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Flashcards in 6 Respiratory Mechanics 1 Deck (19)
1

Respiratory mechanics

  • For ventilation to occur, the respiratory system must...
  • This is possible only when...
  • Respiratory mechanics

  • For ventilation to occur, the respiratory system must alternately expand above and then return to its resting volume
  • This is possible only when sufficient pressure is applied to overcome two factors opposing its movement
    • Elastic recoil
    • Viscous forces
  • The interaction of these applied and opposing forces is referred to as the mechanics of respiration, or respiratory mechanics

2

Opposing forces:
Elastic recoil

  • When removed from the thoracic cavity...
    • The lungs...
    • The isolated chest wall...
  • Any change in volume above or below these equilibrium positions requires...
  • In essence, the lungs and chest wall behave like...
  • The elastic recoil of the excised lungs...
  • Similarly, increasing or decreasing the volume of the isolated chest wall requires...

  • When removed from the thoracic cavity...
    • The lungs deflate far below their resting volume in the chest
      • The equilibrium of the lungs is close to zero
    • The isolated chest wall expands outward
  • Any change in volume above or below these equilibrium positions requires...
    • An increasing amount of applied pressure to overcome elastic recoil
    • Increasing lung volume requires increasing pressure to overcome elastic recoil
  • In essence, the lungs and chest wall behave like...
    • Metal springs, each with its own resting length and elastic recoil
  • The elastic recoil of the excised lungs...
    • Increases with each volume increment
    • Must be exactly balanced by an applied pressure
  • Similarly, increasing or decreasing the volume of the isolated chest wall requires...
    • Positive and negative pressures, respectively

3

Pressure-length relationships of the "lungs" and "chest wall"

  • "Lungs"
  • "Chest wall"
  • "Respiratory system"

4

Opposing forces:
Pressure-volume relationships

  • Lungs & chest wall
  • How we can separately evaluate their elastic recoil
  • Pressure-volume curves are generated by measuring...

  • Lungs & chest wall
    • Useful from a conceptual standpoint to discuss them as if they were isolated structures
    • Actually linked together as a single unit
  • How we can separately evaluate their elastic recoil
    • Having a subject perform a step-wise exhalation following a maximal inspiration
    • At each point, the subject relaxes against an occluded external airway
      • Both the volume change and the pressure gradient across the lungs and chest wall are measured
  • Pressure-volume curves are generated by measuring...
    • Transmural pressures and volume in the absence of airflow

5

Opposing forces:
Pressure-volume relationships

  • Transmural pressures
  • Transpulmonary pressure (PL)
  • Gradient across the chest wall (PW)

  • Transmural pressures
    • Generated by the elastic recoil of the lungs and chest wall
    • Calculated by subtracting the pressure "outside" from the pressure "inside" each structure
      • "Inside" minus "outside"
  • Transpulmonary pressure (PL)
    • Difference between alveolar (Palv) and pleural pressure (Ppl)
    • PL = Palv - Ppl
  • Gradient across the chest wall (PW)
    • Pleural pressure minus body surface (atmospheric) pressure (Pbs)
    • PW = Ppl - Pbs

6

Opposing forces:
Pressure-volume relationships

  • Alveolar pressure (Palv)
  • Pleural pressure

  • Alveolar pressure (Palv)
    • Since these measurements are performed under static conditions (i.e. there is no airflow), Palv is equal to the pressure measured at the airway opening (Pao) proximal to the site of airway occlusion
  • Pleural pressure
    • Approximated by measuring the pressure within the lower two-thirds of the esophagus (Pes) using a balloon catheter
    • Since all pressures are referenced to it, atmospheric pressure is considered to be zero
    • PL = Pao - Pes
    • PW = Pes

7

Opposing forces:
Pressure-volume relationships

  • Elastic recoil pressure of the entire respiratory system (PRS)
  • Pressure-volume curve of the respiratory system
  • Elastic recoil pressure of the respiratory system at any volume

  • Elastic recoil pressure of the entire respiratory system (PRS)
    • Since the lungs & chest wall are in sereis at any volume, PRS is equal to the sum of the pressures generated by its two components
    • PRS = PL + PW
  • Pressure-volume curve of the respiratory system
    • Can be determiend directly by measuring PRS at each volume
    • PRS = Palv - Pbs = Pao
  • Elastic recoil pressure of the respiratory system at any volume
    • Equal to the pressure recorded at the airway opening

8

Opposing forces:
Lung volumes

  • Several commonly measured lung volumes are determined primarily by...
  • Functional residual capacity (FRC)

  • Several commonly measured lung volumes are determined primarily by...
    • The elastic recoil of the lungs and chest wall and by the strength of the respiratory muscles
  • Functional residual capacity (FRC)
    • The volume remaining in the lungs at the end of a passive expiration
    • The point at which the inward recoil of the lungs is exactly balanced by the outward recoil of the chest wall
    • Since PRS equals zero, no respiratory muscle activity is required to maintain this volume, and FRC represents the resting or equilibrium position of the respiratory system
    • FRC: |PL| = |PW|
    • PRS = 0

9

Opposing forces:
Lung volumes

  • Total lung capacity (TLC)
  • Residual volume (RV)
  • Vital capacity (VC)

  • Total lung capacity (TLC)
    • The volume present in the lungs after a maximal inspiration
    • Reached when the combined elastic recoil of the lungs and chest wall is balanced by the maximum pressure that can be generated by the inspiratory muscles
    • PRS = maximum inspiratory pressure
  • Residual volume (RV)
    • The volume of gas remaining in the lungs after a maximal expiration
    • In the absence of obstructive lung disease, RV is determined by the balance between the elastic recoil of the chest wall and maximal expiratory effort
    • PRS = maximum expiratory pressure
  • Vital capacity (VC)
    • The volume of gas that can be exhaled after a maximal inspiration
    • The difference between TLC and RV
    • VC = TLC - RV

10

Opposing forces:
How the elastic recoil of the lungs, chest wall, and respiratory system can be quantified

  • Pressure-volume relationships can be plotted by performing sequential measurements during stepwise exhalation
    • Pros
    • Cons
  • Alternatively, a single measurement of pressure and volume can be made
    • Pros
    • Cons
  • A compromise between these approaches is to measure...

  • Pressure-volume relationships can be plotted by performing sequential measurements during stepwise exhalation
    • Pros
      • Provides a great deal of information
    • Cons
      • Time-consuming
      • Requires both cooperation and practice on the part of the subject
      • The information obtained cannot easily be converted to numerical form
  • Alternatively, a single measurement of pressure and volume can be made
    • Pros
      • Rapid and relatively easy to perform
    • Cons
      • Provides information about elastic recoil only at a specific lung volume
  • A compromise between these approaches is to measure...
    • Elastic recoil pressure at two volumes

11

Opposing forces:
Compliance

  • Compliance
  • The compliance of the lungs, chest wall, and respiratory system
  • Compliance vs. elastic recoil

  • Compliance
    • C = ∆V / ∆P
      • The slope of the volume-pressure curve
    • ​Used to quantiyf elastic recoil
  • The compliance of the lungs, chest wall, and respiratory system
    • Fairly constant (i.e. there is little change in slope) over a large volume range
    • Near TLC and RV, however, compliance falls (i.e. the slope decreases) as the elastic elements within the lungs and chest wall reach the limits of their dispensability
  • Compliance vs. elastic recoil
    • Compliance varies inversely with elastic recoil
    • When compliance is low, a given pressure increment produces only a small increase in volume (i.e. the structure is stiff)
    • A large increase occurs when the compliance is high

12

Opposing forces:
Compliance

  • Compliance equation
  • By measuring the appropriate transmural pressures during relaxation against an occluded airway at two different volumes, we can determine...
    • Compliance of the lungs (CL)
    • Compliance of the chest wall (CW)
    • Compliance of the respiratory system (CRS)

  • Compliance equation
    • C = ΔV/ΔP
  • By measuring the appropriate transmural pressures during relaxation against an occluded airway at two different volumes, we can determine...
    • Compliance of the lungs (CL)
      • CL = ΔV / Δ(Pao - Pes)
    • Compliance of the chest wall (CW)
      • CW = ΔV / ΔPes
    • Compliance of the respiratory system (CRS)
      • CRS = ΔV / ΔPao

13

Opposing forces:
Origin of elastic recoil

  • Tissue forces
  • Surface forces

  • Tissue forces
    • Result from the deformation of elastic elements in the...
      • Lung parenchyma (i.e. elastin and collagen)
      • Chest wall (i.e. bone/ribs, muscles, and cartilage)
    • Easily understood using the analogy of a metal spring
  • Surface forces
    • Unique to the lung parenchyma
    • Produced by surface tension, which is present at all air-liquid interfaces
    • Produced by the layer of surfactant that coats the alveolar epithelium
    • Like all air-liquid interfaces, surfactant generates surface tension which tends to reduce alveolar size and increase the pressure required to maintain a given lung volume

14

Opposing forces:
Origin of elastic recoil:
Surface tension

  • Alveoli
  • Laplace equation
  • Alveolar pressure due to surface forces

  • Alveoli
    • Similar to soap bubbles, in which surface tension acts to both decrease volume and increase internal pressure
  • Laplace equation: P = 2T/r
    • P = alveolar pressure
    • T = surface tension
    • r = alveolar radius
    • Pressure is inversely related to radius
    • The pressure generated by surface tension within a sphere is governed by this equation
  • Alveolar pressure due to surface forces
    • Should vary inversely with alveolar size
      • Elastic recoil due to surface tension must decrease with increasing lung volume
      • Alveolar pressure in small alveoli must be greater than in large alveoli
    • If this were true, elastic recoil would decrease during inspiration, and during expiration, increasing elastic recoil would lead to diffuse alveolar collapse
    • This, of course, does not occur

15

Opposing forces:
Origin of elastic recoil:
Surface tension

  • The solution to this problem comes from a unique property of surfactant
  • Pulmonary surfactant

  • The solution to this problem comes from a unique property of surfactant
    • Unlike any other liquid, the surface tension of surfactant is not constant but varies directly with the size of the air-liquid interface
    • As alveolar volume increases, surface tension rises
    • This more than offsets the effect of increasing alveolar radius and allows surface forces and elastic recoil to increase with lung volume
  • Pulmonary surfactant
    • ​Composed primarily of phospholipids
    • Produced by alveolar type 2 cells
    • Reduces surface tension and lung elastic recoil
      • ​Varies directly with alveolar volume
      • Prevents alveolar collapse

16

Viscous forces

  • Under static conditions, maintaining the respiratory system at any point above or below its equilibrium volume requires...
  • During the dynamic processes of inspiration and expiration, however,...

  • Under static conditions, maintaining the respiratory system at any point above or below its equilibrium volume requires...
    • Only enough pressure to balance its elastic recoil
  • During the dynamic processes of inspiration and expiration, however,...
    • Additional pressure must be supplied to overcome...
      • Frictional forces at the airway surface
      • Viscous forces produced by the flow of gas through the airways

17

Viscous forces:
Airway pressure-flow relationships

  • As gas moves through the airways...
  • A pressure gradient (ΔP) must, therefore, exist between...
  • The magnitude of this pressure gradient depends primarily on...
  • Other factors that determine ΔP are...
  • The most important determinant of ΔP 

  • As gas moves through the airways...
    • Pressure is required to overcome...
      • Friction at the airway surface
      • Cohesive forces between gas molecules
  • A pressure gradient (ΔP) must, therefore, exist between...
    • The mouth and the alveoli
    • This gradient may be thought of not only as the pressure needed to overcome viscous forces but also as the driving pressure required to produce airflow
  • The magnitude of this pressure gradient depends primarily on...
    • The radius of the airway
    • ∆P is proportional to 1 / r4
  • Other factors that determine ΔP are...
    • The radius (r) and length (L) of the airways
    • Flow rate (V) and viscosity* (η) of the gas
      • Viscosity
        • Measure of the resistance of a gas or liquid to movement
        • Directly proportional to the strength of the cohesive forces between its molecules
    • ΔP = 8VηL / πr4
  • The most important determinant of ΔP
    • The caliber of the airway, since ΔP varies inversely with the 4th power of the airway radius
    • This means that viscous forces and the pressure required to overcome them will increase by 16 times when airway radius is reduced by one-half

18

Viscous forces:
Airway pressure-flow relationships

  • Because of irregularities, protuberances, and abrupt changes in diameter, the upper airway (mouth, pharynx, and larynx) accounts for...
  • The viscous forces contributed by the lower airway (below the vocal cords) occur predominantly...
  • The small airways (less than 2 mm in diameter) account for...
  • Given the marked dependence of driving pressure on airway radius, it may appear counterintuitive that...
  • Since the same volume of gas must pass through each generation of airways, flow through each individual airway must...

  • Because of irregularities, protuberances, and abrupt changes in diameter, the upper airway (mouth, pharynx, and larynx) accounts for...
    • ~35% of the viscous forces during ventilation
  • The viscous forces contributed by the lower airway (below the vocal cords) occur predominantly...
    • In the first six generations
  • The small airways (less than 2 mm in diameter) account for...
    • A very small proportion of total airway viscous forces
  • Given the marked dependence of driving pressure on airway radius, it may appear counterintuitive that...
    • Viscous forces should decrease as the airways become smaller
    • However, as the airways divide and become smaller and smaller, their total number increases dramatically
  • Since the same volume of gas must pass through each generation of airways, flow through each individual airway must...
    • Progressively slow until flow stops completely at the level of the alveolus
    • This reduction in gas flow results in a progressive decrease in viscous forces

19

Viscous forces:
Resistance

  • Resistance
  • Resistance equation
  • Other resistance equation
  • The resistance of an airway depends on...

  • Resistance
    • Used to quantify viscous forces
  • Resistance equation
    • R = ΔP / V
    • ΔP = pressure gradient required to overcome viscous forces
    • V = flow generated by the applied pressure gradient
  • Other resistance equation
    • ΔP = 8VηL / πr4
    • R = ΔP / V
    • R = 8ηL / πr4
  • The resistance of an airway depends on...
    • Its length and radius
    • The viscosity of the gas flowing through it

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