Ventilation-Perfusion Relationship - Trachte

Question 1

What are the four causes of hypoxemia?

Answer 1

1. Hypoventilation
2. Diffusion limitation
3. Shunt (right-left)
4. Ventilation-perfusion mismatch

Question 2

How do O₂ levels change from air → airway?

Airway → alveoli?

Alveoli → lung?

Lung → tissue?

Answer 2

• O₂ levels decrease in all
  
  • Humidified air = 150 mmHg
  • Alveoli = 100 mmHg
  • Tissues = 1-100 mmHg
• Oxygen is constantly being removed by blood if alveolus is perfused and it is functioning normally
  
  • P_O2 will be higher if blood is not removing it (such as in the apex of the lung)
• Oxygen is constantly being replenished by inhalation of fresh air

Question 3

What happens in a pulmonary shunt?

Answer 3

• Alveolus collapse → doesn’t oxygenate blood
• Unoxygenated blood mixes with oxygenated blood
  
  • results in hypoxia that cannot be rectified by breathing 100% O₂

Question 4

What are alveolar and blood gases determined by?

Answer 4

• The balance of ventilation and perfusion
  
  • ventilation → provides O₂
  • perfusion → removes O₂
    
    • mismatch of these processes will cause abnormal values

Question 5

What happens if there is no perfusion, but the alveoli are still ventilated?

Answer 5

• P_O2 and P_CO2 will approach that of inspired gas
  
  • P_O2 = 150
  • P_CO2 = 0

Question 6

What happens if there is perfusion but no ventilation (aka obstruction to airflow)?

Answer 6

• P_O2 and P_CO2 will approach that of venous blood
  
  • P_O2 = 40 mmHg
  • P_CO2 = 46 mmHg

Question 7

What are the normal ventilation:perfusion (VQ) ratio differences occurring in normal lungs at the apex and at the base?

Answer 7

• Apex has HIGH ventilation-perfusion ratio
  
  • P_O2 = 132 mmHg
  • P_CO2 = 28 mmHg
• Base has a LOW ventilation-perfusion ratio
  
  • P_O2 = 89 mmHg
  • P_CO2 = 42 mmHg

Question 8

What are the causes of Hypoventilation or inadequate ventilation?

Answer 8

• O₂ is not replenished fast enough in the alveoli → alveolar P_O2 declines
• Causes:
  
  • Drugs → opiates
  • Chest wall damage
  • Paralysis
  • Brain injury
  • Pneumothorax
  • Impaired phrenic nerve

Question 9

What causes diffusion limitations?

Answer 9

• Reducing the surface area available for gas exchange
• Increasing the diffusion distance
  
  • can result in hypoxia

Question 10

How do you calculate the partial pressure of oxygen in the alveoli?

Answer 10

PA_O2 = PI_O2 – (PA_CO2 / R)

• NORMALLY EQUALS 100!!!!
• PA_O2 = alveolar P_O2
• PI_O2 = inspired air P_O2
• PA_CO2 = alveolar P_CO2
• R = respiratory quotient
  
  • produced CO₂ / consumed O₂
  • dependent on substrate
    
    • R=1 with carbohydrate as substrate
    • R=0.7 with fatty acid as substrate

Question 11

Why is hypoxia more difficult to rectify than hypercapnia?

Answer 11

• Hypoxia = too little O₂
  
  • cannot be corrected by more rapid breathing
  • O₂ dissociation curve is sigmoidal and almost flat at the tope of the range
• Hypercapnia = too much CO₂
  
  • CO₂ elimination is a function of rate of breathing → breath more rapidly to dilute out CO₂
  • CO₂ dissociation curve is linear