Fact of the Day .CA

I was recently told that the little device put on a patient’s finger in the hospital not only measuring heart rate, but also measuring blood oxygen levels.  I then became curious of how the oxygen levels are measured.  I knew that blood oxygen levels can be measured based on the colour of the blood.  We all know about our veins and arteries and that they are the vessels used for blood circulation.  The veins are blue for a reason; they contain deoxygenated blood.  Blood contains a protein called hemoglobin that is responsible for carrying oxygen; when oxygen is bound the hemoglobin is red and if oxygen is not bound then it is blue.  Alright, so how do they measure the blood oxygen levels without taking a blood sample?  Answer: light absorption.

Now I have talked about light absorption before (see Colour Vision) so I won’t go over the details.  The device used is called a Pulse Oximeter; it has a pair of LED lights that shine a red visible light and an infared light through the finger.  There is a detector on the other side to measure how much of the light was absorbed.  Since oxgenated blood is red, then the light  picked up by the detector should also be red. If the oxygen level is high, the red blood will reflect the red light.  So now this is making sense, but there are also veins in the finger too.  So how does it measure just the blood in the arteries? Well that’s were the “pulse” part of the name comes in.

The device only measures when there is a pulse.  Pulses only occur in the arteries because this is where blood is sent after being pumped from the heart.  The flow in the veins does not have a pulse, it is basically a passive flow of blood back to the heart.  So, the Pulse Oximeter only measures the colour of the blood that pulses through the arteries; therefore capable of measuring the heart rate as well as the blood oxygen levels.

Sources:
http://www.pulseox.info/pulseox/how.htm
http://en.wikipedia.org/wiki/Pulse_oximeter
http://www.nda.ox.ac.uk/wfsa/html/u05/u05_003.htm