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Understanding Airway Resistance in Physiology

Mar 1, 2025

ICU Primary Prep Class: Airway Resistance

Introduction

  • Speaker: Dr. Mike Clifford from Melbourne
  • Focus: Understanding physiological factors affecting airway resistance

Key Concepts of Airway Resistance

  • Airway resistance refers to the frictional resistance to gas flow in the airway.
  • Formula: Driving pressure divided by the flow.
  • Normal airway resistance: ~2 cm of water per liter per second.
  • Highest resistance found in medium-sized bronchi.

Airway Resistance in Adults vs. Infants

  • Adults: Peak resistance in higher order branching.
  • Infants: Peak resistance in the nasopharynx due to small nasal passages.

Laminar vs. Turbulent Flow

  • Laminar Flow: Described by Hagen–Poiseuille's equation (R = 8nl / πr⁴)
    • 'n' = viscosity, 'l' = length, 'r' = radius
  • Turbulent Flow: Described by Reynolds number (Re = (density x diameter x velocity) / viscosity)
    • Laminar flow: Re < 2000
    • Turbulent flow: Re > 4000
  • Most airflow in humans is turbulent due to short airways and branching.
  • Laminar flow typically seen in bronchioles beyond the 11th generation of airway branching.

Physiological Factors Influencing Airway Resistance

  • Airway Diameter: Influenced by:
    • Lung volume: Resistance inversely related to lung volume.
    • Radial traction: Increases airway diameter during inhalation.
    • Position: Supine position near closing capacity increases resistance.
    • Nervous System:
      • Sympathetic Tone: Causes bronchodilation, reducing resistance.
      • Parasympathetic Activation: Causes bronchoconstriction, increasing resistance.

Non-Physiological Factors

  • Pathological factors like foreign bodies, mucus plugging, and diseases like COPD or asthma are not considered physiological factors.
  • The focus should be on physiological factors in a normal, healthy adult.

Exam Focus

  • Avoid discussing pathology or pharmacology unless specifically asked.
  • Focus on core physiological explanations for exam questions on airway resistance.

Conclusion

  • Maintain focus on physiological aspects to score well in exams.
  • Avoid discussing drugs or pathophysiological states unless it ties back to physiological effect.

Note: Stick to physiological aspects for questions on airway resistance to maximize exam scores.

Full transcript