Respiratory Physiology: Mechanics of Ventilation and the Effect of IPPV on the Lungs

Question 1

Work of breathing

What forces are overcome in inspiration?

What happens in expiration?

why dont alveoli collapse at end of expiration?

Answer 1

• Inspiration
  
  • lung elastic forces
  • chest wall elastic forces
  • lung tissue resistive forces
  • chest wall resistive forces
  • airway resistive forces
• expiration
  
  • passive due to elastic recoil
• elastic tendency of lungs to collapse ballanced by elastic tendency of chest wall of expand
• WOB = area inside pressure-volume loop

Question 2

Draw a whole lung pressure-volume loop

Answer 2

• If spont vent pressure is negative
• If IPPV pressure is positive
• Large curve represents vital capacity breath
• Inspiration
  
  • sigmoid
  • initially flat as negative pressure needed before colume change happens
  • midsection steepest around FRC
  • end section flat as lungs maximally distendend so poor compliance
• Expiration
  
  • smooth curve
  • high volumes compliance poor so curve flat
  • steep part around FRC
  • pressure returns to baseline
• Todal breath
  
  • demontrates compliance at tidal ventilation
  • startes and ends with FRC
• Regional differences
  
  • A=alveoli at top of lungs distended by traction so less compliant
  • B= alveoli at base of lungs. greater compliance.

Question 3

Lung compliance

What is compliance?

Causes of decreased compliance

Causes of increased compliance

What is specific compliance

Answer 3

• Compliance is the volume change per unit change in pressure (ml.cmH₂O^-1 or l/kPa^-1)
• Normal = 200 ml/cmH2O
• A measure of how easy it is to inflate the lungs
• Poor compliance = stiff lungs
• Measured by calculating the gradient of the pressure-volume curve
• Causes of decreased compliance:
  
  • extremes of volume
  • increased pulmonary venous pressure
  • alveolar oedema
  • fibrotic lung disease
• Causes of increased compliance:
  
  • increaseing age
  • emphysema
• Specific compliance:
  
  • compliance per unit volume
  • independent of body size

Question 4

What causes surface tension?

Answer 4

• forces between adjacent liquid molecules greater the between liquid and gas molecules
• liquid surface area becomes small as possible (in bubble this measn sphere)
• responsible for meniscus

Question 5

Laplace’s Law

Answer 5

P= (4xT)/r

P= pressure

T= surface tension (the force acting accross an imaginary 1cm line on the surface of a liquid)

r= radius

When only ONE surface involved the numerator is 2

Laplace’s law states that smaller spheres (smaller r) generates higher pressure then larger spheres

If the spheres connect then sphere with higher pressure would empty into larger one

This doesnt happen with alvoeli due to surfactant

Question 6

Surfactant

Answer 6

• produced from type II alvoelar epithelial cells
• phospholipis containing dipalmitoyl phosphatidyl choline
• reduces surface tension by opposing the normal attraction between surface molecules
• means Laplace’s law no longer applies
• different sized alveoli are more balanced

Question 7

Alveolar stability

Answer 7

• Surfactant
  
  • low lung volumes molecules are pushed together -> greater repellent forces
  • stabilise alveoli to prevent complete emptying and collapse
  • reduces compliance
  • kelps keep lungs dry
• Connective tissue between alveloi
  
  • supoprts
  • prevents over distension

Question 8

Poiseuille’s law

Answer 8

Question 9

Laminar flow

Answer 9

• gas flows in parallel layes with no disruptio within layers
• occurs in airways with smooth walls and low flow velocity (small airways)
• driving pressure (ΔP) through the airway is the alveolar pressure minus barometric pressure
• alveolar pressure is negative during inspiration due to the activity of the respiratory muscles
• the viscosity of the gas rather than its density has a direct impact on the resistance to flow

Question 10

Transitional flow

Answer 10

• has the some of the characteristics of turbulent flow and some of laminar flow
• occurs at each bifurcation and whenever the radius of the airway is reduced
• Due to Poiseuille’s law small changes in the radius can have a marked effect on airway resistance

Question 11

Turbulant flow

Answer 11

• movement of gas is chaotic
• occurs when flow of gas is high and in larger diameter airways
• the lung medium sized airways are the main site of resistance to flow
• density of the gas (d) is more important than the viscosity (η)
• Reynolds number (Re)
  
  • Re = pvd/η
  • where v = velocity, p = density, d=diameter
  • η = viscosity
  • When Reynolds number is about 2000 turbulent flow begins
  • turbulence is more likely in larger diameter airways with high velocity flow
  • If a lower density gas is inhaled, e.g. helium, the Reynolds number will be reduced and hence laminar flow is also more likely to occur

Question 12

Flow volume curve

Answer 12

• used in spirometry
• flow initially high then declines due to compresion of airways by intrathoracic pressure

Question 13

Respiratory function tests

FEV₁

FVC

obstructive/restrictive disease

Factors affecting respiratoy funtion test

Predicted values

Answer 13

• FEV1 = the forced expiratory volume during the first 1 second of a forced vital capacity (FVC) breath
• FVC = the maximum volume of air that can be forcibly expired from the lungs
• FEV1/FVC ratio
  
  • normally 75-80%
• Obstructive disease
  
  • decrease FEV1, increase FVC
  • decrease FEV1/FVC
• Restrictive disease
  
  • decrease FEV1 and FVC
  • same or increased FEV1/FVC ratio
• Respiratory funtion affcted by
  
  • gender
  • age
  • height
  • ethnicity
• predicted
  
  • 50-80% mild disease
  • 30-50% moderate disease
  • <30% severe disease