Pulmonary Function Tests Flashcards
What is the tidal volume?
- Normal breathing in and out
What is the inspiratory reserve volume?
- After breathing normally, further breathing until the lungs are full
What is the expiratory reserve?
- After breathing out normally, blow all air out of lungs
What is the residual volume?
- Volume of air that resides in the lungs but cannot be blown out
What is air flow driven by mechanically?
Pressure changes
Describe how pressure changes in the lungs work
- Requires a pressure gradient
- Air flow moved from high pressure to low and so air is sucked into the lungs
- Relative to atmospheric pressure
What does negative intrapulmonary pressure lead to?
Inspiration
What does positive intrapulmonary pressure lead to?
Expiration
Describe the mechanics of inspiration
- Air flow in
- Due to contraction and contraction (flattening) of the diaphragm and external intercostal muscles
- Created negative pressure and draws air in
Describe the mechanics of expiration
Air flows out
- Largely passive due to elastic recoil that drives air out (up until the functional residual capacity is reached)
- Beyond FRC expiratory muscles need to contract
- Also, to achieve higher pressures and therefore flows
- A higher pressure generated by squeezing thorax
What is the alveolar pressure the sum of?
- Pleural pressure determined by muscular effort
- Elastic recoil generated by the elastic properties (and surface tension)
What increases the elastic recoil pressure?
Volume
- Collapse alveoli and increase alveolar pressure
Therefore how does alveolar pressure become greater than atmospheric?
- Increasing either pleural pressure or elastic pressure
- But only up to a point
- Expiratory effort is limited
In what circumstance does increase in pressure not necessarily increase flow?
Flow envelope
What limits peak expiratory flow?
Volume
What determines air flow?
- Resistance
- Affected by diameter of small airways
- Resistance increases with effort as small airways are compressed by raised external pressure
Is there interdependence between alveoli?
Yess
How do large volumes affect peak flow?
- increase since elastic recoil increases with volume
- While resistance decreases due to radial traction
- As you breathe in, everything stretches
- Always are wide and have lower resistance
What does maximal output require?
- Maximal drops in resistance
- Harder when you breathe out, more pressure is applied across airways
What is the benefit of airways with cartilage?
- Will not collapse
Which airways would collapse when pressure increases and when?
- Lower airways with no cartilage
- Only occurs in forced expiration
What opposes airway collapse in lower airways?
- Radial traction from interdependence
What does puffing fo on expiration (pursing lips)?
- Critical closing point can be moved into area of airway held open by cartilage
- Often done by patients with emphysema that lack interdependence
What is Ohm’s Law?
Voltage = Current x Resistance
How does Ohm’s Law apply to movement of air?
- Voltage- pressure difference
- Current- perfusion/flow (Q)
- Resistance (R)
- Pressure difference- QR
What increases resistance?
Force of breathing
What is lung compliance?
- How hard it is to stretch the lung and chest wall
- Decrease compliance in pulmonary fibrosis and CVD, e.g. heart failure
- Restrictive lung disease
- Fibrotic lungs are stiff and hard, therefore not very compliant
What is lung resistance?
- Resistance to movement of air through the airway
- Increased in asthma and COPD (obstructive lung disease)
- Airflow becomes problematic
- Investigated through spirometry
What is spirometry?
- Measurement of pattern of air movement into and out of lungs
- During controlled ventilatory manoeuvres
- Often does as a maximal expiratory manoeuvre
What can spirometry measure?
- Both resistance and compliance
- Will record FEV1 and FVC
What are spirometry results dependent on?
- Results dependent on mechanical ability of lung and factors which alter resistance
- Which can also be mechanical
What is the forced vital capacity?
- Total volume of air that can be exhaled forcefully from full inflation (TLC) measured in litres
How long does it normally take for the majority of FVC to be exhaled?
- less than 3 seconds
- Often prolonged in obstructive diseases
What is FEV1?
- Volume that can be forcefully expired from full inflation (TLC) in the first second
- Measured in litres
- Resistance
What is the normal FEV1?
- 70-80% of FVC in the first second
- Athletes and physically may achieve more
- Those with respiratory disease may do less
What is the classification of severity of FEV1 interpretation?
- > 75%- Normal
- 60-75%- mild obstruction
- 50-59%- moderate obstruction
- <49% severe obstruction
What is the classification of severity of FVC interpretation?
- 80-120%- Normal
- 70-79%- mild reduction
- 50-69%- moderate reduction
- <49% severe reduction
Why is full inflation important?
- Maximal can allow for a reproductive test
- Also leads to the least possible resistance
- Best possible flow
What are the patterns in spirometry that reflect obstruction?
- Long flow
- All air will come out due to higher resistance
What are the patterns in spirometry that reflect restriction?
- FEV1 drops in proportion to normal
- Smaller amount of air exhaled
- Appropriate percentage FVC1 is removed
What are obstructive disorders characterised by?
- Low expiratory flow
- Typically due to increased resistance
How does obstructive disorders present?
- Low FEV1 with normal FVC
- FVC may be lower but nor substantially
- Crucially, FEV1 will be less than 80% of the FVC
- E.g. asthma and COPD
Describe asthma
- Characterised by bronchoconstriction- increased airway resistance
- Increased resistance impairs expiratory flow leading to obstruction
- Expiratory flows are more greatly affected since resistance to inspiration is less
- Excess mucous is also present in these smaller airways which leads to increased obstruction
Describe chronic obstructive pulmonary disorder (COPD) (bronchitis)
- excessive mucous production in airway
- leads to mucous plugging, inflammatory cell infiltration and oedema
- All of which can impede airflow, leading obstruction
Describe chronic obstructive pulmonary disorder (COPD) (emphysema)
- Excessive protease activity
- Loss of elastic recoil removes large drive of expiration- makes forced expiration more important
- Loss of radial traction increases airway collapse and air trapping
- Presents as obstruction
What are restrictive conditions characterised by?
- Diminished lung volume
- Typically this is due to decreased compliance and so decreasing inspiration
Why is compliance decreased in restrictive conditions?
- Changes in lung parenchyma (interstitial lung disease)
- Diseases of pleura, chest wall (e.g. scoliolosis), or neuromuscular apparatus (e.g. muscular dystrophy)
- Seen as decreased FVC but normal FEV1/FVC ratio
Describe pulmonary fibrosis
- A number of conditions lead to pulmonary fibrosis
- Here, parenchyma is replaced by fibrous tissue
- Leads to stiffer, less compliant lungs that are harder to inflate and have less elastic recoil
Give examples of conditions that lead to pulmonary fibrosis
- Interstitial lung disease, asbestosis, idiopathic pulmonary fibrosis, farmer’s lungs and pigeon fancier’s lung
What is heart failure?
- Failure to eject sufficient blood from the left ventricle
- Also fails to clear it from lungs
What happens to the lungs during heart failure?
- Lungs become engorged with blood as it becomes congested
- Lungs become stiffer and harder to inflate
- Increased resistance- obstruction
- Decreased compliance- restriction
What do flow volumes loops provide?
- Graphical illustration of a patients spirometric
Describe flow volume loops
- Flow (y-axis) is plotted against volume (x-axis) to display a continuous loop from inspiration to expiration
- Overall shape of flow volume loop is important in interpreting spirometric results
Why would having a non-maximal inspiration result in invalid results?
- Maximal inspiration was vital to get best flow
- Here flows will appear reduced and may present, wrongly, as restrictive
Why could coughs result in invalid results?
- Can give a false indication of both FEV1 and FVC
- Any variable including a cough should be ignored
Why would sub-maximal result in invalid results?
- By definition obstruction is a reduced expiration
- As such poor technique appears obstructive
Why would early termination result in invalid results?
- Most adults breathe out for more than 6 seconds
- In obstruction it takes longer to reach plateau
- Ending early gives a false, lower FVC and might suggest restriction
- Equally, it might inflate the FEV1/FVC ratio and hide an obstructive disorder
Why would a partially obstructive mouthpiece result in invalid results?
- May cause obstruction and falsely report an obstructive disorder
- May also (depending on obstruction) lead to air loss reducing FVC, an so, giving a false restrictive
Why would a leak result in invalid results?
- Whether in the tubing or by subject failing to seal
- Similar to early termination
- Since air is not being recorded the FVC will be taken as low
- Lower FVC suggests restriction
- Equally, it might inflate FEV1/FVC ration to hide an obstructive disorder
