Updated: 12/6/2019

Response to High Altitude

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Overview
 
  • High altitude ↓ atmospheric pressure (Patm) and ↓ alveolar PO2
  • Ventilation
    • ↓ alveolar PO2↑ respiratory rate (hyperventilation)
      • ↓ alveolar PO2 stimulates peripheral chemoreceptors in aortic bodies and carotid bodies to instruct medullary inspiration center to increase respiratory rate
  • Arterial blood
    • ↑ ventilation rate → ↑ PaO2 and ↓ PaCO2 respiratory alkalosis  
      • CO2 + H2O  H2CO3  H+ + HCO3-
        • ↓ CO2 shifts equilibrium left
          • ↓ H+↑ pH
    • respiratory alkalosis ↑ renal HCO3- excretion
      • carbonic anhydrase inhibitors (e.g., acetazolamide) ↑ renal HCO3- excretion and create a metabolic acidosis, which can be used to "acclimate" someone going to a higher altitude because they will naturally increase ventilation to compensate for metabolic acidosis
  • Pulmonary blood flow
    • ↓ alveolar PO2 hypoxic vasoconstriction of pulmonary vasculature ↑ pulmonary vasculature resistance ↑ pulmonary arterial pressure
      • pulmonary arterial pressure increases to maintain a constant blood flow
        • right ventricle pumps against ↑ pulmonary arterial pressure (↑ afterload)
          • chronic hypoxic vasoconstriction of pulmonary vasculature causes cor pulmunale (right ventricular hypertrophy (RVH))
  • Cerebral blood flow 
    • primarily controlled by PCO2
      • ↓ PCO2 → vasoconstriction
      • therapeutic hyperventilation (↓ PCO2→ ↓ intracranial pressure by decreasing cerebral perfusion
        • used in cases of cerebral edema
        • effect is usually only temporary
    • PO2 can have effects during severe hypoxic events
      • ↓ PO2 → vasodilation and ↑ cerebral perfusion pressure
  • Cellular changes
    • chronic hypoxia ↑ erythropoietin → ↑ hemoglobin concentration and ↑ hematocrit
      • synthesis of erythropoietin in kidneys
        • erythropoietin acts on bone marrow to stimulate RBC synthesis
      • ↑ hemoglobin concentration↑ O2 carrying capacity and ↑ total O2 content
    • ↑ mitochondria
  • Oxygen-hemoglobin dissocation curve
    • ↑ 2,3-BPG shifts to right
      • ↓ hemoglobin affinity for O2 ↑ O2 unloading
 

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Questions (3)
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(M1.PL.17.4695) A healthy 20-year-old male college student attempts to climb Mount Everest and travels to the Tibetan plateau by plane. Upon landing, he feels increasingly dizzy and fatigued. He notices that he is breathing faster than usual. What is the initial stimulus for the most likely acid-base disorder? Tested Concept

QID: 107937
1

Hypoxic pulmonary vasodilation

6%

(11/171)

2

Decreased partial pressure of alveolar oxygen

64%

(110/171)

3

Increasing arterial partial pressure of carbon dioxide

22%

(37/171)

4

Worsened diffusion limitation of oxygen

4%

(7/171)

5

Undiagnosed atrial septal defect

1%

(1/171)

M 1 C

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(M1.PL.13.117) A 31-year-old female with a history of anxiety has a panic attack marked by dizziness, weakness, and blurred vision. Which of the following most likely accounts for the patient’s symptoms? Tested Concept

QID: 100920
1

Oxygen toxicity

9%

(5/58)

2

Increased arterial CO2

12%

(7/58)

3

Decreased cerebral blood flow

79%

(46/58)

4

Decreased respiratory rate

0%

(0/58)

5

Carotid artery obstruction

0%

(0/58)

M 1 D

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