Lecture 29 - Resistance Flashcards
Condition for there to be flow through a tube?
Must be a pressure difference on either side
What does the pressure gradient through a tube determine? Explain.
Rate and pattern of flow
Stream lines of gas flow through tube at low flow? What flow does this correspond to?
Parallel => laminar flow
Stream lines of gas flow through tube at high flow? What flow does this correspond to?
Disorganized and disturbed => turbulent flow
Poiseuille’s Law? Under what conditions?
R = 8.η.L/π.r^4
η = viscosity L = length
Condition: laminar flow
If the radius of an airway is halved, how will the resistance be affected?
Resistance increased 16 fold
Implication of Poiseuille’s Law in terms of blood transfusions?
To get blood in as fast as possible, use a short wide tube placed into a vein (e.g. dialysis lines, not central lines)
What determines whether flow will be laminar or turbulent?
Reynold’s number, Re
Re = 2.r.v.ρ/η
v = velocity ρ = density η = viscosity
Re > 2000 => probable turbulent flow
Re < 2000 => probable laminar flow
Is helium gas more likely to produce turbulent or laminar flow? Why?
Laminar because it has low density (less dense than N2)
How can we clinically reduce turbulent flow in the airways?
Replace N2 with helium gas in the mixture with O2
Why does inhaling helium gas make your pitch high?
Because less turbulent flow at the vocal cords
Equation for airway resistance?
R = Palv - Pmouth/flow rate
Palv at beginning of inhalation?
0 cm H2O
Palv during inhalation?
Decreases (negative) and then increases back to 0 cm H2O
Palv during at beginning of expiration?
0 cm H2O
Palv during expiration?
Increases (positive) and then decreases back to 0 cm H2O
Ppleura at beginning of inhalation?
-5 cm H2O
Is inspiratory flow positive or negative?
Negative
Is expiratory flow positive or negative?
Positive
Ppleura during inspiration?
Decreases as elastic recoil increases + further drop due to Raw to a value more negative than at resting state
Describe the cycles of volume, flow, intrapleural pressure, and alveolar pressure in regards to one another?
- Flow and alveolar pressure are on same cycle
- Volume and intrapleural pressure are on same cycle
- Both are 1/2 a cycle apart
Where does most of the airway resistance occur? What does this mean?
In the 4th to 7th divisions of the airways => small airways disease is difficult to detect
What airways diseases cause increase airway resistance? What do we call these?
- Asthma
- COPD: chronic bronchitis + emphysema
=> airway obstructive diseases
Explain asthma.
Increase in inflammatory cells around airways causing them to narrow
Explain chronic bronchitis.
Upper and lower airways are inflamed and produce a lot of mucus and pus, which impede on the respiratory cilia and narrow the airways
Explain emphysema due to cigarette smoking.
Cigarette smoking deactivates an enzyme in lungs (anti-protease) that is responsible for keeping in check a protease that is responsible for getting to infections to allow WBCs to access them => protease attacks the elastic skeleton of the lung (parenchyma) => destruction of the alveolar walls + enlarged air sacs due to destruction of alveolar walls + damaged pulmonary capillaries by tearing, fibrosis, or thrombosis => compliance increases but there is no elasticity to keep airway open with expiration => airways close and have high resistance
What are bronchi supported by?
Supported by radial traction of surrounding tissue
How do bronchi change as the lungs expand?
Their caliber/diameter increases as lung expands
Factors determining airway resistance?
- Different volumes cause airways to close/open affecting the resistance
- ANS innervation of smooth muscle
- Inflammation of bronchial walls increases Raw
- Density and viscosity of gas
Describe the parasympathetic innervation of the airways.
Parasympathetic activity causes bronchoconstriction
Describe the sympathetic innervation of the airways.
Sympathetic activity causes bronchodilation
3 treatments for airway obstructive diseases?
- Beta-agonists to cause bronchodilation
- Steroids to decrease inflammation of bronchial walls
- Anti-cholinergic agents to inhibit the parasympathetic activity
Describe the filling of two alveoli: 1 normal and 2 partially obstructed. What does this mean overall?
- Alveolus 2 takes longer to fill with air during inspiration because air first fills up alveolus 1 (path of least resistance)
- When we pause to hold our breath, both will be filled with the same amount of air (air from 1 will move to 2)
- Alveolus 2 takes longer to deflate during expiration because air first leaves alveolus 1 (path of least resistance)
=> Overall, alveolus 2 has a longer time constant to ventilate
What happens to patients with airway obstructive diseases when they hyperventilate?
If they breathe faster then the time constant of obstructed alveoli then these will not be fully ventilated and will have a V/Q imbalance (<1)
Why are patients with airway obstructive diseases hypoxemic?
V/Q imbalance of obstructed alveoli
Peak flow during expiration: effort dependent or independent?
Dependent
Flow during expiration following peak flow: effort dependent or independent? What does this mean? Why?
Independent: after peak flow flow decreases back to 0 and the decelerating slope of this part of the curve is constant regardless of what the peak flow was
BECAUSE of the choke point, which limits flow
Other name for choke point?
Equal pressure point
What is the choke point worsened by? What does this mean?
Worsened by:
- Any increase in airway resistance => decrease in endobronchial pressure
- Reduced driving force (e.g. emphysema)
Palv-Ppleu at the end of inspiration?
+ 8 cm H2O
Describe the pressures in the alveoli and the pleural cavity during a force expiration and how it explains the choke point.
High positive pressure created by the recoil of the lung and the chest muscles so the Palv>Ppleura, but from the alveoli to the outside of the mouth the pressure decreases and there comes a point where positive Pairway this is a choke point that limits expiration
Describe the flow-volume curve of respiration and the important points on it to note.
- X-axis: volume
- Y-axis: flow
- Inspiration: bottom half and moving to the left
- Expiration: upper half and moving to the right
- Point on dotted line to the right of the curve: RV
- Point on dotted line to the left of the curve: TLC
- Between 2 points: vital capacity
How is the flow-volume curve affected by a restrictive lung disease?
Lungs are less compliant and cannot get to the full TLC so the curve will be more narrow although peak flows are normal
How is the flow-volume curve affected by an obstructive lung disease?
Peak flows are lower and the expiratory decelerating portion of the curve is convex due to the airways being narrowed
How do the extrathoracic airways react to inspiration and expiration?
- Inspiration: negative pressure is experienced so they tend to collapse
- Expiration: positive pressure is experienced so they extend
How is the flow-volume curve affected by an obstruction in the trachea outside of the thorax? What is this called? Classic example?
- Expiratory loop is normal
- Inspiratory loop is flattened as the peak flow is lower and constant for a certain period of time = obstruction of inspiratory flow
= VARIABLE EXTRATHORACIC OBSTRUCTION
Classic example: paralyzed vocal cords that collapse during inspiration OR intubated patient with tracheal tube causing inflammation and narrowing of airway
What can you assume if there is an abnormality on only one phase (inspiration/expiration) of the flow volume curve?
Usually due to obstruction of main upper airway
How is the flow-volume curve affected by an obstruction in the trachea inside of the thorax? What is this called?
- Inspiratory loop is normal
- Expiratory loop is flattened as the peak flow is lower and constant for a certain period of time = obstruction of expiratory flow
= VARIABLE INTRATHORACIC OBSTRUCTION
How is the flow-volume curve affected by a FIXED obstruction in the trachea? What is this called?
- Inspiratory loop is flattened as the peak flow is lower and constant for a certain period of time = obstruction of inspiratory flow
- Expiratory loop is flattened as the peak flow is lower and constant for a certain period of time = obstruction of expiratory flow
= FIXED INTRA or EXTRATHORACIC OBSTRUCTION (cannot distinguish between the 2 with the curve)
When you place an asthmatic patient who is having an asthma attack on a mechanical ventilator, how are the peak and plateau pressures affected? What about the difference between the 2?
- High peak pressure because asthma causes increased airway resistance and therefore peak pressure will be higher
- *Normal plateau pressure because asthma does not cause a problem with lung compliance
- Difference between peak and plateau pressures should be larger due to the high peak pressure
* One could also argue that the plateau pressure is increased because: increased airway resistance => increased FRC => movement to the right to a less compliant portion on the compliance curve => less compliance = higher pressure => higher plateau pressure
Are the alveolar membranes affected in chronic bronchitis?
NOPE
Is the damage to the alveolar walls due to emphysema reversible?
NOPE
Are expiratory peak flow and inspiratory peak flow equal in normal individuals?
It all depends on the force applied to inhale or exhale:
- If you use your muscles to rapidly inhale a breath you will generate higher flow rates than if you passively exhale
- However if you are using maximal effort to inhale and maximal efforts to exhale, you can generate much higher flow rates on exhalation than inhalation
Why is only the deceleration curve of expiration affected in obstructive lung diseases? Aka why is the inspiratory curve shape normal (other than the low peak flow)?
In obstructive lung diseases the obstruction occurs in the intra thoracic airways:
- On exhalation these airways narrow thus bringing out the obstruction
- On inhalation the intrathoracic airways are pulled open and increase in diameter
