Lung Mechanics Flashcards
What is the work of breathing deterimined by
lungs
Thoracic compliance
Airway resistance
What is compliance
Ease with which lungs can be expanded
What is embedded in elastic tissue
Lungs
Airway tree
Vascular tree
What is trans-pulmonary pressure
P difference between alveoli and pulmonary wall
If lungs are easy to inflate, what sort of compliance do they have
Increased compliance
Formula for compliance
Change in vol/change in P
Transpulmonary pressure difference from apex to base
APEX = -10
MIDDLE = -5
BASE = -2.5
-ve P is the suction that keeps the lungs open
- Where is there greater alveolar volume
- Where is there greater alveolar ventilation
- Apex
- Base
Which part of the lung is most compliant
Base of lung
Smaller alveoli at base => easier to expand alveoli at base
What does lung compliance depend on
- Distensibility - easy to stretch/deflate - ELASTIN
- Elasticity - w/o it, lungs would enlarge with every breath and compliance would be 0 - COLLAGEN
- Surface tension
Consequence of decreased compliance
Increased stiffness, increased work to inflate lungs
Background of pneumonia
Fluid in lung tissue, alters surface tension of alveoli, changes surfactant and makes it more difficult to control lungs
conditions affected by compliance
Fibrosis
COPD
Emphysema
Chronic bronchitis
Factors that influence compliance
- Posture
- Age
- Post-laproscopic surgery
- Occasional deep breaths
- Pregnancy
- High standing diaphragm - obesity, scoliosis
- Lung disease
- Pneumothorax - trauma, diving, underlying lung pathologies
DECREASED COMPLIANCE WHEN:
Lung is unventilated for long period, increased surface tension within alveoli, lungs engorged with blood (increase pulmonary venous pressure)
High or low compliance with EMPHYSEMA
HIGH CL
High or low compliance with FIBROSIS
Low CL
2 determinants of lung compliance
R due to elasticity of lung (25%)
- Elastic fibres in interstitium of alveolar walls
- Thick alveolar wall => increased elastic fibres => increased R to stretch
Surface tension @ air-water interface in alveoli (75%)
- Alveolar spaces lined with a thin layer of water
- Attractive forces between water molecules (= ST) counteracts expansion of water lining
- Energy is required to overcome ST of this water layer in order to expand
When is surfactant produced and how
What is its composition
Produced by type II alveolar cells from week 28 gestation
90% lipids, 10% proteins => AMPHIPHATIC
Infant respiratory distress syndrome
Decreased CL
Atelectasis
Oedema
Exogenous surfactant
#AD
Name the 2 large hydrophilic surfactant proteins
- SP A
- SP D
Name the 2 small hydrophobic surfactant proteins
- SP B
- SP C
Name the 7 lipids in surfactant
- Dipalmitoylphosphatidylcholine (DPPC - 60%)
- Phosphatidylglycerol
- Phosphatidic acid
- Phosphatidylinositol
- Phosphatidylethanolamine
- Phosphatidylcholine
- Cholesterol - most abundant neutral lipid present
- How much surface tension does DPPC produce
- What does it form
- Generates near 0 surface tension at interface during compression
- Forms tightly packed monolayer which can generate these low ST values w/o collapsing
intermolecular repulsive forces oppose attractive forces between liquid surface H2O molecules
EFFECT IS STRONGER AT LOWER ALVEOLAR VOLUMES
Deficiency of SP A
More susceptible to pathogens
SP B
Decreases surface tension
SP B deficiency in humans leads to RDS (resp distress syndrome)
Stabilises lipid monolayers
anti-bacterial
SP C
found only in lungs
Linked to SP B
Physiological roles of surfactant proteins
- Decrease ST to increase compliance - decreased work of expanding lungs at each breath
- Increased stability of alveoli - decrease small alveoli emptying into larger ones
- Helps to keep alveoli dry because ST of alveolus liquid layer would draw water into alveolus
Law of Laplace
P = 2T/r
P is greater in smaller alveolus
Hysteresis
V/P relationship is different during expiration
What is vol at any pressure greater for
Greater for deflation than inflation
What does most of hysteresis and CL appear due to
Surface tension of alveoli
How does surface tension affect alveoli
Inflation of lungs with saline eliminates air-liquid interface and surface tension
Musch easier to inflate lungs with saline
- elastic recoil is diminished due to lack of ST
- not much difference between inflation and deflation => no HYSTERESIS
3 reasons for hysteresis
- Changes in surfactant activity - ST greater in inspiration
- Stress relaxation - inherent property of elastic tissue - crinkled structure of collagen in lungs
- Re-distribution of gas - fast and slow alveoli
Relationship between lung and chest wall
Lung and chest wall in series with each other - total compliance
- Lung compliance
- Chest wall compliance
- Total compliance
- Change in lung vol/change in Ptp
- Change in lung vol/change in trans-chest wall P
- 100 ml/cmH2O
