Lecture 6 - Normal Lung Function

Question 1

Lung is expandable and retractable

Answer 1

Expandable

• compliant
• low resistance network of branching conduits

Retractable

• elastic
• millions of tiny alveolar sacs

Question 2

Pleural space

Answer 2

• potential space
• Few mL fluid only

Pressure

• lung, chest wall
• gravity
• Position

Question 3

Mechanics of lung function

Answer 3

• 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

Question 4

What determines lung volume

Answer 4

• 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

Question 5

Height and lungs

Answer 5

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

Question 6

Age and alveolar size

Answer 6

• 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?

Question 7

Functional residual capacity

Answer 7

• 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

Question 8

Residual volume

Answer 8

• 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

Question 9

How is residual volume affected by diseases

Answer 9

• increased in obstructive lung disease (gas trapping)

- decreased in restrictive neuromuscular disease

Question 10

Measuring lung volume - body plethysmography

Answer 10

• 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

Question 11

Once MRC is measured, how do you measure other capacities?

Answer 11

• TLC = FRC + inspiratory capacity

- RVL TLC - VC

Question 12

Simpler method of lung capacity measurement: Gas dilution

Answer 12

• inspire a known concentration of particles.
• measure the expired concentration
• Cinsp x Vinsp = Cexp x Valv

Question 13

Body plethysmography characteristics

Answer 13

• 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

Question 14

Gas dilution characteristic

Answer 14

• 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

Question 15

Measuring air flow: spirometry

Answer 15

• FEV1: Forced expiratory volume in 1 sec
• FVC : Forced Vital capacity
• FEV1/FVC = 0.8
• this is influenced by age (reduces) and diseases)

Question 16

How does restrictive disease affect spirometry

Answer 16

• FVC reduced» FEV1
• FEV1/FVC increased
• Faster
• shorter

Question 17

Obstruction disease

Answer 17

• FEV1»_space; FVC

- FEV1/FVC reduced (

Question 18

Spirometry

Answer 18

• 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

Question 19

Effect of obstruction on flow-volume curve

Answer 19

• reduced peak flow
• often reduced FV
• scooped (Concave) expiratory loop

Question 20

Restrictive disease and flow volume curve

Answer 20

• Increased peak flow
• Reduced FVC
• peaked expiratory loop

Question 21

Spirometry characteristic

Answer 21

• 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

Question 22

Airway hyperresponsiveness

Answer 22

• 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

Question 23

Indirect tests for airway hyperresponsiveness

Answer 23

• Adenosine monophosphate (AMP) - measure presence of increased cellular inflammation, eosinophils
• mediator release from cellular inflammation

Question 24

Directe tests for airway hyhperresponsiveness

Answer 24

• Bronchial smooth muscle sensitivity

- bronchial smooth muscle contractionand airway narrowing

Question 25

Measuring gas transfer capacity

Answer 25

- 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

Question 26

Barriers that need to be crossed in gas exchange

Answer 26

- airway surface liquid - alveolar epithelium - alveolar basement membrane - interstitial space - capillary endothelium - plasma - Red blood cell membrane - red blood cell cytoplasm - Hb molecule

Question 27

Rate of gas transfer is highest when

Answer 27

- large surface area - thin membrane - high diffusivity (high solubility, low MW) - large pressure gradient

Question 28

Measurement of gas transfer: uptake of carbon monoxide

Answer 28

- measurable - no back pressure - high diffusivity - rapidly taken up by Hb

Question 29

Reasons for Reduced DCLO

Answer 29

- 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)

Question 30

Increased DCLO

Answer 30

- in exercise (because of increase CO) | - pulmonary hemorrhage (increased availability of Hb)