Pulse oximetry

Arterial oxygenation can be easily measured, and provides an estimate of the level of oxygenation of the blood arterial oxygen saturation (SaO2). It is measured with a pulse oximeter placed either on a finger or an earlobe. The oximeter uses a red and infrared light to detect the absorbed characteristics of oxygenated and deoxygenated haemoglobin. The results are expressed as a percentage of haemoglobin saturated with oxygen. Normal oxygen saturation is usually around 95–98% when breathing room air. Pulse oximetry is accurate to within + or –3 to 5% but the measurement is unreliable at saturations below 70%. It can be inaccurate if the patient has poor circulation, such as peripheral vascular disease or Raynaud’s disease. An SpO2 of 90% would be equivalent to a PaO2 of approximately 8kPa. If the SpO2 is <92% on air, arterial blood gases should be taken to check CO2 levels. General considerations • Explain to the patient that the test assesses the oxygen content of their haemaglobin • Patients should remove nail polish prior to the test • If false nails are in situ an ear probe should be used • Place the probe or clip over the finger or ear lobe, so that the light beams and sensors are opposite each other • Protect the transducer from strong light If the probe is to be kept in situ for long periods, check the site frequently and rotate at least every four hours to avoid skin irritation