Respiratory mechanics Flashcards
What is contained in the thoracic cavity?
lungs thoracic muscles ribcage sternum thoracic vertebrae connective tissue
Where does the respiratory tract begin?
nose/mouth to the trachea then to the pharynx and larynx
Where does the trachea branch?
bifurcates at the sternal angle
- branches at the major carina into the left and right bronchi which connect their respective lungs
How many generations are there in the lungs?
24
What are the conducting airways known as?
dead space- poorly or not perfused
What provides the structural support in the larger airways?
cartilage
- as generations increase and airways become smaller, the cartilaginous support is progressively lost and is replaced by smooth muscle and elastic connective tissue
The lungs are elastic in nature. What does this mean?
They comprise of elastin and collagen fibres
- have a tendency to collapse inwards
- in contrast the chest wall has a tendency to spring outwards
- these 2 opposing forces balance each other out at rest thereby maintaining the overall shape of the thoracic cage
What are the functional units of the lungs?
Acini - meaning “little cavity” in latin
What do the pneumocytes do?
Type 1: form the alveolar wall
Type 2: secrete surfactant - surface tension and are moist for gas exchange
What is surfactant?
Phospholipoprotein that reduces surface tension in the alveoli
- facilitates expansion and relaxation
How does ventilation differ at rest and during exercise?
Relaxed tidal breathing is relatively passive and under subconscious control by rhythmic stimulation of the diaphragm
During exercise or in disease ventilation becomes more active and accessory muscles for inspiration and expiration are recruited
What happens during inspiration?
- Pulmonary system relaxed. The forces of the chest wall and lungs are at equilibrium
- Diaphragmatic contaction induces lung expansion (external intercostals also contract) and this causes a negative pressure in the airways
- Due to the negative pressure air flows into the lungs
What happens during expiration?
- After tidal inspiration, the atmospheric and alveolar air pressures are equal
- Diaphragmatic relaxation causes an increase in airway pressure
- Positive air pressure within the airways causing air to flow out of the lungs
What are the differences between laminar and turbulent flow?
Laminar = linear flow in the smaller airways which is slow
Turbulent = air bounces around and occurs in the larger airways and is much faster
Airflow in the alveolar ducts occurs by diffusion
What does the Reynolds No. refer to?
the transition from laminar to turbulent flow
= pVD/n
p= gas density
V= linear velocity (inversely proportional to the cross-sectional area)
D= diameter
n= gas viscosity
What are the ranges of the Reynolds No.?
<2300 = completely laminar 2300= onset of turbulent flow (transition) >4000= completely turbulent
What is the flow of gas/liquid inversely proportional to in a hollow tube?
the resistance
= greater resistance = slower the flow
- pulmonary airways exert a resistance to airflow during inhalation and exhalation
What is Pouseuille’s law?
R=8nl/pie r(power of 4)
- n= viscosity
- l= length of the tube
Dominant factor in determining resistance is radius
How does pousueille’s law apply to the lungs?
Resistance in the small airways is relatively low due to the large number of small bronchioles (large cross-sectional area)
- majority of airway resistance occurs within the larger bronchioles W
What is the relationship between cross sectional are and resistance in the lungs?
Resistance- increases initially up to lung generation 5 and then decreases as the lung generations increase
Cross sectional area increases as the lungs generations increase- steady increase at the start and more substantial the further down the respiratory tract
What is used to test airway resistance?
Body plethysmography
- airway resistance (Raw)- the pressure change between the alveoli and the mouth divided by the flow
= Raw is the pressure that must be applied between the alveoli and lips to secure a rate of flow through the airways
What is the reciprocal to resistance?
conductance - Gaw = I/Raw
- when conductance is measured during relaxed ventilation it is referred to as “specific airway conductance” or sGaw
- sGaw is a constant (independent of lung volume) and can be easier to interpret -its linear
What is oscillometry?
it looks at the structure of the airways during normal breathing
- forced oscillations (5-35Hz) generated by a loudspeaker are applied at the mouth during ordinary tidal breathing
- differentiate between large and small airway resistance
What does reactance comprise of?
Inertia: resistance to change in state of motion
Capacitance: ability to store energy
How is neural regulation involved in ventilation ?
controlled by respiratory centers= series of interconnected brain cells within the lower and middle brain stem that coordinate respiratory movements
- respiratory centres send out impulses to the diaphragm and external intercostal muscles inducing contraction and subsequent inspiration -this ceases after about 2 seconds and the intercostal muscles and diaphragm relax = expiration
What are the different regions of the respiratory centre and what are their functions?
Pneumotaxic centre: regulates the volume inspired with each breath
Caudal pons: Controls the facial muscles - important in talking and during exercising
Retrotrapezoid Nucleus: regulates the drive to breathe
Pre-Botzinger complex: Controls respiratory rate
What regulates ventilation?
Central and peripheral chemoreceptors
- actually driven by blood pH(related to CO2) and not by O2 - the CO2 response is much quicker than the O2 response
- normally an increased concentration of CO2 is the strongest stimulus to breathe more deeply and more frequently
Where are the central chemoreceptors located and what do they do?
Located in the ventrolateral medulla in the vicinity of exit of the 9th(glossopharyngeal) and 10th (vagal) nerves
- detect changes in pH in the CSF
- however a change in plasma pH alone will not stimulate them because H+ can’t cross the BBB, therefore they are only responsive to CO2
- the CO2 diffuses across the BBB and reacts with H20 forming carbonic acid which decreases pH
Where are the peripheral chemoreceptors located and what do they do?
Carotid and aortic bodies= sensory extensions of the PNS into blood vessels
- detect changes in CO2 and O2
- Afferent nerves carry signals back from the carotid (glossopharyngeal) and aortic (vagus) to the respiratory centre
What is the Hering-Breuer reflex?
More important in babies than in adults
- Pulmonary stretch receptors are present in smooth muscle throughout the lungs
- during large inspirations, they feedback to the pneumotaxic centre via the vagus nerve to prevent over inflation
- doesn’t seem to have a role in relaxed tidal volume in adults
What are the juxtacapillary (J)-receptors?
Sensory nerve endings within alveolar walls in juxtaposition to the pulmonary capillaries
- respond to pulmonary congestion (oedema, emboli, pneumonia)
- stimulation induces rapid shallow breathing and a cough
What are irritant (cough) receptors?
in the epithelium respiratory tract
- sensitive to both mechanical and chemical stimuli
- stimulation produces a cough which is necessary to remove the foreign material from the upper respiratory tract
What are the two different types of sleep and what occurs in both?
Non-REM sleep
- regular breathing
- minute ventilation decreases by 13% in stage 2 and 15% in stages 3/4
- contribution of the external intercostals increases
- 4 stages - go deeper into sleep
REM sleep
- breathing becomes less regular
- fluctuations in frequency and amplitude, central apnoeas - hyperventilation can occur
- contribution of the external intercostal muscles decreases
In respiratory disorders wha are the most important effects that happen to the airways?
Airway calibre= resistance
compliance
- these altered lung mechanics can lead to impairment of function which initially presents symptomatically as breathlessness
What are the four factors that need to be taken into account when predicating normal values for lung function ?
- size of an individual’s lungs is determined by HEIGHT and SIZE
- proportion of lung size to height also varies with RACR
- lung function gradually deteriorates as healthy adults AGE = lose elastic support as we age
- clinical interpretation involves comparison of obtained results to an individuals predicted normal range (based on 4 factors)
What are the two values use to represent normal values?
% of predicted values
Standard residuals ("z-scores")- including Lower limit of normal
Neither are ideal, they both have pros and cons, but the standard residuals more accurately reflect what is likely to be normal (based on height, size, age and race)
What are the 3 basic lung function tests?
1) spirometry
2) lung volumes
3) gas transfer
- collectively these tests provide a good overview of general lung function
What does spirometry assess?
Assesses ventilation
- measuring = VC, FVC, FEV1, PEF and FEV1/FVC ration
FEV1= forced expired volume in 1 second
FVC= forced vital capacity
PEF=peak expiratory flow (L/s)
What happens in obstructive ventilatory defects?
decrease in airway calibre leading to reduced airflow on exhalation and/or inhalation Example conditions: - COPD - Asthma - Bronchiectasis - Cystic fibrosis - upper airway obstruction
What happens in restrictive ventilatory defects?
impaired ability of the lungs to expand due to extra pulmonary, pleural or parenchymal abnormality Example conditions - fibrosis - sarcoidosis - systemic lupus erythematosus - muscle weakness - chest wall deformity
What happens to the flow-volume loop in obstructive pulmonary disease?
- the FEV1 is substantially reduced and the FVC is also reduced, this causes a reduced ration
- concavity = the more sever the obstruction the more severe the concavity
- PEF reduced
- inspiratory tends to be relatively normal
What are the FEV1 % predicted values/FEV1.FEV1 ratio of diagnosis for obstructive pulmonary disease?
% predicted values
- > 80%: mild
- 50-79%: moderate
- 30-49%: severe
- <30%: very severe
FEV1/FVC ratio
- <70% for all
What happens to the flow-volume loop in restrictive pulmonary disease?
PEF is reduced
FEV1 reduced
FVC is reduced proportional to FVC therefore ratio is normal or raise
there is a kink in the loop if the lungs are stiffer
What are some additional uses of spirometry?
Test physiological response to pharmacological agents
- bronchodilators - first line treatment
- antibiotics - check if patient is allergic to antibiotics
- Airway challenge agents - mannitol - inhibits release of histamine
- drugs - methotrexate / chemo
Pre-operative assessment Home monitoring (post-transplant)= monitor spirometry daily
What is spirometry useful in detecting?
a reversal (partial or total) of airflow obstruction by bronchodilators - can be a good differentiator of COPD and asthma
What are the different guidelines for reversibility criteria?
ARTP/BTS: 160ml FEV1 and/or 330ml FVC= based on absolute changes in volume therefore this the worse measures
ERS: >12% predicated FEV1 and 200ml = normally the one used in clinical practice
ATS: 12% and 200ml FEV1 or FVC
What are some of the technical errors in spirometry?
- slow start
- poor effort
- straining throat which causes a cough
- glottic closure- normally towards the end of expiration
- some patients simply can’t do spirometry- dementia, muscular weakness
What are the diagnostic values of spirometry?
- robust and simple assessment of ventilation
- relates well to pathology and mortality
- useful for monitoring
What are the limitations of spirometry?
- insensitive to early disease (esp. COPD)
- May not detect a clinically useful BD response
What are the different lung volume indices?
VC (L) FRC (L)- functional residual capacity ERV (L) - expiratory reserve volume RV (L) - residual volume TLC (L) - total lung capacity
What happens to lung volumes in obstructive respiratory disease (severe)?
VC- reduced FRC- increased ERV- reduced RV- increased (air trapping) TLC - increased (less so than FRC) due to increased compliance
What happens to lung volumes in restrictive respiratory disease (severe)?
VC- reduced FRC- reduced (less than TLC) ERV- reduced RV- redcued TLC - reduced
What is the diagnostic value of lung volume tests?
- provides information about the functional status of the lungs
- useful in assessing degree of air trapping
- useful for pre-transplant
What are the limitations of lung volume tests?
- not particularly informative
- plethysmography technically difficult - pant in and out against closed blockages
What are the gas transfer test indices?
TLco= transfer factor of the lungs for CO
- an estimate of overall efficiency of O2 uptake
- also dependent upon Hb
VAeff= effective alveolar volume
- the surface area over which gas transfer occurs
- > 85% of TLC in absence of airflow obstruction - healthy lung
Kco= transfer coefficient (TLco/VAeff)
- estimate of efficiency of O2 uptake at alveolar level
How do you interpret the gas transfer values?
Least well understood of all routine lung function tests
consider all parameters and interpret in conjunction with other tests and clinical information
understand non-pulmonary factors can affect gas transfer- cardiac shunt, anaemia, drugs
What happens to the TLco and Kco in obstructive COPD and obstructive asthma?
COPD
- TLco= reduced
- Kco= reduced
Asthma
- TLco= normal/increased
- Kco= normal/increased
What happens to the TLco and Kco in restrictive intrapulmonary and restrictive extrapulmonary?
Restrictive intrapulmonary
- TLco= reduced
- Kco= reduced
Restrictive extra pulmonary
- TLco= reduced
- Kco = normal
What is the diagnostic value of gas transfer tests?
- provides information about oxygen uptake
- clinically very useful in aiding diagnosis - can differentiate between ventilatory and interstitial impairment
- more sensitive to early disease than spirometry
What is the limitations of gas transfer tests?
- unable to perform with very small VC = patients simply don’t have the volume to do this if they have severe restrictive disease
- not entirely representative of gas exchange during tidal ventilation
- results do not correlate to arterial blood gases
