Pulmonary Mechanics Flashcards
Describe the movement of the diaphragm in ventilation
- Flat sheet of skeletal muscle fixed at edge of thoracic wall
- Separates abdomen from thorax
- At rest- concave
- Phrenic nerve somatic innervation
- Fibres shorten and flatten, increasing volume of thoracic cavity
Describe the movement of the thoracic wall
- Various skeletal muscles move ribs
- Outwards for inspiration
- Inwards for expiration
Which muscles contract during inspiration?
- Diaphragm
- External intercostal muscle
- Shoulder girdle muscles
Which muscles contract during expiration?
- Internal intercostal muscles
- Abdominal wall muscles
Why does inspiration need muscle activity?
- Requires muscle activity to overcome elastic tissue of lungs
What occurs in airways during inspiration?
- Breathing in draws airways open
- As lung volume increases, airways are pulled by inflation
- From middle outwards- radial traction
Why can expiration be passive down to FRC?
Due to natural elastic recoil of thoracic and lung tissue
In what circumstances are expiratory muscles needed?
- To go below FRC
- To achieve high pressures
- To achieve high flow rates (flow limitation due to dynamic collapse of airways)
- Forced expiration
- Smoking
How are expiratory muscles used in forced expiration?
- Increased pressure flattens small airways, flow limitation
- Airways are connected together- interdependence which prevents collapse
Why are expiratory muscles used more in smokers?
- Smoking causes neutrophils flooding which may lead to emphysema
- This means more muscle activity is required for adequate ventilation
How does elastic recoil occur?
- Elastic properties of tissue
- Also by surface tension of air-fluid interface in alveoli
What are the functions of elastic fibres?
- Elastic recoil
- Keeping small airways without cartilage open
What secretes pulmonary surfactant?
- Alveolar type 2 cellS
What is pulmonary surfactant?
- Covers the surfaces of alveoli
- Helps expiration
What two pressures contribute to alveolar pressure?
Alveolar pressure is the sum of
- Pressure acting on outside alveolus
- Pressure generated by elastic recoil of alveolus
What is pleural pressure determined by?
Muscular effort
How can pleural pressure me measured?
By passing a balloon into the oesophagus
How can stretch ability of the lungs be expressed as?
Compliance
- This can be altered in some disease
What is the relationship between elastic recoil and lung volume?
Elastic recoil pressure will increase as lung volume increases
Why does an increase in peak flow with lung volume occur?
- Elastic recoil increases with volume
- Airways have a lower resistance at larger lung volumes due to increased outward radial traction as lung expands
Why does flow limitation occurs?
- Resistance to flow determines air flow
- This is affected by the diameter of small airways
Describes how forced expiration might cause airway collapse
- Higher pressure in alveoli
- Less in airways further down
- Once pressure in airways is less than pressure in pleural cavity, airway flattens and pressure inside airway becomes less than outside
- Pressure continuously builds up in alveoli
- Leads to airway flutter
What does airway flutter lead to?
Airway collapse
How might a smoker overcome airway flutter?
- Overcome by pursing lips (pink puffer)
- Reduces pressure inside the mouth
- Trying to move ‘critical closing point’ of airway to part that is held open by cartilage
- Normal ventilation is maintained
What is a characteristic of all obstructive orders?
- Low maximal expiratory flow rates
What diseases might affect inspiration?
Abnormally stiff lungs - Fibrosis and pulmonary oedema Respiratory apparatus damaged - Chest wall trauma - Neuromuscular disease
Where does airway resistance come from?
- Mainly from upper airways
- In smaller airways, ir=t is decreased at high lung volumes
What might increase airway resistance?
- Contraction of smooth muscle (bronchoconstriction)
- By swelling of the airway wall
- By excretions/exudations
Where is lung compliance decreased?
- Restrictive conditions such as asbestosis or fibrosis
- When lung vascular pressure is raised like in left heart failure
When is lung compliance increased?
- In emphysema
- Partly as a result of elastin
- Possibly due to loss of surface area, and therefore some of surface tension
Describe emphysema
- ‘Floppy’ lungs- due to loss of elastic tissue
- Very compliant as inflation occurs to dramatically high volumes
- Difficult to breathe out
Describe left heart failure
- Lungs engorge with blood and oedema fluid in interstitial
- Become stiff and non-compliant
- When people lie supine
Describe the composition of surfactant
- 90% lipid
- 70-80% Phosphatidylcholine
- Lipids are stored intracellularly as lamellar bodies
What are the mechanical functions of surfactant?
- Lowers surface tension to 0 at FEC
- Less effect as it is stretched at larger lung volumes
- Enables small alveoli to stabilise when communicating with larger ones
- Reduces oedema formation
What are the consequences of having a lack of surfactant?
- RDS
- In new-borns, common in premature babies as foetal surfactant develops late in gestation
Which mechanoreceptors control ventilation?
- Limbs- feet-forward stimulation of ventilation in exercise
- Chest wall
- Airways
- Pulmonary vascular system
Where are mechanoreceptors found in the chest wall?
- Costo-vertebral joints
- Respiratory muscle spindle
(NOT in diaphragm) - Golgi tendon organs
What do central chemoreceptors do in ventilation control?
- Detects changes in CSF pH and provide most of normal CO₂ response
- Raised arterial PCO₂ can lead to desensitisation to mechanism of chemical control of ventilation
- Patient shifts into hypoxic drive
What happens when patient shifts into hypoxic drive?
- Drive to breathe comes from low arterial PO₂
- If supplementary oxygen given, arterial PO₂ will improve but ventilation will be suppressed and arterial PCO₂ may rise dangerously
Where are peripheral chemoreceptors found?
- Aortic and carotid bodies
- Not sinus
What is the role of peripheral chemoreceptors in ventilation control?
- Normally contribute little to respiratory chemosensory drive
- Vital when central CO₂ sensitivity lost
- Responds very rapidly, but have a very non-linear response curve under normocapnic conditions
- In hypercapnia, becomes more sensitive and more linear, therefore more vital
Describe restrictive lung diseases
- Usually involves decreased lung compliance (i.e. ‘small lungs’
- Restricting inflation but increasing expiratory drive
- May also involve impaired gas diffusion (fibrosis)
Describe obstructive lung diseases
- Impaired airflow, especially in expiration
- May involve airway narrowing and/or obstruction
- Or loss of elastic recoil
Which type of lung disease is more common? (obstructive or restrictive)
Obstructive
Describe what is meant by ‘shortness of breath’
- Clinically defined as ‘an awareness of breathing’
- May result from increased ventilation with exercise/disease
- Often arises from disparity between effort put into breathing and result in terms of chest movement or lung inflation
- May arise from stimulation of lung
Why might restrictive lung disorders occur?
- Exposure to chronic allergens or irritants
- Often results in interstitial lung disease with infiltration and scarring of lung (fibrosis)
How might restrictive lung disorders affect ventilation?
- May cause diffusion impairment, especially on exercise
- Breathlessness with exercise
What is asthma?
Obstructive respiratory disease characterised by airway resistance due to inflammation and broncho-constriction
What is asthma presentation like?
Episodic
Describe airway resistance in asthma
- Less in inspiration than expiration
- Ratchet-like effect on lung volumes which may increase greatly during an attack
What occurs in the lungs in emphysema?
- Enlargement of air spaces distal to terminal bronchiole
- Excessive protease activity- smoking-induced neutrophil
- Loss of alveolar walls and related capillary bed
- Loss of elastic recoil- reduced SA- loss of surface tension component
- Loss of radial traction- increased airway collapse on expiration- air trapping
- Chronic over-inflation- increased TLC, RV, FRC
- Decreased VC, FEV1 and ratio
What is an observable characteristic of emphysema?
Dyspnoea/breathlessness
What is bronchitis?
- Chronic over-production of mucus in bronchial tree
- With periods of infection and excessive expectoration
- On most days for at least 3 months in 2 successive years constitutes as chronic bronchitis
- Usually co-exists with emphysema in COPD
What happens to mucus glands in bronchitis
- Hypertrophy
- Chronic inflammatory changes in larger airway walls
- Inflammatory cell infiltration
- Oedema of walls
What happens to lung volumes in chronic bronchitis?
- Reduced FEV1, FCV, FEV1:FVC ratio
What is a significant characteristic of bronchitis?
Productive cough
Describe COPD
- Chronic Obstructive Pulmonary Disease
- Smokers
- Features of bronchitis and emphysema in varying proportion
What are the two broad presentations of COPD
Blue bloater and pink puffer (first will often progress into the second)
Describe what is meant by ‘pink puffer’
- Better oxygenation
- Normal PCO₂
- More dyspnoea
- More emphysematous spirometric changes
Describe what is meant by ‘blue bloater’
- Hypoxemic
- Cyanotic
- Hypercapnic
- Sluggish and drowsy
- Right heart failure
Describe the Dynamic Respiratory Test
- Breathe out as hard and fast as they can
- Volume of gas breathed out- forced vital capacity
- How much breathed out in one second (FEV1)- anything above 70% is normal
What are the results of a test for someone with an obstructive disease?
- Cannot breathe in as much (vital capacity is low)
- Slow expiration- low FEV1
What are the results of a test for someone with an restrictive disease?
- Lungs behave normally
- Small amounts for FEV or FVC
What is forced expiratory time as a lung assessment?
- Time taken for expiration
- Severe obstructive diseases lead to long FET
