Pulse Oximetry Basics

Pulse oximetry estimates peripheral capillary oxygen saturation, written as SpO₂. A probe shines red and infrared light through a pulsating vascular bed, usually a fingertip. Oxyhaemoglobin and deoxygenated haemoglobin absorb those wavelengths differently, allowing the device to estimate the percentage of pulsatile arterial haemoglobin carrying oxygen. That estimate is useful, but narrow. SpO₂ is not the same as PaO₂, the partial pressure of oxygen measured on an arterial blood gas (ABG). It also does not measure carbon dioxide, ventilation, haemoglobin concentration, or tissue oxygen delivery. A patient with severe anaemia may have a normal saturation because the available haemoglobin is well saturated, while the total amount of oxygen carried in the blood is still inadequate. A normal number therefore cannot reassure you about every cause of hypoxia. The oxygen–haemoglobin dissociation curve makes low readings especially significant. Around a PaO₂ of 60 mmHg, saturation is approximately 90%; below that point, a small fall in PaO₂ produces a much steeper fall in saturation. A change from 94% to 88% is not simply a six-point change on a scale. It may represent a...