Updated: 2/13/2019

V/Q Mismatch

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This illustration compares ventilation (V) and blood flow (Q) in the three zones of the lung. 

V/Q Ratio
  • "Normal" V/Q = 0.8
    • V = alveolar ventilation
    • Q = pulmonary blood flow (perfusion)
  • "Normal" V/Q depends on "normal" respiratory rate, tidal volume, and cardiac output
    • PaO2 = 100 mm Hg
    • PaCO2 = 40 mm Hg
  • Ventilation/perfusion matching is essential for ideal gas exchange of O2 and CO2
    • "ideal" V/Q = 1.0
    • exercise  ↑ cardiac output  vasodilation of apical arteries
      • V/Q approaches 1.0
Distribution of V/Q in Lung
  • Ventilation and perfusion are nonuniformly distributed in normal, upright lung
  • Zone 1
    • apex of lung 
    • decreased ventilation and decreased perfusion
    • V/Q is highest (= 3.0) because of relatively greater decrease in perfusion 
      • wasted ventilation
      • PaO2 is highest and PaCO2 is lowest
        • organisms that thrive in high O2 (e.g., TB) flourish in apex of lung
  • Zone 3
    • base of lung
    • V/Q is lowest (= 0.6)
      • wasted perfusion
      • PaO2 is lowest and PaCO2 is highest
V/Q Defects
  • Dead Space (V/Q → ∞) 
    • e.g., pulmonary embolism (blood flow obstruction)
    • ventilation of lung regions that are not perfused
      • wasted ventilation
      • physiologic dead space
      • no gas exchange occurs
        • PAO2 has same composition as humidified, inspired air (= 150 mm Hg)
        • PACO2 = 0 mm Hg
    • 100% O2 improves PaO2
  • Shunt (V/Q = 0)
    • e.g., airway obstruction, 
    • dependent portion of lung in ARDS can act as "shunt" 
    • perfusion of lung regions that are not ventilated
      • wasted perfusion
      • no gas exchange
        • pulmonary capillary blood has same composition as venous blood
      • 100% O2 does not improve PaO2

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(M1.PL.13.27) In which of the following pathological states would the oxygen content of the trachea resemble the oxygen content in the affected alveoli?

QID: 100830
1

Emphysema

23%

(15/65)

2

Pulmonary fibrosis

12%

(8/65)

3

Pulmonary embolism

43%

(28/65)

4

Foreign body obstruction distal to the trachea

9%

(6/65)

5

Exercise

12%

(8/65)

M 1 C

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(M1.PL.13.15) A child accidentally aspirates a small pebble that lodges in the lower portion of the right inferior lobe. A radiograph is shown in Figure A. What changes are expected to occur in the arterial blood supply for this portion of the lung?

QID: 100818
FIGURES:
1

Increased, secondary to arterial vasodilation

6%

(5/84)

2

Increased, secondary to active hyperemia

4%

(3/84)

3

No change, due to autoregulation

5%

(4/84)

4

Decreased, secondary to arterial vasoconstriction

82%

(69/84)

5

Decreased, secondary to a decrease in surfactant synthesis

0%

(0/84)

M 1 D

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