause
  • increase airway resistance
  • Reduce airflow
  • At this point - airflow depends on elasticity of lung = effort independent
  • Airflow stops in the small airway establishing residual volume
• In non-pathological states the EPP occurs in the larger, cartilaginous airway which are protected from collapse
• During forceful expiration, intrapleural pressure become positive because of contraction of expiratory muscles during effort dependent phase
• Equal pressure point (EPP) - level of airway where the intrapleural pressure is equal to airway pressure

  
• FEV1 - volume of air exhaled in the first second is called forced expiratory volume in one second
• Normal FEV1/FVC = 0.8
• PFTs measure flow (FEV1/FVC) they can be utilized to distinguish obstructive vs restrictive lung disease
• Obstructive lung disease characterized by an increase in airway resistance
• Obstructive lung disease Measured as decrease in expiratory flow
• Obstructive lung disease

  • Examples are
  • Emphysema
  • Chronic bronchitis
  • Asthma
• Obstructive disease

  • Increase compliance
  • EPP occurs in small airway resulting in gas trapping
• Obstructive lung disease (OLD)  diagnosis

  • Cannot get air out due to increase airway resistance
  • FEV1 is reduced - greater reduction
  • FVC is reduced - smaller reduction or normal
• To diagnosis OLD

  • Calculate FEV1/FVC ratio >70%
  • Then look at FEV1 alone, not the ratio, to determine severity