3.6.4. Shunt and V/Q Relationships

Question 1

How can we categorize respiratory failure?

Answer 1

1. Hypoxemic (Type I): oxygenation failure

2. Hypercarbic (Type II): ventilation failure (resp. pump fails in CO2 elimination)

Question 2

What are the 5 causes of hypoxemia?

Answer 2

1. low inspired oxygen content
2. hypoventilation
3. diffusion abnormality
4. V/Q mismatch
5. Shunt

Question 3

Define ventilation:

Answer 3

rate of air entering alveoli

Question 4

Define perfusion:

Answer 4

rate of transfer of oxygen and carbon dioxide through respiratory membrane, determined by blood flow

Question 5

What does a ventilation-perfusion ratio (V/Q) of zero mean?

Answer 5

• this means that there is no alveolar ventilation and alveolar gases equilibrate with blood gases
• in other words, there is more blood flow than ventilation at the lung base (shunt)

Question 6

What does a high ventilation-perfusion ratio (V/Q) mean? (we talk about this in terms of infinity)

Answer 6

• there is no capillary blood flow and alveolar gases equilibrate with humidified inspired air
• in other words, there is more ventilation than blood flow at the lung apex (V/Q=infinity=dead space)

Question 7

What are some causes of shunt?

Answer 7

1. Pulmonary: pneumonia, ARDS, near drowning, alveolar filling processes like blood, pus and cells
2. Cardiac: atrial septal defects, ventricular septal defects
3. Extracardiac: arterial venous malformations, hepatopulmonary syndrome

Question 8

What could be causing hypoxia with a white x-ray?

Answer 8

• Pneumonia
• ARDS
• Pulmonary Edema
• Interstitial Lung Disease

Question 9

What could cause V/Q mismatch?

Answer 9

• Pulmonary embolism
• Asthma
• COPD
• CHF

Question 10

How is Type II Hypercarbic/Ventilatory Failure defined?

Answer 10

-defined by an elevated level of CO2 in the blood

Question 11

What are some causes of respiratory failure due to abnormal airways?

Answer 11

• COPD
• Asthma
• Cystic fibrosis

Question 12

What are some causes of respiratory failure due to peripheral neuromuscular weakness?

Answer 12

• Guillain-Barre Syndrome
• spinal cord injury
• polio
• botulism

Question 13

What causes increased dead space (no chance for gas exchange in that unit)?

Answer 13

1. Anatomic: large airways, masks (if not vented)

2. Physiologic: pulmonary embolism, asthma

Question 14

What causes increased CO2 production?

Answer 14

• Fever
• Sepsis
• Malignant Hyperthermia
• High Carbohydrate diet? (not really)
• Overfeeding (RQ>1.0)

Question 15

Why is Ventilation/Perfusion matching important for respiratory function?

Answer 15

Effective gas exchange critically depends on the relationship between ventilation and perfusion in the gas exchange units.

Question 16

What is a normal physiologic adaptation to adjust V/Q (matching)?

Answer 16

Hypoxic vasoconstriction