Question 17

Describe the principles of measurement of arterial haemoglobin O2 saturation using a pulse oximeter (60% marks). Outline the limitations of this technique (40% marks).

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College Answer

Most candidates provided a reasonable structured sequence of how a pulse oximeter generates a value. Nearly all candidates described the Beer-Lambert laws correctly, but few specifically described the basic principles of absorption spectrophotometry. Most candidates had a reasonable list of extrinsic factors that can interfere with pulse oximeter performance, but few described the intrinsic/inherent limitations of the device that can cause SpO2 to be different to SaO2, such as functional versus fractional saturation.

Discussion

  • Principles fundamental to pulse oximetry
    • Different absorption of different light wavelengths by haemoglobin species
    • Isolation of the pulsatile arterial signal because of pulse-related changes in optical distance 
  • Different light absorption by haemoglobin species:
    • Two wavelengths (660 and 940 nm) are used in pulse oximettry
    • Deoxyhaemoglobin absorbs more light at 660nm and oxyhaemoglobin absorbs more light at 940 nm.
  • Quantification of haemoglobin species concentration
    • Beer Law: the concentration of a given solute in a solvent is determined by the amount of light that is absorbed by the solute at a specific wavelength
    • Thus, concentration of oxyhaemoglobin and deoxyhaemoglobin can be determined from their absorption of the two wavelengths
  • Determination of pulsatile signal
    • Absorption-over-time signal from arterial blood is pulsatile, whereas signal from venous haemoglobin and tissue is not.
    • When the arteries pulsate, the distance travelled by light though them changes
    • One can therefore use Lambert's Law (equal parts in the same absorbing medium absorb equal fractions of the light that enters them).
    • Thus, one can compare the ratio of pulsatile and nonpulsatile absorbance to produce R, the ratio of absorbance at any given time 
    • R = (AC660 / DC660) / (AC940/DC940)
  • Calibration with empirically measured data
    • R is meaningless unless it can be related to oxygen saturation;
    • A series of saturation measurements and R values have been collected from healthy individuals in the 100-75% saturation range, and extrapolated to 0%
    • This array of data is used by the pulse oximeter control circuit as a lookup table to p
  • Correction for ambient light
    • The pulse oximeter LEDs strobe at a high frequency (400-900 Hz)
    • When the LED is off, the photometer measures the absorption of ambient light, and subtracts  this from the signal measured when the LEDs are on.
    • This eliminates the contribution of (most) ambient light
  • Essential design elements of a pulse oximeter include:
    • LED light sources
    • A photometer
    • A control circuit
    • A user interfce with display and alarm functions
  • Limitations of pulse oximetry are:
    • Inevitable difference with ABG oximetry due to processign artifact
    • Inabiulity to detect PO2 or discriminate between haemoglobin species e.g carboxyhaemoglobin
    • Spurious results in the presence of carboxyhaemoglobin and methaemoglobin
    • Errors to detect pulse with poor perfusion, nonpulsatile ECMO flow or patient movement
    • Increasing inaccuracy in the extrapolated range of calibration values (low oxygen saturation, below 50%)

References

Sinex, James E. "Pulse oximetry: principles and limitations." The American journal of emergency medicine 17.1 (1999): 59-66.

Jubran, Amal. "Pulse oximetry." Critical care 3.2 (1999): R11.

Jubran, Amal. "Pulse oximetry." Applied Physiology in Intensive Care Medicine 1. Springer, Berlin, Heidelberg, 2012. 51-54.

Jubran, Amal. "Pulse oximetry." Critical Care 19.1 (2015): 272.

Kamat, Vijaylakshmi. "Pulse oximetry." Indian J Anaesth 46.4 (2002): 261-8.

Kyriacou, Panayiotis, Karthik Budidha, and Tomas Y. Abay. "Optical techniques for blood and tissue oxygenation." Encyclopedia of Biomedical Engineering, ed R. Narayan (Oxford: Elsevier) (2019): 461-472.

Severinghaus, John W., and Shin O. Koh. "Effect of anemia on pulse oximeter accuracy at low saturation." Journal of clinical monitoring 6.2 (1990): 85-88.

Fluck, Robert R., et al. "Does ambient light affect the accuracy of pulse oximetry?." Respiratory care 48.7 (2003): 677-680.

Amar, David, et al. "Fluorescent light interferes with pulse oximetry." Journal of clinical monitoring 5.2 (1989): 135-136.

Block, Frank E. "Interference in a pulse oximeter from a fiberoptic light source." Journal of clinical monitoring 3.3 (1987): 210-211.

American Association for Respiratory Care. AARC Clinical Practice Guideline: Pulse oximetry. Respir Care 1991;36(12):1406–1409

Ralston, A. C., R. K. Webb, and W. B. Runciman. "Potential errors in pulse oximetry: I. Pulse oximeter evaluation." Anaesthesia 46.3 (1991): 202-206.

Webb, R. K., A. C. Ralston, and W. B. Runciman. "Potential errors in pulse oximetry: II. Effects of changes in saturation and signal quality.Anaesthesia 46.3 (1991): 207-212.

Singh, Anupam Kumar, et al. "Comparative evaluation of accuracy of pulse oximeters and factors affecting their performance in a tertiary intensive care unit." Journal of clinical and diagnostic research: JCDR 11.6 (2017): OC05.

Severinghaus, J. W. "Pulse oximetry uses and limitations." ASA Convention. 1989.

Ralston, A. C., R. K. Webb, and W. B. Runciman. "Potential errors in pulse oximetry III: effects of interference, dyes, dyshaemoglobins and other pigments." Anaesthesia 46.4 (1991): 291-295.

Severinghaus, John W. "Takuo Aoyagi: discovery of pulse oximetry." Anesthesia & Analgesia 105.6 (2007): S1-S4.

Tremper, Kevin K. "Pulse oximetry." Chest 95.4 (1989): 713-715.

Feiner, John R., John W. Severinghaus, and Philip E. Bickler. "Dark skin decreases the accuracy of pulse oximeters at low oxygen saturation: the effects of oximeter probe type and gender." Anesthesia & Analgesia 105.6 (2007): S18-S23.

Pologe, Jonas A. "Pulse oximetry: technical aspects of machine design." International anesthesiology clinics 25.3 (1987): 137-153.

Webster, John G. Design of pulse oximeters. CRC Press, 1997.