Ventilation- Perfusion Relationships In The Lungs Flashcards
Define pulmonary ventilation
The movement of air into and out of the lungs
Define pulmonary gas exchange
The diffusion of O2 from the lungs into the blood and the diffusion of CO2 from the blood into the lungs
Define pulmonary perfusion. How is it typically expressed?
The flow of blood through the pulmonary capillaries surrounding the alveoli
Q ml/min
Define alveolar ventilation. How is it typically expressed?
Amount of air reaching alveoli in a given amount of time - function of the tidal volume, the amount of air in the anatomical dead space and the respiratory rate
Expressed as V
E.g. TV: 500ml
RR: 14 /min
ADS: 150ml
14X (500-150) = 4900 ml air/ min = V
What is V/Q? What is the normal range and ideal ratio?
Ventilation/ perfusion ratio ranges 0.8-1.2 ideally = 1
What is meant by ventilation- perfusion mismatch?
In healthy lungs:
Alveoli with increased ventilation have increased perfusion. When pulmonary capillary PaO2 is low- hypoxic vasoconstriction occurs so blood is diverted to better ventilated alveoli. When alveolar PACo2 is low bronchoconstriction occurs to divert air to better perfused lung.
But in some diseased states poorly ventilated alveoli still have significant perfusion e.g. obstructed airway/ blood clot in the lung/ asthma/ COPD bc structural changes are made chronically to match normal V/Q
What is the alveolar gas composition typically? How is it kept steady?
PAO2 = 13.3KP PACO2 = 5.3 KPa
Amount of O2 brought by ventilation = amount diffusing into the blood and CO2 removed is equal
Equilibrium between partial pressures in air and blood
What happens if the V/Q is <1? What are some causes?
PACO2 rises
PAO2 falls
A new steady state is established blood equilibrates to new partial pressures
Causes:
- asthma
- COPD
- pneumonia (acute inflammatory exudate in affected alveoli)
- RDS in newborn
- pulmonary oedema
-> hypoxia
How do you convert from mmHg to KPa?
Multiply mmHg by 0.133322 (one 1, three 3s, two 2s)
Why is CO2 in blood directly proportional to alveolar ventilation but O2 not?
O2 has a carrier molecule as not very soluble- after a certain point can’t load more Hb so sigmoid
CO2 highly soluble, no carrier molecule
What is the effect of PO2 and PCO2 in MIXED blood from hyperventilation?
Effect on O2 is negligible
(small increase)
Effect of CO2 is significantly lowered
What happens if the V/Q is >1? What are some causes?
Ventilation is is higher relative to perfusion so
Alveolar PO2 is higher
Alveolar PCO2 lower
Causes:
Pulmonary embolism
Emphysema
