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
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 mmHgArterial 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
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 | Normal or high | Carboxyhemoglobin absorbs light like oxyhemoglobin |
| Methemoglobinemia | Stuck near 85% | Methemoglobin absorbs at both wavelengths |
| Severe anemia | Normal | Saturation is normal though O₂ content is low |
| Hypoventilation on O₂ | 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.
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.
Related topics
- Oxygen transport and the dissociation curve
- The causes of hypoxemia
- The alveolar gas equation and the A–a gradient
- Carbon monoxide, methemoglobin and cyanide
- Hypoxemic and hypercapnic respiratory failure
- High altitude and acclimatization
Related conditions
- Chronic obstructive pulmonary disease (COPD)
- Carbon monoxide poisoning
- Methemoglobinemia
- Acute respiratory distress syndrome (ARDS)
- Iron deficiency anaemia
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 (MedlinePlus, US National Library of Medicine, 2024); 22.5 Transport of Gases (OpenStax (Rice University), 2022); Carbon Monoxide Poisoning: MedlinePlus (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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Representative model, for educational purposes only.