Pitfalls in pulse oximetry in the critically ill

• Pulse oximeters are calibrated by manufacturers using data collected from healthy volunteers
• In general, pulse oximeters are accurate  within +/- 2% for sats > 70%
• In the critically ill, however, the accuracy of pulse oximetry diminishes when sats drop below 90%
• Also, there may be a significant lag time between a hypoxic event and the actual display of the event - most commonly seen in low flow states, hypotension, mild hypothermia, and when using vasoactive medications
• Prolonged lag times are more common with finger probes
• Pitfall - pulse oximetry does not provide any assessment regarding ventilation (P_aCO₂) or acid-base status (pH) - it is simply an estimate of arterial oxgyen saturation
• Pearl: anemia does not affect the accuracy of pulse oximetry