Ventilation-Perfusion Relationships

Question 1

What are the 4 physiological causes of hypoxemia?

Answer 1

• Hypoventilation
• Diffusion impairments
• V/Q mismatch
• Shunt

Question 2

As you go from the top to the bottom of the lung, the Ventilation/Perfusion (V/Q) ratio…

Answer 2

gets lower

because there’s more blood flow than airflow the further down you get

Question 3

Alveolar CO₂ level is equal to

Answer 3

Arterial CO₂

We can make thsi assumption becuase CO₂ is so extremely soluble and diffusible; whereas, O₂ has an A-a gradient

Question 4

Respiratory quotient

Answer 4

0.8

Corrects for the pCO₂ produced depending on the source - carbs or fats

Question 5

What is the pO₂ in inspired air as it reaches the alveoli?

(Alveolar gas equation)

Answer 5

pO_{2, alveolar} is the fraction of inspired O₂ multiplied by atmospheric pressure (minus water vapor pressure) minus pCO_{2, alveolar}

1. Atmospheric pressure at sea level is 760, so pO₂ at the tip of your nose is (760)(0.21) = 160mmHg
2. Warm and humidify with 47mmHg of water vapor, so pO₂​ at the trachea is (760-47)(0.21) = 150mmHg
3. Add CO₂ into the total mix: (760-47)(0.21) - pCO₂/0.8
   
   1. 0.8 is the respiratory quotient
   2. Don’t forget: pCO_{2, alveolar} = pCO_{2, arterial}

Question 6

How does hypoventilation cause hypoxemia?

Answer 6

Breathing too little.

• The less we breathe, the more CO₂ builds up in the blood
  
  • –> Because pCO_{2, arterial} = pCO_2,alveolar, CO₂ will build up in the alveoli, too
  • –> there is much less pO_{2, alveolar,} decreasing the gradient for oxygen diffusion into the blood

Question 7

Diffusion impairments

Answer 7

Increases thickness of the membrane between alveolar air and blood in the capillaries –> decrease oxygenation of blood

Ex) edema, lung disease, pulmonary fibrosis, pulmonary proteinosis, pulmonary infections

Question 8

Shunt: blood passes through the lungs w/o encountering outside air at all

What are situations that shunt can occur?

Answer 8

• Alveoli filled with a fluid or substance
• Large blood vessel in the lung that has no alveoli
• Intracardiac shunt: Hole between two sides of the heart allows deoxygenated venous blood to flow directly through

Indicated when giving supplemental oxygen fails to improve the pt’s blood oxygen level

Question 9

Usually, ventilation and perfusion are about equal. However,

Answer 9

There’s a little more airflow at the top of the lung and a little more bloodflow at the bottom

Note: this is not the same as ventilation, which also requires bloodflow.

Question 10

What’s the difference between V/Q mismatch and shunt?

Why does this matter?

Answer 10

Shunt involves complete occlusion of airflow -> V=0

Important because giving supplemental oxygen will increase the diffusion gradient of pO_{2, alveolar} if there’s a V/Q mismatch, but won’t do anything fora shunt.

Question 11

Describe the two forms of V/Q mismatch

Answer 11

• Decreased alveolar ventilation (left)
  
  • Ex) asthma, bronchitis, pneumonia impede air flow to regions of the lung
  • When there’s no airflow at all, it’s shunt
• Decreased perfusion to alveoli (right)
  
  • Ex) West’s zone 1, compressed lung blood vessels, pulmonary embolism
  • If there’s no perfusion at all, V/Q becomes infinity and we have dead space

Question 12

According to the alveolar ventilation and alveolar gas equation, what are the consequences of failure to match ventilation to CO₂ production (e.g. drug overdose, impaired lungs, emphysema)?

Answer 12

CO₂ build up in the blood (hypercapneic respiratory failure)

• changes in serum pH
• causes CO₂ to build up in the alveoli -> decreased gradient for O₂ to enter blood
  
  • Can be compensated for with supplemental oxygen

Question 13

Hypoventilatoin, diffusion limitation, shunt, and V/Q mismatch all have what effect on P_aCO₂?

Answer 13

Decreases the partial pressure of arterial CO₂

When there’s a gas exchange problem, hypoxemia occurs before CO₂ problems. The initial increases in CO₂ that occur at first are compensated for by increasing ventilation.

Question 14

Why do people hyperventilate when they have hypoxemia?

Answer 14

The initial increases in CO₂ that occur are compensated for by increasing ventilation.

Question 15

Explain the effect of supplemental oxygen on patients with hypoventilation using the alveolar gas equation

Answer 15

Increases the fraction of inspired oxygen from 0.21 to 0.28 –> increases the 𝑃_IO2 from 150 to 200

Thus, an increase in CO₂ can easily compensated for by increasing the fraction of inspired oxygen.

Question 16

Which has more variation from the top to the bottom of the lung - air flow or blood flow?

Answer 16

Blood flow (perfusion)

Question 17

Why does the V/Q ratio decrease further down the lung?

Answer 17

Both V & Q INCREASE as you go down, but variation in perfusion is greater.

Thus, V/Q decreases because Q increases faster than V does as you go down the lung

Question 18

Explain the regional difference in gas exchange in the lung

Answer 18

• Top
  
  • Little or no blood flow
  • High V/Q ratio
  • High oxygen
    
    • Makes it easier for reactivation tuberculosis to survive in the top of the lung
  • Low carbon dioxide
• Bottom
  
  • More blood flow relative to ventilation
  • Lower oxygen
  • Higher carbon dioxide

Question 19

Two ways to calculate predicted A-a gradient based on age in adults

Answer 19

• Divide patient’s age by 3.
  
  • Ex) 10 for a 30 year old
• Divide age by 4, then +4
  
  • Ex) 11.5 fo ra 30 year old