5 - Matching Ventilation & Perfusion

Question 1

What is the definition of hypoxemia?

Answer 1

Low PaO2, typically < 85 mmHg

Question 2

What are some causes of hypoxemia? Which cause an increased a-ADO2?

Answer 2

• alveolar hypoventilation (normal a-ADO2)
• reduced PIO2 from high altitude (normal a-ADO2)
• reduced FIO2 from fire (normal a-ADO2)
• shunt (increased a-ADO2)
• diffusion abnormality (increased a-ADO2)
• V/Q mismatch (often increased a-ADO2)

Question 3

What is the definition of hypercapnia?

Answer 3

PaCO2 > 45 mmHg

Question 4

What are some causes of hypercapnia?

Answer 4

• alveolar hypoventilation
• increased dead space from shallow, rapid breathing
• increased CO2 production with fixed ventilation
• severe V/Q mismatch (blood going to poorly ventilated alveoli)

Question 5

What is the ideal V/Q? What is it actually in a normal healthy person at rest?

Answer 5

Ideal V/Q =1, Normal V/Q = 0.84

**This reflects an average across the lungs; there are differing regional distributions of perfusion and ventilation

Question 6

When upright, V/Q is highest at the [base/apex] of the lungs and lowest at the [base/apex}.

Answer 6

Highest at apex (high ventilation and low perfusion)

Lowest at base (low ventilation and high perfusion)

Question 7

How do you differentiate between a shunt and low V/Q?

Answer 7

Low V/Q will respond drastically to supplemental oxygen; shunt will not

Question 8

What are some causes of low V/Q?

Answer 8

decrease in lung compliance or increased airway resistance from:

• asthma
• pulmonary edema
• pulmonary fibrosis
• prolonged anesthesia (with atelectasis)

Question 9

What are some causes of high V/Q?

Answer 9

impaired pulmonary circulation from:

• emphysema
• pulmonary embolism
• hemorrhage
• positive pressure ventilation