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[Physiology](https://durgical.com/en/physiology/) · [Respiratory](https://durgical.com/en/physiology/#s-resp)

# Oxygenation: SpO₂, PaO₂, cyanosis and oxygen therapy

- Measurement
- Exam priority: high

A pulse oximeter shows how full of O₂ hemoglobin is; a blood gas measures O₂ pressure. Each has its traps.

Updated 2026-11-01

Also called: Pulse oximetry, Cyanosis, Physiology of oxygen therapy

## Key points

- Normal SpO₂ at sea level ≈ 95–100%; in Bogotá ≈ 92–96% at rest.
- Normal PaO₂ at sea level ≈ 80–100 mmHg; in Bogotá ≈ 60–70 mmHg.
- SpO₂ 90% corresponds to PaO₂ ≈ 60 mmHg: below that the curve turns steep.
- Standard oximeters read carboxyhemoglobin as oxyhemoglobin; methemoglobin drags the reading toward ≈ 85%.
- Central cyanosis appears with ≈ 5 g/dL of capillary deoxyhemoglobin: late in anemia, early in polycythemia.
- Supplemental O₂ corrects altitude, hypoventilation, V/Q and diffusion well; shunt poorly.

## What it is

SpO₂ estimates saturation from red and infrared light absorption; PaO₂ measures the partial pressure of O₂ in arterial blood. They are linked by the dissociation curve, which is flat at the top: a 'good' SpO₂ can hide a PaO₂ that has already fallen. In Bogotá normal resting SpO₂ is somewhat lower than at sea level, ≈ 92–96%.

## Key facts

- Hypoxemia = low PaO₂; desaturation = low SaO₂; hypoxia = too little O₂ in the tissue.
- Poor perfusion, cold, movement and dark nail polish disturb the oximeter signal.
- Oximeters can overestimate saturation in darker skin; a blood gas settles the doubt.
- Supplemental O₂ does not replace ventilation: it can hold SpO₂ up while PaCO₂ climbs.

### Oxygen–hemoglobin dissociation curve

**P50** 26.8 mmHg **Arterial blood** 97 % **Mixed venous blood** 75 % **O₂ given up to the tissues** 23 %

Normal curve. Tap a point (arterial, venous or P50) or move a control.

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

Tap the curve at PaO₂ 60 mmHg: saturation is about 90%.

## Reference values (resting adult, room air)

| Parameter | Value | Note |
| --- | --- | --- |
| SpO₂ at sea level | ≈ 95–100% | |
| SpO₂ in Bogotá (~2600 m) | ≈ 92–96% | lower in older adults and during sleep |
| PaO₂ at sea level | ≈ 80–100 mmHg | |
| PaO₂ in Bogotá | ≈ 60–70 mmHg | |
| Normal PaO₂/FiO₂ ratio (sea level) | > 400 mmHg | lower at altitude |
| SaO₂ at PaO₂ 60 mmHg | ≈ 90% | |
| SaO₂ at PaO₂ 40 mmHg | ≈ 75% | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## When SpO₂ misleads

| Situation | What the oximeter shows | Why |
| --- | --- | --- |
| [CO poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/) | Normal or high | Carboxyhemoglobin absorbs light like oxyhemoglobin |
| [Methemoglobinemia](https://durgical.com/en/conditions/methemoglobinemia/) | Stuck near 85% | Methemoglobin absorbs at both wavelengths |
| [Severe anemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/) | Normal | Saturation is normal though O₂ content is low |
| [Hypoventilation on O₂](https://durgical.com/en/conditions/respiratory-acidosis/) | Normal | SpO₂ says nothing about a rising PaCO₂ |

## Clinical notes

- ARDS: graded by the PaO₂/FiO₂ ratio; at altitude the cut-offs are adjusted for barometric pressure.
- COPD with chronic hypercapnia: lower saturation targets are used because high O₂ can raise PaCO₂.
- CO poisoning (charcoal braziers, heaters in closed rooms): normal SpO₂ in a hypoxic patient; CO-oximetry is needed.
- Nursing (NCLEX): with a low SpO₂, check the patient and the sensor signal before trusting the number.

## Memory aid

The oximeter counts 'occupied' hemoglobin, not who the tenant is: CO also counts as full.

## Test yourself

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

## A patient pulled from a house fire has SpO₂ 99%. Why isn't the team reassured?

A standard oximeter reads carboxyhemoglobin as oxyhemoglobin; CO-oximetry is needed to measure COHb.

## Why can cyanosis be absent in a patient with severe anemia and hypoxemia?

Cyanosis needs an absolute amount of deoxyhemoglobin (≈ 5 g/dL); with little total hemoglobin that amount is never reached.

## A healthy adult in Bogotá has PaO₂ 65 mmHg and SpO₂ 93%. Is this abnormal?

No: it reflects the lower barometric pressure (≈ 560 mmHg); the A–a gradient calculated with that PB is normal.

## Why doesn't an SpO₂ of 97% on supplemental O₂ rule out respiratory failure?

O₂ keeps saturation up even as ventilation fails; PaCO₂ may be rising without SpO₂ showing it.

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## Related topics

- [Oxygen transport and the dissociation curve](https://durgical.com/en/physiology/oxygen-transport-and-the-dissociation-curve/)
- [The causes of hypoxemia](https://durgical.com/en/physiology/the-causes-of-hypoxemia/)
- [The alveolar gas equation and the A–a gradient](https://durgical.com/en/physiology/the-alveolar-gas-equation-and-the-a-a-gradient/)
- [Carbon monoxide, methemoglobin and cyanide](https://durgical.com/en/physiology/carbon-monoxide-methemoglobin-and-cyanide/)
- [Hypoxemic and hypercapnic respiratory failure](https://durgical.com/en/physiology/hypoxemic-and-hypercapnic-respiratory-failure/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)

## Related conditions

- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Carbon monoxide poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/)
- [Methemoglobinemia](https://durgical.com/en/conditions/methemoglobinemia/)
- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)

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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: [Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/arterial-blood-gas-abg-test/) (MedlinePlus, US National Library of Medicine, 2024); [22.5 Transport of Gases](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-5-transport-of-gases) (OpenStax (Rice University), 2022); [Carbon Monoxide Poisoning: MedlinePlus](https://medlineplus.gov/carbonmonoxidepoisoning.html) (MedlinePlus, US National Library of Medicine, 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 43). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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