Oxygen FAQ

Up to date, expert answers to frequently asked questions (FAQ) about oxygen supply systems, respiratory care and pulse oximetry written by OCC & collaborators.

Physiology of pulse ox

PaO2 is a measurement of the partial pressure of oxygen in arterial blood and requires an arterial blood gas (ABG). In many settings an ABG may not be practical or feasible, and thus the established mathematical relationship between SpO2 and PaO2 can be used. There is a distinct relationship between PaO2 and SpO2 which is illustrated by the Oxygen Dissociation Curve (see image). This relationship can be altered by many factors (e.g. temperature, FDG, pH) but generally follows the classic “S-shape” graph with PaO2 on the x axis and oxygen saturation on the y axis. This online calculator can be used to convert SpO2 to PaO2.

References: Lifebox Pulse Oximetry Learning Module; Oxygen Calculator Website

Keywords: PaO2, oxygen dissociation curve, conversion

SpO2 is the functional oxygen saturation measured by pulse oximetry. A normal SpO2 reading is usually considered to be above 94%. An SpO2 of 90-94% can signal that a patient may have a new or chronic respiratory problem, or that they may be progressing to hypoxemia. Many clinical definitions of hypoxemia (i.e. low oxygen concentration in the blood) use a cutoff of 90%, though depending on the context, SpO2 values above 90% may be abnormal (e.g. a healthy, young adult at sea level should be 96-100%. Similarly, depending on the context, an SpO2 less than 90% may be physiologically appropriate (e.g. at high altitude). Read more about the “Optimal SpO2 target for Patients with Respiratory Failure”

References: Lifebox Pulse Oximetry Learning Module

Keywords: normal SpO2, hemoglobin

The term CO-oximetry refers to devices that use at least four wavelengths of light to measure not only oxy and deoxy-hemoglobin, but also other forms of hemoglobin (e.g. CO-Hb, Met-Hb). While most conventional pulse oximeters use two wavelengths of light and can only detect oxyhemoglobin or deoxy-hemoglobin (to provide a functional saturation, sO2), some pulse oximeters contain many more wavelengths and the ability to function as ‘pulse CO-oximeters.’ With more wavelengths, these devices can measure dyshemoglobins and provide “fractional” oxygen saturation – see image below. (Note: these devices are clearly marked with this function and often are considerably more expensive than other devices). 

Despite this, the term ‘CO-oximeter’ is often used synonymously with ‘arterial blood gas analyzer’ because historically it was only multi-function arterial blood gas analyzers that possessed CO-oximetry functionality.  Some pulse CO-oximeters have ~10 wavelengths of light, whereas benchtop CO-oximeters may use 128 to 256 wavelengths of light.

Of note, the terms “CO-oximeter” and “Hemoxemeter” are brand names coined by the companies Instrumentation Laboratories and Radiometer, respectively. The generic term for a device with these capabilities is a “multi-wavelength oximeter,” but the term “CO-oximeter” is widely used to refer to these devices, regardless of brand.

Keywords: CO-oximetry, arterial, carboxyhemoglobin, methemoglobin

illustration describing functional and fractional oxygen saturation

There are many different terms that are frequently (and sometimes incorrectly) referred to as ‘oxygen saturation.’ Here we define them:

  • Oxygen saturation’ – A quantification of the proportion of hemoglobin inside red blood cells that is bound by oxygen. This can refer to multiple measurements, but most commonly is used to refer to functional arterial oxygen saturation.  
  • SO2 or ‘functional oxygen saturation’ – The fraction of effective hemoglobin (e.g. excluding Hb species like CO-Hb or met-Hb and only measures oxy- and deoxy-Hb). This is expressed as (HbO2/[Hb+HbO2]).
  • SpO2 – Functional, arterial oxygen saturation measured by pulse oximetry.
  • FO2Hb or ‘fractional oxygen saturation’ – The fraction of total hemoglobin (Hb) that is carrying oxygen. Total Hb includes not only oxy- and deoxy-Hb, but also other hemoglobin species like Met-Hb, CO-Hb, S-Hb. Fractional oxygen saturation can be measured by relatively few pulse oximeters, and usually requires blood gas analysis and a capable co-oximeter. This is expressed as (HbO2/[Hb+HbO2+MetHb + COHb + SHb]).
  • SO2 or SaO2 – Arterial oxygen saturation measured by arterial blood gas. 
  • SvO2 – Venous oxygen saturation. 
  • ScvO2 – Central venous oxygen saturation. 

Keywords: oxygen saturation, SpO2, SaO2, SvO2, SO2


Spectrophotometry is a type of quantitative measurement technique that is used to measure the reflection or transmission properties of a substance as a function of wavelength. Every type of substance absorbs light over a specific range of wavelengths. This type of measurement allows us to assess the intensity of light that a substance (such as hemoglobin in blood) absorbs, and therefore has clinical and bioengineering applications. 

Clinically, spectrophotometry is used in pulse oximeters to determine the proportion of oxygenated hemoglobin in arterial blood. Since different wavelengths of light are absorbed by oxygenated and deoxygenated blood, pulse oximeters can use this technique to determine a patient’s peripheral oxygen saturation (SpO2).

Keywords: spectroscopy, spectrophotometry, wavelength, light

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