V/Q Mismatch & Hypoxemia

VQmismatch

V/Q Mismatch Confusion

V/Q mismatch is a confusing subject because the terms “dead space” and “shunt” are not very intuitive definitions. Moreover, there is a lot of confusion in textbooks and online from what I’ve seen. I read Marino’s The ICU Book and several articles on Up-to-date and came up with this outline that I think will be helpful for residents like myself.

In essence, V/Q mismatch is a spectrum: with dead space (no perfusion) and shunt (no ventilation) on either end. Some other people tell a narrative of V/Q mismatch vs. shunt (but that to me seems imprecise). Some pathologies have a primarily dead space defect problem, and therefore oxygen can help; other pathologies have a primarily shunt defect problem and therefore oxygen cannot help.

My hypoxemia pdf has the full version of these notes: http://www.henrydelrosario.com/wp-content/uploads/2016/07/Hypoxemia.pdf


Definitions

  • Hypoxemia
    • Definition
      • Low partial pressure of oxygen (PaO2) in the blood (low level of oxygen in the blood)
      • It does not always cause tissue hypoxia
    • Causes
      • Hypoventilation
      • V/Q mismatch
        • Primarily dead space defect **(often called V/Q mismatch)**
        • Primarily shunt defect
      • Diffusion limitation
      • Reduced inspired O2 tension
  • Hypoxia
    • Definition
      • Insufficient oxygen to meet a tissue’s metabolic demand (low level of oxygen in a tissue or organ)
      • Hypoxemia can lead to tissue hypoxia, but not always
  • Oxygenation
    • Definition
      • Process of oxygen diffusing from alveolus to pulmonary capillary to bind to hemoglobin or dissolve in plasma

Causes of hypoxemia

  • Hypoventilation
    • Mechanism
      • Lung alveolus is a space of 100% gas → if the partial pressure of one gas increases the partial pressure of another gas must decrease
      • In hypoventilation there is decrease air movement → alveolar increase of carbon dioxide (PACO2) → alveolar decrease of oxygen (PAO2)
      • A-a gradient is normal
    • Causes
      • CNS depression (drug overdose, opiates, CNS lesions)
      • Obesity hypoventilation (Pickwickian) syndrome
      • Impaired neural conduction (ALS, GB)
      • Muscular weakness (myasthenia gravis, hypothyroidism, critical illness myopathy)
      • Poor chest wall mechanics (kyphoscoliosis)
    • Tx
      • Responds to supplemental oxygen
  • V/Q mismatch
    • Definition
      • Imbalance of ventilation and perfusion
      • A-a gradient is almost always elevated
    • Causes (two opposing forms; per Marino and UpToDate: Mechanical ventilation article)
      • Primarily dead space defect
        • COPD, asthma, PE
      • Primarily shunt defect
        • PNA, pulm edema, ARDS
    • Normal lung
      • A normal lung has V/Q mismatch: V/Q ratio is higher in the apices and lower at the bases (higher ventilation in the apices, more perfusion in the bases)
    • Dead space
      • Definition
        • Ventilation is excessive to perfusion (V/Q >1)
        • Ventilated lung but no blood flow → no gas exchange
        • ***(when the pathology is of mostly dead space defects = people tend to call this simply a V/Q mismatch)***
        • Memory cue: When I see DEAD, I think NO BLOOD = DEAD LUNG. There is SPACE, because alveoli are ventilated and open.
      • Anatomic dead space
        • Large conducting airways have no contact with capillary blood
        • Pharynx, trachea
        • Using a snorkel :)
      • Physiologic dead space
        • Poor perfusion
        • PE
        • Reduced blood flow (low CO)
        • COPD (emphysema destroys alveolar septae and pulm capillary bed → limited blood flow to a fairly well oxygenated lung)
        • Positive pressure ventilation (can inc ventilation to alveoli that do not have corresponding inc in perfusion → worsens dead space)
      • Tx
        • Responds to supplemental oxygen
    • Shunt
      • Definition
        • Ventilation is inadequate to perfusion (V/Q <1)
        • When blood passes from the right to the left side of the heart without being oxygenated
      • Anatomic shunts
        • When blood bypasses alveoli
        • Can cause extreme V/Q mismatch (V/Q=0)
        • Intracardiac shunts (ASD, VSD), AVMs
      • Physiologic shunts
        • When non-ventilated alveoli are perfused
        • Atelectasis
        • Disease with alveolar filling (PNA, pulm edema, ARDS)
        • Obesity
      • Tx
        • DOES NOT respond to supplemental oxygen
          • Blood is not in contact with an alveolar membrane that can exchange oxygen → so breathing 100% will not correct hypoxemia
        • ICU
          • Particularly in the ICU, for ARDS, a shunt is created where lungs are perfused but ventilation is limited due to alveoli filling → thus, increasing FiO2 has limited benefit → thus, you can decrease FiO2 without causing more hypoxia
          • Positive pressure ventilation, esp with PEEP, can tx shunt caused by atelectasis, by opening more alveoli (presumably perfused)
  • Diffusion limitation
    • Definition
      • Impaired movement of oxygen from the alveolus to the pulmonary capillary due to problem with diffusion through the alveolar membrane
      • Exercise induced-hypoxemia
      • A-a gradient is elevated
    • Mechanism
      • During rest, oxygen diffuses slowly, allowing even impaired diffusion to oxygenate sufficiently
      • During exercise, there is less time for oxygenation → oxygenation is impaired
    • Causes
      • ILD, pulmonary fibrosis
    • Tx
      • Responds to supplemental oxygen
  • Reduced inspired O2 tension
    • Definition
      • Decreased FiO2 or atmospheric pressure will decrease PiO2
      • PiO2 = FiO2 x (Patm – PH2O)
      • A-a gradient is normal
    • Mechanism
      • Body naturally hyperventilates → PaO2 inc but PCO2 dec
    • Causes
      • High altitude

Sources

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