HRR: ventilation-perfusion matching

Question 1

What is a typical ventilation-perfusion ratio?

Answer 1

0.8

Question 2

Where does most fresh ventilation go? Why?

Answer 2

The base of the lungs; the alveoli are more compliant whereas the alveoli toward the apices are swollen and less compliant.

Question 3

Physiologically, why are the alveoli toward the apices more open?

Answer 3

Gravity when we’re upright leads to a more negative pressure toward the apices, causing the alveoli to be more expanded.

Question 4

Describe dead space/wasted ventilation as a ventilation-perfusion mismatch.

Answer 4

If alveoli don’t receive pulmonary flow, the ventilation reaching these alveoli is wasted. This adds to the overall dead space.

Question 5

What happens in lungs without compensation when ventilation-perfusion mismatch occurs due to halted perfusion?

Answer 5

The altered lung’s alveoli gas concentration takes on the composition of atmospheric air. The normal lung receives increased blood flow, resulting in lower PO2, higher PCO2, and thus a lower V/Q ratio.

Question 6

Overtime, how can we compensate for wasted ventilation?

Answer 6

Lower PCO2 in the lung that is not perfusing but is ventilating results in constriction of the bronchioles and vessels. This shunts more air to the normal airways and reduces blood flow to the lung.

Question 7

What happens to compliance in a lung with wasted ventilation?

Answer 7

Lower production of surfactant from a lack of O2 reduces compliance and leads to shrinking alveoli.

Question 8

What happens in a lung without compensation during ventilation-perfusion mismatch caused by a shunt?

Answer 8

The V/Q ratio becomes 0, and alveolar gas assumes composition of mixed venous blood. The ventilation of the other lung increases and V/Q increases.

Question 9

How do we compensate for ventilation-perfusion mismatch from a shunt?

Answer 9

Local alveolar hypoxia causes vasoconstriction of arterioles feeding the alveoli. This allows more blood to flow to areas where there is good ventilation to normalize the V/Q ratio.

Question 10

What conditions leading to hypoxemia can be improved with exogenous O2?

Answer 10

Low V/Q ratio, decreased FiO2, dead space, diffusion abnormality, hypoventilation.

Question 11

Which conditions leading to hypoxemia will not be improved with exogenous O2?

Answer 11

Anatomical and physiological shunts.

Question 12

what hypoxemic conditions lead to norml A-a gradient

Answer 12

hypoventilation and FiO2 change (altitude changes)