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Updated: Aug 10 2020

Airflow, Pressure, and Resistance

  • Overview
    • Airflow, Pressure, and Resistance
      • Q = ΔP / R
        • airflow is directly proportional to pressure difference between mouth/nose (atmospheric) and alveoli
        • airflow is inversely proportional to airway resistance
    • Airway Resistance
      • R = 8ηl / πr4
        • η = viscosity of inspired air
        • l = length of airway
        • r = radius of airway
      • total resistance depends on the aggregate cross-sectional area of a zone
        • highest in the bronchi between the 4th and 8th generations of branching
    • Changes in Airway Resistance
      • changes in airway diameter provide major mechanism for altering airway resistance and airflow
      • Autonomic Nervous System
        • sympathetic stimulation
          • bronchial smooth muscle is innervated by sympathetic adrenergic nerve fibers
          • sympathetic stimulation → (+) β2 receptors → relaxation → ↑ airway diameter, ↓ resistance to airflow
          • β2 agonists (e.g., epinephrine, isoproterenol, albuterol) relax bronchial smooth muscle and dilate airway in treatment of asthma
        • parasympathetic stimulation
          • bronchial smooth muscle is innervated by parasympathetic cholinergic nerve fibers
          • parasympathetic stimulation → (+) muscarinic receptors → constriction → ↓ airway diameter, ↑ resistance to airflow
    • Pulmonary Vascular Resistance (PVR)
      • PVR = [P(pulmonary artery) - P(left atrium)] / cardiac output
        • P(pulmonary artery) is pressure in pulmonary artery
        • P(left atrium) is pulmonary capillary wedge pressure
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