In one breath
Oxygen therapy works best when the fault lies in getting O₂ into arterial blood, as in most hypoxic hypoxia, and it does much less when Hb, blood flow or the cells are the problem. Hyperbaric O₂, which is 100% O₂ at 3 atmospheres, dissolves about 6 mL of O₂ in each 100 mL of blood, enough for resting needs, and is especially useful in CO poisoning. Pure O₂ is itself toxic: free radicals injure the brain, lungs and eyes, and the higher the pressure, the sooner the damage starts.
Builds on: Hypoxia · Oxygen transport · Chemical control of breathing · Leads to: Hazards of deep sea diving · Artificial respiration
When it is used
- O₂ therapy is indicated in hypoxia.
- It helps most in acute, severe hypoxia, particularly when the patient is also dyspneic.
Methods
- Oxygen tent: suits children, who rarely tolerate a mask or cannula. A tent is also a way of giving hyperbaric O₂.
- Oxygen mask: O₂ enters the mask at high velocity (see the note below on the side holes).
- Mechanical ventilator: for the semiconscious or comatose patient, through an endotracheal or tracheostomy tube.
- Intranasal tube (cannula): a cannula in the nostril, connected to the O₂ cylinder. The inspired O₂ never reaches 100% this way, so it suits patients whose breathing depends on the hypoxic drive.
Your book's mask line is garbled
Your book says the fast inflow draws oxygen through the holes in the mask. Worth knowing, though not in your pages: in the standard (Venturi) mask, the fast jet of O₂ pulls room air in through the side holes, and the ratio of the two fixes the inspired O₂ concentration.
O₂ therapy in each type of hypoxia
| Type | Benefit | Why |
|---|---|---|
| Hypoxic | Great, except shunts | Raises alveolar PO₂ |
| Anemic | Some | Adds dissolved O₂ |
| Stagnant | Little | Flow is the problem |
| Histotoxic | Little | Cells cannot use O₂ |
Hypoxic hypoxia
- A low PaO₂ is the problem, so O₂ therapy is very useful. It is essential when the cause is low inspired PO₂, hypoventilation or impaired diffusion.
- Extra O₂ steepens the pressure gradient from alveoli to blood, which drives more O₂ into the blood.
- It does not help hypoxia due to A-V shunts: the shunted blood never meets alveolar gas, and it mixes with the oxygenated blood only after the lungs.
Oxygen can stop a patient breathing
When a high PCO₂ has depressed the respiratory centre, breathing is kept going only by the stimulus of hypoxia on the chemoreceptors (the hypoxic drive). Giving 100% O₂ removes that stimulus, and breathing is depressed further. This is why a nasal cannula, which never delivers 100% O₂, suits such patients (see Chemical control of breathing). Worth knowing, though not in your pages: the everyday example is the patient with chronic obstructive lung disease who retains CO₂. Such patients get controlled, low-concentration O₂ and are watched closely.
Anemic hypoxia
- O₂ therapy raises the O₂ content of blood by adding to the dissolved O₂ in plasma.
- The extra amount is small, but it improves tissue oxygenation to some extent.
- In CO poisoning, hyperbaric O₂ is useful: it speeds the release of CO from Hb and carries more O₂ in solution.
Stagnant hypoxia
- Of little use. The arterial blood is already well oxygenated; the trouble is that too little of it reaches the tissue.
Histotoxic hypoxia
- Of little use, because the tissue cannot use O₂.
- Your book adds that hyperbaric O₂ still helps, by displacing the poison bound to Hb.
An unclear line in your book
Cyanide acts on cytochrome oxidase inside the cells, not on Hb, so “displacing the poison from Hb” does not fit cyanide well. For the exam, keep the book’s point that hyperbaric O₂ benefits cyanide poisoning, which it also makes where it covers cyanide (see Hypoxia).
Hyperbaric O₂ therapy
Hyperbaric O₂ means giving 100% O₂ at a raised pressure. It works by raising the O₂ dissolved in plasma.
| Quantity | Value |
|---|---|
| Dissolved O₂ that meets resting needs | 6 mL/100 mL |
| PaO₂ needed for that | 2000 mmHg |
| Pressure that achieves it, on 100% O₂ | 3 atmospheres |
- It is very useful in CO poisoning.
- Because the O₂ is at high pressure, O₂ toxicity sets in sooner.
A slip in your book: the solubility figure
Your book prints the solubility of O₂ in plasma as 0.03 mL/100 mL per mmHg. Its own chapter 107 uses 0.003 (see Oxygen transport), and only 0.003 makes this page’s arithmetic work: 0.003 × 2000 mmHg = 6 mL/100 mL. Write 0.003.
Effects of 100% O₂
How toxic pure O₂ is depends on how long it is given and at what pressure.
- Given for more than 3 days, 100% O₂ produces side effects.
- The higher the pressure, the sooner they appear:
| Pressure | Symptoms appear in |
|---|---|
| 4 atmospheres | About half an hour |
| 6 atmospheres | A few minutes |
CNS effects
- Nausea, irritability, dizziness, disorientation, muscle twitching and convulsions; coma in severe cases.
- Hyperbaric O₂ lowers the brain’s content of ATP and GABA.
Respiratory system
- Congestion and irritation of the airways.
- More tracheobronchial secretion.
- Less surfactant synthesis.
- Pulmonary edema and atelectasis.
- With long use, bronchopulmonary dysplasia and lung cysts; newborns given oxygen for respiratory distress syndrome are most at risk. Your book lists this under the special senses.
Special senses
- Tinnitus (ringing in the ears).
- Blurred vision and loss of equilibrium.
- In the newborn, retrolental fibroplasia, which leads to retinopathy of prematurity. Visual defects also come from opaque vascular tissue forming in the eye.
- Worth knowing, though not in your pages: this is why O₂ for premature babies is given at the lowest level that keeps them adequately oxygenated, and is monitored.
Mechanism of toxicity
- On pure O₂, oxidizing free radicals accumulate in excess: the superoxide anion (O₂⁻) and hydrogen peroxide (H₂O₂).
- They oxidize polyunsaturated fatty acids and destroy cell enzymes.
- The result is the toxic picture above. Free radicals and the antioxidant defences in general are on Oxygen transport.
Exam-answer skeleton: "Oxygen therapy: methods, its value in each type of hypoxia, and the effects of 100% O₂" (short note)
- Indication: hypoxia, above all when acute and severe with dyspnea.
- Methods: tent, mask, ventilator, nasal cannula, and who each suits.
- Hypoxic hypoxia: very useful because it raises the alveolar-to-blood gradient; no help with A-V shunts; the danger of removing the hypoxic drive.
- Anemic: a small gain in dissolved O₂; hyperbaric O₂ for CO poisoning.
- Stagnant and histotoxic: little benefit; hyperbaric O₂ in cyanide poisoning.
- Hyperbaric O₂: 100% O₂ at 3 atm gives a PaO₂ of 2000 mmHg and 6 mL/dL dissolved.
- Toxicity of 100% O₂: onset by duration and pressure; CNS, respiratory and special-sense effects; free radicals as the mechanism.