Ventilation Flashcards
How does intrapleural pressure and alveolar pressure change in inspiration?
Reduced intrapleural pressure (-11cmH2O)
Reduce alveolar pressure by (5cmH2O)
What does ventilation allow?
Inspired air reaches alveoli and blood gas barrier, expired gases are removed from the alveoli
What is total ventilation?
Total rate of flow in and out of the lung during normal tidal breathing
Units for total ventilation
L/min
What do changes in rate and depth of ventilation cause?
Composition of alveolar gas, and therefore composition of gases entering/exiting blood
What is alveolar ventilation?
Volume of air that reaches the alveoli and is avaliable for gas exchange with blood (vol/min)
What is the resting O2 consumption and CO2 production?
Resting O2 consumption = 250ml/min
Resting CO2 production= 200ml/min
What zones do airways compromise and what is their role?
1) conducting airways: delivery of gas to alveoli
2) exchange zones: exchange to and from pulmonary circulation
What are the volumes of the conducting and exchange zones?
Conducting = 150ml Exchange = 3000ml
What is the anatomical dead space?
Volume of conducting airways
What is the physiological dead space?
Anatomical dead space plus the alveolar dead space (volume that doesn’t take part in gas exchange)
How many times do airways bifurcate to reach the alveolar ducts?
23 times
After how many divisions do we reach respiratory bronchioles?
17th is respiratory bronchioles
What are normal alveolar partial pressures of O2 and CO2?
PAO2 = 13.3.KPa (100mmHg)
PACO2= 5.3 KPa (40mmHg)
What happens as total cross sectional area increases going down the airways?
Velocity (of gas flow) decreases
Describe how cross sectional area changes across conducting zone and respiratory zone
Increases in conducting zone and then rapidly increases in respiratory zone
Describe how the mechanisms of convection and diffusion change going further into the lungs
In conducting zone, convection dominant
In respiratory zone convection slows (velocity decreases due to larger cross-sectional area) so diffusion becomes dominant
Describe the oxygen percentage: airway generation graph
From generation 1-16 oxygen percentage 20% (dead space)
Then from 16-17 large decrease in percentage to 13% and stays there for subsequent generations (exchange is occuring)
In exhalation the O2% remains at 13%
How are lung volumes measured?
Spirometer (subject breathes into a sealed container)
The changes in the spirometer are equal and opposite to the the changes in the lungs of the subject
Lung volumes vary with…
Sex, size and gender
What is functional residual capacity (FRC)?
Volume of gas in lungs after normal expiration
What is normal FRC value?
2.5-3 L
What is the tidal volume?
0.5 litre
What is the vital capacity
Amount of gas that can be inhaled by a maximal inspiratory effort following maximal expiration
Normal value for vital capacity
5L
What is intrapleural pressure, what is it normally?
Pressure of fluid within the pleural cavity. Normally negative (less than atmospheric)
What is the pleural cavity?
Fluid filled space between each pleura (visceral and pareital)
What generates the normally negative intrapleural pressure after expiration?
1) Ribcage has natural tendency to spring upwards
2) Lungs have intrinsic tendency to collapse
What happens to intrapleural pressure when pleura damaged?
Air introduced into pleural space, intrapleural pressure may exceed/equal atmospheric pressure leading to pneumothorax
What happens to lungs and chest wall in a pneumothorax?
Lungs collapse inwards, chest wall outwards
What is an assumption when we consider normal intrapleural pressure value?
Static mechanics (after normal expiration and no gas flow in/out) so pressure outside lung and in alveoli is the same
How does inspiration change intrapleural pressure and alveolar pressure (mechanism)?
Lift ribcage and increase intra-thoracic volume (stretch lungs)
Reduces intrapleural pressure (holds lungs in more stretched position)
Alveolar pressure is reduced and inspiraiton initiated
Describe how alveolar pressure compares to atmospheric pressure when no air flowing in
Equal
Describe the mechanism behind expiration in terms of pressures
Muscles of chest wall and diaphragm relax
Decrease intra-thoracic volume
Increased intrapleural pressure and alveolar pressure (above atmospheric)
Hence expiration initiated
Describe the mechanism for forced expiration in terms of pressures
Contraction of chest wall muscles results in even higher increase in intrapleural pressure, increasing expiratory rate further
Which part of the lung receives more ventilation per unit?
Lower part of the lung
Why does lower part of lung experience more ventilation?
Gravity (due to the weight of the lung) means increased pleural pressure at the base (making it less negative).
Reduce the alveolar volume so are smaller percentage of max volume (pre-inspiration).
This gives each alveoli as greater ventilation potential so more compliant and so capable of more oxygen exchange whereas the apex ventilates less efficiently since its compliance is lower and so smaller volumes are exchanged.
What happens to regional ventilation differences when person is supine?
Apical and basal ventilation become same
Posterior aspects of lung are best ventilated
How is FRC measured?
Can’t use spirometer
Use helium dilution
How does helium dilution work?
Helium gas used as indicator (known volume and concentration)
Use concentration of helium in expired air to work out difference
How can regional ventilation be detected?
Analyse distribution of inhaled radioactive gas (133Xe)
Single breath of 133Xe taken and breath held while counts taken at different levels of lung
133Xe breathed to and from until evenly mixed in lungs to estimate volume at each level
What is meant by compliance?
Measure of the pressure required to inflate lungs by a certain incremental volume
What are the units for compliance?
ml/cmH2O
Equation for compliance
delta V/delta P
Which forces work to deflate the lungs (acting against pressure to inflate lungs)?
Inherent elasticity of lungs
Forces which arise due to surface tension
What happens to compliance in obstructive diseases, why?
Increases
Poor elastic recoil
What reduces compliance in normal breathing?
Airways resistance and other frictional forces
What is total compliance equal to?
Chest compliance and lung compliance
What is meant by surface tension?
Tendency of fluid surfaces to shrink into minimum surface area possible
What does high surface tension mean for lungs?
They are more stiff and less compliant (more pressure needed for same volume change)
Why is surfactant beneficial?
Normal water air surface, tension -70mN/m (lungs too stiff and need too much pressure to inflate).
Surfactant reduces surface tension (to 20mN/m) renders lungs more compliant, reduces work of breathing
From when and which cells is surfactant secreted?
From 30 weeks gestation, type 2 alveolar cells
What happens to surfactant and surface tension upon alveolar inflation?
Surface area of surfactant film increases, surface tension is increased
What is the benefit of surface tension increasing when alveoli inflate?
Prevents smaller alveoli entering into larger ones (Smaller alveoli require greater transmural pressures to remain inflated) so increasing surface tension increases pressure (p=2T/r)
What happens when insufficient surfactant in premature babies?
IRDS - lungs collapse as surface tension too high on exhalation
Lungs damaged by high pressures used to ventilate them, may cause atelectasis (lung collapse)
How does pulmonary oedema lead to difficulty inflating lung?
Air water interfaces, so compliance decreases
Harder to inflate lungs and struggle to breathe.
What happens in simple pneumothorax?
Non expanding collection of air around lung occurs spontaneously (air enters through damaged visceral pleura)
What are the symptoms of tension pneumothorax?
Resonant percussion and hyperexpanded chest on affected side
Lung collapse, absent breath sounds on affected side
Tracheal deviation to opposite hemithorax
Tachypnoea
Obstructed venous return to heart, decrease CO, tachycardia
What are the two types of air flow in the lungs?
Turbulent vs laminar flow
In the lungs, the conditions are normally … flow?
Laminar
Normal bronchial tree exhibits both laminar and turbulent flow, true or false?
True
What sound effect arises due to turbulent flow?
Wheezing
What determines if flow is turbulent or laminar?
Reynolds number
<2000 then Laminar
>2000 then Turbulent
What does equation of laminar flow suggest?
Flow directly proportional to pressure gradient (constant=Poiseuille’s law)
How does turbulent flow relate to pressure gradient?
Flow directly proportional to square root pressure gradient
How is air resistance calculated?
Flow=pressure difference/resistance
Three facts needed to calculate the airways resistance
Mouth pressure
Alveolar pressure
Rate of air flow
How is mouth pressure calculated?
Manometer
How do we calculate (estimate) alveolar pressure?
Body plethysmograph/a sealed box, need to know volume of lung and box, and measure pressure of box, then P1V1=P2V2
Equation for airways resistance
delta pressure/flow
At rest describe flow between alveoli and atmosphere
No flow
What is the peak expiratory flow rate?
Measure of maximum speed of expiration (L/min)
How do we measure peak expiratory flow rate?
Flow meter
In forced expiration, why is there a certain PEF rate above which expiratory effort doesn’t increase expiratory rate?
In forced expiration, intrapleural pressure rises this compresses airways which increases resistance and decreases airflow
How does peak flow rate vary with lung volume, why?
Decreases as lung volume decreases due to increased resistance
Compare and contrast flow in submaximal and maximal effort for exhalation
Maximal effort: higher initial flow but then gas can’t come out at higher rate (effort independent) so gas flow becomes same as rate of submaximal effort
Why may pressure in alveoli be greater than pressure in airways during quiet expiration?
Resistance to flow as you go along airways
Why is the pressure difference of inside and outside of alveoli always +8?
Pressure outside alveoli (in pleural space) is more -ve than in alveoli but difference always +8 because of innate tendency of lung to collapse (prevent collapse).
When do the airways collapse?
Pressure difference inside airways and outside is -11 so airways collapse
As airways collapse, describe the resistance in the same segment of airways in submaximal and maximal effort, what’s the effect?
Same resistance thus same pressure gradient through same segment of airways so same flow occurs
Describe how intrapleural and alveolar pressure change in maximal effort exhalation compared to submaximal effort?
Intrapleural pressure but also alveolar pressure increase (by the same value)
Why is high airways resistance clinically significant? Give an example.
High resistance gives low maximal expiratory rate (e.g. asthma) so limits flow rate out of lung
How can emphysema lead to less flow out of lungs?
Low lung elasticity gives poor airways support and easy collapse
Less pressure difference across alveoli at given volume so less support to wall and collapses at lower pressure
Difference across airways also decreases and pressure gradient from alveoli to airways decreases so less flow
How does airways close physiologically?
At residual volume, lower part of lung in state of compression which closes airways so no further gas exhaled (gas trapping)
What happens to airways as you begin inspiration from residual volume?
Initially upper part of lung only will inflate until negative pleural pressures lower done
How does emphysema lead to intermittent ventilation of lower parts of lung?
Loss of elasticity leads to positive intrapleural pressures and airway closure lower down at higher lung volumes
What are the symptoms of emphysema?
Barrel chest, increased FRC, intermittent ventilation of lower parts of lung, hyperventilation
How can airways be closed pathologically?
Bronchiolar constriction (e.g. asthma, anaphylaxis, COPD e.g. chronic bronchitis)
Increases airways resistance
Why do emphysema patients have broader chests?
Increased FRC as lungs more compliant and equilibrium between inward recoil of lung and outward recoil of chest wall disturbed
Relatively unopposed outward recoil of chest wall
What is forced expiratory volume in one second (FEV1)?
Volume of air exhaled during forced breathe in one second
What is FEV1/FVC ratio comparing, what is the normal value?
Volume of air expelled in one second to volume of air expelled in total after deepest breath possible.
Normally above 70% (around 80%)
What is the FEV1/FVC ratio used for?
Distinguish between obstructive and restrivtive diseases
Indicate degree of obstruction in obstructive diseases
How can you diagnose obstructive airways disease?
If FEV1/FVC ratio less than 70% then may have COPD or asthma
How can you use a spirometer to measure resistance?
Calculate change in volume over time
What could lower lung volume?
Damage to phrenic nerve
Kyphosis
Pneumothoax
Restrictive airways disease
Examples of obstructive and restrictive airways disease
Obstructive: reversible e.g. asthma, or irreversible e.g. COPD
Restrctive: pulmonary fibrosis, Duchenne’s, kyphscoliosis
How can chance of pulmonary oedema be reduced (surface tension)?
Lower surface tension, reduce chance of alveoli collapse and pulmonary oedema
Compare the compliance of lungs and thorax together in intact animal compared to just lungs themselves
The compliance of lungs and thorax together in intact animal is half that of the lungs on their own
Describe IP pressure relative to atmospheric pressure in normal expiration
Still lower than atmospheric pressure (-1cmH2O)
Which gases diffuses better within the alveolus?
Oxygen (lighter)
How can diffusion of gases be increased voluntarily?
Hyperventilation to increase SA of alveoli
Does anatomical dead space change? When?
Inspiration, it increases
What affects FRC, what doesn’t?
Height, age doesn’t
Compare IA pressure and IP pressure
IA pressure normally greater
What does oxygen diffusion in pulmonary capillaries depend on?
Perfusion dependent, need alveolar capillary gradient
What is the main component in the work of breathing?
Overcome elastic forces of lungs
What sort of molecule is surfactant?
Phopholipid and protein (not glycoprotein)
Why is surfactant less effective at reducing surface tension at larger lung volumes?
Surfactant concentration is decreased in larger alveoli
True or false and why: low oxygen tensions aggravate V/Q mismatches?
False, hypoxia induces pulmonary vascular vasoconstriction to improve V/Q mismatching
How much is the physiological dead space?
Just >150ml
Alveolar PO2 is greater at the … of the lung?
Apex
What is the main lipid component of surfactant?
Dipalmitoylphosphatidylcholine
In pulmonary emphysema, what happens to lung compliance ?
Increases
What is the intrapleural pressure at the base vs the apex of the lung?
Apex = -15cmH2O
Base = -7cmH2O
Gravity
Where does airways resistance arise mainly from?
Trachea and main bronchi
The compliance of the lungs and thorax together in an intact animal is
Half that of the lungs on their own.
