Factors Affecting Pulse Oximeter Readings

Perfusion at the site of monitoring

  • Blood pressure generally needs to be >80 SBP
  • Vascular impingement from any cause
  • AV fistula can decrease distal flow
  • Elevation with respect to the heart
  • Compression by the probe
  • Cardiac arrest (don't use during arrest)
  • Heart Rate extremes <30 or >200

    • Cold
    • Fear (Endogenous catecholamines)
    • Medications

Technique

  • Applied to the best site ie. index finger (thumb ok in kids)
  • Sensor attachment flush with tissue surface
  • Local vasodilator such as methyl salicylate 10-30% and menthol 2-10% can be used if long transport
  • Don't forget to remove or deflate BP cuff or band
  • SpO2 Heart Rate MUST match actual pulse rate!!!!!
  • Waveform or LED Color needs to confirm clean sample
  • Take all readings when the probe is at heart level

Dyshemoglobinemia (Dysfunctional hemoglobin)

  • Carboxyhemoglobin readings unreliable
  • Methemoglobinemia " "
  • Sickle Cell Anemia " "
  • Thalassemias " "
  • Anemias " "

Interference - readings unreliable

  • Electronic/Electrical - RF, High tension lines rarely
  • Movement - patient, sensor, stretcher, vehicle, active seizure
  • Radiated light - infrared, visible and ultraviolet
  • Nail polish, heavy, or dark
  • Artificial Fingernails
  • Dark pigmentation (not too much of a problem with finger robe)
  • Intravenous Dyes such as Cardiogreen (within 48 hrs)
  • Pulsatile venous system ie. CPR or tricuspid regurgitation

Normal Values (at sea level)

  • Healthy patient on room air 94-100%
  • COPDer on room air 90-100%
  • 92% Sat is roughly equal to a Pa02 of 80 mm Hg, the threshold of hypoxia. (these values are relative and can change based on shifts of the Oxyhemoglobin Dissociation Curve or your elevation above or below sea level)

Caveats

  • Use as tool to worry you, not to make you feel better
  • Don't triage with a pulse oximeter
  • Don't deny a SOB patient O2 while waiting for an SpO2
  • Don't forget to document all readings and changes on the PCR. 

Examples:

  • 10:42 SpO2 = 97% on 100% oxygen.
  • 23:00 SpO2 = 98% on room air.