2017A11 Draw an expiratory flow-volume curve obtained from a maximal expiratory effort after a vital capacity breath,
for a person with a) normal lungs b) restrictive lung disease b) obstructive lung disease (10 marks).
Explain how and why these curves (and the derived parameters) are different for each disease state (15 marks).

 

List:

        Diagramme

        Relevant physiology

        Normal flow-volume curve

        Obstructive pathophysiology

        Restrictive pathophysiology

 

Diagramme:

 

Relevant physiology:

Flow determinants

   Ohmís law: Expiratory flow rate (Q) = (PAlv-Patm)/airway resistance

   Normal values

o PAlv: 38cmH2O

o Patm: 0cmH2O by definition

o AWR: 2cmH2O/L/s

   Factors ↑Q: ↑PAlv, ↓Patm, ↓AWR

Resistance

   Laminar flow (small airways): R = (8 x length x viscosity) / (π x radius4)

   Turbulent flow (large airways): (P1-P2) (length x density) / radius5

   Factors ↓AWR

o Radius (most important since raised to power 4 or 5)

o ↓Viscosity (e.g. temp)

o ↓Density (e.g. heliox cf. air)

o ↓Length (not under control)

Dynamic airways compression (DAC)

   Pressure drop occurs between alveolus and mouth due to airway resistance

   Where airway pressure = intrapleural pressure, unsupported airways collapse

o i.e. equal pressure point (EPP)

o Starling resistor mechanism; P2 = PIP not Patm

o Cartilage absent after generation 11

 

Normal flow-volume curve:

TLC

   Lung volume after maximal inspiratory effort

   Note unable to measure with flow-volume curve

Upward slope

   Effort-dependent

   Radial traction distends airways

Max PEFR

   Occurs at beginning of forced expiration.

o Highest lung elastic recoil

o Highest airway radius

o ? High expiratory muscle mechanical advantage

   Reflects larger airway function

   Mainly effort-dependent

   Limited by onset of DAC

Linear decline

   Mainly effort-independent, especially FEF25-75%

   Decline due to increasing DAC as lung volume and airway radii fall

   Mostly effort independent

   Reflects smaller airway function

RV

   Maximal expiration

   Limited by small airway closure

   Note unable to measure with flow-volume curve

FVC

   = TLC - RV

 

Obstructive pathophysiology:

Example

   Asthma, COPD

?TLC

   COPD with emphysema: ↓elastic recoil

   (? Asthma: gas trapping Ė not clear to me if this is true)

↓Upward slope

   Cause: ↓airway radius

↓PEFR

   ↓Airway radius

   Earlier and exaggerated DAC, distal migration of EPP

Linear decline

   Scooped out appearance, ↓FEF25-75%

o Earlier and exaggerated DAC, distal migration of EPP

o (if emphysema: ↓radial traction, ↓lung recoil)

   Changes severity

↑RV

   ↓Airway radius -> earlier airway closure

   (if emphysema: ↓lung recoil, earlier airway closure)

↓FVC

   ∆RV > ∆TLC

 

Restrictive pathophysiology:

Example

   Pulmonary fibrosis

↓TLC

   ↓Lung compliance

↔↑ Upward slope

   ↑Lung recoil

↔↓Max PEFR

   (affected in severe disease)

   ↓Lung compliance

Linear decline

   ↔↑Slope, especially at end-expiration

o ↑lung recoil -> ↑ transpulmonary pressure

o Stiff, less collapsible tissue

↓RV

   ↑Lung recoil -> airway closure at lower lung volume

↓FVC

   ∆TLC > ∆RV

 

 

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