L6, L7: Mechanisms Of Breathing Flashcards
Inspiration and expiration
Inspiration: ↑ thoracic cage volume —> ↓ intrapleural pressure (always -ve)—> ↑ transpleural pressure (always +ve) —> lung expansion —> ↓ alveolar pressure —> air in
Expiration: ↓ thoracic cage volume —> ↑ intrapleural pressure (always -ve) —> ↓ transpleural pressure (always +ve) —> lung recoil —> ↑ alveolar pressure —> air out
Elasticity: property that cause a matter to return to original size after being deformed by external force
- Recoil pressure of lungs
- Recoil pressure of chest wall
- Recoil pressure of lung-chest system
Recoil pressure of lungs
- natural position: collapse
- larger the volume —> larger the recoil pressure
- slope decreases as recoil pressure increases —> 越來越難distend
- TLC —> maximum recoil pressure
- Origin: 1/3 elastic lung tissues, 2/3 surface tension of alveoli
1. Elasicity (Hooke’s law): pressure applied ↑ —> volume change ↑
2. Surface tension (Laplace’s law): P = 2T/r - smaller r —> greater p (alveolar instability: alveolar emptying into large alveoli)
- however, small alveolar area + larger [surfactant] in smaller r —> smaller T —> smaller P (balance greater P due to smaller r)
—> reduce pressure different between small and large alveoli
—> prevents alveolar instability - Surfactant: protein-carbohydrate-phospholipid complex
—> reduce surface tension
1. Increase distensibility of lung, decrease work of breathing
2. Reduce pressure difference in large and small alveoli —> stabilise alveoli
3. Prevent transudation of fluid into alveoli due to surface tension sucking from capillary
—> loss of surfactant
1. Stiff lungs
2. Alveolar collapse
3. Filled with transudate (IRDS/Hyaline membrane disease)
Recoil pressure of chest wall
- natural position of chest wall: 60% VC
- origin: tissue elasticity only
Residual volume: tendency to recoil outward
TLC: tendency to recoil inward
Recoil pressure of lung-chest system
- Natural position
- EEP: 35% VC
- Volume of air remained: FRC
- No energy required to maintain this position:
—> outward recoil of chest = inward recoil of lung
(Pneumothorax (chest wall puncture / alveolar rupture): air leak into pleural space —> increase intrapleural pressure to atmospheric level —> chest recoil outward, lung collapse) - Inhaling beyond EEP
- energy spent to overcome inward lung recoil, helped by outward chest recoil - Inhaling beyond relaxed position of chest wall (60% VC) —> TLC
- EXTRA energy spent to overcome inward lung recoil + inward chest recoil - Exhaling beyond EEP —> RV
- energy spent to overcome outward chest recoil, helped by inward lung recoil
Changes of EEP in diseased state
- Emphysema: EEP ↑
- decrease in lung elasticity —> decrease in inward lung recoil —> lung curve shift left
- balance outward chest recoil at higher volume - Airway obstruction: EEP ↑
- energy loss in overcoming airway resistance (Raw) —> air trapping - Pulmonary fibrosis: EEP ↓
- increase in lung elasticity (stiffness: hard to distend) —> increase in inward lung coil —> lung curve shift right
- balance outward chest recoil at lower volume
Compliance
Slope of static volume-pressure curve (volume change against transmural pressure change)
Reciprocal of elasticity
(0. 1) Total compliance = change in volume / transthoracic P gradient (Palv-Patm)
(0. 2) Pulmonary / Lung compliance = change in volume / transpulmonary P gradient (Palv-Ppl)
(0. 2) Chest compliance = change in volume / transchestwall P gradient (Pcw-Patm)
1/total compliance = 1/pulmonary compliance + 1/chest compliance
—> total compliance < pulmonary compliance and chest compliance
—> either lung compliance / chest compliance ↑ —> total compliance ↑
Factors affecting lung compliance (CL)
- Lung volume (very low / very large lung volume —> ↓CL (flat slope))
- Lung size (adult CL > children CL: less overall change in alveolar volume)
- Posture (supine ↓ lung volume —> ↓CL (flat slope in sigmoid curve))
- Pulmonary blood volume (vascular congestion in heart failure —> ↑CL)
- Age (↑CL due to loss of elasticity)
- Pathological condition (Emphysema ↑CL, Fibrosis/Oedema ↓CL)
Factors affect chest wall compliance (Ccw)
- Posture (supine ↓Ccw)
- Skeletal muscle disease (spasticity, rigidity ↓Ccw)
- Chest wall deformation (kyphoscoliosis ↓Ccw)
Airway resistance
Pressure difference between mouth (atmosphere) and alveoli / air flow rate
- 50% upper airway (nose, mouth, pharynx, larynx)
- 40% medium airway (trachea, bronchi)
- 10% small airway (bronchioles) —> due to enormous combined cross-sectional area by dichotomous branching —> therefore small airway disease cannot be detected by airway resistance
Factors affecting airway resistance (Raw)
- Lung volume and Radial traction (↑vol —> ↑lung recoil pressure —> ↑radial traction —> 扯開airway —> ↓Raw)
- Bronchomotor tone
- PNS: ACh stimulation —> ↑tone —> bronchoconstriction —> ↑Raw
- SNS: NE stimulation —> ↓tone —> bronchodilation —> ↓Raw
- ↓PaCO2 + inflammatory mediator —> ↑tone —> ↑Raw - Bronchial secretion, oedema, vascular congestion (↑Raw)
- Air density / viscosity (in deep sea dive) (↑Raw)
- Transmural pressure (↑Ptp —> ↓Palv —> ↓Raw + air flows in)
Work of breathing
Pressure x Volume
- elastic work (against elastic force)
- resistive work (against airway and tissue resistance / viscous force)
Total work of breathing:
Work against elastic force (greater portion) + Work against Raw (smaller portion)
Inhalation: work is required to
- overcome elasticity —> stored as potential energy —> recovered during exhalation
- overcome Raw
Exhalation: no work is required
1. Stored potential energy is released —> elastic recoil
Factors affecting work of breathing
- Frequency of breathing
- Tidal volume
- Normal breathing rate (12-20) —> minimal work is required
—> changing breathing pattern, in order to maintain same level of ventilation
1. higher breathing frequency + decreased depth of breathing
—> ↓elastic work but ↑resistive work
2. lower breathing frequency + increased depth of breathing
—> ↓resistive work but ↑elastic work
***Emphysema, Fibrosis
Emphysema: Static - ↓elasticity, ↑CL Dynamic - ↓Radial traction / ↑Raw - ↑EEP / ↑FRC / air-trapping - ↑TLC (↓IRV, ↑ERV, ↑RV) - no change/↓VC ***- ↓FEV1 + no change/↓FVC —> FEV1/FVC <80%
Fibrosis: Static - ↑elasticity, ↓CL Dynamic - ↑Radial traction / ↓Raw - ↓EEP / ↓FRC / harder to distend lung - ↓TLC (↓IRV, ↓ERV,↓RV) - ↓VC ***- no change/↓FEV1 + ↓FVC —> FEV1/FVC>80%
