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

Pulse oximeters estimate the SpO2 of arterial blood, isolated from other tissues (including venous blood), by applying the Beer-Lambert Law as arterial blood volume fluctuates throughout the cardiac cycle. The Beer-Lambert Law of absorbance demonstrates that the transmission of light is attenuated as it passes through different optical filters (tissue types).  Oxygenated hemoglobin and deoxygenated hemoglobin have unique molar extinction coefficients for different wavelengths of light. As arterial blood volume increases during systole the amount of oxygenated hemoglobin increases and ratio of red to infrared light changes (known as the modulation ratio).  

Conversely, the blood volume and therefore the amount of oxygenated hemoglobin changes very little throughout the cardiac cycle in other tissue types.  By taking the amount of change in absorbance over the change in time (pulse rate), the non-arterial tissues are effectively eliminated mathematically (see below) and the amount of oxygenated hemoglobin in arterial blood remains.

Images: Chan et al, Respiratory Medicine 2013

Mathematically (for transmittance pulse oximeters):

Light from the light-emitting diode is absorbed as it passes through various tissue types before being measured by the photodiode detector.  

Absorbance(total) = sum(Absorbance(all tissue types)) Formula 1

Absorbance(total) = Absorbance(arterial blood) + Absorbance(venous blood) + Absorbance(skin, varies by pigment type or melanin content) + Absorbance( other tissue types fat, bone, vessel walls, etc)  Formula 1a

Taking the derivative, or measuring the change in Absorbance(total) over the change in time gives the following:

dAbsorbance(total)/dt = dsum(Absorbance(all tissue types))/dt Formula 2

dAbsorbance(total)/dt = dAbsorbance(arterial blood)/dt + dAbsorbance(venous blood)/dt + dAbsorbance(skin)/dt + dAbsorbance(other tissue types)/dt  Formula 2a

As dAbsorbance for non arterial tissue types is essentially zero throughout the cardiac cycle, dAbsorbance(total)/dt reduces to:

dAbsorbance(total)/dt = dAbsorbance(arterial blood)/dt Formula 3

Absorbance = the negative log of transmission, and Formula 3 can be written as:

R = [dlog(transmission)/dt]wavelength1/[dlog(transmission)/dt]wavelength2 Formula 4

(R = Modulation Ratio, wavelength 1 and 2 are wavelengths of red light and infrared light)

or alternatively, as:

R = (AC/DC)wavelength1/(AC/DC)wavelength2  Formula 5

(AC = alternating component and DC = constant(direct) component)

Without an adequate pulse, pulse oximeters will either be unable to measure SpO2 or will report inaccurate SpO2 levels. 

References: Chan et al, Respiratory Medicine 2013 Mannheimer Anesth Analg. 2007 

Keywords: modulation, ratio

A transmittance pulse oximeter contains two small light-emitting diodes (LED) which each emit a specific wavelength of light, 660 nm (red) and 940 nm (near infrared). Oxygenated Hemoglobin (O2Hb), absorbs a greater amount of 940 nm wavelength light and a lower amount of 660 nm wavelength light than does deoxygenated hemoglobin (HHb).  Opposite from the LED, a transmittance pulse oximeter utilizes a photodiode to measure the amount of 940 nm and 660 nm wavelength light that is transmitted through the body part between the LED and photodiode.  A microprocessor then calculates SpO2 using the ratio of the measured absorbances over a series of pulses based on individual device calibration to determine the modulation ratio and its corresponding SpO2.

Figure: Jubran, Critical Care, 2015

References: Chan et al, Respiratory Medicine 2013 Lifebox Pulse Oximetry Learning Module; WHO Using Pulse Oximeters Jubran, Critical Care, 2015 How Equipment Works – Pulse Oximeter 

Keywords: wavelengths, IR, infrared, red, LED, photodiode, hemoglobin

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

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

Conventional pulse oximeters assume that the major absorbers of light within arterial blood are deoxygenated hemoglobin (HHb) and oxygenated hemoglobin (O2Hb) and calculate SpO2 based on the ratio of measured absorbance of only two wavelengths of light (660nm and 940nm).  Methemoglobinemia (MetHb) occurs when the iron in hemoglobin is oxidized, altering its ability to bind and offload oxygen. 

The molar extinction coefficient of MetHb at 660 nm red light is approximately 1 and approximately equal to the molar extinction coefficient of HHb (see image below).  The molar extinction coefficient of MetHb at 940 nm infrared light is also approximately 1 and significantly higher than the molar extinction coefficient of O2Hb.  As a result of MetHb absorbing 660 nm and 940 nm light approximately equally, the Modulation Ratio (R-Value) approaches 1, this corresponds to an SpO2 of 80-88%. This can result in either an overestimation or an underestimation of SaO2 levels. 

When a patient is cyanotic and methemoglobinemia is suspected, a co-oximeter capable of measuring the absorbance of greater than 2 wavelengths of light is necessary to determine fractional oxygen saturation and MetHb levels. 

Figure: Jubran, Critical Care, 2015

Keywords: Dyshemoglobin, Methemoglobin, Methemoglobinemia, 

References: Jubran, Critical Care, 2015 Chan et al, Respiratory Medicine 2013