Pulmonary mechanics
From Iusmphysiology
- started here on 02/09/11 at 9AM.
Pulmonary mechanics
- FRC set by expansion and contraction
- trans pulmonary pressure determines degree of how fast air gets in and out.
- That is, flow is dependent on the pressure gradient.
- The pleural pressure is negative.
- If you get stabbed, the air flows down it's pressure gradient and fills that side of the thorax.
- So pressure in the lung on stabbed side will go to atmospheric pressure
- pneumothorax
- Then there will even be pressre on the mediastinum because of the increased pressure as pressure gets above the atmospheric pressure
- Interpleural pressure is very important for the lung function
Pressure during breathing cycle
- Alveolar pressure at fu
- Solid line is from normal tidal volume breath.
- Gives an estimate of the resistance to air flow and the resistance to tissue being expanded (elastic recoil).
- Dashed line is taken very slowly.
- Resistance to airflow goes away
- So this line represents just resistance to recoil.
Compliance
- Defined as a change in volume over a change in pressure.
- Normal is 200 ml / cm of H20.
Elastance
- The inverse of the compliance.
- Can hold a lot of volume at a low pressure.
Static vol-pressure relationship
- Volume on y, pressure on x; opposite of the heart!
- Residual volume is where you've blown out all you can blow out.
- Transpulmonary (pleural) pressure has to go up for lungs to get air.
- The more air you want to take in, the more this has to increase.
- So degree of inflation of the lungs is determined by transpulmonary pressure.
- Historesis is due to variable surface tension in the lung.
- It is easier for the lung to expel air than to inhale air.
- You need a higher transpulmonary pressure to get air in than to get it out.
Compliance masurement
- To measure, as one exhales, we measure the pressure; the slow of the curve is the compliance.
Pulmonary compliance
- What are the factors?
- Lung size
- Lung volume
- Elastic / fibrous tissues
- Alveolar surface tenstion
- What decreases?
- high lung volume
- fill a balloon and it gets harder to put air in
- fibrotic disease
- alveolar membranes become thickened and fibrous
- harder to inflate them
- alveolar edema
- surfactant reduced
- vascular congestion
- high lung volume
- what increases?
- COPD
- emphasyma: elastic tissue destroyed
- COPD
Compliance and pulmonary disease
- So more or less compliance changes the volume-pressure relationship.
- More compliant means more volume held at a lower pressure.
- Less compliant means less volume held at a higher pressure.
Compliance respiratory system
- Take the lungs out, look at chest wall and lungs, separately.
- Measure how their volume changes relative to pressure.
- Chest wall by itself is close to that of the lung by itself.
- But together, the slope is lower, so the compliance of the two together is lower than each alone.
- Why?
- FRC is from chest wall expansion and elastic recoil.
- As you inspire, these two factors work together to increase pressure (?) of the system.
Surface forces and lung recoil
- 300 million alveoli
- Each has a radius of 110 micrometers (at FRC)
- We want the air to transfer through watery surface to the blood.
- The size of the alveoli is determined by the translung pressure.
Surface tension (laplace's law)
- Note that the wall pressures of these two alveoli are the same because pressure is higher in the smaller alveoli.
- Air will flow from higher pressure (little alveolus) to higher and the little will collapse.
- Bad! this is why we need surfactant.
Pulmonary volume-pressure relationship hysteresis
- We use submersion in saline to reduce air-surface tension.
- With lower surface tension the lung is much more compliant (more volume at lower pressures).
- It is the variable surface tension during inspiration that is responsible for the hysteresis seen in the volume-pressure relationship.
- hysteresis: "he lagging of an effect behind its cause; especially the phenomenon in which the magnetic induction of a ferromagnetic material lags behind the changing magnetic field" per wordent
Surfactant
?
Surfactant
- The smaller the alveolus, the closer together are the surfactant molecules so the more effect they have on pushing outward.
- They resist compression.
- As they are spread out, elastic recoil increases.
Advantages of surfactant
- Wall tension in larger alveolus is lower than in the small alveolus.
- Surfactant, then, causes the pressures to equalize.
- Thus the alveoli do not collapse.
- Advantages of Surfactant:
- Stabilizes alveoli that tend to deflate at different rates.
- Lowers elastic recoil and thus helps prevent alveolar collapse.
- Reduces the work of breathing.
- Decreases muscular effort needed to expand the lungs.
- compliance
- work of breathing
- Plays a role in host defense.
Respiratory Distress Syndrome
- In this case, there is no surfactant.
- Perhapse Type 2 alveolar epithelial cells do not secrete surfactant.
- Means that greater pressures will have to be generated to get air in.
- This will shift the volume-pressure relationship to the right.
- Need higher pressures to get air into the lungs.
Infant respiratory distress syndrome
- Two surfactant treatments help baby off vent.
Dynamic Volume-Pressure Relationship
- The dotted curve is the max.
- Green is where healthy humans live.
- Red arrows with strenuous exercise.
Airway resistance
- We have to make negative pressures and there is resistance to the airflow caused by the negative pressure.
- Resistance factors:
- size of airways
- smooth muscle tone of airway
- density of gas you choose to inhale
- dynamic compression
- Airway resistance is highest at the medium size bronchii.
- Airway resistance can be calculated by ohm's law.
Airway size and patterns of airflow
- Laminar: in small peripheral flow
- Turbulant flow: high flow rates in trachea and larger aiways
- Resistance is higher
- Transitional flow – occurs in larger airways (branches).
Lung volume and airway resistance
- The resistance to airflow goes down as the tubes get bigger because the lung is filling with air.
Control of bronchial smooth muscle
- Controled by lots of things.
- Dilation
- Sympathetic stimualation
- Primariliy betas, especially beta2
- epinephrine
- NO
- Increased Pco2 in the small airways
- Decreased PO2 in the small airways
- These happen when pt isn't breathing well.
- Sympathetic stimualation
- Constriction:
- Parasymp
- contstricts
- causes mucus production
- Alpha adrena receptors
- Ache
- Noreepie
- Decreased Pco2
- Parasymp
- Parasympathetics > sympathetics
Dynamic airway compression
- -8 is the interpleural pressure.
- At the end of a normal inspiration.
- +10 is wall tension or recoil of the lung.
- +2 is the transpulmonary pressure
- 10-8 = 2
- Because recoil pressure is > interpleural pressure is the air goes out.
- As we expire forcefully, the interpleural pressure is made very positive to push air out.
- Inter pleural pressure is +28
- Note that pressures decrease from the alveoli to the outside.
- There is apoint where the pressure is equal.
- So the airway closes.
Why would the tube collapse when pressures are equal.
Dynamic airway compression
- In emnphysema...
- Was thinking too hard to type.
flow-volume relationship
- Small circle is normal tidal breathing.
- Big line is total inhale with fast, forceful exhale.
flow-volume relationship
- A is healthy
- Note that they all fall onto the "effor independent" portion of the curve.
- This is the location of the dynamic comnpression's effect.
Flow-volume relationship
- This is used clinically to diagnose COPD.
- The scoop on the right (red) shows that the pt is having a hard time expelling air.
- Emphysema:
- Recoil is high
- Total lung capacity is reduced.
Flow-volume relationship
- Guarenteed test quation on this.
Lung volume and pulmonary disease
- See cartoon.
Work of breathing
- read on own
Efficiency of breathing
- Read it.
- stopped here on 02/09/11 at 10AM.
