Pulse Oximetry: Basics and Biases

Topics Discussed in the Podcast:

  • Provider bias and the pulse oximeter.
  • CO oximetry.
  • Pulse oximeter lag.
  • Approach to the well-appearing patient with a low SpO2 reading.
  • Relation of vascular tone to pleth wave amplitude and variability.
  • Using the pulse ox waveform to confirm mechanical capture during transcutaneous pacing.
  • Odds and ends…

Pulse oximetry is more than just a measurement of oxygenation:

The pulse oximetry plethysmograph is a pulsatile waveform that can be thought of as an arterial waveform. However, this waveform is influenced by cardiac output and systemic vascular resistance. 

Thinking of the SpO2 pleth as a pulsatile waveform,  variations in the patients cardiac output should be transmitted to the fingertip arteries, which results in variations in the pulse ox waveform amplitude.

The implications of this are far reaching. In patients who are mechanically ventilated, cyclical changes in intrathoracic pressures can produce measurable variations in the pulse oximetry waveform of a preload dependent patient, which suggesst a potentially fluid-responsive patient.

Do we really need to remove fingernail polish?

You can save your department some money because it’s unlikely to affect the SpO2 measurement to any clinically relevant degree. However, if you are concerned, just turn it sideways.

Is this the silver bullet of measuring fluid responsiveness, non-invasively?

No. At least not yet. The majority of studies evaluating pulse oximetry utilization in this application were conducted under controlled conditions, such as mechanically ventilated patients with consistent respiratory changes in a proper sedation. Additionally, pulse oximetry waveform variations can be difficult to measure during increased systemic vascular resistance (How many patients on mechanical ventilation also require vasopressor support?).

Then there’s the issue with variations in vascular tone. The fingertip is much more susceptible to these changes as opposed to the vessels in the forehead, nose, or ear.

To visualize what pulse wave variability looks like, here’s a helpful picture:

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Extreme respiratory variation in a mechanically ventilated patient

Give it a listen!

Bendjelid, K. (2008). The pulse oximetry plethysmographic curve revisited. Current Opinion in Critical Care. http://doi.org/10.1097/MCC.0b013e3282fb2dc9
Cannesson, M., Besnard, C., Durand, P. G., Bohé, J., & Jacques, D. (2005). Relation between respiratory variations in pulse oximetry plethysmographic waveform amplitude and arterial pulse pressure in ventilated patients. Crit Care. http://doi.org/10.1186/cc3799
Cannesson, M., & Talke, P. (2009). Recent advances in pulse oximetry. F1000 Medicine Reports. http://doi.org/10.3410/M1-66
Chan, E. D., Chan, M. M., & Chan, M. M. (2013). Pulse oximetry: Understanding its basic principles facilitates appreciation of its limitations. Respiratory Medicine. http://doi.org/10.1016/j.rmed.2013.02.004
DeMeulenaere, S. (2007). Pulse Oximetry: Uses and Limitations. Journal for Nurse Practitioners. http://doi.org/10.1016/j.nurpra.2007.02.021
Gesche, H., Grosskurth, D., Küchler, G., & Patzak, A. (2012). Continuous blood pressure measurement by using the pulse transit time: Comparison to a cuff-based method. European Journal of Applied Physiology. http://doi.org/10.1007/s00421-011-1983-3
Goldman, J. M., Petterson, M. T., Kopotic, R. J., & Barker, S. J. (2000). Masimo signal extraction pulse oximetry. Journal of Clinical Monitoring and Computing. http://doi.org/10.1023/A:1011493521730
Hinkelbein, J., & Genzwuerker, H. V. (2008). Fingernail polish does not influence pulse oximetry to a clinically relevant dimension. Intensive and Critical Care Nursing. http://doi.org/10.1016/j.iccn.2007.04.007
Jubran, A. (2012). Pulse oximetry. In Applied Physiology in Intensive Care Medicine 1: Physiological Notes – Technical Notes – Seminal Studies in Intensive Care, Third Edition. http://doi.org/10.1007/978-3-642-28270-6_12
Mendelson, Y. (1992). Pulse oximetry: Theory and applications for noninvasive monitoring. In Clinical Chemistry.
Nitzan, M., Romem, A., & Koppel, R. (2014). Pulse oximetry: Fundamentals and technology update. Medical Devices: Evidence and Research. http://doi.org/10.2147/MDER.S47319
Petterson, M. T., Begnoche, V. L., & Graybeal, J. M. (2007). The effect of motion on pulse oximetry and its clinical significance. Anesthesia and Analgesia. http://doi.org/10.1213/01.ane.0000278134.47777.a5
Pizov, R., Eden, A., Bystritski, D., Kalina, E., Tamir, A., & Gelman, S. (2010). Arterial and plethysmographic waveform analysis in anesthetized patients with hypovolemia. Anesthesiology. http://doi.org/10.1097/ALN.0b013e3181da839f
Pretto, J. J., Roebuck, T., Beckert, L., & Hamilton, G. (2014). Clinical use of pulse oximetry: Official guidelines from the Thoracic Society of Australia and New Zealand. Respirology. http://doi.org/10.1111/resp.12204
Sinex, J. E. (1999). Pulse oximetry: Principles and limitations. American Journal of Emergency Medicine. http://doi.org/10.1016/S0735-6757(99)90019-0

Curbside to Bedside

Curbside to Bedside is a blogcast created by front line EMS Clinicians to provide simple and relevant initial and continuing prehospital EMS education.

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