Pulse oximetry

• pulse oximetry is a simple non-invasive method of monitoring the percentage of haemoglobin (Hb) which is saturated with oxygen
  • pulse oximetry provides estimates of arterial oxyhemoglobin saturation (SaO2) by utilizing selected wavelengths of light to noninvasively determine the saturation of oxyhemoglobin (SpO2)
  • the pulse oximeter consists of a probe attached to the patient's finger or ear lobe which is linked to a computerised unit
  • unit displays the percentage of Hb saturated with oxygen together with an audible signal for each pulse beat, a calculated heart rate
  • safe use of pulse oximetry requires knowledge of its limitations, which include motion artifacts, poor perfusion at the site of measurement, irregular rhythms, ambient light or electromagnetic interference, skin pigmentation, nail polish, calibration assumptions, probe positioning, time lag in detecting hypoxic events, venous pulsation, intravenous dyes, and presence of abnormal hemoglobin molecules
    
    
    
• how pulse oximetry works
  • a source of light originates from the probe at two wavelengths (650nm and 805nm)
    • light is partly absorbed by haemoglobin, by amounts which differ depending on whether it is saturated or desaturated with oxygen
    • via calculation of the absorption at the two wavelengths the processor can compute the proportion of haemoglobin which is oxygenated
    • oximeter is dependant on a pulsatile flow and produces a graph of the quality of flow
    • where flow is sluggish (eg hypovolaemia or vasoconstriction) the pulse oximeter may be unable to function
    • the oximeter is capable of distinguishing pulsatile flow from other more static signals (such as tissue or venous signals) to display only the arterial flow
      
      
• contraindications:
  • presence of an ongoing need for measurement of pH, PaCO2, total hemoglobin, and abnormal hemoglobins may be a relative contraindication to pulse oximetry
    
    
• hazards/complications
  • pulse oximetry is considered a safe procedure, but because of device limitations, false-negative results for hypoxaemia and/or false-positive results for normoxaemia or hyperoxaemia may lead to inappropriate treatment of the patient
  • tissue injury may occur at the measuring site as a result of probe misuse (eg, pressure sores from prolonged application or electrical shock and burns from the substitution of incompatible probes between instruments).
    
    
• oximeters are calibrated during manufacture and automatically check their internal circuits when they are turned on. They are accurate in the range of oxygen saturations of 70 to 100% (+/-2%), but less accurate under 70%
  • pitch of the audible pulse signal falls with reducing values of saturation
  • size of the pulse wave (related to flow) is displayed graphically
  • some models automatically increase the gain of the display when the flow decreases and in these the display may prove misleading
  • alarms usually respond to a slow or fast pulse rate or an oxygen saturation below 90%. At this level there is a marked fall in PaO2 - this represents serious hypoxia

In the following situations the pulse oximeter readings may not be accurate:

• a reduction in peripheral pulsatile blood flow
  • produced by peripheral vasoconstriction (hypovolaemia, severe hypotension, cold, cardiac failure, some cardiac arrhythmias) or peripheral vascular disease
  • result in an inadequate signal for analysis
    
    
• inability to detect saturations below 83% with the same degree of accuracy and precision seen at higher saturations
  
  
• inability to quantitate the degree of hyperoxemia present
  
  
• venous congestion
  • increased noise because of pulsations of venous blood (ie, significant tricuspid regurgitation, hyperdynamic circulation states)
  • particularly when caused by tricuspid regurgitation
  • lower or less reliable SPo2 readings
  • suggested to use new-generation pulse oximeters to overcome this (1)
    
    
• venous congestion of the limb may affect readings as can a badly positioned probe
  • when readings are lower than expected it is worth repositioning the probe. In general, however, if the waveform on the flow trace is good, then the reading will be accurate
    
    
• if intravascular dyes have been used
  • Intravenous dyes such as methylene blue, indocyanine green, and indigo carmine interfere with light absorption
  • lower SPo2 readings
  • in this situation, do not use pulse oximetry or interpret pulse-oximetry readings with caution
    
    
• exposure of measuring probe to ambient light during measurement
  • intense external light energy (as in phototherapy) may interfere with the photodetector ('flooding' effect) - may result in lower SPo2 readings
  • the signal may be interrupted by surgical diathermy
    
    
• shivering may cause difficulties in picking up an adequate signal
  
  
• pulse oximetry cannot distinguish between different forms of haemoglobin
  • COHb presents red-light absorption similar to oxyhemoglobin
  • In carboxyhemoglobinemia pulse oximetry overestimates blood oxygenation
  • check arterial Sao2 if abnormal hemoglobin molecules are suspected (ie, carbon monoxide intoxication)
    
    
• irregular rhythms
  • increased noise caused by tachyarrhythmias
  • lower or less reliable SPo2 readings
    
    
• nail varnish may cause falsely low readings. However the units are not affected by jaundice or anaemia
  • anaemia does not seem to affect the accuracy of pulse oximetry, at least for hemoglobin levels of >5 g/dL and if cardiovascular function is preserved
  • polycythemia does not seem to interfere with pulse oximetry
  • bilirubin has no effect on pulse oximetry, because it presents a different spectrum of light absorption
    
    
• skin pigmentation - may produce lower or less reliable SPo2 readings at lower Sao2 values
  • probably caused by calibration assumptions for dark skin pigmentation
  • suggested to use new-generation pulse oximeters to overcome this (1)

Normal Oxygen Saturation

• in adults less than 70 years of age when awake at rest and at sea level: 96% - 98% (3)
• aged 70 and above when awake at rest and at sea level: greater than 94% (3)
  • NB: Patients of all ages may have transient dips of saturation to 84% during (3)
• in healthy infants and children, mean SPo2 values at sea level have been reported to be 97% to 99% and they might be lower in neonates and young infants (range: 93%-100%)

Reference:

• Fouzas S, Priftis KN, Anthracopoulos MB.Pulse oximetry in pediatric practice. Pediatrics. 2011 Oct;128(4):740-52. Epub 2011 Sep 19.
• Respir Care 1992;37(8):891-897
• Portsmouth Hospitals NHS Trust 010). Policy for the prescription and administration of Oxygen in Adults