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# Hyperbaric oxygen therapy: how it works and when it helps

- Mechanism
- For completeness

Breathing pure oxygen in a pressurized chamber dissolves enough O₂ in plasma to feed tissues, shrinks gas bubbles and clears carbon monoxide faster.

Updated Nov 13, 2026

Also called: HBOT, Hyperbaric chamber

## Key points

- Dissolved O₂ = 0.003 mL/dL per mmHg PaO₂: about 0.3 mL/dL on room air, about 2 mL/dL on 100% O₂ at 1 atm, and about 6 mL/dL at 3 atm, enough to cover resting tissue needs without hemoglobin.
- Boyle's law: higher pressure shrinks bubbles, and the high O₂ gradient washes out their nitrogen: the basis for treating decompression sickness and gas embolism.
- CO elimination half-life falls from about 4–5 h on room air to about 1–1.5 h on 100% O₂ and to roughly 20–30 min under hyperbaric oxygen.
- High tissue PO₂ kills anaerobes such as Clostridium, stops alpha-toxin production and improves neutrophil killing; it also stimulates angiogenesis in hypoxic wounds.
- Main risks: ear and sinus barotrauma, temporary myopia, oxygen-induced seizures and, rarely, pulmonary barotrauma; an untreated pneumothorax is the classic absolute contraindication.

## What it is

Hyperbaric oxygen therapy is breathing 100% oxygen at a pressure above 1 atm, usually around 2–3 atm, in a chamber. Pressure works in two ways: it raises dissolved plasma O₂ many times over (Henry's law), and it shrinks gas bubbles (Boyle's law). These effects explain its accepted uses: decompression sickness and gas embolism, carbon monoxide poisoning, gas gangrene and some necrotizing infections, and certain hypoxic or irradiated wounds. Its risks come from the same physics: barotrauma and oxygen toxicity.

## How it works, step by step

- Chamber pressure rises; alveolar PO₂ on 100% O₂ climbs far above 600 mmHg.
- Plasma dissolves O₂ in proportion to PO₂ (Henry's law), so arterial O₂ content rises even when hemoglobin is already saturated or blocked.
- The diffusion distance of O₂ from capillaries into tissue lengthens, reaching hypoxic wound edges and swollen tissue.
- High PO₂ competes CO off hemoglobin and cytochrome oxidase, speeding its elimination.
- Pressure shrinks bubbles at once; the high O₂ and absent N₂ in the breathing gas create a steep gradient that removes N₂ from them.
- Hyperoxia causes vasoconstriction, which lowers edema while O₂ delivery stays high.
- Repeated exposures drive fibroblast work, collagen formation and new capillaries in poorly perfused tissue.

### Oxygen–hemoglobin dissociation curve

**P50** 17.2 mmHg **Arterial blood** 59 % **Mixed venous blood** 54 % **O₂ given up to the tissues** 5 %

With 40% HbCO those sites no longer carry O₂ (the plateau falls) and the rest release it less well (left shift): that is why the harm is greater than in an anaemia of the same size. A standard pulse oximeter may still read normal.

Left shift (P50 17.2 mmHg): Hb holds O₂ more tightly and gives up less in the tissues.

- Current curve
- Normal curve (pH 7.40 · 37 °C)
- Arterial blood (PO₂ 100)
- Mixed venous blood (PO₂ 40)
- O₂ given up to the tissues

Carbon monoxide preset: less hemoglobin carries O₂ and the rest gives it up less easily; dissolved O₂ under pressure bypasses this.

## Generally accepted indications and the physiology behind them (educational summary; criteria differ between societies)

| Indication | Main mechanism |
| --- | --- |
| [Decompression sickness, arterial gas embolism](https://durgical.com/en/conditions/decompression-sickness-and-diving-injuries/) | Bubble compression and nitrogen washout |
| [Carbon monoxide poisoning (selected cases)](https://durgical.com/en/conditions/carbon-monoxide-poisoning/) | Faster CO removal; dissolved O₂ bypasses blocked hemoglobin |
| Gas gangrene (clostridial myonecrosis) | High tissue PO₂ halts toxin production; aids surgery and antibiotics |
| [Necrotizing soft-tissue infections (adjunct)](https://durgical.com/en/conditions/necrotizing-soft-tissue-infection/) | Better neutrophil killing and tissue oxygenation |
| [Diabetic foot ulcers not healing with standard care](https://durgical.com/en/conditions/diabetic-foot/) | Angiogenesis and fibroblast activity in hypoxic tissue |
| Delayed radiation injury, compromised flaps, crush injury | O₂ to hypoxic, poorly perfused tissue; less edema |

## Clinical notes

- Hyperbaric oxygen is an adjunct: in infections it never replaces surgery and antibiotics, and in wounds it follows standard care.
- Pulse oximetry does not detect carboxyhemoglobin; CO poisoning is confirmed by CO-oximetry.

## Memory aid

Pressure packs plasma with oxygen and squeezes bubbles small.

## Test yourself

Think of the answer, then tap the question to see it.

## How much O₂ is dissolved in plasma at an arterial PO₂ of 2000 mmHg, and why does it matter?

About 0.003 × 2000 = 6 mL/dL, close to the resting arteriovenous O₂ difference, so tissues can be supplied even without functional hemoglobin.

## Which two gas laws explain the use of hyperbaric oxygen in decompression sickness?

Boyle's law (pressure shrinks bubble volume) and diffusion along a gradient (pure O₂ removes N₂ from bubbles and tissues); Henry's law explains the extra dissolved O₂.

## Why does hyperbaric oxygen help in clostridial gas gangrene?

Clostridia are anaerobes: high tissue PO₂ stops their growth and alpha-toxin production and improves neutrophil killing, buying time for surgery and antibiotics.

## What is the classic absolute contraindication to hyperbaric oxygen, and why?

An untreated pneumothorax: gas trapped in the pleura can expand on decompression and become a tension pneumothorax.

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## Connections

## Related topics 3

- [Diving, decompression and hyperbaric oxygen](https://durgical.com/en/physiology/diving-decompression-and-hyperbaric-oxygen/)
- [Oxygen content, delivery and extraction](https://durgical.com/en/physiology/oxygen-content-delivery-and-extraction/)
- [Carbon monoxide, methemoglobin and cyanide](https://durgical.com/en/physiology/carbon-monoxide-methemoglobin-and-cyanide/)

## Related conditions 4

- [Carbon monoxide poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/)
- [Decompression sickness and diving injuries](https://durgical.com/en/conditions/decompression-sickness-and-diving-injuries/)
- [Necrotizing soft tissue infection (necrotizing fasciitis, gas gangrene)](https://durgical.com/en/conditions/necrotizing-soft-tissue-infection/)
- [Diabetic foot](https://durgical.com/en/conditions/diabetic-foot/)

## In Pharmacology 1

- [Oxygen](https://durgical.com/en/pharmacology/oxygen/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Hyperbaric oxygen therapy: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002375.htm) (MedlinePlus, US National Library of Medicine, 2024); [Therapeutic effects of hyperbaric oxygen: integrated review](https://pmc.ncbi.nlm.nih.gov/articles/PMC8103971/) (Medical Gas Research (PMC), 2021); [Carbon Monoxide Poisoning: MedlinePlus](https://medlineplus.gov/carbonmonoxidepoisoning.html) (MedlinePlus, US National Library of Medicine, 2024); [Diabetic Foot Ulceration and Complications](https://www.ncbi.nlm.nih.gov/books/NBK499887/) (StatPearls (NCBI Bookshelf), 2025).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/hyperbaric-oxygen-therapy-how-it-works-and-when-it-helps/