Physiology: Ventilation/Alveolar Gases Flashcards
What effect does dynamic compression of airways have on normal subjects?
Limits airflow during a forced expiration
What diseases exaggerate Dynamic Compression of Airways?
Some lung diseases which have reduced elastic recoil and loss of radial traction on airways e.g. emphysema
How do people counteract dynamic compression of airways?
Purse-lipped breathing - maintain airway pressure which splints airway open & limits the flow rate.
Slows the flow of air and maintains a filling pressure in airways so they cannot collapse.
Avoids dynamic compression.
Why is the work of breathing performed?
Done to overcome the elastic properties/stiffness of the lung system = compliance and surfactant.
Overcome airways resistance (friction) = bronchoconstriction and dynamic collapse
Why is the work of breathing increased in restrictive lung diseases?
Increased work due to decreased lung compliance (stiff lungs) - work harder to change the lung volume
Why is the work of breathing increased in obstructive lung diseases?
Increased work due to increase in airway resistance (narrow pipes)
Why is the work of breathing increased in pulmonary oedema?
Increased work due to decreased lung compliance (stiff lungs) - functional restrictive disease
GO over graph of work of breathing!!
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What is the equation for the work of breathing?
Work of breathing = P x change in volume
How do we know if a patient is Hypoventilating?
Carbon dioxide is greater than 40
Ventilation is the only thing that affects carbon dioxide
What causes a work of breathing too high?
Decreased compliance, increased airways resistance (asthma, COPD)
What could be reasons as to why a patient cannot do the work of breathing?
Fatigue
Muscle Weakness
Depressed central control e.g. opiates
Why might a patient not do enough work of breathing?
Behavioural (sleep), panic attack
What happens to carbon dioxide when someone hyperventilates?
Carbon dioxide goes down.
What is the respiratory exchange ratio?
VCO2/VO2
Should be about 0.8
Where does gas exchange occur?
Diffusion across the alveolar membrane - large SA
What drives gas exchange?
Pressure gradients - alveolar pressures are important e.g. PAO2, PACO2
What else is gas exchange dependent on?
Alveolar perfusion - need the right balance of ventilation and perfusion (V/Q)
What determines the partial pressures?
Composition of inspired air
Alveolar ventilation
O2 consumption or CO2 production
Matching of alveolar ventilation to pulmonary capillary blood flow
What is R/RER and what is it dependent on?
Respiratory Quotient/Resp Exchange Ratio
Depends on diet - mixed diet is about 0.8
What is the A-a gradient?
Difference between PAO2 and PaO2 = indicates a problem with gas exchange.
Normally <10mmHg
Increased A-a = decreased gas exchange
Go over examples in lecture slides.
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