Question 5

College Answer

A lack of understanding of the physics behind pulse oximetry was a common area of weakness amongst most candidates. Candidates were expected to mention the underlying principle of the “Beer Lambert” Law, absorption spectra and that the differential absorption of light at different wavelengths by different haemoglobin species is used to determine the fractions of haemoglobin types. Limitations should include mention of errors due to calibration as well as sources of false positive and false negative readings.

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 40 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 
• 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%)

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