Respiratory Monitoring - An Introduction to Pulse Ox and Capnography

First a bit of physics....

Both pulse oximetry and capnometry rely on the Beer-Lambert Law. 

  • In 1760, Johann Heinreich Lambert proved that the absorbance of light through a material is proportional to the thickness of the material.  
  • In 1852, August Beer proved that the absorbance of light through a material is proportional to the concentration of the attenuating substance in the material.

Together, we know that given:

  • intensity of light transmitted (Io)
  • distance light travels through a substance (d)
  • absorbance of light by the substance (ε)
  • intensity of light received (I1)

We can solve for: the concentration (c) of the substance of interest.

I'm lost. How again does this relate to respiratory monitoring equipment?

In pulse oximetry, the goal is to measure arterial oxygen saturation. Light is transmitted from an emitter and collected at a receiver. The received light is determined to be either pulsatile or non-pulsatile in nature. Simply stated, the pulsatile component is assumed to be arterial, and is the component we are interested in. The non-pulsatile component is assumed to be everything else– venous blood, bone, tissue – and is by and large ignored. 

Two wavelengths of light are transmitted in order to solve for both the concentration of HbO2and deoxyhemoglobin. Red light is preferentially absorbed by deoxyhemoglobin, whereas infrared is preferentially absorbed by HbO2. Having determined the concentration of each, we use the ratio of the concentrations to calculate SpO2.

What are the Limitations of Pulse Oximetry?

  • Pulse oximetry is generally considered accurate only to around 70% SpO2
  • Hemoglobinopathies (methemoglobinemia, carboxyhemoglobinemia) result in erroneous readings – use a co-oximeter in the case of suspected carbon monoxide poisoning
  • Motion artifact
  • Poor probe placement - location affects response time
  • Averaging takes place over 5-20 seconds
  • Tricuspid regurgitation may cause pulsations in venous blood that may be interpreted in the pulsatile component
  • And perhaps most importantly… pulse oximetry tells you nothing about ventilation...

A Brief Introduction to the Capnogram

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The capnogram is divided into phases, as follows: 

  • I: The inspiratory baseline
  • II: The expiratory upstroke – this represents the transition from anatomical deadspace to alveolar gas
  • III: The alveolar plateau – the last gas sample of this phase is generally considered the ETCO2
  • 0: The inspiratory downstroke

The angle between phases II and III is defined as α. Typically, α is approximately 108°. In the case of bronchospasm, the angle is increased, and results in a “shark fin” appearance of the capnograph.

Written by Jeremy Liebman, MD.  PGY-1 at the University of Cincinnati, Dept of Emergency Medicine Residency Training Program

Edited and uploaded by Jeffery Hill, MD MEd, Editor TamingtheSRU, Assistant Professor of Emergency Medicine at the University of Cincinnati, Dept of Emergency Medicine