However, in reality, the pulsatile signal is very small. Typically , only about 2 % of the total signal is pulsatile ! Drawn to scale, 2 % of the total signal will look like the diagram below. The orange part represents the “non changing” light absorbed by the tissues. The red shows the changing absorbance due to pulsatile arterial blood. See how small this pulsatile signal is. Off all the light that passes through the finger, it is only the small pulsatile part that the pulse oximeter analyses. Because it is such a small amount of the total light, the pulse oximeter is very susceptible to errors if for an example, the probe is not placed properly or if the patient moves the probe.