2 Lung volumes and Lung function testing Flashcards
Define lung compliance
Lung compliance is a measure of the lung’s ability to stretch and expand
Explain lung compliance + its features
It can be understood as a change in volume/change in pressure
- At higher, the lung is stiffer, hence compliance is lower and vice versa
- Lung bases are more compliant (more volume) than the apex. - better ventilation
Compliance is followed by elastic recoil
What can cause decreased lung compliance?
e. g.
- Pulmonary fibrosis
- Alveolar oedema
What influences compliance?
- Thoracic cage (expands during inhalation)
- Elasticity (influences stretchiness of lungs)
What is needed for efficient ventilation?
- High lung compliance
- Low alveolar surface tension due to surfactant
What is a surfactant?
A mixture of phospholipids, proteins, and ions, that work to reduce the surface tension in the alveoli
Which cells secrete surfactant?
Type II alveolar epithelial cells
How does surfactant work?
They reduce surface tension and increase the stability of the alveoli, preventing the lungs from collapsing
GIve an example where lack of surfactant has serious consequences
(NOT IMPORTANT, but useful)
Infant respiratory distress syndrome (IRDS)
- In the clinic, it is seen that there is insufficient surfactant production,
- the air sacs (alveoli) collapse,
- and so the lungs are difficult to inflate
Surfactant is secreted into the alveoli between the 6th and 7th month of gestation
- IRDS is observed in premature birth and is fatal if not treated
Management:
- Pre-birth - corticosteroids (betamethasone) given to mother to speed up lung development
- Surfactant replacement therapy immediately after birth
- High levels of O2 (with caution; the baby must be monitored closely for O2 toxicity > free radicals which may damage any surfactant produced)
What is the need for lung function testing?
Tests need to assess:
- Mechanical condition of lung (compliance)
- Resistance of the airways (narrowing)
- Diffusion across the alveolar membrane
What can a spirometer measure?
dependent on the lung’s elastic properties and properties of the muscles of the chest wall
Tidal volume
Vital Capacity
Inspiratory Reserve Volume
Expiratory reserve volume
What can’t a spirometer measure?
Total lung capacity
Residual Volume
Define tidal volume
TV - the volume of air moved in and out of lungs at rest (L)
Define vital capacity
VC - the maximum amount of air moved in and out of the lungs following forceful inhalation and exhalation
Define inspiratory reserved volume
IRV - the extra volume of air that can be forcefully inhaled above normal tidal volume
Define expiratory reserved volume
ERV - extra volume of air that can be forcefully inhaled above normal tidal volume
Define total lung capacity
TLC - the total amount of air that is held in the lungs
Define residual volume
RV - the total amount of air left in the lungs following forceful exhalation
Define functional residual volume
FRV - the air left in the lungs following normal exhalation
What can a vitalograph measure?
spirometer is more modern way
- Forced Vital capacity
- Forced expiratory volume
Define Forced vital capacity
FVC - the toal volume of air exhaled forcefully (approx 5L)
Define forced expiratory volume
FEV1 - the volume of air expired forcefull in the first second
typically, >70% of air forcefully expired is done in this time
What is FVC/FEV1 used for
If this ratio is lower than 70%, diagnosis can be made, along with an indication of airway size, and type of deficit
What physiological/environmentl factors affect lung volumes
- Anthropometric measurement (age, height, sex)
- Occupation (physical fitness; office vs athlete)
- Environment - altitude
- Standing up right vs sitting down
How is functional residual capacity meausred? (TLC/RV)
as residual air cannot be directly expired into the spirometer
- Helium detection
- Nitrogen washout
(helium used as it is not metabolised in the blood)
(can be measured, as it will not get lost)
after measuring FRC, TLC and RV can be calculated
How does Helium detection work?
In a closed system, there is a spirometer with a known concentration and volume of helium
- Before equilibration (before breathing)
- C1 (conc. of helium before breathing in)
- Then participant breathes in (with helium)
- Known conc. of helium is no more
- So C2 is given, conc. of helium after equilibration
- V1 is the volume of helium/air before equilibration (trying to find V2 – FRC representation, reflected in helium measurements)
- C2 measured by spirometer equipment
- Using the formula, rearrange to find V2
V2 = V1 * ([C1-C2]/C2)
V2 = FRC
How does nitrogen washout work?
Procedure:
- Patient expires 100% O2
- Expires into the spirometer system
- Procedure repeated until N2 in lungs is replaced with O2
- FRC calculated from exhaled N2 and estimated alvoelar N2
Define restrictive deficit, and give some features of it
When lung expansion is compromised - may be due to alterations in the lung parenchyma, disease of the pleura, or chest wall
(lungs do not fill to capacity, hence they are less full before expiration)
What happens to FVC, FEV1, and FEV1/FVC when there is a restrictive deficit?
- FVC is reduced
- FEV1 is relatively normal
- FEV1/FVC normal
Define Obstructive deficit, and give some features
Characterised by airway obstruction
- If airways are narrowed, lungs can still fill to capacity
- e.g. asthma, COPD
What happens to FVC< FEV1 and FEV1/FVC when there is an obstructive deficit?
- FEV1 is reduced
- FVC is normal
- FEV1/FVC is reduced
What is a vitalograph?
A lung function test that measures the ability to move air out of the lungs
- FVC and FEV1
What is Miller’s prediction quadrant
It is used to predict the type of deficit, based on the FEV1 and FVC measurements
What is peak expiratory flow rate? (PEF)
A measure of a person’s maximum speed of expiration, as measured
with a peak flow meter
How is PEF measured + list some features of a flow-volume curve
The flow-volume curve consists of expiratory flow rate and expiratory volume
- A patient takes a deep breath in (TLC measured)
- Then they take a deep and fast breath out (RV is measured)
- TLC is when lungs full, PEF is when highest rate of expiration, where flow falls and ceases at RV
TLC to PEF: is effort dependen - increase in effort increases flow rate
PEF to RV: is effort independent (vice versa)
What is a flow-volume loop?
It is a flow-volume curve, that is done using the PEF rate method, but where the patient inspires again (loop)
What does a large, wide, and tall loop show
Normal expiration - no deficit
What does a smaller loop, starts at TLC, which is shifted to the left (larger RV), which has a change in the flow rate (it becomes less steep) show?
Obstructive deficit
- patient is able to get to TLC
- air can be moved from large airways, harder from small airways (compromised), giving the scoop effect (change in flow rate)
- More air is also retained, which increases the RV, as smaller airways compromised, and retain the air
What does a smaller loop, which does not start at TLC (lower TLC), ends at RV, and has the same flow rate of expiration show?
Restrictive Deficit
- normal. shape, smaller, TLC reduced (less than 5L)
- Airways normal
- Issue is compliant - lungs cannot be filled as much as normal
What is gas transfer - diffusion conductance?
It is a measure of how easily carbon monoxide crosses from the alveolar air to blood.
How is the gas transfer test done? (diffusion conductance)
The test is done by:
- Patient inhales a single breath of dilute carbon monoxide followed by a breath-hold of 10s
- The diffusion capacity is calculated from lung volume and CO% in the alveoli in the beginning and end of the 10s breath-hold
Clinical relevance -
- e.g. in fibrosis of the lungs where gas diffusion is compromised
