Ventilation-Perfusion Relationships

Question 1

Basal and Apical Alveoli VA/Q Ratio

Answer 1

Basal gets a lot more perfusion and more ventilation, but not enough to offset increased perfusion. So low VA/Q and thus low pO2 and high pCO2 (towards mixed venous blood)
Opposite in apical (towards dead space)

Question 2

3 Mechs to Minimize VA/Q Differences w/in Lung

Answer 2

Constriction of arterioles if pO2 falls below 50 mmHg
Constriction of bronchiolar smooth muscle of pACO2 falls
Slow reduction of surfactant/ventilation in alveoli receiving inadequate blood flow

Question 3

2 Reasons in Reduction of O2 Transfer w/ VA/Q Mismatch

Answer 3

1. Alveoli w/ low VA/Q produce less O2 saturation, bc Hb saturation falls off steeper from values lower than 100 mm Hg than it increases from values higher than 100 mm Hg
2. Low VA/Q units with their lower pO2 and O2 content contribute more to total blood flow than high VA/Q units

Question 4

Clinical Calculation of Shunt

Answer 4

Qt x CaO2 = (Qs x CvO2) + ((Qt - Qs) x CiO2)

So Qs/Qt = (CiO2 - CaO2)/(CiO2 - CvO2)

Question 5

Alveolar Gas Equation

Answer 5

pAO2 = piO2 - (pACO2/R) + F
R = VCO2/VO2, usually about 0.8
Ignore F

Question 6

5 Possible Causes of Hypoxemia (and how to rule them out)

Answer 6

Increased VA/Q nonuniformity - breathe 100% O2. If it does increase arterial O2, it’s this
Shunt - breathe 100% O2. If doesn’t increase arterial O2, it’s shunt
Hypoventilation - opiate/barb overdose, will have CO2
Diffusion Limitation - measure DLco
Breathing low Inspired pO2 - would need increased elevation