Pulse oximetry and Oxygen saturation Prepared by Shane Barclay MD

Definitions Pulse oximetry uses means to measure peripheral arterial oxygen saturation (SpO2). It thus provides an estimate of PaO2 from ABGs. Overall, most oximeters will be within +/- 2% of arterial oxygen levels when between 70 – 100%. Accuracy drops with decreasing true arterial oxygen levels.

Indications for oximetry Any clinical setting where hypoxia could occur – all resuscitations, asthmatics, COPD, airway obstruction, when medications are being given that could reduce cardiorespiratory drive, procedural sedation, RSI …

Advantages Non invasive. Fast Provides continuous readings. Can reduce the number of arterial blood gases.

Limitations and Sources of Error Unable to measure SaO2 when over 100% (ie the intubated patient) Can under estimate PaO2 from ABGs. Even under normal conditions can take up to a minute to recalibrate and measure changes in SpO2. Even longer times if anemic or hypothermic. Can be inaccurate with pigmented skin, nail polish, inaccurate probe position, ambient and light artifact, with low saturations (<80%) and motion.

What’ s a normal Spo2? For most people without any lung disease > 95%. For someone with COPD 88 – 92% could be ‘normal’.

A few words on oxygen toxicity There is ample evidence that high inspired FiO2, especially greater then 60% for several hours or days, can be detrimental to the lungs - tracheobronchitis, atelectasis and airway damage.

A few words on oxygen toxicity However there is no consensus on ‘how long is too long’? Many authors feel and FiO2 > 60% for more than 6 hours is ‘too long’. Of interest, astronauts breath 100% oxygen for up to 4 hours before going on a space walk, with no deleterious effects.

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