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ABEM UHM Practice Questions — 100 Original Items
Hyperbaric Physics & Physiology (1–20)
1. According to Boyle's law, what happens to gas volume as ambient pressure increases
during descent?
A) Volume increases proportionally
B) Volume decreases inversely with pressure
C) Volume remains constant
D) Volume decreases exponentially
Answer: B. Boyle's law states gas volume varies inversely with pressure at constant
temperature. This explains barotrauma of gas-filled spaces during descent .
2. A diver at 20 meters seawater (msw) is exposed to approximately what total pressure?
A) 1 ATA
B) 2 ATA
C) 3 ATA
D) 4 ATA
Answer: C. At 20 msw, water pressure contributes ~2 ATA plus 1 ATA atmospheric = 3 ATA
total. Approximately 1 ATA is added for every 10 msw .
3. Which gas law explains why nitrogen dissolves into tissues during a dive?
A) Boyle's law
B) Dalton's law
C) Henry's law
D) Charles' law
Answer: C. Henry's law states that the amount of gas dissolved in a liquid is proportional to
its partial pressure. Increased nitrogen partial pressure at depth drives more nitrogen into
solution .
4. What is the partial pressure of oxygen in air at 1 ATA?
A) 760 mmHg
B) 160 mmHg
C) 100 mmHg
D) 40 mmHg
Answer: B. At 1 ATA, oxygen comprises 21% of air: 0.21 × 760 = ~160 mmHg .
5. A patient breathing 100% oxygen at 2.8 ATA will have an arterial oxygen tension of
approximately:
A) 760 mmHg
B) 1540 mmHg
C) 2200 mmHg
D) 4000 mmHg
Answer: C. 2.8 ATA × 760 mmHg × ~1.0 = ~2128 mmHg (approximately 2200 mmHg) .
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6. Which mechanism explains hyperbaric oxygen's vasoconstrictive effect?
A) Increased carbon dioxide
B) Direct smooth muscle contraction
C) Reduced nitric oxide
D) Decreased oxygen delivery
Answer: B. Hyperoxia causes direct vasoconstriction without reducing oxygen delivery to
tissues, which helps reduce edema .
7. The primary mechanism by which HBO enhances wound healing is:
A) Direct bactericidal effect only
B) Increased tissue oxygen tension enhancing fibroblast function and angiogenesis
C) Increased blood pressure
D) Decreased perfusion
Answer: B. Higher oxygen levels improve collagen synthesis, angiogenesis, and leukocyte
function in ischemic wounds .
8. What is the threshold for CNS oxygen toxicity seizures?
A) >1.0 ATA
B) >1.6 ATA
C) >2.5 ATA
D) >3.0 ATA
Answer: B. CNS oxygen toxicity (Paul Bert effect) occurs at oxygen partial pressures >1.6 ATA,
with seizure risk increasing at higher pressures .
9. Pulmonary oxygen toxicity is tracked using:
A) PaO2 levels
B) OTU/UPTD units
C) SpO2 monitoring
D) Venous oxygen saturation
Answer: B. Oxygen toxicity units (OTU) and unit pulmonary toxic dose (UPTD) track
cumulative pulmonary oxygen exposure .
10. The Lorrain-Smith effect refers to:
A) CNS oxygen toxicity
B) Pulmonary oxygen toxicity
C) Nitrogen narcosis
D) Decompression sickness
Answer: B. The Lorrain-Smith effect is pulmonary oxygen toxicity from prolonged oxygen
exposure .
11. Hyperbaric oxygen therapy is defined as breathing near 100% oxygen at a pressure of at
least:
A) 1.0 ATA
, 3
B) 1.4 ATA
C) 2.0 ATA
D) 2.5 ATA
Answer: B. The UHMS defines HBO2 as breathing near 100% oxygen at ≥1.4 ATA .
12. All UHMS-approved indications require a minimum pressure of:
A) 1.4 ATA
B) 2.0 ATA
C) 2.8 ATA
D) 3.0 ATA
Answer: B. UHMS currently approved indications require breathing near 100% oxygen at
minimum 2.0 ATA .
13. Low-pressure fabric chambers are FDA-approved only for:
A) Diabetic wounds
B) Acute mountain sickness
C) Carbon monoxide poisoning
D) Radiation injury
Answer: B. Soft chambers are cleared only for acute mountain sickness .
14. The partial pressure of nitrogen in air at 3 ATA is approximately:
A) 0.8 ATA
B) 2.4 ATA
C) 3.0 ATA
D) 4.0 ATA
Answer: B. Nitrogen is 79% of air: 0.79 × 3 = ~2.4 ATA.
15. Which gas is considered biologically inert?
A) Oxygen
B) Carbon dioxide
C) Nitrogen
D) Carbon monoxide
Answer: C. Nitrogen is biologically inert under normal conditions .
16. What is the pressure equivalent of 1 ATA?
A) 600 mmHg
B) 700 mmHg
C) 760 mmHg
D) 800 mmHg
Answer: C. 1 ATA = 760 mmHg .
17. A depth gauge reads 99 FSW. What is the total pressure in ATA?
A) 1 ATA
B) 3 ATA
ABEM UHM Practice Questions — 100 Original Items
Hyperbaric Physics & Physiology (1–20)
1. According to Boyle's law, what happens to gas volume as ambient pressure increases
during descent?
A) Volume increases proportionally
B) Volume decreases inversely with pressure
C) Volume remains constant
D) Volume decreases exponentially
Answer: B. Boyle's law states gas volume varies inversely with pressure at constant
temperature. This explains barotrauma of gas-filled spaces during descent .
2. A diver at 20 meters seawater (msw) is exposed to approximately what total pressure?
A) 1 ATA
B) 2 ATA
C) 3 ATA
D) 4 ATA
Answer: C. At 20 msw, water pressure contributes ~2 ATA plus 1 ATA atmospheric = 3 ATA
total. Approximately 1 ATA is added for every 10 msw .
3. Which gas law explains why nitrogen dissolves into tissues during a dive?
A) Boyle's law
B) Dalton's law
C) Henry's law
D) Charles' law
Answer: C. Henry's law states that the amount of gas dissolved in a liquid is proportional to
its partial pressure. Increased nitrogen partial pressure at depth drives more nitrogen into
solution .
4. What is the partial pressure of oxygen in air at 1 ATA?
A) 760 mmHg
B) 160 mmHg
C) 100 mmHg
D) 40 mmHg
Answer: B. At 1 ATA, oxygen comprises 21% of air: 0.21 × 760 = ~160 mmHg .
5. A patient breathing 100% oxygen at 2.8 ATA will have an arterial oxygen tension of
approximately:
A) 760 mmHg
B) 1540 mmHg
C) 2200 mmHg
D) 4000 mmHg
Answer: C. 2.8 ATA × 760 mmHg × ~1.0 = ~2128 mmHg (approximately 2200 mmHg) .
, 2
6. Which mechanism explains hyperbaric oxygen's vasoconstrictive effect?
A) Increased carbon dioxide
B) Direct smooth muscle contraction
C) Reduced nitric oxide
D) Decreased oxygen delivery
Answer: B. Hyperoxia causes direct vasoconstriction without reducing oxygen delivery to
tissues, which helps reduce edema .
7. The primary mechanism by which HBO enhances wound healing is:
A) Direct bactericidal effect only
B) Increased tissue oxygen tension enhancing fibroblast function and angiogenesis
C) Increased blood pressure
D) Decreased perfusion
Answer: B. Higher oxygen levels improve collagen synthesis, angiogenesis, and leukocyte
function in ischemic wounds .
8. What is the threshold for CNS oxygen toxicity seizures?
A) >1.0 ATA
B) >1.6 ATA
C) >2.5 ATA
D) >3.0 ATA
Answer: B. CNS oxygen toxicity (Paul Bert effect) occurs at oxygen partial pressures >1.6 ATA,
with seizure risk increasing at higher pressures .
9. Pulmonary oxygen toxicity is tracked using:
A) PaO2 levels
B) OTU/UPTD units
C) SpO2 monitoring
D) Venous oxygen saturation
Answer: B. Oxygen toxicity units (OTU) and unit pulmonary toxic dose (UPTD) track
cumulative pulmonary oxygen exposure .
10. The Lorrain-Smith effect refers to:
A) CNS oxygen toxicity
B) Pulmonary oxygen toxicity
C) Nitrogen narcosis
D) Decompression sickness
Answer: B. The Lorrain-Smith effect is pulmonary oxygen toxicity from prolonged oxygen
exposure .
11. Hyperbaric oxygen therapy is defined as breathing near 100% oxygen at a pressure of at
least:
A) 1.0 ATA
, 3
B) 1.4 ATA
C) 2.0 ATA
D) 2.5 ATA
Answer: B. The UHMS defines HBO2 as breathing near 100% oxygen at ≥1.4 ATA .
12. All UHMS-approved indications require a minimum pressure of:
A) 1.4 ATA
B) 2.0 ATA
C) 2.8 ATA
D) 3.0 ATA
Answer: B. UHMS currently approved indications require breathing near 100% oxygen at
minimum 2.0 ATA .
13. Low-pressure fabric chambers are FDA-approved only for:
A) Diabetic wounds
B) Acute mountain sickness
C) Carbon monoxide poisoning
D) Radiation injury
Answer: B. Soft chambers are cleared only for acute mountain sickness .
14. The partial pressure of nitrogen in air at 3 ATA is approximately:
A) 0.8 ATA
B) 2.4 ATA
C) 3.0 ATA
D) 4.0 ATA
Answer: B. Nitrogen is 79% of air: 0.79 × 3 = ~2.4 ATA.
15. Which gas is considered biologically inert?
A) Oxygen
B) Carbon dioxide
C) Nitrogen
D) Carbon monoxide
Answer: C. Nitrogen is biologically inert under normal conditions .
16. What is the pressure equivalent of 1 ATA?
A) 600 mmHg
B) 700 mmHg
C) 760 mmHg
D) 800 mmHg
Answer: C. 1 ATA = 760 mmHg .
17. A depth gauge reads 99 FSW. What is the total pressure in ATA?
A) 1 ATA
B) 3 ATA
