V/Q Mismatch & Hypoxemia

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

Good (stating V/Q mismatch consists of two opposing forms: dead space and shunt)
Okay (really good explanations, but sometimes confusing)
- https://www.youtube.com/watch?v=RJ-H8_0-8wk
- https://www.youtube.com/watch?v=pRIkwjlFRgo
Hella confusing, read with caution
- https://www.uptodate.com/contents/oxygenation-and-mechanisms-of-hypoxemia