Ventilation Perfusion Matching Flashcards
Ventilation is greater at the base of the lung than at the apex is a standing vertical patient, why is this the case?
A vertical gradient in pressure is established due to the weight of the lung. Pleural pressure is less negative at the base. The lung is more inflated at the apex at rest and has a lower compliance to new air coming in. The higher compliance is at the base of the lung.
Where is V/Q ratio greatest?
Towards the top of the lung. This is because the effect of gravity in expanding the blood vessels at the base of the lung is greater than the effect of gravity on the vertical distribution of ventilation. Consequently the ratio of V/Q increases towards the apex of the lung.
What is a shunt?
Perfusion without ventilation. The shunt has a low V/Q ratio.
What is alveolar dead space?
ventilation without perfusion. There is a high V/Q ratio
How are alveolar PO2 and PCO2 related to V/Q?
PO2 is directly proportional to V/Q. A higher V/Q means a higher PO2. PCO2 is inversely related and thus a high V/Q means a low PCO2
What is a normal A-a Gradient?
P_A O2 = P’‘c’’ O2. PaO2 is slightly less than P”c”O2 due to normal shunt.
What happens to the A-a gradient in inspiratory hypoxia?
It is a normal A-a gradient but PaO2 is decreased. Breathing gas enriched with O2 improves blood oxygenation.
What happens to the A-a gradient with a diffusion problem?
There is an increased A-a gradient with hypoxemia. P_AO2 is normal but P”c”O2 is less than it. PaO2 is decreased (hypoxemia). A-a gradient is increased at level of pulmonary capillaries. Breathing gas with O2 will improve blood oxygenation.
What happens to the A-a gradient with a Right-to-Left shunt?
The A-a gradient is increased and there is hypoxemia. P_AO2 is normal and P”c”O2 is equal to it. Pa O2 is decreased. A-a gradient is increased at level of shunt admixture. V/Q ratio is greatly decreased in shunted blood (perfusion without ventilation). Breathing gas enriched with O2 will NOT improve blood oxygenation
What happens to the A-a gradient with alveolar dead space?
The A-a gradient increases with hypoxemia. End-tidal P_A O2 is elevated and P”c”O2 is less than it. Functional alveolar P_AO2 is decreased due to decreased alveolar ventilation. PaO2 is decreased. A-a gradient is increased if using end-tidal PO2. V/Q ratio is greatly increased. Breathing gas enriched with O2 will improve blood oxygenation.
