Respiratory Mechanics Flashcards
Describe the physiologic and anatomic components affecting lung mechanics and work of breathing.
Chain of command = negative pressure ventilation o CNS o PNS o Muscles o Chest wall compliance o Pleural space o Lung compliance o Airway
- When normal FRC = potential energy is leveraged from the chest wall
- Two main forces to be overcome are elastance and resistance
- Obstructive disorders = problems with both decreased elastance and increased resistance
- Restrictive disorders = problems with elastance (never with resistance)
Describe lung elastance
Sum of elastic forces trying to collapse the lung
o The change in pressure for every unit change in lung volume during expansion with positive pressure
o The inverse of lung compliance: 1/CRespSystem = 1/Clung + 1/CChestWall
o Elastic tissues of airways, visceral pleura, alveolar ducts and alveoli are interconnected
o Resting state
• Lung elastic recoil force = collapses lung inward
• Chest wall elastic recoil force = outward
o Two opposing forces are balanced at rest
• Linked by pleura
• Lungs are tightly coupled to chest wall
Describe compliance
Change in volume/ change in pressure (L/cm H2O)
o Normally about 0.15 = 0.150 L/ 1 cm H2O
o How much pressure to generate a unit of change in volume
Define affecting airway resistance and how it is distributed along the tracheobronchial tree.
Airflow resistance = force dissipating the pressure driving airflow
o Sum of resistance to airflow in airways (aprox. 90%) and in lung parenchyma (10%)
o 80% of resistance from “large” airways >2 mm
o As lung volume increases = tethering effect increases cross-sectional area of airway = airway resistance decreases
Factors affecting airflow resistance and flow of airways
Resistance affected by:
o Poiseuille’s law: R = (8ηL)/(πr4)
o Lung volume, due to tethering of airways to parenchyma
o Bronchial smooth muscle tone
o Inspired gas properties (density and viscosity η)
o Flow patterns (turbulent vs. laminar)
• Factors dynamically change during respiratory cycle!
o Forced expiration → dynamic airway collapse
o When pleural P > airway P = collapse (positive pleural pressure during forced expiration compresses intrathoracic airways)
o Extra effort does NOT increase airflow!
Factors affecting flow of airways
o Narrowing of airways → increased pressure drop during forced expiration = will collapse earlier in expiration and closer to alveoli → decreased flow
o Decreased elastic recoil = less tethering action → more collapse of small airways during expiration → decreased flow
o Decreased elastic recoil = reduces static elastic recoil pressure of lung → less driving force → decreased flow
Pneumothorax and lung mechanics
o Air in pleural space
o Result: uncoupling of lungs and chest wall
o Chest wall and lung come to rest at new FRC values
• Chest wall recoils outward
• Lung recoils inward
o Volume of the pneumothorax = difference between two FRCs
Fibrosis and lung mechanics
o Smaller lung, more stiff
o Replacement of lung units with fluid, tissue, etc.
o Decreased compliance of lung, increased elastic recoil
Emphysema and lung mechanics
o Larger lung
o Alveolar destruction
o Increased compliance of lung, decreased elastic recoil
o Increased TLC
Lung mechanics with reduced chest wall compliance
o Stiff chest wall (reduced distensibility)
o Ex: kyphoscoliosis, ankylosing spondylitis
Lung mechanics with reduced respiratory muscle forces
o Normal lung and chest wall compliance
o Ex: muscular dystrophies, diaphragm paralysis
Changes in airflow resistance in disease
Airway obstruction o Debris or mucus o Remodeling of airway wall (thickening) o Loss of tethering forces → dynamic compression and decreased alveolar pressure Examples: • Asthma • Bronchoconstriction or small airways disease • Episodic • Reversible • Often includes allergic component • Relatively younger population • Chronic bronchitis • Bronchoconstriction or small airways disease • Relatively older patients • Smokers • Persistent productive cough • Emphysema • Dynamic airway compression • Older patients • Smokers • Dyspnea on exertion
Decreased resistance from increased tethering in fibrosis
o Tethered opened airways
TLC
greatest lung volume with maximum inspiration
o Equal and opposite static balance between inspiratory muscle forces and elastic recoil forces in respiratory system
o Lung contributes most of elastic recoil forces
o Reduced TLC is hallmark of restrictive lung disease
1) Due to lung or thorax abnormality
2) Due to respiratory muscle weakness
Causes of increaed TLC:
emphysema
asthma
acromegaly
Causes of decreased TLC: neuromuscular disease rib cage or thoracic spine abnormalities morbid obesity, ascites, pregnancy fluid or exudate in alveoli parenchymal fibrosis
RV
lowest lung volume with maximum expiration
o Static balance between expiratory muscle forces and elastic recoil in respiratory system
o Chest wall contributes most of elastic recoil forces
o With lung disease: also have dynamic mechanism
• With low airflow rates = prolonged expiration
• Need to stop expiration before static equilibrium is reached → increased RV
o See increased RV (“air trapping) with airflow obstruction, emphysema, and neuromuscular weakness
o See decreased RV with parenchymal lung diseases that cause a reduced TLC
• FRC: volume of air in lungs at end-expiration during tidal breathing
FRC
volume of air in lungs at end-expiration during tidal breathing
o Static equilibrium volume when elastic recoil pressures of lung and relaxed chest wall are equal and opposite
o With lung disease: dynamic mechanisms
• Increased inspiratory muscle activity during expiration
• Decreased expiratory flow rates
• Result: increased FRC
o Increased FRC associated with emphysema (due to decreased lung elastic recoil)
o Decreased FRC associated with restrictive lung disease secondary to parenchymal or some thoracic causes
