📌 Professor’s Notes: Quick Revision
High-yield revision notes for rapid review before university examinations.
- High altitude causes hypoxia because barometric pressure falls with increasing altitude, reducing the partial pressure of inspired oxygen. The percentage of oxygen in air remains about 21%. Symptoms usually appear above 10,000 feet (3,000 m) and become severe above 15,000 feet. Slow ascent allows acclimatization.
- The most important compensatory mechanism is hyperventilation, which is triggered by peripheral chemoreceptors (carotid bodies). Initially, hyperventilation causes hypocapnia and respiratory alkalosis, which partially suppress ventilation. With continued exposure, ventilation progressively increases due to ventilatory acclimatization.
- Ventilatory acclimatization develops over several days to two weeks by:
- Increased sensitivity of carotid body chemoreceptors.
- Removal of bicarbonate from cerebrospinal fluid, restoring its pH.
- Increased renal bicarbonate excretion, correcting respiratory alkalosis and allowing sustained hyperventilation.
- Respiratory adaptations include persistent hyperventilation, increased minute ventilation, increased pulmonary diffusion capacity, and increased maximal voluntary ventilation. The ventilatory equivalent also increases with altitude.
- Hematological adaptations:
- Increased erythropoietin secretion stimulates erythropoiesis, increasing red blood cell count and hemoglobin concentration.
- Increased 2,3-bisphosphoglycerate (2,3-BPG) shifts the oxygen-hemoglobin dissociation curve to the right, facilitating oxygen unloading to tissues.
- Cardiovascular changes:
- Early increase in heart rate, cardiac output, and blood pressure due to sympathetic stimulation.
- Increased tissue blood flow improves oxygen delivery.
- Excessive polycythemia increases blood viscosity, cardiac workload, and may cause ventricular hypertrophy during prolonged exposure.
- Tissue adaptations include increased capillary density, increased number of mitochondria, enhanced oxidative enzyme activity, increased myoglobin, and angiogenesis, all of which improve tissue oxygen utilization.
- High-altitude illnesses:
- Acute mountain sickness (AMS): Develops after rapid ascent. Symptoms include headache, fatigue, dyspnea, nausea, vomiting, insomnia, impaired coordination, and confusion. Severe cases may progress to high-altitude pulmonary edema (HAPE) or high-altitude cerebral edema (HACE).
- Chronic mountain sickness (Monge’s disease): Characterized by excessive erythrocytosis, persistent hypoxemia, pulmonary hypertension, right ventricular hypertrophy, and eventually right-sided heart failure.
- Mechanisms of HAPE and HACE:
- HAPE results from hypoxia-induced pulmonary vasoconstriction, pulmonary hypertension, and leakage of fluid into alveoli.
- HACE results from increased cerebral capillary permeability, causing cerebral edema that may progress to coma if untreated.
- Treatment:
- Immediate descent to a lower altitude is the most effective measure.
- Supplemental oxygen rapidly corrects hypoxemia.
- Acetazolamide promotes bicarbonate excretion and accelerates acclimatization.
- Dexamethasone is useful for cerebral edema.
📝 Practice MCQs
🎙️ Frequently Asked Viva Questions
High-yield viva questions with concise answers for rapid university examination preparation.
Q1. Why does hypoxia occur at high altitude even though the oxygen percentage in air remains unchanged?
Answer: The oxygen concentration remains about 21%, but barometric pressure falls, reducing the partial pressure of inspired oxygen and causing hypoxemia.
Q2. What is the most important immediate compensatory response to high altitude?
Answer: Hyperventilation is the earliest and most important response. It is triggered mainly by stimulation of the carotid body peripheral chemoreceptors.
Q3. Why is hyperventilation initially limited at high altitude?
Answer: Hyperventilation lowers arterial PCO₂, causing respiratory alkalosis, which suppresses the respiratory center and partly opposes the hypoxic drive.
Q4. What is ventilatory acclimatization?
Answer: It is the gradual increase in ventilation during continued exposure to high altitude, allowing better adaptation to chronic hypoxia.
Q5. How do the kidneys help in acclimatization?
Answer: The kidneys increase bicarbonate excretion, correcting respiratory alkalosis and allowing sustained stimulation of the respiratory center.
Q6. What is the physiological purpose of acclimatization?
Answer: Acclimatization improves oxygen delivery to tissues despite reduced atmospheric oxygen by adapting the respiratory, cardiovascular, hematological, and tissue systems.
Q7. Why does erythropoietin increase at high altitude?
Answer: Hypoxia stimulates the kidneys to release erythropoietin, which increases red blood cell production and improves oxygen-carrying capacity.
Q8. Why does 2,3-bisphosphoglycerate (2,3-BPG) increase at high altitude?
Answer: Increased 2,3-BPG shifts the oxygen–hemoglobin dissociation curve to the right, facilitating oxygen unloading to tissues.
Q9. What cardiovascular changes occur during early exposure to high altitude?
Answer: Heart rate, cardiac output, and arterial blood pressure increase because hypoxemia activates the sympathetic nervous system.
Q10. What tissue adaptations occur during long-term acclimatization?
Answer: There is increased capillary density, more mitochondria, higher oxidative enzyme activity, increased myoglobin, and angiogenesis, improving tissue oxygen utilization.
Q11. What is the physiological basis of acute mountain sickness?
Answer: Rapid ascent causes hypoxemia before acclimatization develops, producing headache, dyspnea, nausea, fatigue, and neurological symptoms.
Q12. Why does pulmonary edema develop at high altitude?
Answer: Hypoxic pulmonary vasoconstriction raises pulmonary capillary pressure, causing fluid leakage into alveoli and producing high-altitude pulmonary edema.
Q13. Why does cerebral edema occur at high altitude?
Answer: Increased cerebral capillary permeability causes brain edema, leading to headache, confusion, ataxia, coma, and possibly death if untreated.
Q14. Why is acetazolamide useful in high-altitude illness?
Answer: Acetazolamide promotes bicarbonate excretion, reduces respiratory alkalosis, and enhances ventilatory acclimatization.
Q15. Why is gradual ascent recommended at high altitude?
Answer: Slow ascent allows time for acclimatization, reducing hypoxemia and lowering the risk of severe high-altitude illness.
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