In one breath
Every 10 m (33 ft) of sea water adds 1 atmosphere to the pressure on a diver, and gas breathed at that pressure dissolves in the body in large amounts. The result is five hazards: nitrogen narcosis, the high-pressure nervous syndrome (HPNS) of helium mixtures, acute O₂ toxicity, decompression sickness and air embolism. Decompression sickness (caisson disease) follows a rapid ascent, when dissolved N₂ comes out as bubbles in tissues and vessels; slow ascent prevents it, and prompt recompression followed by slow decompression treats it. Holding the breath on the way up can burst lung vessels and let air into the circulation (air embolism).
Builds on: Gas exchange in the lungs · Oxygen transport · Oxygen therapy
Pressure under water
- At sea level the air presses at 760 mmHg (1 atmosphere), and the lungs sit at that same pressure.
- Under water the pressure rises in proportion to depth: 1 atmosphere for every 10 m (33 ft), on top of the 1 atmosphere of air above.
| Depth | Total pressure |
|---|---|
| Sea level | 1 atm |
| 10 m (33 ft) | 2 atm |
| 20 m | 3 atm |
| 100 ft (about 30 m) | About 4 atm |
| 50 m | 6 atm |
Why this matters:
- A diver must breathe gas at high pressure to balance the water pressure on the chest and abdomen, so that the chest can still expand properly with each breath.
- Breathed at high pressure, a gas goes into solution in the body fluids in large amounts, and most of the hazards start here.
- Caisson workers meet the same pressures. A caisson is a watertight chamber for building work under water, and its air is kept at high pressure to hold the water out.
- Worth knowing, though not in your pages: two gas laws explain the chapter. Henry’s law (see Oxygen transport) says the amount of gas that dissolves rises with its partial pressure, so N₂ loads the tissues at depth. Boyle’s law says a gas expands as its pressure falls, so on the way up the gas in the lungs and any bubbles in the tissues swell.
The five hazards
| Hazard | Culprit | Hallmark |
|---|---|---|
| Nitrogen narcosis | N₂ at high pressure | Drunk-like, intellect dulled |
| HPNS | Helium mix on deep dives | Tremor, poor dexterity |
| Acute O₂ toxicity | O₂ at high pressure | Convulsions |
| Decompression sickness | N₂ bubbles on rapid ascent | Pain in joints and muscles |
| Air embolism | Breath held on ascent | Air enters the circulation |
Nitrogen narcosis
- 100% O₂ at high pressure is harmful (your book says it depresses the CNS), so divers normally breathe 20% O₂ with 80% N₂, at high partial pressures.
- At high pressure the N₂ itself turns narcotic, through its anesthetic effect.
| Depth | Effect of dissolved N₂ |
|---|---|
| Up to 100 ft (4 atm) | Euphoria |
| From about 120 ft | Narcosis begins |
| 250 ft (8.5 atm) or deeper | Severe narcosis |
- The picture resembles alcohol intoxication. Intellectual functions are altered, but manual agility is preserved.
High-pressure nervous syndrome
Because N₂ turns narcotic at depth, deep divers breathe helium–O₂ instead:
| Property | Helium | Nitrogen |
|---|---|---|
| Narcotic effect | One-fifth of N₂’s | Reference |
| Density | 2 | 14 |
| Solubility in body fluids | Lower | Higher |
- Being lighter, helium meets less resistance to airflow, so the work of breathing is less.
- But breathing helium mixtures on deep dives causes the high-pressure nervous syndrome (HPNS).
- Your book puts HPNS down to helium’s anesthetic effect at high pressure. Like xenon, krypton and neon, helium is inert at atmospheric pressure but anesthetic at high pressure, because of its lipid solubility and a direct action on neuron membranes.
- Features: tremors, drowsiness, incoordination, depressed EEG α activity, impaired manual dexterity.
Narcosis spares the hands; HPNS does not
In N₂ narcosis the thinking suffers but manual agility is kept. In HPNS, tremor and poor manual dexterity are central. Pair N₂ with narcosis and helium with HPNS. Worth knowing, though not in your pages: diving-medicine texts put HPNS down to the high pressure itself acting on nerve cells, rather than to helium, which is chosen precisely because it is barely narcotic. Adding a little N₂ back to the helium (trimix) lessens HPNS.
Acute O₂ toxicity
- CO₂ is normally removed from the diving mixture, because its build-up in the body would be fatal.
- 100% O₂ at high pressure is toxic too. It acts mainly on the CNS: nausea, dizziness, irritability, disorientation, disturbed vision and convulsions. The lungs are damaged as well.
- So the O₂ in high-pressure mixtures is cut to about 20%.
- How O₂ toxicity works (free radicals) is on Oxygen therapy.
Decompression sickness
Also called dysbarism or caisson disease.
Definition: the illness that follows a sudden move from high to low pressure.
Who gets it:
- deep-sea divers who return quickly to the surface
- caisson workers who come up quickly
- people carried rapidly from sea level to about 9000 m in an unpressurized aircraft cabin
Mechanism
- Divers usually breathe about 80% N₂. On descent, N₂ dissolves in the body fluids in proportion to the depth. Being lipid-soluble, it also dissolves in the fatty structures of the tissues.
- On a slow ascent, the N₂ coming out of solution diffuses into the blood and is breathed out through the lungs.
- On a rapid ascent (rapid decompression), it comes out too fast and forms bubbles in the tissues and blood, which damage the tissues.
| Bubbles in | Result |
|---|---|
| Tissues, mainly muscles and joints | Severe pain |
| Vessels of brain and spinal cord | Paresthesia, weakness, paralysis, respiratory failure |
| Pulmonary vessels | Dyspnea, a sense of chest compression |
| Coronary vessels | Myocardial ischemia |
- The muscle and joint pain is the classic picture, the one your book calls caisson disease. Damage to the brain and spinal cord marks the severe cases.
- Features usually appear within half an hour of surfacing.
- Worth knowing, though not in your pages: the joint pain is nicknamed “the bends” and the chest symptoms “the chokes”.
How big a pressure drop is safe
- Severity depends on the percentage fall in pressure, not the absolute fall.
- A rapid fall to half the original pressure is safe; a fall to below 45% of it is harmful. (Your book’s wording here is loose; this is its sense, and it matches its rule that the pressure should never be cut by more than half.)
- By this rule, a quick rise from 30 m (4 atm) to 10 m (2 atm) halves the pressure, while a quick rise from 30 m to the surface (1 atm) leaves only 25% of it.
- Worth knowing, though not in your pages: this is Haldane’s classic 2:1 rule, and it is why divers come up in stages, pausing at set depths (stage decompression).
Prevention
- Ascend gradually, so the N₂ has time to leave through the lungs.
- The pressure should not be cut by more than 50%.
A slip in your book: recompression or decompression?
Your book’s prevention line says “slow recompression”. It means slow decompression, that is, a gradual ascent. Recompression is the treatment, once the disease has started.
Treatment
- Recompress at once in a pressurized chamber as soon as symptoms appear.
- Follow with slow decompression.
- Hyperbaric O₂ helps.
- Recovery is usually complete, though some neural deficits may remain.
- Worth knowing, though not in your pages: recompression works by the same two gas laws. The raised pressure shrinks the bubbles and drives the N₂ back into solution, and the slow decompression that follows lets it leave through the lungs.
Draw it: decompression sickness as a flow chart
Top box: “deep dive, breathing 80% N₂”. Arrow down to “N₂ dissolves in body fluids and fat, in proportion to depth”. Split into two arrows. Left: “slow ascent → N₂ diffuses into blood → breathed out → no harm”. Right: “rapid ascent → N₂ bubbles in tissues and blood”, then four small boxes: “joints, muscles: pain”, “brain, cord: paralysis”, “lungs: dyspnea”, “heart: ischemia”. Across the bottom write “Treat: recompress, decompress slowly, hyperbaric O₂”.
Air embolism
- A diver breathing from a tank at raised pressure rushes to the surface holding his breath. The lung gas expands quickly, ruptures pulmonary veins, and air escapes into the circulation.
- The same happens when the outside pressure falls suddenly from atmospheric to subatmospheric, as when the pressurized cabin of an aircraft or rocket ruptures at high altitude.
- It can happen on a rapid ascent from as little as 5 m.
- Prevention, in your book: take a breath at the surface, dive holding it, and come up still holding that same breath.
- Worth knowing, though not in your pages: the breath-hold trick works because a lungful taken at the surface only shrinks on the way down and returns to its starting volume on the way up. A diver breathing compressed gas at depth has no such margin, so must breathe out steadily while ascending.
SCUBA
- SCUBA stands for self-contained underwater breathing apparatus: the diver carries the gas in a tank, with valves that deliver it.
- In your book’s account, its cylinders hold compressed helium and O₂.
- The valves let the diver breathe in from the cylinders and breathe out.
- Helium–O₂ mixtures are used because they are less toxic on deep dives (the reasons are under HPNS above).
- Worth knowing, though not in your pages: everyday recreational scuba uses compressed air. Its valve is a demand regulator, which delivers gas only when the diver inhales, at the pressure of the surrounding water.
Exam-answer skeleton: "Decompression sickness (caisson disease): cause, features, prevention and treatment" (short note)
- Definition: a sudden move from high to low pressure, in divers, caisson workers or an unpressurized climb to 9000 m.
- Pressure rises 1 atm per 10 m; the 80% N₂ breathed dissolves in body fluids and fat in proportion to depth.
- Slow ascent: N₂ leaves by the blood and lungs. Rapid ascent: bubbles form in tissues and blood.
- Features by site: joints and muscles (pain), brain and cord, lungs, heart; onset within half an hour.
- Severity follows the percentage fall in pressure: halving is safe, below 45% is harmful.
- Prevention: gradual ascent, never cutting the pressure by more than 50%.
- Treatment: immediate recompression, then slow decompression; hyperbaric O₂; possible residual neural deficits.
Asked in exams
- Decompression sickness (caisson disease, dysbarism): Oct/Nov 2018, 2 marks · Apr 2021, 3 marks · May 2022, 4 marks · Jan 2023, MCQ · Aug 2024, essay
- Nitrogen narcosis: Aug 2024, essay
- SCUBA diving: Jan 2021, 3 marks