V/Q 2

Question 1

What are the two compensation mechanisms for low V/Q mismatch?

Answer 1

Local smooth muscle - alveolar hypoxia causes vasoconstriction to divert perfusion, Low PACO2 causes bronchial smooth muscle contraction to divert ventilation

Compensation by respiration control system -hypercapnia and hypoxia

Question 2

What is the compensation paradox?

Answer 2

Compensatory mechanisms will generally cause the PaCO2 to return to normal, resulting in normal PaCO2 and low PaO2

However, the increases in ventilation goes mostly to the unaffected unit where there is no obstruction. Since the Hb in this unit is already almost 100% saturated, there is only a very small increase in PaO2

*Patients with severe low V/Q disorders will not result in this finding

Question 3

How does restrictive disease affect V/Q?

Answer 3

Regional changes in compliance from patch disease distribution cause local reductions in ventilation

These units have low V/Q ratios because ventilation is reduced but perfusion remains the same

Question 4

What are 5 general causes of hypoxemia?

Answer 4

Breathing air with low PO2

Hypoventilation

Shunts*

Low V/Q mismatch*

Diffusion Problem*

*Large A-a gradient

Question 5

What are the physiological concerns of a pulmonary emoblism?

Answer 5

Initially, increased workload on the right heart to force CO throughout the rest of the lung

The embolus attracts immune cells that release inflammatory agents that result in inflammation, altered capillary permeability

This results in edema, destroys surfactant, causing local alveolar collapse or atelectasis

Question 6

What are the compensatory mechanisms in a pulmonary embolism?

Answer 6

Lower V/Q means PO2 is low and PCO2 is high

Hyperventilation results to address the hypercapnia, this brings the V/Q ratio back to around 0.8 in the perfused alveolus