Compliance and work of breathing Flashcards
What is pulmonary compliance?
The distensibility and elasticity of the lungs and thorax
How is compliance measured?
Ratio between change in volume and change in the distending pressure
i.e. C = ΔV/ΔP
What is hysteresis?
Lung compliance is higher during expiration than inspiration
Why is the lung more compliant during expiration than inspiration?
During expiration, lung surface area decreases faster than surfactant can be removed from the liquid lining
The density of surfactant molecules rapidly increases, which decreases surface tension and increases compliance
What conditions increase lung compliance?
Emphysema
What conditions decrease lung compliance?
Pulmonary congestion, fibrosis and restrictive lung diseases
What factors decrease the thoracic wall compliance?
Deformity of the thoracic wall
Arthritis
Skeletal muscle diseases
Obesity
Describe the innervation of the smooth muscles in the trachea and bronchi
Controlled by the autonomic nervous system:
* Sympathetic nervous system causes bronchial dilatation
* Parasympathetic nervous system causes bronchial constriction and increases mucus secretion
What is airway resistance?
It is the resistance produced by the respiratory passages to air flow during breathing
How can flow of air through the respiratory system be calculated?
Flow = ∆pressure (∆P)/resistance (R)
How can air resistance be calculated?
Resistance = (8 x length of airway x viscosity of air)/(π x radius^4)
Since the length of the airways and the viscosity of the air does not change, the variable is radius.
Bronchodilatation = Increase radius → decrease resistance → increase flow
Why is epinephrine given to asthmatics?
Epinephrine is a mediator of the sympathetic nervous system that causes bronchodilatation
Why are asthma attacks more frequent in the morning?
Early morning accompanied by increase parasympathetic effect that causes bronchoconstriction
Types of work during inspiration
ETA
Elastic work: Compliance work that is required to expand the lungs against the lung and chest elastic forces. 65% of work of breathing
Tissue resistance work: Work required to overcome the viscosity of the lung and chest wall structures. 7% of work of breathing
Airway resistance work: Work required to overcome airway resistance to movement of air into the lungs. 28% of work of breathing
What diseases increase the work of breathing?
Diseases of the lung such as pneumonia, pulmonary fibrosis and pulmonary edema.
Increased surface tension due to decreased surfactant secretion
Narrowing of the air passages by bronchial asthma.
