Respiratory Physiology - Ventilation Flashcards
Weibel’s model of airways
Idealised model of the airways with 23 levels of branching (23 airway generations)
Total cross sectional area of lung with subsequent airway generations
Exponentially increases
Significantly increased area as reach respiratory zone
Implication of significant increase in total cross sectional area (eg as reach respiratory zone)
Decreased forward flow of air through airways
Mechanism of gas transfer from conducting zone to respiratory zone
Diffusion of air due to large increase in cross sectional area
Implication of gas diffusion and lung disease
Pollutants collect in junction between anatomical dead space and respiratory zone as larger particles diffuse less easily
Typical tidal volume
6-8 ml/Kg
Average size adult ~500 ml
Typical Alveolar gas volume
3000 ml
Typical anatomical dead space volume
150 ml
Typical pulmonary capillary blood volume
70 ml
Pulmonary blood flow per minute definition and typical value
Cardiac output
~ 5 L/min
Alveolar ventilation calculation
(Tidal volume - Anatomic dead space) x RR
Eg. typically:
(500 ml - 150 ml) x 15 = 5250 ml
Alveolar ventilation definition
Volume of fresh gas reaching blood/air barrier each minute
Typical Ventilation perfusion ratio
Alveolar ventilation per minute / Pulmonary blood flow per minute
As per previous flashcards roughly = 5250 / 5000 ml/min
Roughly = 1
Spirometry measurements and definitions diagram
Measurements that cannot be measured using spirometry
Functional residual capacity
Residual volume
How to measure Functional Residual Capacity and Residual Volume
Helium dilution
Why is helium used for helium dilution measurement of FRC and RV
Helium very insoluble to not taken up by blood
Helium dilution calculation graphic
Helium dilution calculation explained
Concentration of helium equilibrates across box and lungs, and as not taken up by lung then concentration can be used to calculate lung volume
Boyle’s Law (for gases)
Pressure x Volume is constant at a constant temperature
I.e if gas is in fixed space and volume decreases, pressure must increase and vice versa
Newer method of calculating FRC and RV
Tidal volume calculation
Dead space volume + Alveolar gas volume
Where does expired CO2 come from
Alveoli - no CO2 comes from dead space as no gas exchange occurs there
Alveolar ventilation equation use
Calculate alveolar ventilation as it is difficult to measure anatomic dead space
Alveolar ventilation equation
Alveolar ventilation = K x CO2 ventilation / PACO2 (alveolar but is essentially equal to arterial PCO2 in healthy individuals)
I.e PaCO2 is inversely proportional to alveolar ventilation
K is a constant
V dot is volume per unit of time
FCO2
Fractional concentration of CO2
Fractional concentration adds up to 1 rather than percentage which adds up to 100
Tidal volume range
6-8 ml/kg
Inspiratory reserve volume range
30 - 40 ml/kg
Expiratory reserve volume range
20 ml/kg
Residual volume range
20 ml/kg
Vital capacity range
60 ml/kg
Functional residual capacity range
35 - 45 ml/kg
Total lung capacity range
90 ml/kg
