Work of Ventilation Flashcards
What is Work of breathing?
- Work is required to move the lung and chest wall
- W = P x ΔV
What happens during normal, quiet breathing?
- Inspiratory muscles do all the work
- Expiration is passive
- Elastic work: 70%
- Non-elastic work: 30%
What increases oxygen cost in breathing?
2-5% normally, 50L/min
- Exercise
- Disease
What is minute volume/ventilation?
L/min
- Respiratory rate per minute x Tidal volume
What is increased with an increase in tidal volume?
- Elastic work
What is increased with an increase with respiratory rate?
- Flow and resistive work
(non-elastic work)
What are the optimal values of RR and tidal volume?
- RR: 15/min
- TV: 500ml
In which lung diseases do you get an increase in elastic resistance?
- Restrictive
In which lung diseases do you get an increase in airway resistance?
- Obstructive
What is Compliance?
- The measure of ease with which the lungs can be stretched or inflated
- CL = ΔV / ΔP
- Normal adult lung compliance = 0.1-0.4
- Total respiratory system compliance < compliance of lung/chest alone
What causes increased compliance?
- Loss of elastin fibres
- Emphysema + aging
decreased elastance
What causes decreased compliance?
- Scoliosis, ankylosing spondylitis
- Pulmonary fibrosis
increased elastance
What is elastance?
- Inversely related to compliance
- elastic recoil force
What contributes to elastic work?
- Tissue elasticity: energy needed to deform elastic tissue
- Surface tension: minimises SA of alveoli
How is surface tension alleviated?
- Surfactant
- Increases compliance
- prevents collapse of alveoli
What is surfactant?
- 10% surfactant proteins, 90% lipid
- synthesized by type II pneumocytes
- stored in lamellar bodies
How does the law of Laplace relate to alveolar surface tension?
absence of surfactant*
- pressure in smaller alveoli is higher
- so air from small alveoli would enter big alveoli
- causes small alveoli to collapse, however with surfactant this would not happen!
What is the surface tension lowering effect of surfactant?
- Surfactant hydrophobic chains face alveolar air
- Hydrophillic ends face alveolar fluid
- Works more at low volumes (small alveoli, expiration)
How is RDS treated?
- corticosteroids given to mother prior delivery
- O2 delivery
- Survanta
We know that surfactant prevents alveolar collapse, but what else does?
- Alveoli interdependence
- surrounding alveoli pull outwards to keep middle alveoli open
What is non-elastic work?
- Resistance to air flow
What components determine airway resistance?
- Diameter of airway + distance air must travel
- Flow type (laminar vs turbulent)
How is airway diameter linked to flow resistance?
- Hagen-Poiseuille Law
- If airway radius is reduced 50%, resistance increases 16-fold
Since a reduced airway radius causes more resistance, how do the bronchioles adapt?
- Large number of bronchioles
- Compensates, so overall airway widens
- Tracheal resistance higher than bronchioles
increase in cross-sectional area as you go down respiratory tract
