Peripheral vasoconstriction – Oximetry relies on detecting a stable pulse. In order for pulsatile flow to be detected, there must be sufficient perfusion (blood flow) in the monitored areas. If peripheral(surface) pulses are weak or absent, readings can be difficult to obtain. This can give false low measurements compared with central saturation, which perfuses the brain and other vital organs. Patients most at risk for low perfusion states are those with hypotension, hypovolemia (low blood volume), and hypothermia, and of course those in cardiac arrest. Patients who are cold but not hypothermic may have vasoconstriction (narrowing of blood vessels) in their fingers and toes that can also compromise arterial blood flow.
If vasoconstriction is a problem, try moving the sensor to the ear lobe or warming the extremity to enhance perfusion (blood flow).