Quiz 111 · Hazards of Deep Sea Diving and Increased Barometric Pressure

Answers to Quiz 110 · Hypoxia and Oxygen Therapy

1. In which type of hypoxia is the arterial PO₂ reduced?

   Answer: Hypoxic hypoxia

   Only hypoxic hypoxia lowers PaO₂. In the other three the arterial PO₂ is normal and the fault is the Hb, the blood flow or the cell's use of O₂. Revise hypoxia

2. Which finding is typical of histotoxic hypoxia?

   Answer: A high venous PO₂

   The tissue cannot use O₂, so the O₂ stays in the venous blood. A low PaO₂ marks hypoxic hypoxia, a low O₂ content anemic hypoxia, and poor flow stagnant hypoxia. Revise hypoxia

3. A man found in a closed garage with the car engine running has a normal PaO₂ but severe tissue hypoxia. Which type of hypoxia is this?

   Answer: Anemic hypoxia

   CO binds Hb about 210 times more strongly than O₂, so the Hb is present but unavailable to carry O₂. The lungs and inspired PO₂ are normal, which is why it is not hypoxic hypoxia. Revise hypoxia

4. Which of these causes hypoxic hypoxia?

   Answer: Pulmonary fibrosis

   Fibrosis is a diffusion defect, which lowers PaO₂. Methemoglobin causes anemic hypoxia, heart failure stagnant hypoxia, and cyanide histotoxic hypoxia. Revise hypoxia

5. Cyanide causes hypoxia by inhibiting which enzyme?

   Answer: Cytochrome oxidase

   Blocking cytochrome oxidase paralyses tissue oxidation, so the cells cannot use O₂ even though the supply is normal: histotoxic hypoxia. Revise hypoxia

6. If the inspired PO₂ falls suddenly below 20 mmHg, as when an aircraft loses cabin pressure at great height, consciousness is lost in about:

   Answer: 15 seconds

   Consciousness goes in about 15 s. If the hypoxia continues, death follows in 4–5 min. Revise hypoxia

7. In hypoxic tissue, hypoxia-inducible factors (HIFs) switch on genes for:

   Answer: Erythropoietin and angiogenic factors

   In hypoxia the α and β subunits dimerize, and the dimer induces the erythropoietin and angiogenic genes. When O₂ is adequate, the α subunits are removed quickly. Revise hypoxia

8. Which of these is NOT another name for stagnant hypoxia?

   Answer: Respiratory hypoxia

   Respiratory hypoxia is the group name for hypoxic hypoxia caused by hypoventilation, diffusion defects or V/Q mismatch. The other three all describe hypoxia from reduced blood flow. Revise hypoxia

9. Why does a patient with moderate anemia cope at rest but become severely hypoxic on exertion?

   Answer: Raised 2,3-DPG covers rest, but delivery cannot rise in exercise

   PaO₂ stays normal in anemia. The rise in red-cell 2,3-DPG helps Hb unload O₂ at rest, but the reduced carrying capacity cannot meet the demand of exercise. Revise hypoxia

10. Oxygen therapy does little to correct hypoxic hypoxia caused by:

    Answer: Large A-V shunts

    Shunted blood bypasses the alveoli and mixes with oxygenated blood after the lungs, so a higher alveolar PO₂ never reaches it. In the other three, extra O₂ raises the alveolar-to-blood gradient. Revise oxygen therapy

11. A patient with a depressed respiratory centre and a high PCO₂ breathes less after being given 100% O₂. Why?

    Answer: It removed the hypoxic drive that kept him breathing

    When a high PCO₂ has depressed the centre, breathing depends on hypoxia stimulating the chemoreceptors. Pure O₂ abolishes that stimulus, so a nasal cannula, which never gives 100% O₂, is safer. Revise oxygen therapy

12. Breathing 100% O₂ at 3 atmospheres gives a PaO₂ of about 2000 mmHg. About how much O₂ does this dissolve in each 100 mL of blood?

    Answer: 6 mL

    At 0.003 mL/dL per mmHg, 2000 mmHg dissolves 6 mL/dL, enough for the body's resting needs. 0.3 mL is the dissolved O₂ at a normal PaO₂. Revise oxygen therapy

13. Hyperbaric oxygen therapy is especially useful in:

    Answer: Carbon monoxide poisoning

    Hyperbaric O₂ speeds the release of CO from Hb and carries more O₂ dissolved in plasma. It does little where flow is the problem or where blood bypasses the lungs. Revise oxygen therapy

14. Retrolental fibroplasia in a newborn is a complication of:

    Answer: Prolonged 100% O₂ therapy

    Pure O₂ damages the newborn eye, causing retrolental fibroplasia and retinopathy of prematurity. Long O₂ therapy for respiratory distress syndrome also causes bronchopulmonary dysplasia. Revise oxygen therapy

15. The toxic effects of 100% O₂ are mainly due to:

    Answer: Free radicals such as superoxide and H₂O₂

    Oxidizing free radicals accumulate on pure O₂; they oxidize polyunsaturated fatty acids and destroy cell enzymes. Revise oxygen therapy

13 questions on chapter 111. Pick one answer for each, then save. Your answers stay in this browser, and the right ones are shown at the top of the next quiz.

1. What is the total pressure on a diver at a depth of 20 m?

2 atmospheres3 atmospheres4 atmospheres20 atmospheres

2. Nitrogen narcosis becomes severe at a depth of about:

66 ft (3 atm)100 ft (4 atm)250 ft (8.5 atm)33 ft (2 atm)

3. A diver at 200 ft breathing 20% O₂ and 80% N₂ behaves as if drunk. Which finding fits his condition?

Severe joint pain before he surfacesIntellect altered but manual agility keptManual dexterity lost but intellect keptAir entering his circulation at depth

4. Which is NOT a reason for using helium instead of nitrogen in deep-diving mixtures?

Helium prevents the high-pressure nervous syndromeHelium is less soluble in the body fluidsHelium has one-fifth the narcotic effect of N₂Helium is lighter, so the work of breathing is less

5. On a very deep dive breathing helium–O₂, a diver develops tremors, drowsiness and incoordination, and his EEG shows depressed α activity. The likely diagnosis is:

Decompression sicknessAir embolismHigh-pressure nervous syndromeNitrogen narcosis

6. Why is the O₂ in gas mixtures for deep diving cut to about 20%?

O₂ at high pressure causes nitrogen narcosisO₂ forms bubbles in the tissues on ascentO₂ at high pressure is toxic, mainly to the CNSMore O₂ would make the gas mixture too dense

7. Decompression sickness is caused by:

N₂ bubbles forming in tissues and blood on rapid ascentFree radicals from O₂ breathed at high pressureExpanding lung gas rupturing the pulmonary veinsThe anesthetic effect of N₂ dissolved at high pressure

8. In decompression sickness, the pain is felt mainly in:

The skin and scalpThe ears and sinusesMuscles and jointsThe abdominal viscera

9. The severity of decompression sickness depends mainly on:

The time spent at the surfaceThe percentage fall in pressureThe absolute fall in pressureThe O₂ content of the gas mixture

10. A diver develops joint pain and paresthesia 20 minutes after a rapid ascent. What is the treatment?

Slow decompression alone, without recompressionImmediate recompression, then slow decompressionRest at sea level while breathing room airA helium–O₂ mixture breathed at sea level

11. Decompression sickness can occur without any diving, in a person who:

Is flown rapidly to about 9000 m in an unpressurized cabinBreathes 100% O₂ at sea level for 3 daysHyperventilates voluntarily for 3 minutesClimbs slowly to 3000 m over several days

12. A diver breathing compressed gas from a tank at 5 m panics and shoots to the surface holding his breath, then collapses. The most likely cause is:

Acute O₂ toxicityNitrogen narcosisHigh-pressure nervous syndromeAir embolism

13. According to your book, the cylinders of a SCUBA set contain:

O₂ with 5% CO₂Pure O₂ at 3 atmospheresCompressed helium and O₂Compressed N₂ and CO₂

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