High Altitude Physiology — MCQs
Compensating mechanism involved in acclimatization to altitude is:
Which drug is given to prevent acute mountain sickness?
Which of the following is seen in high altitude climbers?
What is the most immediate hematological adaptation that occurs during high-altitude exposure to improve oxygen delivery to tissues?
A hyperventilating patient has the following ABG values: pH=7.53, pCO2=20 mmHg, HCO3= 26 mEq/L. What is the most likely diagnosis?
Which equation is used to calculate physiological dead space?
What is the primary effect of acetazolamide at high altitudes?
Which physiological adaptation does not happen at high altitudes?
Which flow volume curve recording is shown below?
During heavy exercise the cardiac output (CO) increases up to five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by
High Altitude Physiology Indian Medical PG Practice Questions and MCQs
Question 1: Compensating mechanism involved in acclimatization to altitude is:
- A. Respiratory depression
- B. Hypoventilation
- C. Hyperventilation (Correct Answer)
- D. Respiratory acidosis
Explanation: ***Hyperventilation*** - **Hyperventilation** is the primary immediate compensatory mechanism at altitude, increasing alveolar ventilation to improve **oxygen uptake** despite lower partial pressures of oxygen. - This response is mediated by the **carotid bodies**, which sense the reduced arterial PO2 and stimulate the respiratory center. *Respiratory depression* - **Respiratory depression** would worsen hypoxia at high altitude by further reducing **oxygen intake**. - This is not a compensatory, but rather a detrimental, response in this setting. *Hypoventilation* - **Hypoventilation** decreases the amount of air reaching the alveoli, exacerbating the **hypoxia** present at high altitudes. - This would further reduce the **partial pressure of oxygen** in the blood, which is counterproductive for acclimatization. *Respiratory acidosis* - **Respiratory acidosis** results from **hypoventilation** and CO2 retention. - Acclimatization leads to **respiratory alkalosis** due to increased CO2 excretion from hyperventilation, which is then partially compensated by renal mechanisms.
Question 2: Which drug is given to prevent acute mountain sickness?
- A. Acetazolamide (Correct Answer)
- B. Diltiazem
- C. Digoxin
- D. Dexamethasone
Explanation: ***Acetazolamide*** - This drug is a **carbonic anhydrase inhibitor** that acidifies the blood and causes compensatory hyperventilation, increasing oxygenation. - It is the **first-line prophylactic agent** for acute mountain sickness (AMS) and is best started 24-48 hours before ascent. - Most effective and widely recommended for AMS prevention. *Digoxin* - This is a **cardiac glycoside** used to treat heart failure and irregular heartbeats. - Its mechanism of action is unrelated to the physiological changes that cause acute mountain sickness. *Diltiazem* - This is a **calcium channel blocker** primarily used for hypertension, angina, and certain arrhythmias. - It has no known role in the prevention or treatment of acute mountain sickness. *Dexamethasone* - While **dexamethasone** can be used for AMS prophylaxis, it is typically reserved as an **alternative agent** when acetazolamide is contraindicated or not tolerated. - It is more commonly used for **treatment** of severe altitude illness including **High Altitude Cerebral Edema (HACE)** and **High Altitude Pulmonary Edema (HAPE)**. - **Acetazolamide remains the preferred first-line prophylactic agent** due to its mechanism of action that directly addresses the underlying pathophysiology of AMS.
Question 3: Which of the following is seen in high altitude climbers?
- A. Hyperventilation
- B. Pulmonary edema
- C. Decreased PaCO2
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - High altitude climbers experience **hypoxia**, which triggers several physiological responses as the body tries to compensate. - **Hyperventilation**, **pulmonary edema**, and **decreased PaCO2** are all common occurrences in individuals exposed to high altitudes. *Hyperventilation* - **Hypoxia** at high altitudes stimulates the peripheral chemoreceptors, leading to an increased respiratory rate and depth. - This increased ventilation is a compensatory mechanism to try and increase **oxygen intake**. *Pulmonary edema* - **High-altitude pulmonary edema (HAPE)** is a potentially life-threatening condition caused by exaggerated hypoxic pulmonary vasoconstriction. - This leads to increased pulmonary arterial pressure, capillary leakage, and **fluid accumulation in the lungs**. *Decreased PaCO2* - The increased respiratory rate due to **hyperventilation** causes an excessive exhalation of carbon dioxide. - This results in a **decreased partial pressure of arterial carbon dioxide (PaCO2)**, leading to respiratory alkalosis.
Question 4: What is the most immediate hematological adaptation that occurs during high-altitude exposure to improve oxygen delivery to tissues?
- A. Increased red blood cell mass
- B. Reduced erythropoietin production
- C. Increased white blood cell count
- D. Increased 2,3-BPG levels (Correct Answer)
Explanation: ***Increased 2,3-BPG levels*** - **2,3-Bisphosphoglycerate (2,3-BPG)** is an organic phosphate that binds to hemoglobin, reducing its affinity for oxygen and thereby facilitating oxygen release to tissues. - This is a **rapid adaptation** in response to hypoxia at high altitudes, occurring within hours to days, providing an immediate improvement in oxygen delivery. *Increased red blood cell mass* - An increase in **red blood cell mass (polycythemia)** is a more chronic adaptation, typically taking weeks to months to develop in response to sustained hypoxia. - While it ultimately improves oxygen-carrying capacity, it is not the most immediate hematological adaptation. *Reduced erythropoietin production* - High-altitude exposure actually leads to **increased erythropoietin (EPO) production** by the kidneys due to tissue hypoxia. - This increased EPO stimulates erythropoiesis, leading to the delayed increase in red blood cell mass. *Increased white blood cell count* - An **increased white blood cell count (leukocytosis)** is primarily associated with infection, inflammation, or stress, not with the physiological response to high-altitude hypoxia for improving oxygen delivery. - It does not directly contribute to the oxygen-carrying capacity of the blood.
Question 5: A hyperventilating patient has the following ABG values: pH=7.53, pCO2=20 mmHg, HCO3= 26 mEq/L. What is the most likely diagnosis?
- A. Metabolic alkalosis
- B. Metabolic acidosis
- C. Respiratory alkalosis (Correct Answer)
- D. Respiratory acidosis
Explanation: ***Respiratory alkalosis*** - The pH of 7.53 indicates **alkalemia**, and the low pCO2 (20 mmHg) is the primary driver, signifying **respiratory alkalosis** - A hyperventilating patient exhales more CO2, leading to a decrease in its partial pressure in the blood and a subsequent rise in pH - The HCO3 is within normal range (26 mEq/L), indicating **uncompensated respiratory alkalosis** *Metabolic alkalosis* - This would be characterized by a high pH and an elevated **HCO3**, but the HCO3 is within the normal range (26 mEq/L) - While it causes alkalemia, the primary disturbance here is respiratory, not metabolic *Metabolic acidosis* - This would present with a **low pH** and a low **HCO3**, which is contrary to the given ABG values - The patient's pH is elevated, indicating an alkalotic state, not acidotic *Respiratory acidosis* - This would be defined by a **low pH** and an elevated **pCO2**, which is the exact opposite of the provided ABG results - The patient's high pH and low pCO2 rule out respiratory acidosis
Question 6: Which equation is used to calculate physiological dead space?
- A. Dalton's law
- B. Bohr equation (Correct Answer)
- C. Charles's law
- D. Boyle's law
Explanation: ***Bohr equation*** - The Bohr equation is used to calculate **physiological dead space**, which is the sum of anatomical dead space and alveolar dead space. - It relates the partial pressure of carbon dioxide in arterial blood to the partial pressure of carbon dioxide in expired air, along with **tidal volume** and expired volume. *Dalton's law* - Dalton's law states that the **total pressure** exerted by a mixture of non-reactive gases is equal to the **sum of the partial pressures** of individual gases. - It is used to calculate partial pressures of gases in a mixture, not dead space. *Charles's law* - Charles's law describes the relationship between the **volume and temperature** of a gas at constant pressure. - It states that the volume of a given mass of gas is directly proportional to its absolute temperature. *Boyle's law* - Boyle's law describes the inverse relationship between the **pressure and volume** of a gas at constant temperature. - It is fundamental to understanding mechanics of breathing, but not dead space calculation.
Question 7: What is the primary effect of acetazolamide at high altitudes?
- A. Increase CO2
- B. Decrease ventilation
- C. Elevate pH
- D. Reduce HCO3- (Correct Answer)
Explanation: ***Reduce HCO3-*** - Acetazolamide is a **carbonic anhydrase inhibitor**, which primarily acts in the renal tubules to prevent the reabsorption of bicarbonate (HCO3-). - This leads to a loss of bicarbonate in the urine, causing **metabolic acidosis**, which in turn stimulates ventilation and counteracts the effects of high altitude. *Increase CO2* - Acetazolamide's action to induce metabolic acidosis through bicarbonate excretion actually **stimulates ventilation**, leading to a *decrease* in CO2, not an increase. - An increase in CO2 would further depress respiration and worsen high-altitude symptoms. *Decrease ventilation* - The primary goal of acetazolamide at high altitude is to *increase* ventilation by inducing metabolic acidosis, which then stimulates peripheral chemoreceptors. - A decrease in ventilation would exacerbate **hypoxia** and symptoms of acute mountain sickness. *Elevate pH* - By promoting the excretion of bicarbonate (a base), acetazolamide effectively *lowers* the blood pH, creating a state of **metabolic acidosis**. - An elevated pH (alkalosis) would suppress the respiratory drive, which is counterproductive at high altitudes.
Question 8: Which physiological adaptation does not happen at high altitudes?
- A. Pulmonary vasoconstriction
- B. Respiratory acidosis (Correct Answer)
- C. Hypoxia
- D. Polycythemia
Explanation: ***Respiratory acidosis*** - At high altitudes, the primary physiological response to **hypoxia** is to increase ventilation, leading to a decrease in **arterial PCO2**. - This reduction in **PCO2** causes **respiratory alkalosis**, not acidosis, as the body tries to compensate for the lower oxygen levels. *Pulmonary vasoconstriction* - This is a significant physiological response to **hypoxia** at high altitudes, leading to an increase in **pulmonary artery pressure**. - Its purpose is to divert blood flow to better-ventilated areas of the lung, but it can also contribute to **pulmonary hypertension**. *Hypoxia* - Reduced **atmospheric pressure** at high altitudes directly results in a lower partial pressure of oxygen (**PO2**), leading to **hypoxia**. - This low **PO2** is the primary trigger for most other physiological adaptations seen at high altitudes. *Polycythemia* - Prolonged exposure to **hypoxia** stimulates the kidneys to release **erythropoietin (EPO)**, which in turn increases **red blood cell production**. - This adaptive increase in **red blood cell count** and **hemoglobin concentration** aims to enhance the oxygen-carrying capacity of the blood.
Question 9: Which flow volume curve recording is shown below?
- A. Parenchymal obstructive airway disease
- B. Restrictive defect
- C. Variable extrathoracic obstruction (Correct Answer)
- D. Variable intrathoracic obstruction
Explanation: ***Variable extrathoracic obstruction*** - This flow-volume loop shows a **flattening** of the **inspiratory limb** (I), while the expiratory limb (E) remains relatively normal. - Variable extrathoracic obstructions, such as vocal cord dysfunction or laryngeal edema, predominantly affect airflow during inspiration because the extrathoracic airway pressure becomes more negative than atmospheric pressure during inspiration, leading to airway narrowing. *Parenchymal obstructive airway disease* - Characterized by a **diminished expiratory flow** and a **scooped-out appearance** of the expiratory limb on the flow-volume loop. - The inspiratory limb is usually well preserved, which is not seen here as the inspiratory limb is significantly affected. *Restrictive defect* - Presents as a **miniature version of a normal flow-volume loop**, indicating reduced lung volumes, but usually with preserved flow rates for the given lung volume. - Both inspiratory and expiratory flows would be proportionally reduced, unlike the isolated inspiratory flattening shown. *Variable intrathoracic obstruction* - This typically causes a **flattening or reduction in the expiratory limb** of the flow-volume loop, with a relatively normal inspiratory limb. - During forced expiration, the positive intrathoracic pressure compresses the compromised intrathoracic airway, leading to obstruction.
Question 10: During heavy exercise the cardiac output (CO) increases up to five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by
- A. Large amount of smooth muscle in pulmonary arterioles
- B. Increase in the number of open capillaries (Correct Answer)
- C. Sympathetically mediated greater distensibility of pulmonary vessels
- D. Smaller surface area of pulmonary circulation
Explanation: ***Increase in the number of open capillaries*** - During heavy exercise, the significant increase in cardiac output is accommodated by the **recruitment of previously closed pulmonary capillaries**. - This recruitment, along with **distension of existing capillaries**, reduces overall pulmonary vascular resistance, allowing blood flow to increase without a substantial rise in pulmonary arterial pressure. *Large amount of smooth muscle in pulmonary arterioles* - While pulmonary arterioles do contain smooth muscle, their primary role is in **regulating regional blood flow** and response to hypoxia, not facilitating large increases in overall blood flow during exercise. - The pulmonary circulation is characterized by **low resistance** and high capacitance compared to the systemic circulation, meaning it has less smooth muscle tone at baseline. *Sympathetically mediated greater distensibility of pulmonary vessels* - The pulmonary vasculature has **limited sympathetic innervation** compared to systemic vessels, and sympathetic activity plays a minor role in its distensibility during exercise. - Changes in pulmonary vascular resistance during exercise are primarily due to **mechanical factors** (recruitment and distension) rather than neurogenic control. *Smaller surface area of pulmonary circulation* - The pulmonary circulation actually has a **vast capillary surface area** crucial for efficient gas exchange. - A smaller surface area would lead to **higher resistance** and a greater pressure increase for a given flow, which contradicts the observation during exercise.
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