Flow Limitation Flashcards
A common adaptation to flow limitation is for an individual to . . .
. . . breathe at a higher end-expiratory lung volume and increase work of breathing. This allows the same ‘resting’ tidal volume at an increased baseline (above FRC)
Total resistance
combination of airway resistance and tissue resistance or viscous impedance - i.e., the lung resists changing volume; there are frictional forces within the tissue as it expands
In the normal lung, ___ is the major component of lung resistance
In the normal lung, airway resistance is the major component of lung resistance
Transmural pressure
PTM = Pin - Pout
A negative PTM indicates infinitely compressible tube collapse
A positive PTM indicates an infinitely compressible tube expansion
Transmural pressure changes in forced exhalation
During a forced exhalation, we will consider Ppleura to be constant throughout the exhalation. Therefore, changes in PTM of the airway are due to changes in pressure inside the airway.
Ultimately, as the pressure drops in airways, there is a possibility that the transmural pressure will become negative, and there will be a tendency for the airway to collapse.
Reasons that pressure decrease as air moves through tubes
- Friction
- Bernoulli’s principle
- Changes in flow pattern (laminar vs tubrulent)
Lung volume loop
Maximal inspiratory flow occurs___. Maximal expiratory flow occurs ___.
Maximal inspiratory flow occurs about one-third to half way between RV and TLC. Maximal expiratory flow occurs very close to TLC
From the point of maximal expiratory flow until the end of expiration, the decline in flow, per change in volume of the lung, is ___.
From the point of maximal expiratory flow until the end of expiration, the decline in flow, per change in volume of the lung, is relatively linear.
Factors that affect resistance in the airways
- Increases in velocity past a certain point (more turbulence)
- Increase in resistance with narrower airways
Equal pressure point hypothesis
During maximal forced exhalation there is a point along the airways where the pressure inside the airway is just equal to the pressure outside the airway. At that point, the transmural pressure gradient is zero.
Downstream of this point, towards the mouth, PTM < 0, and airways would collapse if they were infinitely flexible. But then, pressure would be re-established. This results in an oscillating airway.
Once equal pressure point (EPP) conditions have been established, exhaling more forcefully does not result in greater expiratory flows, rather, the driving pressure is Pel, recoil pressure of the lung
Critical transmural pressure
Some negative transmural pressure that accounts for the low compliance of airway tubes and re-establishes the conditions of the equal pressure point hypothesis for thick-walled, resilient airways.
The driving pressure relationship at the beginning of exhalation and under the equal pressure point hypothesis
Beginning of exhalation: Driving pressure is Palv - Pmouth , so increaseing Palv will result in increased flow
EPPH: Driving pressure is Palv-Ppleura , and under these conditions, exhaling harder increases Palv and Ppl the same amount. But since Palv = Ppleura + Pelastic , elastic recoil becomes the true driving force.
As one continues to exhale, lung volume decreases. What effect does this have on the EPP?
As lung volume decreases, the elastic recoil pressure of the alveolus decreases. This reduces the difference between alveolar and pleural pressures; hence, one reaches equal pressure point “sooner” during the exhalation, i.e., closer to the alveolus
Flow-volume loop showing different efforts of exhalation
Flow volume loop for an emphysema patient
