Oximetry

• Photoplethysmography is the general term for the non-invasive optical study of the pulsatile waveform and was originally used in simple pulse rate meters in the 1960’s & 1970’s.
• The visual detection of hypoxia is unreliable and is not reliably detected above SaO2 90%.
• A co-oximeter, measures light absorbance using at least four light frequencies.
• There are four haemoglobin species which may be present in blood, (oxy-, deoxy-, carboxy-, & met-) and each has its own absorption spectra.
• The co-oximeter can measure all four haemoglobin species and the result displayed is the SaO2.
• This ‘fractional saturation’ is the ratio of oxyHb to the sum of all Hb species (oxyHb, deoxyHb, CoHb, metHb) and is measured by the co-oximeter.
• The pulse oximeter uses only two frequencies and the result displayed is the SpO2.
• This ‘functional saturation’ refers to the ratio of oxyHb to the sum of oxyHb and deoxyHb.
• The pulse oximeter probe has two light emitting diodes (LEDs) of different wavelength and a photodetector (usually a silicon photodiode).
• There are 3 measurement phases which continuously cycle with a pulsing frequency of approx 400 Hz.
• Both the pulsing frequency & exact LED light wavelengths vary slightly between manufacturers.
• In the interval between the two light pulses, background (ambient) light level is measured and its effect on signal intensity is compensated for electronically.
• In the first measurement phase, the difference in absorption between oxy- and deoxy-haemoglobin is maximal at 660 nm.
• The algorithm used in pulse oximetry was based on a few healthy subjects breathing known (safe) oxygen concentrations, whose arterial oxygen saturation was determined accurately by a co-oximeter.
• Extinction Coefficient describes the amount of absorbance at a particular wavelength for a given concentration of absorbant in pulse oximetry.
• The original ‘stand-alone’ pulse oximeters used Light Emitting Diode (LED) or Liquid Crystal (LCD) displays, but oximetry is now incorporated into modular monitors and uses the monitor’s multiparameter display.
• The ratio of light transmission at the two frequencies (660 & 940 nm) is calculated and compared with ratios derived empirically that are stored in the microprocessor to convert this information to SpO2 in pulse oximetry.
• Greatly improved artefact rejection technology has reduced false alarms, particularly diathermy interference, however distracting false alarms caused by movement, NIBP cycling or neuromuscular monitors can still occur in pulse oximetry.
• The accuracy at low saturations was extrapolated from this data, but has progressively been improved in pulse oximetry.
• Signal processing in pulse oximetry involves a filter, amplifier and analogue to digital converter.
• Finger is the most popular monitoring site, but earlobe and flexible multi-site probes are also available.
• Measurement of absorbance in pulse oximetry shows two components; a constant background level due to skin pigmentation, tissue, venous blood, non pulsatile arterial blood and ambient light, and a pulsatile component due to arterial blood flow.
• The plethysmograph displayed on the monitor shows the absorption by the arterial pulse against time in pulse oximetry.
• Absorbance departs slightly from that predicted by the Beer-Lambert Law due to some ‘scattering’ of light by red blood cells in pulse oximetry.
• Isoestic point (804 nm), where oxy & deoxyHb absorb light equally, is not used in pulse oximetry.
• If the constant component is subtracted from the total, then the pulsatile component (arterial component) is isolated in pulse oximetry.
• There are two main probe types in pulse oximetry: Transmissive, where light shines through the digit (or ear lobe) to the sensor, and Reflectance, where the LED and the detector are next to each other.
• In the second measurement phase, the absorption curve is relatively flat, so slight variation between diode output is observed.
• The alarm can be disabled for prolonged periods while blood pressure is measured, but placing the oximeter probe on the opposite arm to the NIBP cuff is preferable.
• Alarm parameters should be visible at all times and some oximeters 'remember' previously set alarm limits whilst others revert to preset values with each 'switch on'.
• The 'Ideal oximeter' should have a small delay (<10 sec) at power up, rapid response time, diathermy & MRI resistance, battery & mains operation, memory (trend capability), and an interface for computers, printers.
• Pulse oximeters should have multiple sites able to be monitored, be shielded from ambient light & RF interference, and have a display that varies with pulse volume.
• Pulse oximeters should have realistic alarm default values, an alarm activated automatically at switch on, and temporary alarm suppression only.
• Pulse oximetry has limitations due to patient factors such as pulseless circulation, hypovolaemia, hypothermia, haemoglobin species like CarboxyHb, MetHb, and Foetal Hb, anaemia, bilirubin, dyes injected like Methylene blue, and other factors like skin pigmentation, nail polish, and false nails.
• Applications of pulse oximetry include anaesthesia & sedation, respiratory care (PACU, ICU, postoperative), neonatal care (prevention hyperoxia), detection of MH, air & amniotic fluid embolism.
• Interference and error in pulse oximetry can be caused by diathermy, MRI, ambient light, motion artefact, and error.
• Pulseless circulation, hypovolaemia, hypothermia, and other factors can lead to the absence of a pulsatile arterial circulation, making pulse oximetry ineffective.
• Plethysmographic applications of pulse oximetry include estimation of blood pressure (using BP cuff), estimation of volume status, estimation of adequacy of circulation, Allen’s test, surgery (digital reimplantation, free flaps), CPR effectiveness, and oximetric - (SpO2) monitoring.
• The high incidence of undetected hypoxic events during anaesthesia drove research in oximetry in the 1980’s.
• Pulsatile venous flow with right heart failure may interfere with oximetry.