pk 5 - Ventilation and Perfusion Flashcards
What is the normal V/Q ratio for the whole lung? Why?
0.8. As the total blood flow is slightly greater than the total alveolar ventilation
Why does V/Q ratio vary throughout the lungs?
It is a consequence of the effect of gravity upon the distribution of ventilation and perfusion
How does ventilation vary throughout the lung differ and why?
Greater at the base. At the apex there is greater retraction of the lung that causes there to be a greater negative intrapleural pressure at the apex which then leads to the alveoli there being inflated towards their maximal values and thus less compliant than alveoli at the base. Ventilation is thus directed towards the base of the lungs.
How does perfusion vary throughout the lung?
Blood flow is greater at the base. Impact of gravity on the hydrostatic pressure within the pulmonary capillaries. Capillaries at the base are more likely to be open and greater distended leading to a reduced resistance to flow at base compared to apex.
Whilst V and Q vary ……….. through the lung, the V/Q ratio varies ………. through the lung with the high V/Q ratios at the ………. and lower V/Q ratios at the …… . The specific V/Q ratio of any particular segment of the lung determines its ……… and ……… . high V/Q ratios tend to draw alveolar gas tensions towards those of ……… …………, whilst low V/Q ratios draw alveolar gas tensions towards those of ………… ………….
linearly non-linearly apex base PAO2 PACO2 room air venous blood
If a regional abnormality in the lungs impacts of the V/Q ratio, it will affect the …….. ……. …….. and this change can give rise to ………….. ………… (including ………….. ………….. ……….. HPV) that will affect the ………. and ……. …….. in that region in an attempt to bring the V/Q ratio back towards its normal value
alveolar gas tensions corrective reflexes hypoxic pulmonary vasoconstriction airflow blood flow
Not all blood entering the left heart has been ……………. in the lungs, with …..% of venous blood being ‘………….’ through the left hear without passing through the lungs. The two natural R-L shunts are the ………. ……. and the ………… ……….. Note that shunts are at the extreme, ……. end of V/Q ratio continuum.
oxygenated 1-2 shunted Thesbian veins bronchial circulation low
What contributes to the normal A-a difference normally observable in health?
How does A-a difference change with age?
shunts
V/Q mismatch
The A-a difference increases gradually with age as the V/Q ratio changes
What are the two systems where pathological shunts are found? Give some examples
Pulmonary disease e.g. airway block, collapse alveoli
Cardiovascular anatomical abnormalities - patent ductus arteriosus, AVS defects
Describe the effect of a high V/Q ratio on alveolar gas tensions.
Describe the corrective reflex response to this.
Describe the corrective result.
Increase alveolar oxygen and decrease alveolar CO2 (hyperoxia and hypocapnia).
Corrective reflex is vasodilation and bronchoconstriction.
This reduces the V/Q ratio.
Describe the effect of a low V/Q ratio on alveolar gas tensions.
Describe the corrective reflex response to this.
Describe the corrective result.
Decrease alveolar oxygen and increase alveolar CO2 (hypoxia and hypercapnia)
Corrective reflex is vasoconstriction and bronchodilation.
This increases the V/Q ratio
What is the term for the lungs response to hypoxia as a result of a low V/Q ratio?
This is usually beneficial but when may it be harmful?
hypoxic pulmonary vasoconstriction
at altitude it may result in pulmonary hypertension
What is the mechanism for hypoxic pulmonary vasoconstriction?
hypoxia inhibits potassium efflux, membrane depolarisation, influx of calcium, pulmonary artery smooth muscle contracts
The alveolar gas equation can be used to asses if a patient hypoxia and hypercapnia are solely due to hypoventilation or if there are other contributing factors. It is found that a patients A-a difference in 2.5kPa. What does this tell us?
There alveolar gas tensions are different not just because of the hypoventilation but also because of:
- increased V/Q mismatch
- increased shunts
- impaired diffusion
