General Respiratory Mechanics I & II Questions Flashcards
What is IP Pressure?
Its’ function?
The pressure within the pleural cavity - should be negative. Created by the negative surface tension exerted by the pleural fluid. This pressure resists the separation of the pleura, thus holding the lungs and thoracic cage together (so that when cage expands, so do the lungs, and vice versa)
What is TP pressure?
Palv - Pip
Essentially, can be thought of as the ‘balanbe’ between opening and collapsing forces.
The greater the lung volume, the higher the TP - this is a determinant
Pressures ‘between breaths’ at end expiration
Palv = 0 (no flow)
Pip = -4 (Minor inward/outward forces, that are temporarily in equilibrium. Thoracic cage is at its’ resting position)
Ptp = 4 (Lower lung volume)
Pressures during mid-inspiration
Patm = -1 (Favouring inward flow)
Pip = -6 (Opening forces are outweighing collapsing forces - increased outward force accounts for more negative pressure)
Ptp = 5 (Moderate lung volume)
Pressures at end-inspiration
Palv = 0 (No flow)
Pip = -7 (Opening and collapsing forces are transiently in equilibrium as collapsing forces of lung become greater with higher lung volumes)
Ptp = 7 (Greater lung volume)
Pressures during mid-expiration
Palv = 1 (Favouring outwards flow)
Pip = -5 (Collapsing forces have outweighed the opening forces, inspiratory muscles have relaxed)
Ptp = 6 (Lung volume is again decreasing)
What is Boyle’s Law?
P1 V1 = P2 V2
The basis of breathing
What are the components compliance?
When is lung compliance generally greatest?
C = change in lung V / change in Ptp
Elasticity of the lung tissue (ability to stretch)
Surface Tension of alveoli (most powerful at high lung volumes) - resisted by surfactant at lower lung volumes
Elastic properties of the thoracic cage - its ability to expand
Usually greatest over the middle ranges of lung volumes: difficult to start it off at lower lung volumes, and at max volumes there isn’t much stretch left
2.3-2.8L
What effect would increased or decreased pulmonary compliance have on Ptp?
Increased compliance: high volumes will be achieved at lower Ptp (e.g. emphysema)
Decreased compliance: Higher Ptp are required to achieve small increases in lung V (e.g. restrictive lung disease)
What is ‘hysteresis’?
The difference between the pressure/volume curve during inhalation and exhalation.
It demonstrates that a higher pressure is required to maintain a given volume during inspiration, compared to during exhalation.
What contributes to hysteresis?
The Lung: Elastic recoil, stretchiness, collagen preventing over-stretch
Chest wall: Elastic properties, where it reaches its natural position (2/3 total lung volume) and collapsing forces
Surface tension: Powerful at higher lung volumes, resisted by surfactant at lower lung volumes
Describe the significance of the lung volumes over which normal quiet breathing occurs
Tidal volume during normal quiet respiration = ~500mls
Usually occurs at 2.3-2.8 L
This is where compliance is greatest - where WOB is the lowest, due to the specific balance of all factors contributing to the WOB, and all factors contributing to pulmonary compliance
Ventilation is affected by which major concepts?
Lung Compliance and Airway Resistance
contribute the most to WOB
What governs ‘flow’?
Airway resistance and Pressure gradient
Boyle’s law = P1 V1 = P2 V2
Flow = Pressure gradient / Airway Resistance
What is Airway Resistance?
How is it expressed as an equation?
When does it occur?
‘Drag’ as air flows through the airways
Poisuelle’s Law” R = (8 x L x n) / (pie)r(to the power of 4)
I.e.(8 x length of tube x gas viscosity) / pie x radius of tube, to the power of 4
Only occurs when there is airflow
What is Poisuelle’s Law?
Of Resistance
R = (8 x l x n) / pie x r, to the power of 4
Where is resistance greatest? Why?
Fist 4-7 divisions
Total cross sectional area increases with increasing divisions
Larger airways tend to be longer, which increases resistance
Flow rates (velocity) tend to be higher in larger airways, which increases resistance
What happens to velocity through the respiratory tree?
Greatest in the trachea
Velocity decreases with increasing cross-sectional area
What are the determinants of Airway Resistance? How does this relate to resistance throughout the breathing cycle?
Determinants include:
Lung Volume
Elastic Recoil - dynamic airway compression
Bronchial Smooth Muscle Tone
Higher Lung volumes = Lower Resistance. Mainly attributable to the greater distension of smaller airways, thus decreases in resistance. *Larger airways, with cartilage etc, are not as prone to large changes in distension with increasing lung volumes
Resistance is then greatest upon expiration, as the smaller airways tend to collapse
What is the Bernoulli Principle?
States that pressure will decrease as air moves towards the mouth, because the air is moving from an area of high pressure, to an area of relatively lower pressure, reaching 0 as it reaches the mouth
What is Dynamic Airway Compression?
Bernoulli Principle
Dynamic compression occurs at the point (of air exiting during expiration) where Ptp exceeds Palv.
Bernoulli principle states that Palv will gradually decrease from its initial value (+) to 0 as it reaches the mouth.
In normal individuals, during normal quiet breathing, the point at which Ptp and Palv equilibrate is usually in the larger, cartilagenous airways - thus, dynamic airway compression doesn’t occur.
Compression of smaller airways only occurs in health people during forced expiration.
In lung disease states, such as emphysema, the damage to the elastic fibres of the parenchyma etc weakens the tethering effect that normally holds the smaller airways open during expiration. Thus, dynamic airway compression can occur in the smaller airways during normal quiet breathing
Describe the relationship between certain lung diseases and airway calibre, and how this affects breathing
Emphysema: destruction of lung parenchyma causes loss of traction in smaller airways. Thus, they tend to collapse during expiration (dynamic airway compression). Thus, inhaling is easier, but exhaling is difficult.
Fibrosis: Thickening/hardening of airways can cause excessive radial traction, so small airways do not collapse during expiration. However, the fibrotic parenchyma does not expand easily. Thus, inhalation is difficult, but exhalation is easier
How can WOB be expressed?
WOB = change in volume / change in pressure
I.e. the WOB relates to the amount of energy required to create the specific pressure change to cause the desired/required change in volume
WOB components and percentages:
Exacerbating Conditions for each?
Elastic Resistance = 65%
Exacerbated by restrictive lung conditions
Airflow Resistance = 28%
Exacerbated by obstructive conditions
Friction between pleural surfaces = 7%
Exacerbated by diseases involving the pleura
How does ventilation pattern affect WOB?
How can ventilation pattern be expressed?
Ventilation = Vt x f
(volume of breath x frequency)
Deep Breathing = Increased WOB due to elastic recoil
Fast Breathing = Increased WOB due to airway resistance
What are the limits of ventilation due to WOB?
Limits to ventilation occur when the O2 required to support WOB cannot be provided by added ventilation
In normal individuals, this does not occur until VERY high levels of ventilation are reached
In emphysemic patients, this may occur at rest. Thus, maintaining O2 may require supplemental O2 so that they can continue to breathe
What is ‘total ventilation’?
Minute volume in L/min
= f x tidal volume
Dead space?
150-200ml normally. May increase slightly with large inspirations as bronchi pulled apart slightly be parenchyma
What is physiological dead space?
Anatomical dead space + Alveolar dead space.
IN normal people, anatomical and physiological dead space should be the same
Regional differences in ventilation, and regional differences in Pip*
…
Why is gas trapping more likely to occur at base of the lung?
Because Pip at base is LESS negative - thus tends to equilibrate with Palv more easily
