By Dr. Daniel Pehböck · Reading time approx. 12 minutes
Pulse oximetry is one of the most widely used non-invasive monitoring procedures in outpatient medicine. Whether in general practice, pulmonology, or pediatric offices – peripheral oxygen saturation (SpO₂) provides clinically relevant information about oxygenation within seconds. However, as ubiquitous as the pulse oximeter may seem, correct interpretation of the values, recognizing typical sources of error, and choosing the right device require expertise. This article summarizes the essential basics – practical, evidence-based, and with concrete recommendations for device selection.
Table of Contents
- Functional Principle of Pulse Oximetry
- SpO₂, Heart Rate and Perfusion Index – What is Measured?
- Typical Sources of Error and Limitations
- Clinical Interpretation: When Does it Get Critical?
- Device Types in Comparison
- Purchase Criteria for Medical Practice
- Special Features in Pediatrics
- Practical Tips for Everyday Use
- Conclusion and Recommendations
1. Functional Principle of Pulse Oximetry
Pulse oximetry is based on the principle of spectrophotometry. Two light-emitting diodes – one in the red (approx. 660 nm) and one in the infrared range (approx. 940 nm) – emit light through perfused tissue, typically the fingertip. A photodetector on the opposite side detects the transmitted light intensity.
The crucial physical background: Oxygenated hemoglobin (O₂Hb) absorbs more infrared light, while deoxygenated hemoglobin (HHb) absorbs more red light. The device's algorithm calculates the functional oxygen saturation (SpO₂) from the ratio of absorption values at both wavelengths.
The device utilizes the plethysmographic method: Only the pulsatile (arterial) component of the signal is evaluated. By subtracting the constant part (venous blood, tissue, bone), the algorithm isolates the arterial signal. This technique makes the measurement independent of tissue thickness and skin pigmentation – at least in theory.
Good to know: Conventional pulse oximeters as two-wavelength devices measure only O₂Hb and HHb. They cannot differentiate dyshemoglobins like carboxyhemoglobin (COHb) or methemoglobin (MetHb). Special CO-oximeters with 7–8 wavelengths are required for this purpose.
2. SpO₂, Heart Rate and Perfusion Index – What is Measured?
In addition to oxygen saturation, modern pulse oximeters provide additional clinically useful parameters:
SpO₂ (Peripheral Oxygen Saturation)
The primary measurement value, expressed in percent. It correlates with arterial oxygen saturation (SaO₂) but deviates under certain conditions. Accuracy is typically ±2% in the range of 70–100% for most certified devices. Below 70%, the values are unreliable as the calibration curves are based on subject studies that for ethical reasons do not enter this range.
Heart Rate
Derived from the pulsatile signal component. Matches the ECG heart rate well in sinus rhythm but can deviate significantly in arrhythmias (especially atrial fibrillation).
Perfusion Index (PI)
The PI describes the ratio of pulsatile to non-pulsatile signal and is an indirect marker for peripheral perfusion. A low PI (< 0.4%) indicates poor perfusion and should prompt questioning the measurement. Devices with PI display are particularly recommended for practice as they objectify signal quality.
Plethysmographic Curve
The graphical representation of the pulse wave allows visual assessment of signal quality. A regular, well-modulated curve indicates a reliable measurement. Devices with a plethysmography curve display are superior in clinical settings compared to purely numeric displays.
3. Typical Sources of Error and Limitations
Pulse oximetry is a robust method but not error-free. The following disturbances should be known by every user:
| Source of Error | Mechanism | Clinical Relevance |
|---|---|---|
| Nail Polish / Gel Nails | Absorption in the measurement range, especially with blue, black, or green polish | SpO₂ falsely low; apply sensor laterally or on the ear |
| Peripheral Vasoconstriction | Reduced pulsatile signal in cold, shock, vasoconstrictors | No or incorrect signal; warm hands, use ear clip |
| Motion Artifacts | Venous pulsations are interpreted as arterial | Falsely low values; calm patient, increase averaging |
| Strong Ambient Light | Foreign light interferes with the photodetector | Implausible values; cover sensor |
| CO Poisoning | COHb is measured as O₂Hb | SpO₂ falsely high despite severe hypoxia – life-threatening! |
| Methemoglobinemia | MetHb absorbs similarly at 660 nm and 940 nm | SpO₂ converges towards 85%, regardless of actual value |
| Severe Anemia | Reduced Hb content, relative signal decreases | SpO₂ can be normal despite low O₂ content (Hb < 5 g/dl) |
| Dark Skin Pigmentation | Altered light absorption by melanin | Tendency for falsely high values by 2–3%; clinical correlation! |
Warning: Recent studies (Sjoding et al., NEJM 2020; Wong et al., JAMA IM 2021) show that pulse oximeters can systematically display higher SpO₂ values in patients with dark skin. The FDA released new guidelines in 2024 for validation with different skin pigmentations. Pay extra attention to clinical signs of hypoxia in these patients.
4. Clinical Interpretation: When Does it Get Critical?
The interpretation of SpO₂ values is always done in the clinical context. The following orientation values apply to adult patients without relevant pre-existing conditions:
| SpO₂ Range | Assessment | Action |
|---|---|---|
| 96–100 % | Normal Finding | No intervention necessary |
| 92–95 % | Mild hypoxemia, requires monitoring | Investigate causes, possibly administer O₂, monitor progress |
| 88–91 % | Moderate hypoxemia | O₂ therapy, consider ABG, close monitoring |
| < 88 % | Severe hypoxemia, potentially life-threatening | Immediate O₂, ABG, consider hospitalization |
Important in COPD: For patients with chronic hypercapnia (e.g., COPD GOLD III–IV), the target SpO₂ range is 88–92 %. Uncritical administration of oxygen until normoxemia can worsen hypercapnia and lead to CO₂ narcosis. LTOT indication should be based on a stable arterial pO₂ < 55 mmHg, not just SpO₂.
The oxygen-hemoglobin dissociation curve (sigmoid shape) has a significant clinical implication: In the steep part of the curve (SpO₂ 75–93%), small changes in paO₂ cause significant changes in saturation. In the plateau part (> 95%), considerable paO₂ changes can occur with almost constant SpO₂. An SpO₂ of 99% does not rule out significant hyperoxia – an argument against uncritical oxygen overdosage.
5. Device Types in Comparison
Depending on the area of use, different device types are suitable:
| Type | Advantages | Disadvantages | Suitability |
|---|---|---|---|
| Finger Clip | Compact, affordable, ready to use | Not suitable for continuous monitoring, small display | Spot check in practice |
| Desk/Handheld Device | Large display, plethysmography, alarms | Less mobile, higher cost | Practice with higher monitoring need |
| Ear Clip | Independent of finger perfusion, faster reaction | Special sensor required, more expensive | Shock patients, intensive care |
| Pediatric | Adjusted sensor size, wrap sensors | Only suitable for children/infants | Pediatric practice, neonatology |
6. Purchase Criteria for Medical Practice
When acquiring a pulse oximeter for professional use, you should systematically examine the following criteria:
CE Marking and MDR Compliance
Since May 2021, the Medical Device Regulation (MDR, EU 2017/745) applies in the EU region. Ensure that the device is certified as a Class IIa medical product and carries a valid CE marking with a Notified Body. Cheap products without correct certification often fail to meet the required accuracy and are unsuitable for professional use.
Measurement Accuracy (Arms Value)
Accuracy is given as Arms (Root Mean Square). For clinical use, the Arms value should be ≤ 2% in the SpO₂ 70–100% range. High-quality devices achieve Arms values of ≤ 1.5%.
Perfusion Index and Signal Quality
A perfusion index (PI) as an additional parameter and the display of the plethysmography curve allow for immediate assessment of measurement quality. Devices with these functions are clearly preferred in professional settings.
Display and Readability
A well-readable OLED or LED display with sufficient brightness, multiple view modes, and large numbers is crucial in everyday practice. Devices with a rotatable display make it easier to read in different positions.
Durability and Hygiene
The sensor must be disinfectable (wiping disinfection with common surface disinfectants). Robust construction, drop resistance, and a solid clamping structure extend the lifespan. Pay attention to suitability for common DACH disinfectants.
Battery Life
Finger-clip oximeters should offer at least 30 hours of operating time with one set of batteries. Automatic shut-off functions conserve battery life.
7. Special Features in Pediatrics
Pulse oximetry in children and infants is subject to specific requirements:
- Sensor Size: Standard finger clips are unsuitable for children under 6 years and infants. Wrap-around sensors (adhesive sensors) for toes, fingers, or foot provide significantly more reliable results here.
- Motion Tolerance: Children rarely cooperate – devices with advanced motion artifact compensation (e.g., Masimo SET technology) significantly reduce false alarms.
- Normal Values: Newborns have physiologically lower SpO₂ values in the first hours of life. The pre-ductal saturation (right hand) is higher than the post-ductal (foot) before ductus closure. This gradient is used in pulse oximetry screening for critical congenital heart defects (CCHD screening).
- CCHD Screening: Pulse oximetry screening for newborns (24–48 hours postnatal) is established in Austria and Germany. It detects relevant cyanotic heart defects with a sensitivity of approx. 75–80% and a specificity of > 99%.
8. Practical Tips for Everyday Use
Practical Tip 1: Always measure on the "right" finger
The best signal quality is provided by the middle and ring fingers of the non-dominant hand. Avoid the thumb (its own pulsation can interfere with the measurement) and the little finger (often too thin for good contact). Ensure the sensor is fully in place and the finger is not cold.
Practical Tip 2: Always correlate reading with clinic
An SpO₂ reading is only meaningful in context. Always assess in parallel: respiration rate, respiratory effort, skin coloration, consciousness, and perfusion index. A "normal" SpO₂ in a tachypneic patient with increased respiratory effort is no reassurance – it may mask impending decompensation.
Practical Tip 3: Don't forget device maintenance
Clean the sensor after each use with an approved surface disinfectant. Regularly check the clip's spring tension, battery performance, and the sensor's function (e.g., by self-measurement as a reference). Document the device check according to the requirements of your QM system.
Practical Tip 4: Measurement technique in poor perfusion
If a stable signal cannot be achieved on the finger: use earlobe, toes or nasal bridge as alternative measurement sites. Warm cold hands before measuring. Always check the perfusion index – if it is below 0.4%, the reading is questionable. In doubt: perform arterial blood gas analysis.
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