Shunt versus Dead space

Variations in the distribution of ventilation in conjunction with distribution of perfusion, can run the gamut of perfusion without ventilation (known as physiologic dead space) to anatomic shunt (blood bypasses the lungs and directly enters the systemic circulation)

Anatomic shunt cause deoxygenated blood to transfer into the systemic circulation without passing through the pulmonary circulation:

Bronchial and Thebesian Veins

Accounts for 75% of the difference between alveolar O2 and arterial O2.

shunt deadspace.jpg

 

 

dj shunt deadspace.jpg

V/Q infinite:

ventilation with 0 perfusion – alveolar dead space

High V/Q:

PAO2 high, PACO2 low

ventilation > normal, or perfusion < normal

Low V/Q: PAO2 low, PACO2 high

Ventilation < normal, or Perfusion > normal

V/Q = 0

0 ventilation with perfusion – alveolar shunt