Respiratory mechanics II week 5 Flashcards
What is airflow? How is it calculated?
Airflow is the rate at which air travels through the airways.
V dot = Δ P/Rairway
ΔP is the pressure gradient btwn the nose/mouth and the alveoli. Just like blood flow, there must be a pressure gradient for airflow to occur and flow is from high to low pressure.
Remember that resistance (for laminar flow) is calculated using Poiseuille’s equation:
R= 8nl/πr4
The larger the resistance, the lesser the airflow. Radius is the largest determinant of resistance!
Describe how resistance changes as you go from segmental bronchi to terminal bronchioles.
Resistance increases with decreasing radius. The total resistance of larger airways is higher than that of smaller airways is a result of branching and increasing number of airways with higher airway generation. Smaller airways collectively have a much larger cross-sectional area.
Total cross-sectional area increases at each successive airway generation. The total area increases enormously in the respiratory zone —> less resistance —> larger airflow
What can cause a decrease in the radius of airways?
What can cause an increase in the radius of airways?
What are the two types of airflow? (just list)
laminar flow
turbulent flow
What is laminar flow? What is turbulent flow?
- Laminar flow
a. Streamlined flow in which the gas molecules move parallel to each other and the walls
b. Has high axial velocity
c. Is efficient - Turbulent flow
a. Chaotic movement with irregular changes in velocity and pressure
b. Cannot be prevented
c. Occurs preferentially in larger airways
d. Is less efficient than laminar flow
e. Is audible (breathing sounds you hear with a stethoscope)
In the attached graph, explain why the pressure/airflow relationship is different for laminar vs. turbulent flow.
With laminar flow, pressure and flow rate are proportional (as predicted with our v dot = delta P/R equation). With turbulent flow, increasing driving pressure does not increase flow rate by as much because this type of flow is inefficient. With turbulent flow, flow rate is proportional to the square root of driving pressure.
What equation is used to predict turbulence?
How does turbulence change as you go down the airway?
How does the likelihood of turbulence in air vs. blood compare?
Reynold’s number predicts turbulent flow:
Re = 2 r V (ρ/η) r = radius of the tube V = flow rate ρ = density η = viscosity
Air flow in trachea (largest airway) is always turbulent. Turbulence and breathing sounds in distal parts of the respiratory tree (small airways), however, are a clinical sign of pulmonary disease. As you go down in the respiratorty system and tubes get smaller, r decreases and the tendency for turbulence decreases (in individual tubes) as predicted by Reynold’s number. Also, as you go down the airway and cross sectional area increases, the flow rate through each tube is smaller (as predicted by v dot=delta P/R equation. Radius decreases so resistance increases resulting in smaller flow rate). These both contribute to the decrease in turbulent flow as you go down the airway.
Note: It makes intuitive sense that increasing flow rate would increase the likelihood of turbulence. If you push air slowly and carefully (flow rate) air flows nicely. If increase flow rate, increase likelihood of turbulence because the air doesn’t “know” where the walls are and molecules bounce off of one another more.
The ratio of ρ/η is 20x greater for air than for blood so there is much more turbulence in the airways than in the cardiovascular system. Air molecules move more independently than liquid molecules and bounce everywhere so you are more likely to have turbulence in air than in blood.
Explain the difference btwn the flow volume curves for expiration and inspiration.
Expiration:
At TLC airways are stretched to larger diameter. Resistance decreases. So get max flow rate is almost immediately after expiratory efforts (peak of expiration curve). At first have a large driving pressure-compressing thoracic cavity trying to push air out. But as push air out, pressure gradient dissipates. Airways also get narrower which increases resistance and limits air flow rate. The bronchioles are inside of the lungs and have no cartilage, so their size changes with lung volume. This is the effort-independent portion of the curve. Independent of effort, flow rate stays the same on this part of the curve. By compressing the thoracic cavity, the airways are compressed and therefore resistance is increased (see slide 14 of notes). If the subject tries to expel the air faster, this causes dynamic compression of the airways. However, the peak expiratory flow rate (PEFR) is effort dependent.
Inspiration:
The inspiratory side of the V-V curve looks different from the expiratory side. From RV, the subject can expand the chest wall rapidly and increase the pressure gradient, but the airways still have a small radius, so Raw is large. A high Raw makes it harder to inflate the lungs. As the lung volume increases, airway diameter increases and hence Raw decreases, making airflow easier, but now the pressure gradient is diminishing, so the flow rate is relatively constant and Peak Inspiratory Flow Rate is smaller than the PEFR (flat part of curve).
These curves are strongly effected by resistance (which is strongly effected by radius)
What is obstructive disease? What are 4 causes of obstructive disease? Name 3 types of this respiratory disease.
Obstructive disease: makes it hard for air to get out. Obstructive disease is characterized by **increased airway resistance. **
Types: chronic bronchitis, asthma, emphysema
Causes:
- Mechanical obstruction (foreign objects, tumors, mucus plug)
- Increased airway resistance (airway thickens from inflammation – chronic bronchitis)
- Increased airway resistance (bronchoconstriction -asthma)
- Increased airway closure (due to dynamic compression – emphysema)
What is the definition of emphysema? In emphysema, the lungs lose tissue and become large and floppy which results in what two things?
Emphysema: “a disease in which the air spaces distal to the terminal bronchioles are enlarged and their walls destroyed”
Lungs lose tissue and become big and floppy resulting in:
i. Lower elastic recoil
ii. Bronchioles are more likely to collapse
- air can’t get out –> hyperinflation and air trapping
- tissue destruction –> surface area for gas exchange decreases
For a pt with emphysema, what parameters change?
- Spirometry:
a. FEV1/FVC ratio decreases in emphysema
i. FEV1- forced expiratory volume in 1 second decreases
Explain the attached pressure volume curve for a pt with emphysema.
Compliance of lung is too high: dynamic compression of airways. Maximal flow rate is less and falls off quickly. The expiratory part of the airflow volume curve is grossly reduced (scooped out appearance) due to dynamic compression of the airwyas. Pts can expand their lungs to greater volumes, but are unable to expire the air. Inspriatory flow rates may be normal.
Note that expiratory flow rate is dismally weak in emphysema, and that at mid-expiration it overlaps the normal breathing loop. This means that expiratory flow is maxed out already during normal breathing – in other words, this person has no possibility of increasing expiratory flow at that volume.
What is restrictive disease? Give 3 examples. What is it characterized by?
What spirometry parameters change? What do the flow volume loops look like for pts with this type of respiratory disease?
In restrictive disease, pts have difficutly getting air in.
Restrictive disease is characterized by increased recoil. Examples are fibrosis (stiff lungs/interstitial lung disease), stiff chest wall, respiratory muscle weakness
Spirometry: FEV1/FVC ratio stays the same or increases. FEV1 and FVC both decrease. These pts have a less than normal TLC bc they cannot inflate their lungs as much as normal (restrictive diseases decrease most volumes for this reason)
Flow volume loops:
- lung volumes are low
- flow is also low. this is because the lung volume is smaller than normal. corrected for the volme the flow rate may be normal or larger than normal. the airways are normal and the greater elastic recoil may help expel air.
Describe the transmural pressure just prior to inspiration.
pg 95 of course notes
Describe the transmural pressure during inspiration.
pg 96 of course notes
Describe the transmural pressure at end inspiration.
pg 97 of course notes
Describe the transmural pressure during normal expiration.
Describe the transmural pressures during forced expiration.
Why does dynamic compression occur during forced expiration?
please see slides 20 and 21.
The key to predicting when dynamic compression will occur is to consider the equal pressure point where PTM =0, the point in the airways where Paw has fallen to Ppl. At the equal pressure point, there is no net pressure, and distal to this point (higher in the airways) there will be a dynamic compression of airways.
Dynamic compression limits the rate at which air can be expelled by forced expiration (decreases radius).
Note that dynamic compression only occurs in airways with no cartilage!
Using your understanding of dynamic compression, explain why expiratory flow rate is effort independent.
