Lecture 6 - Normal Lung Function Flashcards
Lung is expandable and retractable
Expandable
- compliant
- low resistance network of branching conduits
Retractable
- elastic
- millions of tiny alveolar sacs
Pleural space
- potential space
- Few mL fluid only
Pressure
- lung, chest wall
- gravity
- Position
Mechanics of lung function
- Changes in pressure inside thorax are generated by changes in lung volume, which are made possible by the moevement of air, which necessarily requires exchanges of gas
What determines lung volume
- height: big persons = big lungs
- age: volume increases up to age 20, but alveolar number is determined in utero/infancy (controversial)
- gender: men>women
- Race: Caucasion> others
- in utero history: what happens in lung at old age is determined before birth
Height and lungs
Height is the most important determinant of a person’s finak TLC (linear relationship)
- kick up at about 160cm because this is when puberty hits -> growth spurt. Dont see this lung growth spurt in females
Age and alveolar size
- alveolar size increases with age up to age 20
- ADC is a measure of alveolar size
- but studies show that alveolar size doesnt increase as predicted by lung volume - must be new alveoli?
Functional residual capacity
- at FRC, breathing is most efficient -> highest lung compliance
- 45% of TLC at rest -> FRC
- equilibrium point when outward recoil pressure of chest wall = inward recoil of lung
- in disease, obstruction (asthma) increases
Residual volume
- in normals, RV is determined by limitation of the chest wall and muscle strength, not by lung.
- the lung itself could still emptyh but the chest wall cant press back down anymore
How is residual volume affected by diseases
- increased in obstructive lung disease (gas trapping)
- decreased in restrictive neuromuscular disease
Measuring lung volume - body plethysmography
- relies on boyles law
- PxV is constant
- man in a box
- when airflow is absent, alveolar pressure = mouth pressure
- volume of air inside the box expands if you empty your lung : inject only a known amount of air particle ion the box and measure the difference
- this measures FRC
Once MRC is measured, how do you measure other capacities?
- TLC = FRC + inspiratory capacity
- RVL TLC - VC
Simpler method of lung capacity measurement: Gas dilution
- inspire a known concentration of particles.
- measure the expired concentration
- Cinsp x Vinsp = Cexp x Valv
Body plethysmography characteristics
- measures entire thoracic gas volume
- not affected by degree of obstruction
- complicated, expensive equipment
- high degree of patient coordination
- rapidly repeatable, but limited by effort/fatigue
Gas dilution characteristic
- single breath
- measures accessible lung volume (alveoli)
- susceptivle to gas trapping/slow gas mixing (obstructive diseases)
- simple and relatively inexpensive
- less coordination required
- need time to washout helium between test
Measuring air flow: spirometry
- FEV1: Forced expiratory volume in 1 sec
- FVC : Forced Vital capacity
- FEV1/FVC = 0.8
- this is influenced by age (reduces) and diseases)
How does restrictive disease affect spirometry
- FVC reduced» FEV1
- FEV1/FVC increased
- Faster
- shorter
Obstruction disease
- FEV1»_space; FVC
- FEV1/FVC reduced (
Spirometry
- gold standard for measurement of air flow
- air flow = rate of change in volume over time
- requires a flow meter: pneumotachograph: pressure transducers either side of resistor
- requires patient effort: Forced volumes
Effect of obstruction on flow-volume curve
- reduced peak flow
- often reduced FV
- scooped (Concave) expiratory loop
Restrictive disease and flow volume curve
- Increased peak flow
- Reduced FVC
- peaked expiratory loop
Spirometry characteristic
- Gold standard measure of airflow obstruction
- severity = % predicted FEV1
- FEV1 and FVC influenced by height, age, gender, race
- Robust, repeatable measurement but requires: coordination, effort
- correlates poorly with clinical outcomes
Airway hyperresponsiveness
- asthma = reversible airway obstruction
- asthmatic airways more sensitive to stimuli => abnormal degree of smooth muscle contraction
- can test for hyperresponsiveness: manifests as fall in FEV1 on provocation
Indirect tests for airway hyperresponsiveness
- Adenosine monophosphate (AMP) - measure presence of increased cellular inflammation, eosinophils
- mediator release from cellular inflammation
Directe tests for airway hyhperresponsiveness
- Bronchial smooth muscle sensitivity
- bronchial smooth muscle contractionand airway narrowing
Measuring gas transfer capacity
- gas exchange is the ultimate function of the respiratory system
- highly efficient process: needs to occur within milliseconds
- passive process that relies on pressure gradients - needs to be maintained
- several barriers need to be crossed
Barriers that need to be crossed in gas exchange
- airway surface liquid
- alveolar epithelium
- alveolar basement membrane
- interstitial space
- capillary endothelium
- plasma
- Red blood cell membrane
- red blood cell cytoplasm
- Hb molecule
Rate of gas transfer is highest when
- large surface area
- thin membrane
- high diffusivity (high solubility, low MW)
- large pressure gradient
Measurement of gas transfer: uptake of carbon monoxide
- measurable
- no back pressure
- high diffusivity
- rapidly taken up by Hb
Reasons for Reduced DCLO
- reduced lung volume - can adjust for this as DL/VA or KCO
- reduced membrane surface area (emphysema)
- increased thickness of membrane (interstitial lung disease)
- pulmonary vascular disease (pulmonary hypertension)
Increased DCLO
- in exercise (because of increase CO)
- pulmonary hemorrhage (increased availability of Hb)
