Obstructive lung diseases (e.g., asthma, emphysema) characterized by obstruction to airflow.
Muscles of Ventilation
Diaphragm: Most important muscle of inspiration; contraction increases the vertical height of the thoracic cavity.
External intercostal muscles: Produce a “bucket-handle” movement of the ribs, increasing the lateral and anteroposterior diameter of the chest.
Expiration is passive during quiet breathing but becomes active during exercise.
Muscles of expiration: Abdominal wall muscles.
Internal intercostal muscles: Assist expiration by pulling the ribs downward and inward.
Neuromuscular diseases (e.g., Guillain-Barré syndrome) can cause respiratory muscle weakness, potentially requiring mechanical ventilation.
Airway Anatomy
Air is distributed through a highly branched airway.
Conducting Zone
Comprises the first 16–17 generations of airway division (from trachea to terminal bronchioles).
Gas exchange occurs in the respiratory zone, distal to the terminal bronchioles.
Lung Acinus
A functional unit formed by the division of a terminal bronchiole into the respiratory bronchioles, alveolar ducts, and alveoli.
Gas moves by diffusion within lung acini.
Alveolar Damage in Emphysema
Centriacinar Emphysema: Damage primarily in the respiratory bronchioles; associated with smoking; most damage in the apical regions of upper lobes.
Panacinar Emphysema: The entire acinus is damaged; associated with α1-antitrypsin disease; typically involves the entire lung, with lung bases most diseased.
α1-Antitrypsin: A serum protein produced by the liver that combats damaging protease activity in the lung; deficiency leads to panacinar emphysema and liver cirrhosis.
Lung and Chest Wall Recoil
Subatmospheric pressure in the intrapleural space allows air to be drawn into the airway during inspiration.
Pressure is created by the opposing recoil of the lungs and chest wall.
Transpulmonary Pressure (P_tp)
The force acting across the wall of the lung to expand it; calculated as the difference in pressure between the alveoli (Pa) and the intrapleural space (P{ip}):
P{tp} = Pa - P_{ip}.
Mechanics of Pressure Changes
During relaxed respiratory muscles:
Alveolar pressure is the same as atmospheric pressure (0 ext{ cm}), intrapleural pressure is about -5 ext{ cm}.
Inspiratory muscles contraction causes P{ip} to become more negative (e.g., P{ip} = -10 ext{ cm}).
Positive value indicates a force for lung expansion (inspiration); negative P_tp indicates a compression force during forced expiration.
The Ventilation Cycle
Air flows in/out of the lung due to pressure differences along the airway between the mouth and alveoli.
Airflow is driven by changes in alveolar pressure, which result from changes in intrathoracic pressure.
Contraction of inspiratory muscles reduces P{ip} and increases P{tp}, expanding the lung.
Lung compliance is low at both high and low lung volumes:
High volume: Lung tissue is already stretched.
Low volume: Many lung acini are collapsed.
Compliance is largest at FRC (Functional Residual Capacity).
Hysteresis: Different path of expiration curve vs. inspiration curve.
Surfactants
Phospholipids (mainly dipalmitoyl phosphatidylcholine) secreted by type 2 pneumocytes in alveolar walls.
Line the inner surface of the alveoli.
As lung volume decreases during expiration, surfactant molecules are forced closer together and repel each other, resisting alveolar collapse.
Functions of surfactant:
Maintains alveoli open during expiration, allowing more time for gas diffusion.
Counters surface tension.
Prevents alveolar atelectasis (collapse).
Law of Laplace predicts smaller alveoli will have higher internal pressure, causing them to empty into larger alveoli unless maintained by lung surfactants that reduce surface tension.
Respiratory distress syndrome of the newborn (hyaline membrane disease) is caused by surfactant deficiency.
Corticosteroids given to the mother prior to delivery can increase surfactant production.
Elastic Properties of the Lung and Chest Wall
Compliance is optimal around FRC, minimizing the work of breathing.
FRC is controlled by the relative strength of lung and chest wall recoil forces.
Lungs tend to collapse; chest wall tends to expand.
Causes of low thoracic compliance includes:
Pulmonary fibrosis
Pulmonary edema
Pleural effusion
Thoracic musculoskeletal pain
Rib fracture
Morbid obesity
Increased abdominal pressure
Interstitial lung disease causes the lung to be stiff and fibrotic, with low compliance.
Airway Resistance
A dynamic property that manifests during gas flow.
Airway resistance (R) determines the rate of gas flow (V) for a given pressure gradient ( riangle P): R = rac{V}{ riangle P}.
Factors Affecting Airway Resistance
Airway radius: Main component; bronchi and bronchioles are sites of variable resistance.
Turbulent gas flow: Occurs in larger airways and at branch points; bronchoconstriction and high velocity increase turbulence, causing wheezing.
Asthma
Key spirometry feature is reversible bronchoconstriction following treatment with a β2 agonist.
Characterized by inflammatory hyperreactive airways.
Triggers include allergens, infections, exercise, cold air, and drugs.
Methacholine can be given during pulmonary function testing to provoke bronchospasm when diagnosing airway hyperreactivity.
Breath Sounds
Wheezing: Musical sound, typically during expiration, created by high-velocity airflow from restricted airways; occurs during bronchospasm, airway edema, or airway partial obstruction.
Rales (crackles, crepitus): Inspiratory sounds created by forceful opening of alveoli; occurs in pulmonary edema, atelectasis, and interstitial lung disease.
Rhonchi: Low-pitched vibration (snoring), often rattling, occurring during inspiration and/or expiration; created by mucus-air interface; occurs in bronchitis or COPD.
Stridor: Harsh high-pitched wheeze during inspiration created by severe upper-airway obstruction; occurs in infants with croup, foreign body obstruction, epiglottitis, or laryngeal tumor or edema.
Dynamic Airway Compression
During forced expiration, intrapleural pressure becomes positive, compressing airways.
Distal bronchioles, lacking cartilaginous support, are at risk of collapsing.
Radial traction force helps resist collapse.
Emphysema patients experience increased airway resistance and gas trapping due to weakened radial traction forces.
Strategies to Reduce Dynamic Airway Collapse
Slow expiration reduces intrapleural pressure.
High resting lung volume increases airway diameter.
Pursed-lip breathing creates positive pressure in the mouth, increasing pressure inside the airways.
Large, hyperlucent lung fields, flattened diaphragm, and increased retrosternal airspace (observed in chest radiography).
Expiratory Flow Limitation
Maximal expiratory flow rate is achieved with relatively little expiratory effort due to dynamic airway compression.
Expiratory flow becomes independent of effort at medium to low lung volumes.
Patients with emphysema experience expiratory flow limitation during normal quiet breathing.
Static and Dynamic Compliance During Mechanical Ventilation
High airway pressure poses a risk of lung rupture.
Peak inspiratory pressure (PIP): Highest pressure recorded when a tidal volume is delivered during ventilation.
Plateau pressure (P_{PLAT}): Airway pressure recorded after a short pause before expiration, reflecting elastic (static) properties.
Static compliance can be calculated as:
C{static} = rac{P{PLAT} - P{EEP}}{Vt}.
Dynamic compliance includes the pressure component due to airway resistance:
C{dynamic} = rac{P{IP} - P{EEP}}{Vt}.
Clinical Spirometry Terms
Forced expiratory volume in 1 second (FEV1): Volume expired in the first second of a forced expiration test.
Forced vital capacity (FVC): Total volume expired under maximum effort.
Normally, FEV1 is about 80% of FVC (FEV1/FVC ratio = 0.8).
Expiratory flow through the middle 50% of the expired breath (FEF25-75) is also determined.
General Disease Patterns
Obstructive lung diseases (e.g., emphysema, chronic bronchitis, asthma): High airway resistance reduces both FEV1 and FVC, lowering the FEV1/FVC ratio.
Restrictive lung diseases (e.g., pulmonary fibrosis): Low lung compliance reduces virtually all lung volumes, particularly TLC and FVC, but the FEV1/FVC ratio is normal or increased.
Chronic Obstructive Pulmonary Disease (COPD)
Applies to patients with either emphysema or chronic bronchitis.