Lung volumes Flashcards
What is a lung capacity
Sum of two or more lung volumes
What is an average tidal volume
7ml/kg
500mls
Define tidal respiration
the volume of gas inhaled or exhaled during normal respiratory cycle
Define inspiratory reserve volume
is the maximum volume of additional air that can be inspired over and above VT after normal end inspiratory level during tidal breathing
What is an average inspiratory reserve volume
45ml/kg
3150mls
What affects IRV
Kyphosis
Rib fractures
COPD
Anything reducing range of movement of diaphragm and chest wall
Expiratory reserve volume define
maximum volume of additional air that can be expired from the end expiratory level following normal tidal exhalation
What is an average value for expiratory reserve volume
15ml/kg
1050mls
Residual volume define
the volume remaining in the lungs at maximum exhalation
What is an average volume for residual volume
15ml/kg
1050mls
What is FRC
‣ The volume of gas present in the lung at end expiration during normal tidal breathing
What is FRC made up of - what is its volume therefore
ERC + RV
2100mls
30ml/kg
Vital capacity
Maximum volume change between full inspiration and full expiration
67ml/kg
4.8L
ERV + VT + IRV
Inspiratory capacity
VT + IRV
Maximum inspiration capacity from FRC
What is an average inspiratory capacity
52ml/kg
3.6L
TLC define
Volume of gas in the lungs after maximal inspiration
What is an average value for TLC
82ml/kg
5.7L
How do lung volumes change with age 4
TLC increases until reaching maximal heigh then remains similar
FRC increases until maximal growth and slowly increases
Residual volume increases until maximal growth and then slowly increase throughout life, is responisble for all of the increase in FRC with a fall in ERV later in life
Draw and label a volume loop
What can spirometry measure in terms of lung volumes
◦ All lung volumes with the exception of residual volume can be measured in this way
What other ways of measuring lung volumes are there 3
Gas dilution
Body plethysmography
Multiple breath nitrogen washout test
What gas is used in the gas dilution method of assessing lung volumes? Why?
5% helium, does not diffuse across alveolar capillary barrier so any drop is due to redistribution in the lung
How does the gas dilution of lung measurements work
◦ Patient breathes 5% helium, through a spirometer (known volume and concentration)
‣ does not diffuse across alveolar capillary barrier (insoluble), any drop is due to redistribution in the lung
◦ At the end of tidal expiration a spirometer containing a known concentration is opened ot the patient
◦ The patient breathes in an out until He equilibrates
◦ The new concentration is then used to work out volume
‣ Concentration of helium x volume in spirometer (prior to opening) = concentration at equilibrium x (volume in spirometer + FRC)
‣ C1 x V1 = C2 x V2 (where V2 is V1 + FRC)
‣ If the same spirometer is opened to the patient after full inspiration the TLC can be calcualted
What equation is the foundation of the gas dilution method
◦ Patient breathes 5% helium, through a spirometer (known volume and concentration)
‣ does not diffuse across alveolar capillary barrier (insoluble), any drop is due to redistribution in the lung
◦ At the end of tidal expiration a spirometer containing a known concentration is opened ot the patient
◦ The patient breathes in an out until He equilibrates
◦ The new concentration is then used to work out volume
‣ Concentration of helium x volume in spirometer (prior to opening) = concentration at equilibrium x (volume in spirometer + FRC)
‣ C1 x V1 = C2 x V2 (where V2 is V1 + FRC)
‣ If the same spirometer is opened to the patient after full inspiration the TLC can be calcualted
What device is used to measure the changes in volume during the gas dilution method
Spirometry
What phase of the respiratory cycle does the gas dilution conventionally measure?
End tidal expiration
FRC
Therefore the FRC is measured as it is V1 + FRC = V2
How might the gas dilution method be altered to assess TLC
Open it to the patient at full inspiration
What are the problems with the gas dilution method 2
‣ Understimate lung volumes as if airways are patient or air trapping equilibration can be impaired - if the tracer has not mixed widely and evenly
‣ No gas is perfectly insoluble so some is lost
What is the principle of body plethysmography
◦ Subject and equipment in a large airtight box with a known gas volume
‣ Patient sits and breaths through a mouthpeice with a shutter and pressure transducer
‣ Pressure transducer in wall of box
What is Boyle’s law
at a constant temperature, the volume of a fixed mass of gas is inversely proportional to its absolute pressure.
‣ Pressure x volume = constant
‣ Pressure (chest) x volume (chest) = pressure (box) x volume (box)
What is the Boyle’s law principle applied to Body plethysmography
‣ Pressure x volume = constant
‣ Pressure (chest) x volume (chest) = pressure (box) x volume (box)
What is required for Boyles law to be applied
at a constant temperature, the volume of a fixed mass of gas is inversely proportional to its absolute pressure.
What does body plethysomgraphy caculate?
◦ As the subject exhales at the end of tidal exhalation shutter closes: (FRC measurement)
‣ Intrathoracic volume decreases, which means the volume of the box increases (as the walls are rigid and there is a finite volume shared by the chest and the box).
‣ Intrathoracic pressure increases, and therefore box pressure decreases proportionally.
Inspiratory occlusionin body plethysomography offers what benefit?
‣ At the end of normal expiration the mouthpeice shutter closes, and inhalation occurs against a closed mouthpeice leading to increased lung volume, with a drop in pressure wthin the lung as air cannot equialise across the presures
‣ The volume within the body plethysmograph decreases by an equal amount
‣ The body box is airtight so a decreased volume within the box must resultin an increased pressure which is measured
‣ The change in volume is measured
* before closure of the mouthpeice
* After inspiration against a closed mouthpeice
* Pressure before close x volume before closure = pressure post closure x (volume before closure - change in volvume)
Conventionally done at end of tidal expiration
‣ Mouthpeice pressure before closure x FRC = mouthpeice pressure after inspiration (FRC + change in volume)
What proportion does nitrogen make up in the air
79%
What is the multiple breath nitrogen washout test?
‣ Patient breathes room air
‣ At the end of tidal expiration i.e. FRC the inspired gas is switched from air to 100% oxygen
‣ From the next exhalation all expired gasses pass through the nitrogen analyser and are collected
‣ as the patient rbeathes in and out all the nitrogen is replaced by O2
‣ The test finishes when the expired nitrogen <1% when all nitrgoen has been exchanged (usually 70-80 tidal volumes - 7 minutes) and the total volume of expired nitrogen is measured and the subsequent total exchgaled volume and its concentration in exhaled gas allows intrathoracic gas volume to be calaculated (total volume of exhaled nitrogen vs nitrogen concentratino of the first breath)
What are the problems with multiple breath nitrgoen washout test 4
- Underestimates lung volume in gas trapping (body plethysmography measures total lung volume whereas helium dilution requires communication with the mouth)
- N2 never reaches zero as it is an exponential
- Nitrogen may not be measured directly
- Leaks in the system or into body tissues and fluids
How does FRC related to elastic recoil
- It represents the point where elastic recoil force of the lung is in equilibrium with the elastic recoil of the chest wall, i.e. where the alveolar pressure equilibrates with atmospheric pressure.
What are the 4 functions of the FRC
Oxygen reservoir
Preventing collapse of alveolar and small airways
Optimisation of respiratory worload
Optimisation of pulmonary vascular resistance
Explain how the FRC is an oxygen reservoir
◦ FRC maintains an oxygen reserve which maintains oxygenation between breaths with ongoing diffusion due to consistent alveolar partial pressure - as 2/3 of the time there is no fresh gas entering the chest
◦ This prevents rapid changes in alveolar oxygen tension and arterial oxygen content - as if intermittent oxugenation only occured with inspiration then cardiac output would intermittently have no oxygenation
◦ This also allows for de-nitrogenation to improe safe apnoea time before de-oxygenation
What % of the time is there no fresh gas flow? How does gas exchange continue?
◦ FRC maintains an oxygen reserve which maintains oxygenation between breaths with ongoing diffusion due to consistent alveolar partial pressure - as 2/3 of the time there is no fresh gas entering the chest
◦ This prevents rapid changes in alveolar oxygen tension and arterial oxygen content - as if intermittent oxugenation only occured with inspiration then cardiac output would intermittently have no oxygenation
◦ This also allows for de-nitrogenation to improe safe apnoea time before de-oxygenation
If the FRC did not exist what would happen to alveoli?
Collapse at end expiration and atelectasis –> V/Q mismatch and hypoxia, reduced duration of gas exchnage and atelectotrauma
At FRC the small airway resistance is at its lowest as they are splinted open
The benefits of all this is seen as you age and closing capacity is greater than FRC
How does FRC affect respiratory workload
◦ At FRC, lung compliance is maximal and small airway resistance is low
◦ The work of breathing required to inflate the lung from FRC is minimum
◦ If the FRC did not exist re-expansion of alveoli would be required more consistently with every breath
How does FRC impact pulmonary vascular resistance
◦ At FRC, pulmonary vascular resistance is minimal
◦ The RV afterload and pulmonary blood flow are therefore optimal
What are the consequences of FRC decreasing
- Gas exchange
◦ Reduced oxygen reserves - as FRC is an oxygen resevoir, so increasing fluctuation in arterial O2 content between breaths
◦ Increased atelectasis - decreasing FRC below closing capacity causes resorption atelectasis on expiration
◦ Increased shunt - due to atelectasis - Mechanism of breathing
◦ Increased airway resistance and reduced lung compliance –> increased work of breathing
◦ Decreased lung compliance due to reducing size of alveoli –> increased surface tension
◦ Airway resistance increases as collapsing alveoli stop providing radial traction keeping small airways open
◦ Poor tolerance of position changes that further reduce FRC - Reduced tidal volume and increased respiratory rate - due to reduced lung compliance
- Increased PVR and right ventricular afterload
◦ Narrowed alveoli on perialveolar vessels and hypoxic pulmonary vasoconstriction
What 3 effects does reducing FRC have on oxygenation
- Gas exchange
◦ Reduced oxygen reserves - as FRC is an oxygen resevoir, so increasing fluctuation in arterial O2 content between breaths
◦ Increased atelectasis - decreasing FRC below closing capacity causes resorption atelectasis on expiration
◦ Increased shunt - due to atelectasis
How does a smaller FRC impact on mechanism of breathing 3
Increased airway resistance - collapsing alveoli provide less radial traction to small airways
Decreased lung compliance - reducing size of alveoli, increased surface tension
Increased WOB as increased resistance AND reduced compliance
This reduces TV and increases RR as a compensatory mechanism
What effect does FRC reducing have on PVR
- Increased PVR and right ventricular afterload
◦ Narrowed alveoli on perialveolar vessels and hypoxic pulmonary vasoconstriction
What factors increase FRC
- Intrinsic3
- Pathology 3
- Controllable 2
- Intrinsic
◦ Male sex
◦ Large body size/taller
◦ Increasing age - reduced elastic tissue so FRC to TLC increases but absolute FRC remains stable - Pathology
◦ Open chest
◦ Emphysema - lung elastic tissue is destroyed so reduced inward elastic recoil, as the balance between the inward elastic recoil and outward springing of the thoracic cage is found at a higher volume
◦ Auto-PEEP - Asthma - Controllable
◦ Erect body position - FRC falls by 1000mls by being supine ; or prone position
◦ PEEP from mechanical ventilation
Factors decreasing FRC
- intrinsic 3
- Pathology 3
- Controllable 2
influence lung size (height and gender) - larger lung size increases FRC
* Intrinsic
◦ Female sex
◦ Small stature
◦ Younger age - neonates, infants, young children
* Pathology
◦ ARDS, pulmonary fibrosis , pulmonary oedema, atelectasis
◦ increased intraabdominal pressure - obesity, pregnancy, acute abdomen, laparoscopic surgery
‣ pregnancy, obesity
◦ Burns
* Controllable
◦ anaesthesia and paralysis - cause unknown and irrespective of spontaneous or controlle dventilation but potentialy due to lost thoracic cage muscle tone and loss of physiological PEEP
◦ supine position - FRC falls by 1000mls; or head down
Define closing capacity
is the maximal lung volume at which airway closure can be detected in the dependent parts of the lungs i.e. point in expiration where lung voluem falls enoughf or small airways to collapse
What areas of the airway are dependent on radial traction to be pulled open?
◦ Airways (especially terminal bronchioles and alveolar ducts) are usually pulled open by radial traction from neighbouring alveolar septa –> with expiration the traction decreases as alveolar volume reduces
Predisposition to collapse in small airways comes from
‣ Collapse when elastic recoil of the lungs overcome the negative intrapleural pressure
‣ Law of Laplace and lack of rigid enhancing cartilage causes predisposition towards collapse
Where in the lung do you find closing capacity first
dependdent areas, due to gravity
Inert gas washout methods can find closing capacity at what point
Transition from phase 3 to phase 4
Closing capacity is composed of which 2 volumes
RV and closing volume
Closing capacity is affected by
2 effort
4 diseases
1 unalterable
◦ Expiratory air flow: (higher flow = higher CC)
◦ Expiratory effort (more effort = higher CC) - the pressure of the chest wall on inflated alveoli is transmitted to smaller airways propping them open
◦ Small airways disease, eg. asthma or COPD - increased musuclarity and mucous content making them more narrow and more prone to collapse at higher lung volumes
◦ Increased pulmonary blood volume, eg in CCF - increased lung weight putting pressure on dependent regions
◦ Decreased pulmonary surfactant
◦ Parenchymal lung disease, eg. emphysema - loss of elastic pull of neighbouring septa
◦ Age (increasing age = increased closing capacity) –> increased residual volume is the cause of the increasing closing capacity rather than increasing closing volume. Due to lost lung elasticity
‣ At age 44, supine FRC is lower than closing capacity
‣ At age 66, erect FRC is lower than closing capacity
‣ Increased in neonates because of highly compliant chest wall and reduce ability to maintain negative intrathoracic pressures
‣ Linear relationship between age and volume
What are the 2 modifiable features of closing capacity
◦ Expiratory air flow: (higher flow = higher CC)
◦ Expiratory effort (more effort = higher CC) - the pressure of the chest wall on inflated alveoli is transmitted to smaller airways propping them open
What 4 disease processes predispose to increased closing capacity
◦ Small airways disease, eg. asthma or COPD - increased musuclarity and mucous content making them more narrow and more prone to collapse at higher lung volumes
◦ Increased pulmonary blood volume, eg in CCF - increased lung weight putting pressure on dependent regions
◦ Decreased pulmonary surfactant
◦ Parenchymal lung disease, eg. emphysema - loss of elastic pull of neighbouring septa
How does age affect closing capacity
◦ Age (increasing age = increased closing capacity) –> increased residual volume is the cause of the increasing closing capacity rather than increasing closing volume. Due to lost lung elasticity
‣ At age 44, supine FRC is lower than closing capacity
‣ At age 66, erect FRC is lower than closing capacity
‣ Increased in neonates because of highly compliant chest wall and reduce ability to maintain negative intrathoracic pressures
‣ Linear relationship between age and volume
What age is closign capacity supine above FRC
44
At what age is closing capacity above FRC erect
66
What is the relationship between closing capacity and age
linear
How can you measure closing capacity 2
Gas bolus measurement
Resident gas method/inert gas washout
What is a gas bolus method of measurement?
◦ Gas bolus measurement, where a subject inhales a small bolus of tracer gas, starting at RV (exhale maximally) then start inhlaing slowly up to TLC over 5-10 seconds and you quickly give them a small bolus of tracer gas
‣ Because at RV the small distal airways in the dependent lung regions are closed the upper alveolar get all the tracer gas - after fillling their lungs they exhale slowly back to RV
‣ Tracer gas comes out in 4 phases e.g. xenon
* Dead space - no tracer gas (phase 1)
* Phase 2 - alveolar tracer gas increases
* Plateau of tracer gas - middle alveoli (phase 3)
* Phase 4 - closing capacity reached as dependent (tracerless gas) stops being exhaled as these alveoli cease to empty and tracer concentration rises (measures closing volume –> need to add measured residual volume to this)
What gas is used for the gas bolus measurement?
Xenon
What are the phases of a gas bolus method of closing capacity measurement
◦ Gas bolus measurement, where a subject inhales a small bolus of tracer gas, starting at RV (exhale maximally) then start inhlaing slowly up to TLC over 5-10 seconds and you quickly give them a small bolus of tracer gas
‣ Because at RV the small distal airways in the dependent lung regions are closed the upper alveolar get all the tracer gas - after fillling their lungs they exhale slowly back to RV
‣ Tracer gas comes out in 4 phases e.g. xenon
* Dead space - no tracer gas (phase 1)
* Phase 2 - alveolar tracer gas increases
* Plateau of tracer gas - middle alveoli (phase 3)
* Phase 4 - closing capacity reached as dependent (tracerless gas) stops being exhaled as these alveoli cease to empty and tracer concentration rises (measures closing volume –> need to add measured residual volume to this)
Inert gas washout or the resident gas method involves?
where a subject inhales a TLC of oxygen, starting from RV
‣ Subjects exhales to RV –> upper lobe alvoli well opena dn full of nitrogen rich air and dependent lung collapsed.
‣ Pure O2 inhaled until TLC - fills the lung with pure oxygen except for upper lobe alveoli where some nitrogen still remains diluting inspired O2
‣ Subject exhales through a nitrogen sensor - initially no nitrogen (dead space)
‣ Then nitrogen begins to leak out and nitrogen concentration increases mixed with nitrogen rich adn nitrogen poor alveoli emptying
‣ At the end of expiration the dependent airways close and now the only gas being exhaled comes from nitrogen rich upper lobe alveoli - increasing expired concentration of nitrogen
4 distinct phases - no nitrogen –> then nitrogen leaks out (nitrogen rich and poor alveoli emptying) the plateau before finally nitrogen rich upper alveoli cause the tracer to go up again reflecting closing capacity of lower alveoli
What is the physiological consequence of an increase in closing capacity?
◦ Higher CC increases dependent atelectasis (airways closure –> absorption of alveolar gas and loss fo volume and collapse) –> shunt and hypoxaemia
‣ It is responsible for the age-related decrease in oxygenation, because of shunt
‣ Higher CC decreases the effect of pre-anaesthetic preoxygenation as it becomes more difficult to denitrgonate the FRC
◦ It aggravates lung injury through cyclic atelectasis
◦ Reduced lung compliance
How do you find out the residual volume
Cannot be measured directly
But if you use the FRC from either helium dilution, nitrogen washout or body plethysmography
You can then calculate the expiratory reserve volume with standard spirometry and subtract this from the FRC
Volume of RV
15ml/kg
ERV volume
15ml/kg
IRV volume
45ml/kg
TLC volume
75-80ml/kg
VC volume
60-70ml/kg
Inspiratory capacity volume
50ml/kg
5 main factors affecting FRC
Height
Weight
position - 30% higher in erect position
Disease - elastic recoil, pregnancy, fibrosis
Muscle relaxation
Explain the gas dilution method of measuring FRC
Rebreathing from a closed circuit commencing at end expiration of normal tidal breathing which contains a known volume V1 and concentraiton of helium
After a period of equilibration the new helium concentration is established
Minimal helium is taken up and therefore FRC can be calculated based on Boyles law C1 x V1 = C2 x V2
If a patient has lung disease and the FRC is measured by both body plethysmography and gas dilution - which will calculate a larger value
Body plethysmopgrahy because gas dilution only meausres communicating volume
Functions of FRC
Oxygen store, buffer to maintain steady PO2
Prevent atelectasis and minimise V/Q mismatch
Keep airway resistance low and minimise work of breathing
Minimise pulmonary vascular resistance
How much oxygen is in the FRC of a 70kg adult breathing room air
290mls
How much oxygen is in the FRC of a preoxygenated 70kg adult
1800mls
How is sowrk of breathing minimised at FRC
HIhg pulmonary compliance - steep part of the pressure/volume curve so low elastic work
Low airways resstance
partial inflation and being above closing capacity means no energy to open colapsed portions of the lung
Define closing capacity
The lung volume at which the small airways fo the lung first start to close
How does the nitrogen washout test differ between Fowlers method closing capacity
- The point of measurement - i.e. halfway through phase 1 vs waiting for phase 4
- Initial starting point
- Fowlers 100% O2 breath is taken from FRC
- Closing capacity breath is taken from RV
PO2 of dry room air
159
PO2 of saturated room air
149
