Pulm Mechanics Flashcards
Which muscles do you use for inspiration and expiration?
Inspiration: diaphragm, external intercostal, accessory muscles (scalenes, SCM)
Expiration: passive during quiet breathing, active during exercise/stress (abdominal muscles, internal intercostals)
What are the pressures during the breathing cycle?
End of normal breath:
Patm=0, P alveolus = 0, P pleural = -5 cm
Transpulmonary pressure = 0-(-5)=+5 (keeps lungs open)
When Patm=Palveolar, there is no air flow
Respiratory system at mid-inspiration:
Patm= -0, P alveolus = -1, P pleural = -6.5
Transpulmonary pressure = -1-(-5.5)=+5.5
End-inspration:
Patm=0, P alveolus = 0, P pleural = -8
Transpulmonary pressure = 0-(-8)= +8 (keeps lung open)
Respiratory system at mid-expiration:
Patm=0, P alveolus = +1, P pleural = -6.5
Transpulmonary pressure = 1-(-6.5) = +7.5
The positive transpulmonary pressures kee the alveoli open throughout the whole cycle; the negative pressure during mid-inspiration is what gets filled with air
What is transpulmonary pressure?
Alveolar pressure - Pleural pressure
Why is pleural pressure negative at rest?
You can confirm it because in a pneumothorax, the air pushes the visceral pleura inward (shrinking the lung) to counteract the -5 partial pressure that is normal in pleural space
At rest, the lung wants to contract and the chest wall wants to expand. Having the pleural pressure -5 “pulls” the lung keeping it open
What are the 3 wais you can get airway resistance/obstruction?
Intraluminal: secretions
Intramural: edema of the walls
Extraluminal: i.e. tumor or loss of radial traction of alveoli pulling on airway walls which happens due to reduced lung volume at end of expiration or during increased lung compliance during emphysema
What is spirometry?
Measure volume/time during forced expiration
What is FEV1? FVC?
FEV1 = amount of gas that comes out after 1 second (first second)
FVC = forced vital capacity, the volume of gas that you exhale during a forced expiratory maneuver after maximal inhalation
What is normal FEV1/FVC?
Normal FEV1/FVC = 80%
If it’s less than 70%, you have airflow obstruction
What are the major underlying causes of airflow obstruction?
Asthma
COPD
Bronchiectasis (CF)
Focal airway obstruction= tumor, foreign body, stenosis
Small airway disease
What is a flow volume loop?
Shows you volume (L) on x axis and flow (rate: L/s) on the y axis
Why is the bottom half of the loop (for inspiration) symmetrical but the top half portion that represents the loop for expiration is not?
During expiration, you exhale most of the volume very quicly & reach the peak fast
Then your flow rate goes down steadily which accounts for the fact that it’s not a mirror image of the first half- becomes a less steep slope
It’s a sawtooth pattern bc you lose traction that holds the lungs & alveoli open; this is normal
What does the lower airway obstruction curve look like?
In mild disease, the expiration portion scoops out becoming concave reflecting that at a given volume, flow rates are lower. Also FEV1/FVC becomes smaller
In severe disease, flow rates are tiny and volumes are tiny = gas trapping, low FVC
Upper airway obstruction loop look like?
Flattened curve
What is the equation for compliance in the lung?
Compliance = change in volume/change in pressure
Compliance = Inverse of elastance
What are the 2 determinants of lung compliance?
Elastic properties of lung parenchyma (tissue)
Surface tension in alveoli (note that this causes hystersis and that surfactant decreases surface tension thus increasing compliance)
What accounts for the elastic properties of lung parenchyma?
Elastic fibers
Fibril forming collagens
Geometric arrangement: “nylon stocking” - nylon stocking is easy to stretch, nylon threads are difficult to stretch
What decreases lung compliance? Examples of diseases that increase it?
Decreased: lung fibrosis, lung inflammation, pulmonary edema
Increased: emphysema= loss of alveolar walls
Why does the pressure/volume loop for deflation and inflation look different?
Hysteresis: at a given transpulmonary pressure, the lung volume is greater during expiration than inspiration
This is because the surface tension of alveolar lining fluid has to be overcome before the alveolus can be inflated - you have to recreate the air/liquid interface
What is surfactant?
Allows alveoli to remain open by lowering surface tension
Makes it easier to open up the lungs
Produced by type II pneumocytes: protein rich lipid layer with DPPC and surfactant proteins ABC&D
Inspiratory reserve volume
Tidal volume
Expiratory reserve volume
Residual volume
Inspiratory capacity
Vital capacity
Total lung capacity
Functional residual capacity
Inspiratory capacity = inspiratory reserve volume + tidal volume
Functional residual capacity = expiratory reserve volume + residual volume
Vital capacity = Inspiratory capacity + expiratory reserve volume
FRC
Functional Residual capacity
Volume of gas in lungs when resp muscles are not acting on the lungs
Determined by lung and chest wall compliance only
Once you know FRC, you can calculate all the other volumes
What are 3 methods to measure FRC?
Wash in: Helium method
Wash out: breath in pure O2, wash N2 out
Body box = body plethysmography
What is restrictive ventilatory defect?
Reduction in total lung capacity
Measure FRC, then calculate TLC
What are causes of restrictive ventilatory defect?
Decreased total lung capacity can be caused by…
Decreased lung compliance (abnormal lung parenchyma)
Decreased chest wall compliance (pleural or chest wall disease)
Lung removal/destruction/collapse
Reduced resp muscle force generation
What is a common pattern in spirometry of restrictive ventilatory defect?
Reduced FVC, reduced FEV1, but NORMAL FEV1/FVC ratio
This pattern does not exclude other possible causes
What can cause normal FEV1/FVC ratio but reduced FVC and reduced FEV1?
Restrictive ventilatory defect (check TLC)
Gas trapping from airway disease
Poor effort/muscle weakness
