Mechanics of Breathing and Compliance

Mechanics of Breathing

Inspiration

• Primary Muscle: Diaphragm
  • Contracts to push abdominal contents down
  • Increases thoracic cavity volume
  • Lifts ribs upward and outward
  • Main mode of respiration during quiet breathing
• Additional Muscles: Used during exercise or respiratory distress
  • External intercostal muscles
  • Accessory muscles

Expiration

• Typically Passive: Due to lung elasticity
• Active Expiration: During exercise or increased air resistance (e.g., asthma)
  • Abdominal muscles
  • Internal intercostal muscles

Compliance in the Respiratory System

• Definition: Compliance = Volume Change / Pressure Change
  • Indicates distensibility of lungs and chest wall
  • Slope of the pressure-volume curve
• Pressure-Volume Curve
  • Steeper curve indicates higher compliance
  • Flatter curve indicates lower compliance

Transpulmonary Pressure

• Definition: Pressure difference across pulmonary structures
• Importance:
  • Highest compliance in the middle pressure range
  • Transpulmonary pressure is alveolar pressure minus intraoral pressure
  • Negative pressure outside lungs causes expansion (inspiration)
  • Positive pressure causes collapse (e.g., pneumothorax)

Lung and Chest Wall Compliance

• Combinations:
  • Blue curve: Compliance of chest wall system
  • Red curve: Compliance of lung system
  • Green curve: Combined lung and chest wall system
• Functional Residual Capacity (FRC):
  • Point where lung and chest wall forces are equal and opposite
  • At rest, results in net zero pressure, no air movement

Factors Affecting Compliance

• Decreased Compliance: Pulmonary fibrosis, pneumonia, pulmonary edema
• Increased Compliance: Emphysema, normal aging (decreased elastance)

Practice Problems Review

Problem 1

• Scenario: Interpretation of interpolar pressure at FRC
• Answer: Interplural pressure is negative, approximately -5 cm H2O

Problem 2

• Scenario: Patient with myocardial infarction and dyspnea
• Diagnosis: Decreased lung compliance due to pulmonary edema from left ventricular failure

Problem 3

• Scenario: Pressure-volume curve analysis of a patient
• Diagnosis: Pulmonary fibrosis indicated by decreased compliance (flatter curve)

• Conclusion:
  • Understanding mechanics of breathing and compliance is crucial for identifying conditions affecting respiratory function.