Mechanics of respiration Flashcards
During spontaneous inspiration how far does the diaphragm move down?
1 - 1.5cm
What forces explain passive expiration
Relaxation of inspiratory muscles that increased the volume of the chest cavity
Passive contraction of elastic lung tissue
Alveolar surface tnesion
Respiratory muscles are efficient or inefficient
Inefficient 90% of the energy is lost as heat
Describe the change in alveoalr pressure over the respiratory cycle
Inspiration varies from -1 to -2 –> expiration +1
Sine wave
◦ As the lung volume expands during inspiration, the alveolar pressure drops to below atmospheric. ◦ This produces a pressure gradient between the upper airway and the alveoli ◦ This pressure gradient produces airflow into the lungs ◦ At the end of inspiration, alveolar pressure equals atmospheric pressure, and there is no flow because there is no pressure gradient.
Intrapleural pressure vs time?
-5cmH20 –> -8cmH20 at peak inspiration
What is transpulmonary pressure
alveolar pressure - pleural pressure
Varies from +5cmH2O at baseline pre inspiration –> 8cmH20 throughout inspiration, and gradually returning to +5 throughout expiration
Airflow over time in the lungs
Inspiraory flow at baseline peaks 20L/min
Expiratory flow 10-15L/min
What is the bucket handle and pump handle mechanisms?
◦ Inspiratory skeletal muscle contract:
‣ “Bucket handle” movement: elevation of the ribs (mainly by the external intercostals)
‣ “Pump handle” movement: elevation of the sternum (by the sternomastoid muscle)
◦ This also increases intrathoracic volume and opposes the effect of atmospheric pressure
What is required for the lung to expand in pressures
Negative pleural pressure overcome the elastic recoil of the lung causing expansion
Describe passive expiration
◦ By relaxing, allows the chest wall and abdominal content to decrease the intrathoracic volume.
◦ Abdominal muscles, by maintaining tone, put pressure on the abdominal contents and push it into the chest cavity, making the diaphragm take on a dome shape.
◦ By intruding back into the chest cavity, the intrathoracic volume is decreased by the relaxation of the diaphragm.
What 4 variants can be used to descirbe inspiratory and expiratory processes
What happens in each fo these zones
- Diaphragm
- Chest wall
- Pleural cavity pressures
- Alveoli - including pressure, gas flows, relative volumes
Define compliance
- The change in lung volume per unit change in transmural pressure gradient (usually 100ml/cmH20)
What are the two methods of describing lung compliance?
Static and dynamic compliance
What is static compliance
Static compliance change in lung volume per unit change in pressure in the absence of flow. This requires time allowed for mobile respiratory elements to relax
What is static compliance composed of? normal values
- Chest wall compliance - 200ml/cm H20
- Lung tissue compliance - also usually cm H20
What is dynamic compliance
- Change in volume divided by change in pressure measured during normal breathing between points of apparent zero flow at the beginning and end of inspiration (PIP and PEEP measurements reached = start and stop); its components
How is dynamic compliance different to static compliance
Flow or no flow
This implicates resistance when flow is invovled
Airway resisatnce is dependent on
Frequency dependent
Pressure contribution of airway resisatnce
Preferential distribution of flow into lung units with shorter time constants –> this tendancy increases with shorter inspiratory times and increasing RR
How does dynamic compliance compare with static compliance
Dynamic compliance is always lower than static compliance as there is an extra form of resistance
What is dyanamic compliance measured between
PIP and PEEP
However there is no pause allowing for equilibrationn means slower lung units not ventilated increasing resistance
What is specific compliance?
Compliance normalised for a lung volume –> usually FRC allowing comparison between people
Factors affecting respiratory complaince
Lung
Chest wall
Lung comppliance is affected by 6
2 wet stuff
2 P’s
2 resistance/elastic
Surfactant
Blood volume
Position
Volume
Resistance/dynamic compliance
Elastic recoil
Describe the inluence of lung surfactant on complaicne
◦ Increases lung compliance, conversely a loss of surfactant reduces lung compliance
◦ Surface tension is reduced by surfactant