Pulse Oximetry Basic Principles and Interpretation

last modified on: Tue, 10/03/2017 - 22:30

Pulse Oximetry Basic Principles and Interpretation

return to: Pulse Oximetry common misconceptions regarding use 

Introduction

  • Pulse oximetry is considered by some as the '5th' vital sign.
  • The pulse oximeter gives a rapid estimation of the peripheral oxygen saturation, providing valuable clinical data in a very efficient, non-invasive and convenient manner. 

Figure 1: An example of one type of pulse oximeter 
(By Teutotechnik, Med. Produktions- und Vertriebs-GmbH, Niedersachsenstr. 7,49186 Bad Iburg (http://www.teutotechnik.de/produkte1/pulsox2.html) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons)

 

Background

Hemoglobin (Hb) exhibits positive cooperativity.

  • When one O2 molecule binds to one of hemoglobin's four binding sites, the affinity to oxygen of the three remaining available binding sites increases; i.e. oxygen is more likely to bind to a hemoglobin bound to one oxygen than to an unbound hemoglobin.
  • This property results in a sigmoidal oxygen dissociation curve allowing for more rapid loading of oxygen molecules in oxygen rich environments (i.e. alveolar capillaries of the lungs) and easier offloading in oxygen-deficient environments (i.e. metabolically active tissues). 

 

Figure 2: Animation demonstrating the oxygenated and deoxygenated configuration of Hb molecule.
(By en:User:BerserkerBen (Uploaded by Habj) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons)

Hemoglobin is composed of 4 subunits (2 alpha, 2 beta in adults) and exists in two forms: 

  • Taut (T): deoxygenated form with low affinity for O2, therefore it promotes release/unloading of O2
  • Relaxed (R):  oxygenated form with high affinity for O2, therefore oxygen loading is favored. 
  • T and R configurations lead to different electromagnetic absorption and therefore different emission of light.

 

Oximeters operate based on this principle of different absorption and light emission of the T and R configurations. 

  • The oximeter utilizes an electronic processor and a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe.
  • One LED is red, with wavelength of 660 nm, and the other is infrared with a wavelength of 940 nm.
  • Absorption of light at these wavelengths differs significantly between blood loaded with oxygen and blood lacking oxygen.
  • Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through.
  • Deoxygenated hemoglobin allows more infrared light to pass through and absorbs more red light.

Figure 3: Oxy and Deoxy Hemoglobin Absorption

  • The LEDs sequence through their cycle of one on, then the other, then both off about thirty times per second. 
  • The amount of light that is transmitted (in other words, that is not absorbed) is measured.
  • These signals fluctuate in time because the amount of arterial blood that is present increases (literally pulses) with each heartbeat. 
  • By subtracting the minimum transmitted light from the peak transmitted light in each wavelength, the effects of other tissues is corrected for allowing for measurement of only the arterial blood. 
  • The ratio of the red light measurement to the infrared light measurement is then calculated by the processor (which represents the ratio of oxygenated hemoglobin to deoxygenated hemoglobin). 
  • This ratio is then converted to SpO2 by the processor via a lookup table based on the Beer–Lambert law.

Photoplethysomography: 

  • An important tool for any SpO2 reading is plethysmography tracings or "pleth" which is a measure of volumetric changes associated with pulsatile arterial blood flow. 
  • Inconsistent or distorted pleth may result in changes to the computer calculated value resulting in artificially HIGH or LOW SpO2 reading.
  • Therefore, plethysomography ensures reliability of the calculated oxygen saturation. 

Figure 4: Representative PPG taken from an ear pulse oximeter. Variation in amplitude are from Respiratory Induced Variation.

(Spl4 [Public domain or Public domain], via Wikimedia Commons) 

Interpretation Tips

  • Always evaluate plethysomograph in conjunction with SpO2 readings to ensure reliability. 
  • The oxygen saturation as determined by the oximeter is calculated using the ratio of Oxy-Hb/Deoxy-Hb. 
  • This is a useful piece of data to determine whether a patient is able to transfer oxygen into the bloodstream, however 100% saturation on the oximeter does not guarantee that tissues are sufficiently oxygenated.
  • Hemoglobin can normally bind approximately 1.34 mL of O2/g Hb and a normal Hb of 15 g/dL making the O2 binding capacity approximately 20 mL O2/dL blood if 100% saturation. 
    • When the concentration of Hb is decreased, there is a decrease in total O2 content of the blood, but no change in the O2 saturation, hence oximetry is not an effective test to evaluate for anemia. 
    • For example, in a patient with normally functioning hemoglobin, but with a Hb concentration of 8 g/dL the Obinding capacity is approximately 10.7 mL O2/dL. Essentially half of the amount of oxygen is being delivered, but the oximeter reading may still read 100%. 
  • Similarly, if a patient has abnormal hemoglobin molecules, such as in the case of sickle cell anemia where the oxygen dissociation curve is right-shifted, pulse oximetry is a poor measure of hypoxemia and may lead to over diagnosis and over treatment.
    • Therefore, arterial blood gas determination of PaO2 and SaO2 is much more accurate in patients with abnormal hemoglobin dissociation curves.  
  • Pulse oximeters are often applied to areas of thin skin such as an ear lobe or finger tip.
    • Fingernail polish and even different types of skin pigmentation may skew pulse oximeter results. 
  • In a patient with carboxyhemoglobin (i.e. carbon monoxide poisoning) or methemoblobinemia (i.e. hemoglobin with an oxidized iron atom resulting in increased O2 binding and reduced unloading), this abnormally bound hemoglobin has similar absorption spectrum as when O2 is bound in the R configuration.
    • Therefore, the pulse oximeter may report a high saturation due to the large number of hemoglobin in the R configuration, but in reality the tissues are not receiving sufficient oxygen. 

 

Summary

  • Pulse oximetry is a valuable non-invasive tool that provides data regarding the percentage of hemoglobin molecules loaded with oxygen in arterial blood in patients with normal oxygen-dissociation curves. 
  • Awareness of the value, nuances, and shortcomings of pulse oximetry will allow a clinician to better understand the true tissue oxygenation status of a patient and be better prepared for making treatment decisions. 
  • In patients with abnormal hemoglobin structure, abnormal hemoglobin levels, or hemoglobin abnormally bound to other molecules such as CO, pulse oximetry is not an accurate representation of oxygenation. 

 

Further Reading

Jubran, A. (2015). Pulse oximetry. Critical Care, 19(1), 272. http://doi.org/10.1186/s13054-015-0984-8

Blaisdell CJ, Goodman S, Clark K, Casella JF, Loughlin GM. Pulse Oximetry Is a Poor Predictor of Hypoxemia in Stable Children With Sickle Cell Disease. Arch Pediatr Adolesc Med. 2000;154(9):900–903. doi:10.1001/archpedi.154.9.900