While quantitative waveform capnography and pulse oximetry are both infrared technologies and may seem like similar measurements, they assess very different things. Capnography is a direct measurement of ventilation and, indirectly, circulatory and metabolic status. Pulse oximetry measures the color of the hemoglobin (Hgb) molecule and then uses mathematic algorithms to estimate the percent of Hgb saturation.
Hemoglobin is a helix-type molecule that coils tighter and appears redder as it becomes saturated and uncoils and appears bluer as it desaturates. The more red/blue the Hgb the higher/lower the saturation percentage estimated by pulse oximetry. An important technological limitation to know regarding pulse oximetry is that it cannot decipher what is bound to the Hgb molecule. This becomes crucial to understand in the context of carbon monoxide poisoning.
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Above photo by Rick McClure