Pulmonary Mechanics

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Pulmonary Mechanics
HuBio 541 The Respiratory System

• Bill Altemeier, M.D.
• billa_at_u.washington.edu

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Primary Function of the Lung

• Bring in oxygen for delivery to tissues and
  remove carbon dioxide from blood
• Accomplished through tidal breathing
• Inspired, fresh air is distributed via the
  airways to the gas exchanging regions or alveoli
• Oxygen depleted, carbon dioxide containing gas is
  exhaled via same airways
• Air, like any gas or fluid, moves from areas of
  higher pressure to lower pressure

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The Engineering Problem

• The lung cannot expand itself, it can only move
  passively in response to external pressures
• Two ways to get air into the lung
• Create a positive pressure at the airway opening
  to push air into the lung
• Orcreate a negative pressure within the lung as
  occurs in free breathing humans

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Goals

• Understand how a pressure gradient is generated
  during respiration
• Negative pressure pump system
• Elastic properties of the lung and chest wall
• Role of surfactant
• Understand the role of resistance in determining
  air flow
• Put it together to begin understanding the work
  of breathing

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Boyles Law - How gas gets into the lungs
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Mechanics of Respiration
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Elasticity - The Capability of a Strained Body to
Return to an Unstressed Position
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Elasticity - The Capability of a Strained Body to
Return to an Unstressed Position
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Recoil Pressure or Transmural Pressure
Recoil Pressure Pressureinside - Pressureoutside
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Elastic Properties of the Lung
Volume
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Inflation of the Lung at Different Transmural
Pressures
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Recoil Pressure
Recoil Pressure Pressureinside - Pressureoutside
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Elastic Properties of the Chest
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Combining Two Elastic Structures
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Putting the Respiratory System Together
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Example of Lung Recoil

• Pneumothorax
• Penetration of chest wall allows Air into pleural
  space, eliminating negative pleural pressure
• Loss of pleural pressure decouples lung and chest
  wall
• Lung collapses to its resting volume

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Compliance

• Is a measure of the distensibility of a
  structure how much pressure need be applied to
  get a given change in volume
• Mathematically
• Compliance ?V/?P
• Graphically it is the slope of the pressure
  volume curve of the structure

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Measurement of Compliance
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Compliance in Disease - Fibrosis
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Compliance in Disease - Emphysema
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The Real World - Alveoli

• So far, we have dealt with the lung as if it were
  a single giant alveolus
• In reality, the lung is made up of millions of
  tiny air sacs, each of which is fluid-lined but
  air filled
• Elastic property of the lung is a combination of
  the elastic fibers throughout the alveolar wall
  structure and surface tension of the alveolar
  lining fluid

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Surface Tension
Molecular interactions resulting from hydrogen
bonds between water molecules in liquid but not
between water and air
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LaPlaces Law

• P 2T/r
• Pressure 2Wall Tension/sphere radius

T
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Surface Tension Alveolar Instability
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Surfactant Solution
Surfactant 1) Reduces surface tension 2)
Prevents alveolar collapse 3) Reduces work of
breathing 4) Prevents alveolar flooding
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The Alveolus
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Surfactant Dynamics

• Surfactant moves in and out of the lining
  monolayer, main-taining intermolecular distance
  and low surface tension across a range of lung
  volume

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Resistance

• The flow of gas or fluid in a tubular structure
  depends on the magnitude of the pressure
  difference and the resistance of the tube
• Flow Pressure difference / Resistance
• V ?P/R

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Resistance

• Resistance to airflow
• increases with increasing viscosity of gas
• is directly proportional to the length of the
  airway
• increases with 1/radius4 (if the radius halves
  resistance increases 16 fold)

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Airway Caliber

• Airway diameter is affected by
• anatomy (the more distal the smaller)
• lung volume
• pressure across the airway wall
• bronchial smooth muscle contraction
• secretions

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Airway Resistance as a Function of Lung Volume
Airway resistance increases as lung volume
decreases
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Recoil of the Lung Tissue Holds Airways Open
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Inspiration
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Passive Expiration
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Forced Expiration
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Airflow as a Function of Lung Volume
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Airflow as a Function of Lung Volume
Flow (L/sec)
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Effort Independent Flow Rates
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Clinical Measurements of Lung Mechanics

• Volumes of the respiratory system
• Flows of gas out of the respiratory system
• Resistance during flow
• Diffusion of gas within the lung

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Spirometry - Measurement of Lung Volumes and Flow
Rates
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Volumes do not overlap Capacities are
combinations of volumes Spirogram is
displacement from FRC
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Measurement of Airflow Obstruction

• FEV1 - volume exhaled in the first second of a
  forced exhalation
• FVC - forced vital capacity or the total volume
  exhaled during forced exhalation
• FEV1/FVC is a measure of airflow obstruction

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Some Diseases We Can Diagnose with Pulmonary
Testing

• Obstructive Lung Diseases
• Asthma
• COPD
• Restrictive Lung Diseases
• Fibrosis
• Muscle Weakness

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Work of Breathing

• In the respiratory system work is proportional to
  the product of the pressure applied and the
  change in volume
• Work in the respiratory system is of 2 kinds
• elastic work to overcome the recoil pressure of
  the lung and chest wall
• resistive work to overcome the resistance to
  airflow in the airway (and a small amount of
  tissue resistance)

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Important Take Home Points

• The elastic properties of the respiratory system
• Calculating recoil pressures of the lung, chest,
  and the combination of the two
• Understanding the interaction of lung and chest
  and how they define various spirometry values
• Divisions of lung volumes and capacities
• The role of surfactant in lung elasticity
• Airway resistance
• Lung volume dependency
• Development of dynamic airway collapse during
  forced exhalation and the resultant effort
  independence of mid-exhalation flows
• The work of respiration
• Contributions of airway resistance and elasticity
  at different lung volumes
• How various diseases will affect the work of
  breathing