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Oxygen Therapy: How Much, How, and the Targets

Oxygen is a drug with a dose, a delivery device, and a target — not a comfort blanket you turn up to the maximum. Learn the devices from nasal cannula to high-flow, why saturation goals differ for COPD, and how content beats partial pressure.

Oxygen is a dose, not a default

Oxygen therapy treats hypoxemia — low oxygen in the blood. It does not treat breathlessness with normal oxygen, and it does not speed up a tiring patient's breathing pump. Giving it is easy; giving the right amount is the skill. The goal is to keep tissues supplied while avoiding the harms of too much — and yes, too much oxygen can harm.

We follow the oxygen with pulse oximetry — the clip on the finger that reads oxygen saturation (SpO2). For most acutely ill adults the target is an SpO2 of 94–98%. There is almost never a reason to chase 100%; saturations above 96% on a sick patient often mean you are giving more oxygen than the body can use, and you lose the early warning that comes when oxygen quietly starts to fall.

The devices, low to high

  1. Nasal cannula (1–6 L/min): the gentle prongs in the nose. Comfortable, lets you eat and talk; delivers a rough FiO2 of about 24–44%.
  2. Simple face mask (5–10 L/min): more oxygen, roughly 40–60% FiO2; needs a minimum flow so exhaled CO2 doesn't accumulate.
  3. Venturi mask: a coloured valve mixes oxygen and air to a *precise* FiO2 (e.g. 24%, 28%) — ideal when you must control the dose carefully, as in COPD.
  4. Non-rebreather mask (10–15 L/min): bag plus one-way valves delivers the highest FiO2 a mask can — up to ~85–90% — for the acutely crashing patient.
  5. [[high-flow-nasal-oxygen|High-flow nasal oxygen]] (up to 60 L/min, warmed and humidified): delivers a controlled FiO2 up to nearly 100%, a little PEEP-like pressure, and washes dead-space CO2 — comfortable and often the step before non-invasive ventilation.

The special case: chronic CO2 retainers

In some people with advanced COPD or type 2 respiratory failure, we aim for a lower SpO2 target of 88–92%. The reason is partly the old idea of hypoxic drive, but mostly that flooding sick lungs with oxygen worsens V/Q mismatch and releases CO2 from haemoglobin, so the PaCO2 climbs further and the patient grows drowsy. The fix is not to withhold oxygen — hypoxia kills faster — but to titrate it, ideally with a Venturi mask, and to recheck a blood gas.