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Chapter 42. Pulse Oximeter

Ruchir Gupta, MD

Measures ratio of oxygenated to deoxygenated hemoglobin (Hb) using light-emitting diodes (LEDs) that emit light (red and infrared) of different wavelengths (660 and 940 nm). Oxygenated Hb absorbs more infrared light, whereas deoxy-Hb absorbs more red.

Figure 42-1. Oxygen Saturation Curve
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Oxyhemoglobin dissociation curve for whole blood.

An increase in temperature or a decrease in pH (as in working muscle) shifts this relationship to the right, reducing the hemoglobin saturation at the same Po2 and thus aiding in the delivery of oxygen to the tissues.

P50 (Po2 resulting in 50% saturation) = 27 mm Hg.

P90 = 60 mm Hg.

Reproduced from Brunton LL, Chabner BA, Knollmann BC. Goodman & Gilman’s The Pharmacological Basis of Therapeutics. 12th ed. Figure 19-8. Available at: www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.

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Causes of Erroneous Pulse Oximetry Readings
Carboxy-Hb (HbCO)Artificially higher reading than actual O2 saturation
Met-HbFixed at about 85%
IV methylene blueMassive drop in measured saturation (artificial)
Poor perfusion (PVD, cold extremities)Nonpulsatile waveform results in inaccurate measurement
Irregular rhythm (e.g., rapid atrial fibrillation)Reading unreliable
Severe anemia (<5 g/dL)Unreliable reading
Nail polish (black, blue, green)Interferes with LED absorption by Hb causing lower reading or absence of reading
OnychomycosisInterferes with LED absorption by Hb causing lower reading or absence of reading

  • Proprietary system that uses “more than 7 wavelengths of light” to acquire data.
  • Measures:
    • Total Hb (SpHb®)
    • Oxygen content (SpOC™)
    • Carboxyhemoglobin (SpCO®)
    • Methemoglobin (SpMet®)
    • Pleth Variability Index (PVI®)
    • Oxygen saturation (SpO2), pulse rate (PR), perfusion index (PI)
  • Estimates Hb concentration, and surrogates of tissue perfusion and cardiac output
  • Time to loss of signal is longer and recovery time shorter with BP inflation if on same limb
  • With good signal conditions, Masimo found to be more reliable than older oximeters
  • In conditions of severe hypoxemia, Masimo found to be equivalent in accuracy to older oximeters

  • Uses several different wavelengths to determine oxy-, carboxy-, and met-Hb percentages
  • Can be used to accurately determine CO poisoning and methemoglobinemia
  • “Fractional” saturation determined by:

    image

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1. Yelderman M, New W. Evaluation of the pulse oximeter. Anesthesiology. 1983 Oct;59(4):349–352.   [PubMed: 6614545]
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2. Barker SJ, Tremper KK. Effect of carbon monoxide inhalation on pulse oximetry and transcutaneous PO2. Anesthesiology. 1979 May;66(5):677–679.   [PubMed: 3578881]
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3. Kawagishi T, Kanaya N, Nakayama M, Kurosawa S, Namiki A. A comparison of the failure times of pulse oximeters during blood pressure cuff-induced hypoperfusion in volunteers. Anesth Analg. 2004 Sept;99(3):793–796.   [PubMed: 15333413]
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4. Robertson FA, Hoffman GM. Clinical evaluation of the effects of signal integrity and saturation on data availability and accuracy of Masimo SET® and Nellcor N-395 oximeters in children. Anesth Analg...

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