Assessment of V/Q

Question 1

Dead space vs. V/Q mismatch vs. shunt

Answer 1

• 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
  • V_phys = V_anat + V_alv
  • Causes work without benefit, doesn’t usually cause hypoxemia unless disease is severe, decreases with exercise
  • ↑ P_aCO2
• ​
  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

Question 2

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

Answer 2

• 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

Question 3

Major causes of low V/Q and shunts

Answer 3

• Low V/Q:
  
  • Regional resistance (bronchitis, asthma)
  • Hypoventilation
  • Diffusion Defec
  • t

Emphysema

* 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)

Question 4

very low V/Q vs. shunt

Answer 4

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

Question 5

Pulse oximetry measurement characteristics and common problems

Answer 5

• = 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.

Question 6

Causes of hypoxemia (5)

Answer 6

• 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

Question 7

Calculation of A-a gradient

Answer 7

• 10 or below is normal
• = P_AO2 - P_aO2