1.3.1 Mechanism of Ventilation 2 Flashcards
What is the formula for resistance?
R = Pressure difference/flow
What is the formula for total airway resistance?
What is the site of highest airway resistance?
Nose
Pulmonary capillaries make up how much of the total resistance?
less than 10% due to high cross sectional area
What are the two factors that control the cross-sectional area of the airways?
Neurohumoral factors affeting the tone of bronchial smooth muscle
Factors influencing the pressure difference across the airway wall
What are the affectors of bronchial smooth muscle tone?
What are each of the shaded areas?
What is the vital capacity?
the volume of air that can be exhaled in a maximal expiration after having inhaled maximally
What is the VC normally measured as?
The vital capacity is usually measured as the Forced Vital Capacity (FVC). In the FVC the patient is not only asked to empty the lungs as completely as possible, but also to do it as rapidly as possible.
What is important about the FEV1/FVC? What can it indirectly determine?
The FEV1 is the volume that can be exhaled during the first second of an FVC maneuver. The normal value for an FEV1/FVC is roughly 0.7 to 0.8. Deviations from this value can provide an indirect estimate of airway resisitance.
For a patient with increased airway resistance, how will this effect their value for FEV1?
They will have a lower value for FEV1 due to the increased airway resistance.
What is the reason for airflow decreasing with lung volume?
The reason that the airflow decreases with lung volume is that the airway resistance increases throughout expiration. This is due to narrowing of the airways as the lungs become smaller. This means that, if airway resistance is increased by disease, it is always more difficult to get air out of the lungs than get air into the lungs.
Is both of these graphs of differing effort levels, which point has the highest flow?
Explain what is also going on between the effort dependent and effort independent section.
At high lung volumes, expiratory flow depends on the expiratory effort, i.e. flow is highest in a, lower in b and lowest of all in c.
As expiration proceeds, and the lungs get smaller, expiratory flow becomes independent of effort. In the effort-independent part of expiration, at any given lung volume, the flow is the same no matter what the effort is.
The mechanism responsible for this is known as dynamic airway compression, and is due to the fact that the high PPL needed to generate a high expiratory airflow tends to narrow the airways and limit the flow.
What is the formula for flow?
Airway resistance is determined by?
Airway diameter - inverse proportion
The curve in the attached image is representative of a person giving a low effort expiration and a max effort expiration. How does a low effort vs max effort expiration affect airway resistance?
First, the red arch in the first image is due to higher flow because of a greater pressure in the Palv due to contraction of muscles. At first this will generate more flow. The greater ‘squeeze’ on the lungs will also narrow the airways thus increasing the resistance. The pressure inside the airway is lower than the pressure outside the airway, compressing the airway.
The same factors that tend to increase flow (higher PB-PALV), also tend to decrease flow (increased resistance).
What does curve B represent in the two graphs? What accounts for these changes?
Curve B represents moderately increased airway resistance.
As airway resistance increases, the slope of the FVC tends to flatten. Vital capacity itself does not change, but the individual is not able to exhale as rapidly, and FEV1/FVC decreases
In the flow volume curve, increased resistance is manifested by a lower expiratory flow for any given lung volume.
What does curve C represent in the two graphs?
Severe increase in resistance
Such as with severe asthma or COPD, expiration is even slower than in the preceding example, and FEV1/FVC is much less than 75%.
The intense expiratory effort, coupled with the elevated resistance, leads to airway closing at a very high lung volume; air is trapped behind the airway occlusion and residual volume increases and vital capacity decreases.
Increase in RV accounts for the barrel chest associated with COPD
What are two mechanisms of increased airway resistance?
The graph above represents a forced vital capacity of a healthy 25 year-old man. After a dinner of clams in a fine New Orleans eatery, he developed an anaphylactic reaction with intense bronchoconstriction. What changes would you expect to see in the volumes and capacities listed below?
a. Increase; b. Decrease; c. No change
Select a, b, or c to answer each of the following:
A. Volume 1-2
B. Volume 2-3
C. Volume 3-4
D. Volume 1-4
E. Volume 4-8
F. Slope of line 5-7
A-D : Answer is B
E : A
F : B
7A: Volume 1-2 (the inspiratory reserve volume will decrease (Answer: B)
7B: Volume 2-3 is the tidal volume, and this will also tend to decrease (Answer: B)
7C: Volume 3-4 is the expiratory reserve volume, and this will also decrease (Answer: B)
7D: Volume 1-4 is the vital capacity, which will decrease since all its components have decreased (Answer B)
7E. Volume 4-8 is the volume and it will increase: as expiration is limited, the amount of air remaining at the end of the forced expiation is greater than normal. (Answer: A)
7F: Slope of line 5-7 will decrease, (more shallow, lower FEV1/FVC) because the increase resistance does not allow air to flow out as rapidly as normal (Answer: B)
Increased airway resistance would not be produced. The stiff lung would help keep the airways open.
The graphs below represent the static pressure volume curves of the right and left lungs of a patient. How would this difference influence ventilation in each lung?
Compliance is lower in the left lung. This means that for a given increase in PTP, the increase in volume will be lower in the left lung, consequently ventilation will be lower in the left than in the right lung.
The graphs below represent the airflow-pressure curves of the right and left lungs of a patient. How would this difference influence ventilation in each lung?
Airway resistance is higher in the right lung. When the person inspires and reduces PALV, the associated flow will be lower than that in the left lung; i.e. the volume of air entering the lungs per unit time will decrease; this will tend to reduce ventilation.
Add the answer e. If surfactant is added, surface tension will decrease proportionately more in A than in B.
Both answer C and E are correct
C - This is correct, surface tension is a characteristic of the liquid; when there is no surfactant the surface tension would be the same everywhere. This is what creates the problem, since, according to Laplace’s law: PALV= 2ST/r. If ST is the same, the value of PALV to maintain the alveoli inflated is higher in the smaller alveolus; since both alveoli are connected the small alveolus collapses and inflates the larger one. If surfactant were present, ST would be lower in the small alveolus and both could remain inflated at the same PALV.
