Pulmonary Mechanics Flashcards
methods of measuring intrapleural pressure
placing a small catheter connected to a pressure measuring device in the intrapleural region
placing a balloon in the thoracic esophagus and measuring the intraesophageal pressure
typical intrapleural pressure at the (passive) functional residual capacity
-3 to -6 cm H2O
this pressure expands the lung and collapses the rib cage by a corresponding amount
transpulmonary pressure
the pressure acting to inflate the lungs
usually negative with respect to the alveolar pressure and thus acts to expand the lungs
PA - Pip
transthoracic pressure
the force acting on the thoracic wall
Pip - Pb
since pleural presssure is generally negative, it “sucks” the chest wall inward
transrespiratory pressure
the potential pressure graident for flow into or out of the alveoli
the difference between alveolar and atmospheric pressure measured with the glottis closed and with respiratory muscles relaxed
PA - PB
if the transrespiratory pressure is negative, gas will flow into the alveoli
if transrespiratory pressure is positive, gas will flow out of the alveoli
compliance
the change in volume with respect to change in pressure
elastance
the inverse of compliance in the lungs
causes of decreased lung compliance
respiratory distress syndrom ( decrease)
edema (decrease)
atelectasis (decrease)
fibrosis (decrease)
causes of increased compliance
age (increase)
emphysema (increase)
increasing body size (increases)
Law of Laplace
P = 2T/R
P is the pressure within a spherical object
T is the tension int he wall of the object
R is its radius of curvature
a smaller bubble would have a greater internal pressure than the larger bubble
constant varies with the geometry of the object, for a sphere it would be 4 instead of 2
implications of Law of Laplace
the smaller the radius of curvature, the stronger the inward force resulting from surface tension
bubbles (alveoli) with smaller radii have larger internal pressures
small alveoli will have a tendency to collapse (instability of alveoli)
surface forces tend to pull intersitial fluid into the alveolus
regulation of lung fluid balance
because of the additional surface tension, there is more hydrostatic pressures in the capillaries in the alveoli, so this favors fluid movement into the interstitium
surfactant reduces alveolar surface tension, and this helps prevent edema
hysteresis
the different relationship between pressure and volume during inflation compared to that during deflation
What is the main effect of introducing saline into the lung?
it eliminates the very large (>70m2) fluid-air interface in the alveoli
this abolishes any effects due to surface tension of the fludis lining the alveoli
this makes the lung much more compliant, almost completely abolishing hysteresis
roles of surfactant in the lungs
reduces alveolar surface tension and varies it with breathing
preserves alveolar integrity
prevents continuous transudate (edema) from pulmonary capillaries to alveoli
reduces work of breathing
composition of surfactant
lipoprotein complex that is approximately 30% protein and 70% phospholipid (mostly dipalmitoyl phosphatidyl choline)
synthesize dnad released from lamellar bodies in type II alveolar epithelial cells
dipalmitoyl lecithin or dipalmitoyl phosphatidyl choline
principle active ingredient of surfactant
hydrophilic head, which aligns with the water surface, and a hydrophobic tail that is more compatible with air
released into the alveoli as a result of lung distention or stimulation of beta adrenergic receptors
quickly spreads in a monolayer along the interior lining of the alveoli at the surface of a thin aqueous lining of the alveolus known as the hypophase
How does surfactant contribute to hysteresis?
during inflation, the surfactant is broken up and micelles beneath the surface help fill in gaps
this reduces the effect of the surfactant and thus increases surface tension
upon deflation, the expanded layer of surfactant is compressed to a surface solid state, which provides the maximal amoutn of action, reducing the surface tension to almost zero
the role of surfactant in the first few breaths
the first breath is very difficult to take as a lot of pressure must build up before the lungs open
once the first few breaths are taken, surfactant is spread across the surfaces, which then makes inspiration much easier
babies with RDS never reach that stage of easy breathing as the lung collapses every time they expire
What is the main difference between the complaince of the lung and chest wall?
at normal lung volumes (up to about 60-80% of vital capacity) the chest wall is attempting to expand
reducing the pressure gradient across the chest wall increases thoracic volume
describe the relationship between the compliance of the lung and the chest wall
the two systems are in series
1/Cresp= 1/Clung+ 1/Cchest
at FRC, the total compliance is less than the individual compliances
What happens to the FRC when the chest wall is less compliant?
the FRC is larger with a normal chest wall
Pip is more negative
What happens ot the FRC when the lungs are less compliant?
FRC decreases - stiff lungs pull chest to a smaller volume
Pip is more negative
describe the relaxation pressure-volume curve of the lung and chest wall
the compliance curves for the lungs and chest wall are considered together when determining the total compliance of the system
Where is ventilation the lowest in the lung and why?
lowest in the apices where the negative intrapleural pressure is the greatest
lung is in the upper portions of the compliance curve, making lung complicance low
therefore, for a given change in intrapleural pressure produced by an inspiratory effory, the volume increase will be small
Describe the ventilation of the lung at FRC and at Residual Volume.
at FRC, the base of the lung is open and ventilation is higher there
at RV, the base of the lung is compressed and closed, so ventilation becomes greater at the apex
ventilation at the apex
intrapleural pressure more negative
greater transmural pressure gradient
alveoli are larger and less compliant
less ventialtion
perfusion at the apex
lower intravascular pressures
less recruitment
distension
higher resistance
less blood flow
ventilation at the base
intrapleural pressure less negative
smaller transmural pressure gradient
alveoli smaller, more compliant
more ventilation
perfusion at the base
greater vascular pressures
more recruitment
distension
lower resistance
greater blood flow
describe the volume-pressure-flow relationships during a breath
the hashed lines represent the additional pressure generated by airway resistance
What happens to maximum flow as lung volume decreases and why?
maximum flow decreases because of compression of the airways
specific factors affecting resistance
lung volume
bronchial smoth muscle tone
gas characteristics such as desnity or viscosity
Why does dependence of airway compression on lung volume occur?
airways are “effectively” tethred to the chest wall
at large lung volumes, tethers are stretched and increased the effective stiffness of the airway - less compressible
diseases such as emphysema increase lung compliance, reduce tethering and increase dynamic compression, thereby increasing flow limitation
effort independent (flow limited) portion of the flow-volume curve
arises from dynamic compression of the airways
as expiratory effort increases, Pip increases, thereby increasing airway compression (although U may increase) and limiting flow
the narrowed airway can oscillate and turbulent flow cuase wheezing sounds
starling resistor
Pi > Pe > Po
tube will flutter open and closed
flow is continuous, but a function of Pi - Pe
