Shunt and V/Q Relationships

Question 1

Anatomic Dead Space

Answer 1

• Volume of the conducting airways
  
  • There is no gas exchange in these regions
• Approximately 150mL

Question 2

Physiologic Dead Space

Answer 2

-Volume of lung that doesn’t participate in gas exchange
-approximately equal to anatomic dead space in normal lung
-May be greater in diseases with a V/Q mismatch
VD=VT[(pACO2-pECO2)/pACO2]
VT is tidal volume
pACO2 is pCO2 of alveolar gas
pECO2 is expired CO2

Question 3

Alveolar ventilation rate

Answer 3

Alveolar ventilation= (Tidal volume-dead space) X breaths/min

Question 4

Dead Space

Answer 4

• V/Q=infinity
• West zone 1 (alveolar dead space)—RECRUITABLE—should be 0 in normal lungs
• Conducting air passages (anatomic dead space)—NOT RECRUITABLE
• 30% normally

Question 5

Distribution of Pulmonary blood flow

Answer 5

• Zone 1: apex-Blood flow is lowest
  
  • alveolar pressure>arterial pressure>venous pressure
  • High alveolar pressure compress capillaries, decrease flow
  • Can occur w/ hemorrhage or positive pressure ventilation
• Zone 2: Middle-blood flow is medium
  
  • arterial pressure>alveolar pressure>venous pressure
  • Inferiorly, lung arterial pressure increases due to gravity
  • arterial>alveolar pressure drives blood flow
• Zone 3: base- blood flow is highest
  
  • arterial pressure>venous pressure>alveolar pressure
  • blood flow driven by difference in arterial and venous beds

Question 6

Regulation of pulmonary blood flow

Answer 6

• Hypoxic vasoconstriction
• Opposite of other organs
• Physiologically important b/c it diverts blood flow away from poorly ventilated regions
• Fetal pulmonary blood flow low b/c so hypoxic

Question 7

Understand the five physiologic reasons for hypoxemia

Answer 7

1. V/Q mismatch
2. Shunt
3. Diffusion abnormality
4. Low FiO2
5. Hypoventilation

Question 8

Right to Left Shunt

Answer 8

• Normally occurs to a small extent b/c 2% cardiac output bypasses the lung
• May be as high as 50% in some congenital defects
• Seen in Tetrology of Fallot
• Results in decrease in arterial pO2 b/c mix blood
• magnitude of RT to LT shunt can be measured by having patient breath 100% O2 and measuring degree of O2 dilution of arterial blood

Question 9

Left to Right Shunt

Answer 9

• More common b/c the pressure is higher on the LT side than RT
• Congenital abnormalities (PDA) or traumatic injury
• DO NOT result in decreased arterial pO2
  
  • pO2 elevated on RT side of heart b/c mixed blood

Question 10

V/Q Ratio

Answer 10

• Ratio of alveolar ventilation to pulmonary blood flow
• Matching is important to achieve maximal O2 and CO2 exchange
• V/Q ratio is normally 0.8 and results in pO2 100 and pCO2 40

Question 11

V/Q Ratios in Different Parts of the Lung

Answer 11

• Blood flow is lowest at the apex and highest at the base of the lung do to gravity
• Ventilation is lower at the apex and higher at the base, but regional differences aren’t as great as profusion
• Therefore, V/Q is higher at the apex and lower at the base
• Apex: pO2 higher and pCO2 lower b/c more gas exchange
• Base: pO2 lower and pCO2 higher b/c less gas exchange

Question 12

V/Q ratio in airway obstruction

Answer 12

• If airway is completely blocked, then ventilation is zero
• If blood flow is normal, V/Q is zero
• This is a shunt
• There is no gas exchange b/c not ventilated
• pO2 and pCO2 of pulmonary capillaries approach values in mixed venous blood
• Increased A-a gradient

Question 13

V/Q ratio in pulmonary embolism

Answer 13

• If blood flow completely blocked, blood flow is zero
• If ventilation is normal, V/Q is infinity
• This is called dead space
• No gas exchange b/c not perfused
• pO2 and pCO2 reach that of inspired air

Question 14

A-a gradient

Answer 14

• A-a gradient
• Alveolar Gas
• (760 mmHg-47).21-(1.2PaCO2)
• Measure arterial O2 (PaO2)
• A-a Alveolar Oxygen-Arterial Oxygen
• Normal ( 20)
• Shunt, ventilation-perfusion inequalities, and diffusion impairment