Updated: 9/11/2020

Oxygen Deprivation

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Hypoxemia

Alveolar Gas Equation

A-a Gradient

  • Alveolar gas equation
    • predicts the change in PAO2 that will occur for a given change in PACO2
    • PAO2 = alveolar PO2
    • PIO2 = PO2 in inspired air
      • normally, PIO2 = 150 mm Hg
    • PACO2 = alveolar PCO2
    • R = respiratory exchange ratio = (CO2 production) / (O2 consumption)
      • normally, R = 0.80
  • A-a Gradient
    • alveolar PO2 (PAO2) - systemic arterial PO2 (PaO2)
    • normal A-a gradient = 15 mm Hg (~Age/4 +4)
      • equilibration of O2 across alveolar gas and pulmonary capillary blood
        • pulmonary capillary blood becomes systemic arterial blood
      • increases with age
    • hypoxemia↑ A-a gradient
      • e.g., shunt, V/Q mismatch, pulmonary fibrosis (diffusion defect)
  • Causes of Hypoxemia (↓ PaO2)
    • High Altitude (normal A-a gradient) 
      • a decrease in barometric pressure (PB) decreases the PIO2 of inspired air
        • FiO2 doesn't change (21%)
        • ↓ PB → ↓ PIO2  ↓ PAO2 → ↓ PaO2
    • Hypoventilation (normal A-a gradient)  
      • a decrease in minute ventilation (respiratory rate x tidal volume) decreases PAO2
        • ↓ ventilation → PACO→ ↓ PAO2
        • FiO2 and PB don't change in alveolar gas equation
    • V/Q Mismatch (↑ A-a gradient)
    • Diffusion Limitation (↑ A-a gradient)
      • e.g., pulmonary fibrosis
        • ↑ diffusion distance for diffusion
      • e.g., pulmonary edema
        • ↓ surface area for diffusion, ↑ diffusion distance
    • Right-to-Left Shunt (↑ A-a gradient)
      • low O2 shunted blood mixes with and dilutes high O2 non-shunted blood
Hypoxia
  • Decreased Oxygen Delivery to Tissues
    • O2 content of blood = (O2 binding capacity) x (% saturation) + dissolved O2
      • (O2 binding capacity) x (% saturation) = O2 bound to hemoglobin
    • O2 delivery = (cardiac output) x (O2 content of blood)
  • Causes of Hypoxia (↓ O2 Delivery to Tissues)
    • circulatory (↓ Cardiac Output)
      • ↓ blood flow
    • hypoxemic
      • ↓ PaO2 → ↓ % saturation → ↓ O2 content of blood
    • anemic (see CO poisoning below)
      • ↓ concentration of hemoglobin→ ↓ O2 content of blood
    • histotoxic (example: cyanide poisoning)
      • ↓ O2 utilization by tissues (mitochondrial poison)
    • CO Poisoning
      • ↓ O2 bound to hemoglobin → ↓ O2 content of blood, ↓ O2 delivery to tissues
        • CO binds to hemoglobin with 200x greater affinity than O2
          • oxygen-hemoglobin dissociation curve left shift
      • early sign of exposure is headache; significant exposure causes coma, death
      • classical presentation at autopsy is cherry-red appearance of skin
        • not common in living patients
Ischemia
  • Causes of Ischemia (↓ Blood Flow)
    • ↓ arterial blood flow
      • e.g., atherosclerosis
    • ↓ venous return
      • e.g., Budd-Chiari syndrome
 

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(M1.PL.14.79) A 64-year-old male with a past medical history of two myocardial infarctions presents to the emergency room with shortness of breath. He notes that he stopped taking his furosemide two weeks prior, because he ran out of pills. On exam, his oxygen saturation is 78%, his lungs have crackles throughout, and jugular venous pulsation is located at the earlobe. EKG and troponin levels are normal. Which of the following is consistent with this man's pulmonary physiology? Tested Concept

QID: 106909
FIGURES:
1

Normal Aa gradient, decreased surface area for diffusion, normal diffusion distance

5%

(3/64)

2

Decreased Aa gradient, decreased surface area for diffusion, normal diffusion distance

3%

(2/64)

3

Decreased Aa gradient, increased surface area for diffusion, decreased diffusion distance

5%

(3/64)

4

Increased Aa gradient, normal surface area for diffusion, increased diffusion distance

27%

(17/64)

5

Increased Aa gradient, decreased surface area for diffusion, increased diffusion distance

59%

(38/64)

M 2 D

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