Altitude and Diving Physiology Practice Questions

Q: What is the barometric pressure at a depth of 10,000 feet in the sea?

523 mmHg. **Explanation:** The barometric (atmospheric) pressure decreases exponentially as altitude increases. This is a fundamental concept in high-altitude physiology. At sea level, the standard barometric pressure is **760 mmHg**. As one ascends, the weight of the air column above decreases, leading to a fall in pressure and a subsequent decrease in the partial pressure of oxygen ($PO_2$), which can lead to hypoxia. At an altitude of **10,000 feet**, the barometric pressure is approximately **523 mmHg**. This is a high-yield value to remember for NEET-PG, as it represents the threshold where compensatory physiological mechanisms (like hyperventilation) become significant. **Analysis of Options:** * **Option A (760 mmHg):** This is the standard pressure at **sea level**. * **Option B (523 mmHg):** **Correct.** This is the pressure at 10,000 feet. * **Option C (349 mmHg):** This is the approximate pressure at **20,000 feet** (e.g., near the summit of Mt. Kilimanjaro). * **Option D (226 mmHg):** This is the approximate pressure at **30,000 feet** (near the summit of Mt. Everest). **High-Yield Clinical Pearls for NEET-PG:** 1. **Fraction of Oxygen ($FiO_2$):** Remains constant at **21%** regardless of altitude; only the total barometric pressure (and thus $PO_2$) changes. 2. **Alveolar Gas Equation:** As barometric pressure falls, $PAO_2$ falls. At 10,000 feet, $PAO_2$ drops to about 60-67 mmHg (compared to 100 mmHg at sea level). 3. **Acute Mountain Sickness (AMS):** Usually begins to manifest at altitudes above 8,000 feet. 4. **Rule of Thumb:** Barometric pressure roughly halves for every 18,000 feet of ascent.

Q: What is the cause of "Raptures of the deep"?

Nitrogen narcosis. **Explanation:** **Nitrogen Narcosis** (Option C) is the correct answer. Often referred to as "Raptures of the Deep," this condition occurs in deep-sea divers who breathe compressed air at depths typically exceeding 100 feet (approx. 4 atmospheres). According to **Henry’s Law**, the solubility of a gas in a liquid is proportional to its partial pressure. At high pressures, nitrogen—which is normally physiologically inert—dissolves into the lipid-rich membranes of neurons. This produces an anesthetic effect similar to nitrous oxide, leading to euphoria, impaired judgment, and disorientation, which can be fatal for a diver. **Why other options are incorrect:** * **Carbon monoxide narcosis (A):** CO has a high affinity for hemoglobin, leading to hypoxia, but it does not cause the specific "rapture" or anesthetic effect associated with deep diving. * **Carbon dioxide narcosis (B):** While high levels of $CO_2$ can cause confusion and coma (often seen in severe COPD), it is not the primary cause of the "raptures" sensation in diving. * **Oxygen toxicity (D):** Also known as the **Paul Bert effect**, high partial pressures of oxygen cause oxidative stress and seizures, but this is distinct from the narcotic effect of nitrogen. **High-Yield Pearls for NEET-PG:** * **The Martini Effect:** A common rule of thumb is that every 50 feet of depth is equivalent to drinking one dry martini on an empty stomach. * **Heliox:** To prevent nitrogen narcosis, deep-sea divers use a mixture of Helium and Oxygen. Helium is used because it has low lipid solubility and lower density. * **Decompression Sickness (The Bends):** Caused by rapid ascent where dissolved nitrogen forms bubbles in tissues/blood. This is different from narcosis, which occurs *at* depth.

Q: A man is planning to travel from sea level to Colorado to climb Mount Wilson (14,500 feet, barometric pressure = 450 mm Hg). Before his trip, he takes acetazolamide. What response would be expected before he makes the trip?

Elevated ventilation. **Explanation:** The correct answer is **Elevated ventilation**. **Mechanism of Action:** Acetazolamide is a carbonic anhydrase inhibitor. By inhibiting this enzyme in the proximal convoluted tubule of the kidney, it prevents the reabsorption of bicarbonate ($HCO_3^-$), leading to **bicarbonate diuresis**. This loss of base results in a mild **hyperchloremic metabolic acidosis**. To compensate for this drop in blood pH, the peripheral and central chemoreceptors trigger the respiratory center to increase the rate and depth of breathing (hyperventilation) to "blow off" $CO_2$. This pre-emptive increase in ventilation improves oxygenation and helps prevent Acute Mountain Sickness (AMS) by counteracting the respiratory alkalosis that typically occurs at high altitudes. **Analysis of Incorrect Options:** * **A. Alkalotic blood:** Acetazolamide causes metabolic *acidosis* due to bicarbonate loss, not alkalosis. * **B. Normal ventilation:** Ventilation will be *increased* as a compensatory mechanism for the induced metabolic acidosis. * **D. Normal arterial blood gases:** The ABG will show a decreased $HCO_3^-$ and a decreased $PaCO_2$ (due to compensatory hyperventilation), thus it will not be normal. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Acetazolamide is the gold standard for the prevention and treatment of **Acute Mountain Sickness (AMS)**. * **Acclimatization:** It speeds up the natural acclimatization process by acidifying the blood, which offsets the alkalosis caused by hypoxic ventilatory drive at high altitudes. * **Side Effects:** Common side effects include paresthesia (tingling in extremities) and a metallic taste when consuming carbonated beverages. * **Contraindication:** Avoid in patients with severe sulfonamide allergies.

Q: Acute cerebral edema at high altitude occurs due to which of the following mechanisms?

All of the above. High Altitude Cerebral Edema (HACE) is a severe form of altitude sickness characterized by a breakdown of the blood-brain barrier (BBB) and brain swelling. The pathophysiology is multifactorial, involving both hemodynamic and cellular changes. **Explanation of the Correct Answer (D):** The development of HACE is driven by a combination of the following mechanisms: 1. **Cerebral Arteriolar Dilation (Option B):** In response to systemic hypoxia, the body triggers **hypoxic cerebral vasodilation** to maintain oxygen delivery to the brain. This autoregulatory response increases cerebral blood flow. 2. **Increased Capillary Blood Pressure (Option A):** As arterioles dilate, the high pressure from the arterial system is transmitted directly to the fragile capillary beds (increased hydrostatic pressure). This "over-perfusion" forces fluid out of the vessels into the brain parenchyma (vasogenic edema). 3. **Hypoxic Damage/Capillary Leak (Option C):** Severe hypoxia triggers the release of inflammatory mediators and vascular endothelial growth factor (VEGF). This increases vascular permeability and causes direct oxidative damage to the capillary endothelium, further worsening the leak. Since all three mechanisms work synergistically to cause cerebral swelling, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Definition:** HACE is typically defined as the onset of ataxia, altered consciousness, or papilledema in a person with Acute Mountain Sickness (AMS). * **Classification:** It is primarily a **vasogenic edema** (leakage of fluid), not cytotoxic edema. * **Management:** The definitive treatment is **immediate descent**. * **Pharmacology:** **Dexamethasone** is the drug of choice for HACE (reduces inflammation and stabilizes the BBB), whereas Acetazolamide is primarily used for prevention/AMS. * **Gold Standard:** Hyperbaric oxygen therapy (Gamow bags) can be used as a bridge until descent is possible.

Question 1

What is the additional amount of oxygen transported in 100 ml of blood in a subject breathing 100% oxygen under hyperbaric conditions of 4 ATA compared to normobaric conditions (1 ATA)?

Accepted Answer

6 ml

Answer

9 ml

Answer

3 ml

Answer

0.3 ml

Question 2

Which of the following illnesses is NOT associated with high altitude?

Accepted Answer

Hypoventilation

Answer

Cerebral edema

Answer

Venous thrombosis

Answer

Refractory cough

Question 3

What is the barometric pressure at a depth of 10,000 feet in the sea?

Accepted Answer

523 mmHg

Answer

760 mmHg

Answer

349 mmHg

Answer

226 mmHg

Question 4

What is the cause of "Raptures of the deep"?

Accepted Answer

Nitrogen narcosis

Answer

Carbon monoxide narcosis

Answer

Carbon dioxide narcosis

Answer

Oxygen toxicity

Question 5

In a person acclimatized to high altitude, O2 saturation is maintained because of which of the following mechanisms?

Accepted Answer

All of the above

Answer

Increased Hb concentration

Answer

Increased blood volume

Answer

Better diffusion in lungs

Question 6

At an altitude of 6500 meters, where the atmospheric pressure is 347 mmHg, what is the inspired partial pressure of oxygen (PO2)?

Accepted Answer

73 mmHg

Answer

63 mmHg

Answer

53 mmHg

Answer

83 mmHg

Question 7

A man is planning to travel from sea level to Colorado to climb Mount Wilson (14,500 feet, barometric pressure = 450 mm Hg). Before his trip, he takes acetazolamide. What response would be expected before he makes the trip?

Accepted Answer

Elevated ventilation

Answer

Alkalotic blood

Answer

Normal ventilation

Answer

Normal aerial blood gases

Question 8

Acute cerebral edema at high altitude occurs due to which of the following mechanisms?

Accepted Answer

All of the above

Answer

Increased capillary blood pressure

Answer

Cerebral arteriolar dilation

Answer

Hypoxic damage to capillary walls leading to capillary leak

Question 9

During acclimatization to high altitude, all of the following physiological changes occur EXCEPT:

Accepted Answer

Shift of the oxygen-hemoglobin dissociation curve to the left

Answer

Increase in erythropoietin

Answer

Increase in minute ventilation

Answer

Increase in the sensitivity of central chemoreceptors

Question 10

Which of the following may be the effect of positive 'g' acceleratory force on the body?

Accepted Answer

Pooling of blood in lower body

Answer

Increased cardiac output

Answer

Initial rise and then fall of blood pressure

Answer

Thrombocytopenia

Altitude and Diving Physiology — MCQs

Altitude and Diving Physiology — MCQs

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