Assessment of V/Q Flashcards Preview

CVPR: Pulmonary > Assessment of V/Q > Flashcards

Flashcards in Assessment of V/Q Deck (7)
Loading flashcards...

Dead space vs. V/Q mismatch vs. shunt

  • V/Q: Ratio of ventilation to perfusion.  Quantitative measure of gas exchange.  Best value would be 1. Normal is .8. 

    • Local regulation of V/Q occurs to limit either ventilation or perfusion to normalize the V/Q 

  • Dead space is the ventilation of un-perfused airway or alveoli

    • Anatomic - trachea, bronchi, bronchioles 

    • Alveolar - ventilation of unperfused alveoli 

    • Vphys = Vanat + Valv 

    • Causes work without benefit, doesn’t usually cause hypoxemia unless disease is severe, decreases with exercise 

    • ↑ PaCO2 

  • Shunt: Blood passing through capillaries that does not get oxygenated.  

    • Bronchial circulation is 1-2% of CO

    • Shunt is the extreme of low V/Q 


Patterns of ventilation impact on amount of dead space (+causes)

  • Rapid shallow breathing will only ventilate trachea, bronchi (minimal alveoli)
    • RR @ 40x/minw/250 ml per breath ==> maximize dead space breathing
  • Breathe deeply 10x/min at 1000 ml per breath ==> perfuse the alveoli maximally thus minimizing your dead space ventilation.
  • Causes:
    • Rapid, shallow breathing

    • PE 

    • Decreased CO 

    • Mechanical Ventilation- snorkeling, being on a ventilator 

    • Emphysema 



Major causes of low V/Q and shunts

  • Low V/Q:

    • Regional resistance (bronchitis, asthma) 

    • Hypoventilation 

    • Diffusion Defec 

    • t


    • Interstitial lung disease 

    • Congenital heart disease 

    • Pulmonary fistula 

    • Vascular lung tumor 

  • Shunt: Filled alveolar space
    • Heart failure (transudate)

    • Pneumonia / ARDS (adult respiratory distress syndrome) (exudate) 



very low V/Q vs. shunt

  • V/Q will respond well to increased FiO2 (fraction of inspired oxygen). 


Pulse oximetry measurement characteristics and common problems

  • = ratio of Deoxygenated Hb : Oxygenated Hb 

    • Ratio = SpO2 

  • Problems:
    Hb may be bound to something other than O2 

    • Carboxy-Hb absorbs in the red band  (where oxygenated Hb absorbs) 

    • Sensitive to movement, temperature, lighting, nailpolish
    • Met-Hb (3+) can’t bind O2 and increases O2 affinity at other sites.  Also absorbs @ red. 





Causes of hypoxemia (5)

  • Normal A-a Gradient

    • Altitude 

    • Hypoventilation (obesity, central apnea, neuromuscular disease, drugs) 

  • Increased A-a Gradient
    • Diffusion limitation (extreme exercise, interstitial lung disease during exertion)

    • Low V/Q 

    • Shunt 



Calculation of A-a gradient

  • 10 or below is normal 

  • = PAO2 - PaO2