Lung Compliance; Thorax Compliance; Airway Resistance
March 8, 2004
NOTE: In several of the lectures, including this one, I have noticed that I didn’t put the units that go with the pressures. I will definitely include units if they are something other than this, but if I don’t include a unit, then I mean for it to be cm H0. 2
Review of Pulmonary Pressures—Recall from previous lectures that all pulmonary
pressures are differences from atmospheric pressure. If the pulmonary pressure is equal to 0, then the pressure is the same as atmospheric pressure. If the pressure is less than 0, it is a subatmospheric pressure. If the pressure is greater than 0, it is greater than atmospheric pressure.
Pleural pressure (P)—This is the pressure in the thin layer of fluid between the pl
visceral and parietal pleura. It is generally a subatmospheric pressure (around –5
cm H0). 2
Alveolar pressure (P)—This is the driving force for airflow into and out of the A
lungs. If it is equal to 0, then there is no airflow. If alveolar pressure is less than
0, then there is inspiration and if it is greater than 0, then there is expiration.
Transpulmonary pressure (Por P)—This is the pressure difference between the L tp
inside and outside of the lungs. It determines the degree of inflation of the lungs.
Lung Compliance—Compliance is the measure of how easy it is to inflate something. If it is easy to inflate something, then compliance is high. If it is hard to inflate something, then compliance is low. Compliance in the lungs is defined as a change in volume
divided by a change in transpulmonary pressure (C = ;V / ;P). A typical value of LL
compliance is 200 ml/cm H0. 2
Specific compliance is defined as the compliance divided by the initial volume of the lung. Specific compliance is a property of the lung tissue itself and is not dependent on the body size. (Compliance depends on how much tissue is being considered).
Lung Volume vs. Transpulmonary Pressure—The lecture slides show a graph which
relates lung volume to transpulmonary pressure. The slope of the line is the compliance. According to the graph, as transpulmonary pressure increases lung volume increases. At a higher P (and volume), compliance is low. In other words, a small change in volume L
elicits a large change in pressure if compliance is low. For a lower P (and volume), L
compliance is high. In this situation, a large change in volume results in only a small change in pressure.
In reality, the pressure-volume curves for inspiration and expiration are not the same. When the forward path is different from the reverse path, then this is referred to as hysteresis. Hysteresis is a common phenomenon in nature and in the lung. Hysteresis is
best observed when starting with a collapsed lung. For normal breathing, starting with a lung inflated to FRC (2-3 L), the forward and reverse paths are much more similar.
Lung Volume vs. Intrapleural Pressure—This type of situation is seen within the
normal thorax if the lung is inflated to various volumes using the respiratory muscles. There is no airflow and the glottis is open, so alveolar pressure is equal to atmospheric pressure, or P= 0. The Pis estimated at each volume using an esophageal balloon. It A pl
is noted (on the lecture slides), that at higher lung volumes, the Pis more pl
subatmospheric and at lower lung volumes, the Pis less subatmospheric. pl
Compliance is again the slope of the graph and is measured by C= - ;V/;P The L pl.
negative sign in the equation accounts for the fact that the Pon the x-axis is pl
subatmospheric. To keep the formulas for compliance straight, it is best to remember that compliance is always a positive number. Compliance is low at higher lung volumes and
more subatmospheric P. Compliance is high at lower lung volumes and less pl
subatmospheric P pl.
Influence of Gravity on Fluid—In any fluid compartment, gravity is pushing against the
top of the fluid. As a result, the pressure at the bottom of the fluid compartment will be greater than it is at the top of the fluid compartment. In other words, the pressure increases as you go toward the bottom of the fluid compartment due to the effects of gravity pushing down.
In a vertical (upright) lung, gravity causes the Pto be less negative at the base of the pl
lung than at the apex of the lung. When the glottis is open and there is no airflow, the PA
at both the apex and the base is 0 (equal to atmospheric pressure).
In the example shown in lecture, the Pat the base of the lung was around –2 pl
while the Pat the apex of the lung was around –10. Since the Pwas 0 in both pl A
situations, the Pat the base of the lung was +2 while the Pat the apex of the L L
lung was +10. This means that the lung units at the top of the lung were more
expanded than those at the bottom of the lung.
Lung compliance is higher at the base of the lung and lower at the apex of the lung. This means that inspiring a tidal volume of air causes more expansion at the base of the lung and that the lung units at the base are more susceptible to collapse.
Compliance vs. Elastic Recoil—If compliance is low, the elastic recoil is high. This
means that the lungs are stiffer and there is a greater tendency of the lungs to collapse. This is seen in various fibrotic conditions. If compliance is high, the elastic recoil is low. This is seen in the case of emphysema.
Passive Elastic Properties of the Chest Wall—At FRC, there is a normal Pof about –5 pl
cm H0 that keeps the thorax at this volume, which is just below its resting position. At 2
this pressure, the elastic recoil of each lung is exactly balanced by an outward recoil of the thorax.
If the lung or thorax is punctured, then air is put into the thorax and a pneumothorax results. This results in a Pthat is atmospheric on the side of the pneumothorax. The pl
lung collapses on that side. The half of the thorax on the side of the collapsed lung (the hemithorax) expands b/c there is no longer any pressure pulling inward from the lung on that side. Finally, the mediastinum shifts away from the side with the collapsed lung. The shift is only minor and the other lung remains inflated b/c there is an airtight seal formed by the mediastinum to prevent disruption of the pressure on that side too.
The compliance of the lungs and thorax combined is less than the compliance of either one alone. This is why a positive pressure ventilation pump pressure doesn’t change Pby a great amount. The ventilator is working against the collapse tendency of L
the lung and the compliance in both the lungs and thorax.
There are three graphs shown in the lecture slides (it would be slides #19, 20, and 21) that shows the passive pressure-volume relationships of the lung alone, the chest wall alone, and the lung and chest wall combined. The 1st slide shows only the relationship due to the passive elastic properties of the chest wall/abdomen (Dr. Jennings pointed out that this can’t really be done inside the body). From a resting volume of the chest wall/abdomen (about 1 L above FRC), a negative pressure must be applied to reach FRC or RV. A positive pressure must be applied to reach TLC.
ndThe 2 slide adds the properties of the lung alone. In this case, at FRC, the pressure is above atmospheric. To reach TLC, the pressure must be increased more and to reach RV, a pressure that approaches 0 must be applied.
rdThe 3 slide shows the combined properties of the chest wall and lungs. This can be demonstrated experimentally. For any volume, the applied pressure is equal to the sum of the pressures necessary to hold the lungs alone or the thorax alone at that volume. If no pressure is added, then the lungs and thorax will be at FRC.
Compliance of the Lung and Chest Wall—The slopes of the curves in the graphs
discussed above represent the compliance (of either the lung alone, the chest wall alone,
or the two combined. It should be noted that the slope of the combined system is always lower than that of either individual curve b/c the compliance of the combined system is always lower than the individual compliances. Total compliance is determined by
adding the reciprocal of the individual compliances (1/ C= 1/ C+ 1/C). total L CW
If either the compliance of the lungs or the chest wall is very small (as is seen with restrictive disease), then the compliance of the total system will also be small. This means that low compliance in either the lung or the chest wall/abdomen results in restricted breathing. At FRC, the typical compliances are: C= 200, C= 200, and L CW
C= 100. total
Compliance and Ventilation—Normally, an applied pressure of around 10 is needed to
increase the lung volume by about 1 L (above FRC). If there is reduced compliance in
either the lung or the thorax, then a higher pressure is needed to cause the same amount of inflation.
In either of these situations, if the respiratory muscles are used instead of the ventilator, then a decrease in the compliance of either the lungs or the thorax would require that more muscular work be done to expand the lungs. This type of situation is seen
in obese individuals. When they are in a supine position, the lung compliance may be normal, but the thorax compliance may be lowered by the weight of the abdomen pushing against t he thorax. This increases the amount of work that is required to expand the lungs. This is the reason that obese people often sleep sitting up. It is simply harder for them to breathe while in a supine position b/c of the greater compressed mass.
Volume vs. Pin Normal Inspiration and Expiration—Up until now, we have only pl
considered a pressure-volume relationship in which there is no airflow. However, in order to drive airflow, the Pwould have to be different from the atmospheric pressure. A
During normal breathing, alveolar pressure must be positive during expiration and negative during inspiration.
Consider the lung volume as a function of Pas is seen in the lecture slides. Beginning pl,
at FRC, Pis atmospheric and Pis –5. Pand elastic recoil are exactly balanced. The A pl pl
pleural pressure must become more subatmospheric in order to overcome the elastic recoil of the lungs. The subatmospheric Pgenerates a negative Pwhich is what pl a
actually drives the airflow inward during inspiration. During expiration, the Ppl
becomes less subatmospheric and is no longer able to overcome the elastic recoil of the lungs. As a result, the P is positive and there is an air outflow during expiration. A
Ultimately, the breathing cycle arrives back at FRC where the elastic recoil and Pare pl
There is an additional graph in the lecture slides which plots volume, pressures, and flow as a function of time. Airflow and alveolar pressure always have to change in parallel to each other b/c alveolar pressure is the driving force for airflow. The pressure to keep the lungs open is always more subatmospheric than it needs to be during inspiration and less subatmospheric than it actually needs to be during expiration.
Airway Resistance—Airway resistance is the determining factor for the instantaneous rate of airflow at a given driving pressure (driving force). The formula for determining airway resistance is based on an equivalent of Ohm’s Law (V = I x R). In the case of airway resistance (R), the driving force is equal to flow times resistance (driving force = flow x resistance). In other words, airflow is equal to driving force divided by distance (airflow = driving force/distance). The driving force in the lungs is the difference between alveolar pressure and pressure at the mouth (P- P). Put simply, airflow = mouthA
(P- P)/R. mouthA
Airflow—Airflow in the lungs can be described as being either laminar or turbulent. Flow is fastest in the middle of the tube and is all in the same direction. This is laminar
flow. Laminar flow is found only in the smallest of the airways where the radius is small
and the velocity of flow is slow. Turbulent flow occurs when there is a net forward flow,
but there are many local eddy currents (little circulations that occur). Turbulent flow of air is observed in the upper airways where the radius is larger and the airflow is more rapid.
The Reynolds number is used as an index to determine whether flow is laminar or
turbulent. It is a unitless number that is defined as: Re = 2rvd/？, where r is radius, v is
velocity, d is density, and ？is viscosity. If the Reynolds number is greater than 2000,
then turbulent flow is very likely. According to this equation, turbulent flow is likely if
the tube has a large radius, a high velocity, a high density, or a low viscosity.
Main Sites of Airway Resistance—The nose is a main site of airway resistance. The
larger and medium bronchi/bronchioles are also a major site of airway resistance. The
small airways only make a minor contribution to overall airway resistance. Because they don’t play a large role in airway resistance, a disease that originates in the small airways will be difficult to detect. The disease would have to spread to the airways were there is a larger contribution to the resistance before the effects would be seen. Rhoades and Tanner shows the resistance for various airway generations in figure 19.26, p. 331.
Factors that Control Airway Resistance—Airway resistance is influenced by a number
of factors. One factor is lung volume. In general, as lung volume increases, resistance
decreases. This is due to radial traction exerted on the airways. When the volume of the lung increases, the radius (AKA caliber) of the conducting airways increases and the result is lower airway resistance. When the lung volume decreases, the resistance of the airways increases.
ndA 2 factor is bronchial smooth muscle activity. The contraction of bronchial smooth
muscle decreases the airway radius, causing an increase in airway resistance. Relaxation of the bronchial smooth muscle increases the airway radius and causes a decrease in airway resistance. The tone of bronchial smooth muscle is determined by autonomic input (by the vagus nerve, remember?). Adrenergic stimulation, mainly by norepinephrine acting on ？ receptors and by nitric oxide, causes bronchial smooth 2
muscle relaxation. Substances such as acetylcholine, histamine, and prostaglandin F 2；
cause bronchial smooth muscle contraction.
Work of Breathing—There are two categories that the physical work of breathing can be broken down into. One type is resistance work in which an increase in resistance
results in an increase in work. The amount of resistance work done is increased by
obstructive disease and is minimized by slow breathing. Compliance work is the other
type of breathing work done. A decrease in compliance of the lungs requires an increase in work of them. Compliance work is increased by restrictive disease and is minimized
by shallow breathing (b/c it is easier to breathe at lower volumes when the lungs can’t be expanded).
In summary, the work of breathing can be increased by increased airway resistance, reduced lung compliance, or reduced thorax compliance.