Lung mechanics Flashcards
Draw and label a flow volume loop (respiratory flow envelope, vital capacity, ERV, IRV, tidal volume, expiration, inspiration)
diagram from notes
What is the effect of mild obstructive disease on the flow volume loop and give an example
Displaced to the left
Indented exhalation curve (coving)
COPD: lungs become larger to move the curve up, gradient is lower, smaller airways -> lower flow rates -> coving
What is the effect of severe obstructive disease on the flow volume loop and give an example
Shorter curve
Displaced to the left
Indented exhalation curve
decreased vital capacity
Coving
Peak expiratory flow rate decreases
What is the effect of restrictive disease on the flow volume loop
Displaced to the right
Narrower curve
Less access to air
Peak is lower or unchanged (filling problem)
What is the effect of variable intra thoracic obstruction on the flow volume loop
Blunted expiratory curve
Otherwise normal
What is the effect of fixed airway obstruction on the flow volume loop
Blunted inspiratory curve
Blunted expiratory curve
Otherwise normal
Describe obstructive lung diseases
Flow of air into and out of the lung is obstructed
Lungs operate at higher volumes -> higher residual volume
Lower IRV, TV and ERV
What are the causes of obstructive diseases
chronic: COPD, emphysema, bronchitis
Acute: asthma
Describe restrictive lung diseases
Inflation/deflation of the lung or chest wall is restricted
Lungs are operating at lower volumes (IRV, TV, ERV)
residual volume greatly decreased
What are the causes of restrictive lung diseases
Pulmonary: lung fibrosis, interstitial lung disease
Extrapulmonary: Obesity, neuromuscular disease
Describe the volume / transmural pressure graph
Sigmoid shape
Lots of effort required to move to either side of the curve
What is the effect of restrictive and obstructive disease on volume/ transmural pressure graph
Obstructive increases gradient and shifts the curve upwards
Restrictive decreases gradient and shifts the curve downwards
What is surface tension in the lungs
No opposing upwards force caused by air-fluid interfaces
Filling the lungs with fluid instead of air makes the lungs more compliant (less ST)
What causes surface tension in the lungs
Surfactant release (lipids) from type II pneumocystis on to the surface of the alveoli -> reduces surface tension Polar phospholipids, non-polar lipids, protein
What is the role of surfactant
Prevents the collapse of small alveoli
Increases compliance by reducing surface tension
Reduces the work of breathing
What is compliance
Tendency to distort under pressure
How is compliance calculated
Volume / pressure (gradient)
How is compliance affected by COPD
More compliant, less elastic
What is elastance
The tendency to recoil to its original volume
How is elastane calculated
Pressure / volume
How does the number of airways vary with airway generations
Exponential increase in cross-sectional area from generation 9 onwards
How does the resistance vary with airway generations
Increases then decreases after generation 4 (inversely proportional to the fourth power of the radius)
How does conductance vary with an increase in lung volume
Increases with lung volume
How does resistance vary with an increase in lung volume
Resistance decreases
Airways are not rigid and they dilate as lung volume increases
Describe airflow in a tube pre-inspiration
No airflow (therefore the volume of air = functional residual capacity) Pressure in the airways and lungs is 0 (no gradient) Recoil of the lung means intrapleural pressure is - 5
Describe airflow in a tube during inspiration
Increase in tension in the intrapleural space -> pressure = -9
Pressure gradient established
Overall airway transmural pressure = -2 - (-8) = +6
Positive -> airway is open and air flows in
Describe airflow in a tube at the end of inspiration
The intrapleural pressure is -8 due to strain
Describe airflow in a tube during hard expiration
Transmural pressure = +20 - 22 = -2
Negative
Airway becomes occluded if not supported by cartilage
