Respiratory Mechanics Flashcards
-Recap mechanisms of respiration before this
What muscles are used during inspiration?
-diaphragm
-external intercostals
-sternocleidomastoids
-scalenes
What muscles are used during expiration?
-none at rest- expiration is a passive process
Forced:
-rectus abdominus
-internal/external obliques
-internal intercostals
-transverse abdominal muscle
What is FRC at rest ?
The point at which chest wall recoil and lung recoil are balanced
What factors influence ventilation?
-compliance
-resistance
-muscle strength and fatigue
What is compliance?
-The ease at which the lungs and chest wall can expand (distensibility) in response to changes in pressure
What is resistance?
the resistance to air flow usually caused by airway diameter, type of flow, lung volume
What is the equation for compliance?
-defined as change in lung volume per unit change in transpulmonary pressure
compliance=change V/ Change P
(The higher the compliance, the less pressure required to inflate the lungs for a given volume)
What factors affect lung compliance?
1-elastic properties of the lung tissue- 35-50%
2-surface tension and surfactant- 35-45%
3-lung volume and airways resistance - 5-10%
4-chest wall mechanics- 10-20%
Define lung elasticity
resistance of tissues to deformation
-opposite to compliance, so if elastance is high, compliance will be low and vice versa
What is lung elastance determined by?
-collagen and elastin fibres in pulmonary parenchyma
What is the equation for elastance?
1/ compliance
How does elastance affect compliance?
is elasticity is too high (pulmonary fibrosis), compliance will be low so more pressure is needed to expand the lungs
if lung elasticity is low emphysema) compliane will be high, the lungs expand easily, but have difficulty recoiling
Define surface tension?
-water molecules at a gas-liquid interface do not have water molecules on all sides so they cohere more stongly to the water molecules adjacent and below them
-these cohesive forces act to reduce the area of the interface
Describe surface tension in the lungs
-alveoli are like millions of tiny bubbles with an air/liquid interface
-the pressure inside the bubble is greater than outside due to surface tension
surface tension in lungs tends to reduce volume to absolute minimum
What is laplace’s law?
pressure within a bubble depends on surface tension and radius of bubble
What is the equation for laplace’s law?
pressure inside an alveolus= 2xsurface tension/radius
What is the effect of surface tension?
-if surface tension was equal all alveoli then pressure would be greater in smaller alveoli
-this creates inherently unstable lungs as small alveoli would empty into large
-this is prevented by surfactant which acts to reduce surface tension in alveoli
What is pulmonary surfactant?
-detergent like substance
-produced by type II alveolar cells
-90% phospholipid, 10% protein
-DPPC makes up about 50% of surfactant
-DPPC also known as lecithin
What property does surfactant have that makes it suitable for its role?
amphillic
-non polar- hydrophobic tail
-polar - hydrophillic head
surfactant molecules move to the air/liquid interface due to their amphillic properties and form a thin film
surface tension is reduced due to the prevention of forces being created between water molecules
Describe the role of surfactant in stabilizing alveoli
-P in smaller alveoli is reduced due to the prescence of surfactant reducing surface tension
-as alveoli increase in V the surface tension increases due to the reduction in surfactant concentration
-once the P exceeds that of adjacent, smaller alveoli, the alveolus will start to empty due to the P gradient from large to small alveoli
-slows rate of expansion of the alveoli and results in alveoli of similar sizes
-all alveoli will tend to increase or decrease in size at the same rate
What is the general role of surfactant?
-stabilizes alveoli
-small alveoli remain open at low inflating pressures
-reduces the work of breathing
-additional role in preventing fluid moving from pulmonary capillaries into the interstitium and so helps prevent pulmonary oedema
Whats the equation for specific compliance?
compliance/FRC
What are the effects of lung volume and age on lung compliance?
-compliance increases with lung volume. In order to standardise measurements, compliance is often measured as specific compliance
-Compliance increases with age dues to structural changes in the lung elastin fibres
Describe the clinical significance of compliance
-emphysema and COPD
-pulmonary fibrosis
-atelectasis
-newborn respiratory distress syndrome
Where does airway resistance come from within the lungs?
-airways resistance-the resistance to flow 80%
-elastic resistance- resistance to expansion 20%
-inertia of the respiratory system- negligible
-Airways resistance = 80% total resistance
What is airways resistance?
Resistance offered to the flow of air into and out of the lungs during breathing
measured in kPa.L-1.s-1
What factors affect airway resistance?
-airway diameter
-lung volume
-viscosity of air
What is the equation for airways resistance?
RAW+ change in P/V
pressure difference driving airflow divided by the airflow
What factors determine airway resistance?
-type of flow
-ariway calibre
-lung volume at which it is measured ( resistance decreases as lung volume increases)
What are the types of flow that determine airway resistance?
-laminar
-turbulent
-transitional
What airway calibre determines airway resistance?
-airway smooth, muscle constriction increases resistance
-dynamic airflow limitation will also increase resistance
What is the equation for resistance as quantified by poiseuille’s law?
8 x n x l/ pi x r4
n-gas viscocity
l-tube length
r-radius
If the radius halves what happens to resistance?
increases 16 fold
What is resistance measured by?
-body plethysmograph - dependent on lung volume at which it is measured
-kPa/L/s
-normal airways resistance- 0.2kPa/L/s
airway resistance drops as lung volume increases
Describe how muscle strength and fatigue affects the lungs
-lung disease, increases the load on the respiratory pump but reduces pressure generating capacity of respiratory muscles
-if respiratory muscles have high loads for a prolonged time they will fatigue
-respiratory muscle fatigue causes respiratory pump failure- resulting in hypercapnic ventilatory failure
