Lecture 27 - Compliance Flashcards
What is the slope of the pressure(x)/volume(y) curve of the lung? What does a steeper slope mean?
Compliance
Steeper slope = more compliant
What is the recoil pressure of the lung? Other name?
Force or tension as a result of distending force exerted on the lung that when released recoils the lung toward a resting volume
= elastic pressure (Pel)
What are static and dynamic properties?
- Static = snapshot or point in time when all muscles are relaxed, lack motion, time, or dynamic forces
- Dynamic = occur over time
What is elastic recoil?
Tendency of the lung to collapse from an inflated volume
Compliance of lungs that expand easily?
High
Compliance of lungs that does not expand well? What do we call this?
Low = stiff lung
Factors influencing STATIC lung compliance?
Tissue structure and composition: 1. Collagen fibers 2. Elastin fibers 3. Cellularity of interstitium Others: 4. Vascular distention/engorgement 5. Surface tension
How do collagen fibers affect static lung compliance?
If they accumulate in the interstitium they will add stiffness and create final restraint at total lung capacity
How do elastin fibers affect static lung compliance?
They contribute to the recoil properties of the lung
In what category can we include sarcoidosis and pulmonary fibrosis?
Restrictive lung diseases that increase the cellularity of the interstitium
Definition of surface tension?
Tendency of surface molecules to pull inward
What is LaPlace’s Law?
P = 4T/R
T = surface tension R = radius P = recoil pressure
LaPlace’s Law for alveolus?
P = 2T/R because only one surface is involved
How does an alveolus with a small radius will compare to an alveolus with a larger radius with regards to surface tension? What is the problem with this? How is it corrected for?
Without surfactant, the surface tension in all alveoli would be the same regardless of size, so the recoil pressure of small alveoli would be higher.
Issue: if lung only relied on surface tension to keep alveoli open, since the lungs do not inflate uniformly (especially at lower volumes), the smaller alveoli would collapse and the larger ones would over inflate => areas of hyperventilation + areas of atelectasis
Solution: surfactant produced by Type II pneumocytes lines the alveolar walls to decrease surface tension that occurs in alveoli
How does surface tension affect static lung compliance?
Loss of surface tension = increased compliance
How can you remove surface tension in a lung?
Fill it with saline
When does surfactant production begin during development?
24-26 weeks
What is surfactant primarily composed of?
Dipalmitoyl lecithin
Good indication of fetus lung development sufficient for child delivery?
Lecithin:sphingomyelin ratio of 2:1
What is hysteresis?
Inspiratory compliance curve is different from expiratory compliance curve because it is easier (requires less pressure) to deflate the lung than to inflate it: lung volume at any given pressure during deflation is larger than during inflation = to achieve a particular volume, more pressure must be applied on the inflation limb of the compliance curve than on the deflation limb
What drives gas to the terminal bronchioles during inspiration in active inflation?
Pressure gradient applied across the thorax that expands the lungs and chest wall = effective transpulmonary pressure = Palv-Ppl
Ppl = pressure in pleural cavity
SO, when the pleural pressure is negative, then the transpulmonary pressure is positive and air will enter and expand the lungs (per a volume on the compliance curve)
What drives gas to the terminal bronchioles during inspiration in passive inflation?
Manual expansion of the lungs (e.g. via a ventilator) pushing air into the lungs with a positive pressure in the alveoli and a pressure of 0 in the pleura
How is the lung’s elasticity counterbalanced under static conditions?
Palv-Ppl counterbalances the elastic forces of lung recoil
When can lung transpulmonary pressure be measured?
Only when a patient is passively inflated: inflate the lungs with positive pressure, hold the breath, and at the end of the breath a positive pressure will be exerted on the pleural space = plateau pressure distending pressure counterbalancing the lung recoil (Palv is 0)
Not possible in a spontaneously breathing patient (aka active patient) because the pressure distending the chest wall cannot be measured since the pressures are generated by the muscles in the chest wall itself
Describe the compliance curve of the lung and chest wall.
- X-axis: airway pressure in cm H2O
- Y-axis: vital capacity %, meaning volume
Describe the compliance curve of the lung in isolation.
- In a resting state the lung is at a volume much lower than when it’s in the chest wall
- Can reach a minimal volume depending on the elasticity of the lung
- Inflates to a maximum where the curve becomes asymptotic and the lung is non-compliant
Describe the compliance curve of the chest wall in isolation. What does it represent?
Represents the compliance of the respiratory muscles, ribs, and diaphragm together:
- In a resting state the chest wall is at a volume much higher than when the lungs are in it
- Chest volume can be expanded to a much higher volume than when the lungs are in it because of how compliant the diaphragm is
- Chest volume can be decreased to a volume limited by the ribs
What is the most compliant structure in the respiratory system?
Diaphragm
Describe the compliance curve of the chest wall and the lung together (aka the respiratory system).
- Exhalation is limited by the chest wall (it sets the residual volume)
- Inhalation is limited by the lung and its elastic skeleton (it sets TLC)
- Resting state of 0 airway pressure at the functional residual capacity: takes -5 cm H2O pleural pressure to pull the chest inward and +5 cm H2O to expand the lung which counterbalance each other out
In healthy individuals what pressure is necessary to reach TLC? What does this mean?
30-35 cm H2O
If a lung is exposed to more, it will rip
What happens to the respiratory system with pneumothorax?
The chest and lungs become uncoupled => air rushes in pleural cavity (from -5 to 0 cm H2O) and lung collapses to its resting volume, chest wall expands to its resting volume
How to find volume of pneumothorax on a compliance curve of the chest wall and the lung together?
Difference in volume between chest and lung at an airway pressure of 0 cm H2O
Which is worst: a sucking blowing chest wound or a sucking wound? When does it often happen?
Sucking wound because the pneumothorax gets bigger and bigger with each breath and the positive pressure will push the lung volume down => tension pneumothorax
This often happens if the patient is on a ventilator (positive pressure breathing)
What is a lung resection? How is it done?
Surgical procedure during which the surgeon removes a small portion of the lung containing the cancerous cells
Need for intubation of both lungs with the one being worked on collapsed down to make space
What is the steepest part of ALL compliance curves (lung, chest, and respiratory system)? Which curves are steeper?
FRC to slightly higher volume
Individually, the curves are steeper than their combined curves at FRC, each with a slope of 200 ml/cm water, meaning the chest wall and lungs are more compliant individually than when combined.
When is hysteresis more pronounced?
When surfactant is lacking
When does the convexity of the compliance curve become apparent?
High volumes
What is a dynamic compliance curve?
Loop curve showing hysteresis
Which curve is more compliant: static or dynamic compliance curve?
Static one
What should compliance be expressed in terms of? Why? What is this called?
Compliance should be expressed in terms of absolute volumes because the uncorrected compliance of respiratory system after a pneumonectomy for example is greatly reduced from pre-op value but the specific compliance is unchanged => SPECIFIC compliance
What can be said of the elasticity of the lung and chest wall? What does this mean? What can this be used for?
They are additive, so the following formula for their compliances can be written:
1/Crs = 1/Cl + 1/Ccw
We can use this information to calculate the slope of our combined curve because the elasticity of the lung and chest wall are additive, and elasticity is the inverse of compliance => the slope of the combined curve = (1/200 + 1/200)^(-1) = (1/100)(^-1) = 100 ml/cm water
What happens to the compliance curve of the respiratory system as the lung or chest wall stiffens? What does this mean for treatment?
Compliance curve of respiratory system shifts to the right and flattens between 2 inflection points: bottom one (PEEP) and top one
Treatment: if patient is on a ventilator, the goal is to keep their respiratory system between the 2 inflection points or else the lung will be damaged
What is the peak pressure? What does it correspond to?
Pressure it takes to get air in thorax => dynamic compliance
What does the difference between static and dynamic compliance account for?
Accounts for the difference in pressure it takes to get the air into the airways and the pressure it takes to hold it there (dynamic is higher than static)
What is the plateau pressure? What does it correspond to?
Pressure it takes to hold air in the thorax => pressure overcoming the elastic recoil of the lung and chest wall = static compliance
Why is dynamic pressure higher than static pressure?
Because the dynamic pressure is not only overcoming the elastic recoil of the respiratory system, but also the airway resistance
Is the fully inflated alveolar pressure static or dynamic pressure?
STATIC
In an ICU, the ventilator pressure is kept below 30 to 35 cm of H2O: is this static or dynamic pressure?
Static
Equation for dynamic compliance?
Dynamic compliance = TV / (peak pressure - PEEP)
TV = tidal volume PEEP = positive end expiratory pressure
What is positive end expiratory pressure? How is it maintained?
Pressure due to FRC in patients on ventilators because without a positive pressure at rest, their FRC would be too low due to the fact that they are sedated (muscles relaxed) and laying down (gravity not helping diaphragm) and we need to keep them between the 2 inflection points of their compliance curve = PEEP RECRUITS NORMAL FRC
Maintained with:
- Ventilator (ends expiration at 5 cm H2O)
- Continuous positive airway pressure mask (does not let patients expire all the way)
Normal end expiratory pressure in healthy individuals that is NOT in hospital?
0 cm H2O
Normal positive end expiratory pressure in healthy individuals that supine and sedated? AKA physiologic PEEP?
5 cm H2O
Equation for static compliance?
Static compliance = TV / plateau pressure - PEEP
TV = tidal volume PEEP = positive end expiratory pressure
Which is larger: static or dynamic compliance? Why?
Static because denominator is smaller (using plateau instead of peak pressure)
Equation of resistance of airways?
R = peak pressure - plateau pressure / flow
Equation for peak pressure - PEEP?
This is dynamic pressure, which needs to overcome both the airway resistance and the elasticity of the respiratory system:
ΔP = 1/Crs x TV + Raw x flow = (1/Cl + 1/Ccw) x TV + Raw x flow
What 4 factors will cause peak pressure to increase?
- Decrease compliance
- Increased airway resistance
- Large tidal volume
- Higher flow
What 2 factors will cause plateau pressure to increase?
- Decrease compliance
2. Large tidal volume
If a patient on a ventilator’s peak pressure is rising, what is the issue? Assume volume mode. How is the plateau pressure affected?
Ventilator keeps TV and flow constant, so either compliance is decreasing (meaning plateau pressure is increased) or airway resistance is increasing (plateau resistance unchanged)
Equation for plateau pressure - PEEP?
ΔP = 1/Crs x TV + Raw x flow BUT flow = 0 so ΔP = 1/Crs x TV
If a patient on a ventilator’s plateau pressure is rising, what is the issue?
Ventilator keeps TV constant, so compliance must be decreasing
If a patient on a ventilator’s peak pressure is decreasing, what is the issue?
- Air leak
2. Hyperventilation
What can cause increased airway resistance?
- Aspiration
- Bronchospasm
- Tracheal tube with kink or biting
- Endobronchial partial obstruction
- Asthma
- Secretions
What can cause decreased compliance?
- Abdominal distention
- Asynchronous breathing
- Atelectasis
- Auto-PEEP
- Pneumonia
- Pulmonary edema
- Pneumothorax
- Pulmonary fibrosis
- Sarcoidosis
What is happening if a patient is in respiratory failure but their peak pressure is unchanged?
- Pulmonary embolus
2. Extrathoracic process
What is aspiration?
Entry of material from the oropharynx or gastrointestinal tract into the larynx and lower respiratory tract
Does compliance decrease with age?
YUP
Compare airway resistance in base and apex of lung.
Airway resistance is higher at base than at apex because they alveoli are more open
At FRC, does inspiration cause a larger increase in volume at base or apex of lungs?
BASE!
Is surfactant a protein?
NOPE, it’s a phospholipid
How does the presence of surfactant affect the hydrostatic pressure in the interstitium around the pulmonary capillaries? What does this mean?
Surfactant reduces inward pull of alveolar wall => hydrostatic pressure in interstitium around capillaries is less negative => surfactant prevents transudation of fluid from capillary to interstitium or alveoli
How does intrapleural pressure change during expiration?
Increases (becomes less negative)
Compare velocity of gas in larger airways and terminal bronchioles during expiration.
Velocity of gas in terminal bronchioles is smaller because of larger cross-sectional area
Compare alveolar pressure and atmospheric pressure during expiration.
Alveolar pressure is higher
If the lung is held at a constant volume, which 2 pressures must be the same? What about the intrapleural one?
Mouth and alveolar are equal
Intrapleural is negative
