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 mmHgArterial 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	Bubble compression and nitrogen washout
Carbon monoxide poisoning (selected cases)	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)	Better neutrophil killing and tissue oxygenation
Diabetic foot ulcers not healing with standard care	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.

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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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 (MedlinePlus, US National Library of Medicine, 2024); Therapeutic effects of hyperbaric oxygen: integrated review (Medical Gas Research (PMC), 2021); Carbon Monoxide Poisoning: MedlinePlus (MedlinePlus, US National Library of Medicine, 2024); Diabetic Foot Ulceration and Complications (StatPearls (NCBI Bookshelf), 2025).

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