Ventilation perfusion relationships Lecture 7

Question 1

Ventilation and pulmonary blood flow (Perfusion) relationships

Answer 1

this relationship determines gas exchange in the lung. Matching of V and Q is critical for adequate gas exchange

Ventiliation is abbreviated to V

Perfusion is abbrevated by Q

V/Q cna be considered for the whole lung

Mismatching of V/Q is the commenest cause of hypoxaemia. Low arterial PO2.

It has much less effect on arterial CO2

Question 2

Describe hypoventilation in the perfect lung

Answer 2

Decreases ventilation/perfusion (V/Q) by decreasing V

Decreases P(A)O2 and PaO2

Increases P(A)CO2 and PaCO2

Decreases HbO2 saturation and arterial O2 content

Increases arterial CO2 content

Question 3

Describe hyperventilation in the perfect lung

Answer 3

Increases V/Q by increasing V

Increases P(A)O2 and PaCO2

Decreases P(A)CO2 and PaCO2

Has little effect on HbO2 saturation or arterial O2 content

Decreases arterial CO2 content

Question 4

What is the difference between the perfect and normal lung?

Answer 4

In a perfect lung you decrease V/Q by decreasing V for the whole lung, decreasinf PaO2 and HbO2 saturation and increase PaCO2 and arterial CO2 content (hypoventilation). Increasing V/Q by increasing V. increases PaO2 with little effect on HbO2 saturation but decreases PaCO2 and arterial CO2 content. (hyperventilation)

Hb in arterial blood is slightly less than 100% saturated with O2 as arterial blood does not perfectly exchange with alveoli blood because of shunts and V/Q mismatches

There are normal anatomical shunts and pathogenic shunts that create a dead space and result in the mixing of blood with can have an effect in arterial blood depending on the size of the shunt

Question 5

Describe the effects of gravity on V/Q in a normal lung

Answer 5

Collapse of apex capillaries, distension of basal capillaries Alveoli at the apex have a larger resting volume but basal alveoli expand more for a give change in intrapleural pressure- gravity affects ventilation less than perfusion When stood upright: Apex- PO2 of blood leaving alveoli is high and PCO2 is low- ventilation is better than perfusion Basal- PO2 of blood leaving the alveoli is low and PCO2 is high- perfusion is better than ventilation If they mix on equal volume it would give a PaO2 of 100mmHg However blood flow from the base is much greater than to the apex so it has more of an influence and the PaO2 which makes the value closer to 97mmHg

Question 6

Describe pathogens of V/Q mismatch

Answer 6

Hypoventilation- global decrease in V/Q Alveolar dead spaces due to a mucous plug Partial blockage lowering the V/Q Obstruction of blood supply- low Q Compensatory mechanisms: Hypoxia- causes pulmonary vasoconstriction and blood redistribution Hypercapnia- relaxes bronchiolar smooth muscle helps to increase V to poorly ventilated alveoli