Pulse Oximetry¶
Category: Core Concepts Last updated: 2026-04-05 San Juan College EMT-B Program — New Mexico
Overview¶
Pulse oximetry gives you a continuous, non-invasive estimate of how well your patient's hemoglobin is saturated with oxygen. It is one of the most useful monitoring tools available to EMT-B — and one of the most misunderstood. The number is real but incomplete. Knowing when to trust it and when to ignore it is the clinical skill.
What It Measures¶
The pulse oximeter measures SpO2: the percentage of hemoglobin molecules in arterial blood that are carrying oxygen (oxyhemoglobin), as opposed to carrying nothing or carrying other substances.
What SpO2 is NOT: - It is not PaO2 (the partial pressure of oxygen dissolved in plasma — that requires an arterial blood gas) - It is not a measure of how much oxygen is being delivered to tissue (that requires cardiac output, hemoglobin level, and perfusion) - It is not proof of adequate ventilation (a patient can have SpO2 99% while severely hypoventilating — CO2 is rising but SpO2 won't show it)
How It Works¶
The probe emits two wavelengths of light: red (~660 nm) and infrared (~940 nm). Oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) absorb these wavelengths differently. The device measures the ratio of light absorbed at each wavelength through a pulsatile vascular bed — typically a fingertip — and calculates SpO2.
The key word is pulsatile: the device uses the pulsatile component of the signal to differentiate arterial blood (which carries the oxygenated blood you want to measure) from venous blood and tissue. No pulse, no reliable reading. Weak pulse, weak signal.
Normal Values¶
| Population | Normal SpO2 | Notes |
|---|---|---|
| Healthy adults | ≥ 94% | Target for most patients in the field |
| COPD patients | 88–92% | Acceptable target to avoid suppressing hypoxic drive |
| Altitude (> 8,000 ft) | May be 92–95% at rest | Farmington (5,300 ft) has minimal effect; higher terrain may |
| Newborns at 10 min | 85–95% | See neonatal-resuscitation for post-delivery targets |
Clinical threshold: SpO2 < 94% in most patients warrants supplemental oxygen. SpO2 < 90% is hypoxemia requiring aggressive intervention.
Limitations — When NOT to Trust the Number¶
This section is the most important part of this article. A normal-looking SpO2 does not mean the patient is fine. Know when the number lies.
Carbon Monoxide Poisoning (False High — Dangerous)¶
CO binds hemoglobin with 200× greater affinity than oxygen, forming carboxyhemoglobin (COHb). The pulse oximeter cannot distinguish COHb from HbO2 — they absorb light similarly. A patient with severe CO poisoning may read 99–100% SpO2 on the monitor while their tissues are functionally asphyxiating.
Clinical rule: In any patient with suspected CO exposure (smoke inhalation, running engine in enclosed space, multiple victims at same scene), do not use SpO2 to guide oxygen therapy. Treat with 100% O2 regardless of the reading.
Poor Peripheral Perfusion (Unreliable)¶
In shock, hypothermia, or severe hypovolemia, peripheral vasoconstriction reduces blood flow to the extremities. The probe cannot detect a strong enough pulsatile signal. The device will display a low perfusion warning, give a fluctuating or absent reading, or simply report an unreliable number.
The patient who needs the oximeter most (the shocked patient) is the one in whom it is least reliable. Use clinical assessment — skin signs, mental status, vital signs — when SpO2 is unreliable.
Anemia (Normal Reading, Abnormal Delivery)¶
A severely anemic patient may have SpO2 of 98% — meaning all their hemoglobin is saturated — but have so little hemoglobin that total oxygen delivery to tissues is critically low. SpO2 measures saturation, not hemoglobin level. A patient with a hemoglobin of 5 g/dL and SpO2 of 98% is not okay.
Dark Nail Polish and Artificial Nails¶
Nail polish, particularly dark colors (black, dark blue, dark green), and acrylic or gel nails can interfere with light transmission through the nail bed. Remove the probe and place it on a different finger, on the earlobe, or across the bridge of the nose if nail polish is present and the reading is questionable.
Motion Artifact¶
A patient who is shivering, seizing, or moving excessively will cause spurious SpO2 readings. The device may alarm or display values that fluctuate wildly. This is motion artifact, not real desaturation. Stabilize the probe, use a different site, or weight the reading against your clinical assessment.
Jaundice (Bilirubin)¶
Elevated bilirubin (jaundice) can interfere with light absorption at certain wavelengths, potentially causing falsely low readings. Less common in the field, but relevant in a patient with known liver disease.
Peripheral Vascular Disease, Raynaud's Phenomenon¶
Patients with chronic peripheral vascular disease or Raynaud's may have chronically low peripheral perfusion, producing unreliable readings even at baseline.
Technique¶
- Select a site: fingertip preferred; earlobe or bridge of nose if fingertip unavailable
- Remove nail polish if present and interfering
- Ensure the finger is warm — if cold, warm it briefly (friction, warm blanket)
- Apply probe: clip probe over fingertip so light source and detector are aligned through the tissue
- Wait for waveform stabilization — most devices take 15–30 seconds for a stable reading
- Read the displayed SpO2 and heart rate; confirm heart rate matches your manual pulse count
- Reassess continuously during transport
A discrepancy between the oximeter's heart rate and your manual pulse count is a sign of an unreliable reading — artifact or poor signal.
Clinical Decision Rule¶
Treat the patient, not the number.
If SpO2 reads 97% but the patient is using accessory muscles, has retractions, has marked tachypnea, and looks frightened → treat for respiratory distress regardless of the number.
If SpO2 reads 85% but the patient is alert, comfortable, and has dark nail polish on the probe finger → assess clinically before initiating aggressive intervention. Move the probe.
The oximeter is a tool that informs your assessment. It does not make decisions — you do.
Related¶
- oxygen — pharmacology reference for oxygen; when and how to administer
- oxygen-administration — delivery device selection (NC vs. NRB vs. BVM) based on SpO2 and clinical picture
- respiratory-distress — when SpO2 values drive treatment escalation vs. clinical picture
- vital-signs — SpO2 in context of the full vital signs set