Saturday, February 24, 2007

Explaining a Medical Device: The Pulse-Ox

I would like to share some interesting chemistry related information that Laura and I received from our EMT class. It’s about the pulse-ox, a medical device that is frequently used on emergency, surgery, and other hospital patients. The output of a pulse-ox gives both the patient’s pulse rate and their arterial blood oxygenation level. If you’ve ever been a patient in a hospital before you have probably had one clipped onto your index finger. The pulse-ox is noninvasive. It simply clips onto the patient’s finger and then seemingly magically measures the pulse rate and blood oxygenation level.

The way it works is by shining two types of light, red and infrared (about 650nm and 900nm), across the tip of the finger; a sensor on the opposite side measures how much light was adsorbed. As you guys probably know, inside the body when hemoglobin is carrying oxygen its color changes from purplish-blue to red. So, when red light is directed across the finger, areas containing red arterial (oxygenated) blood reflect the red light while non-red-colored areas either adsorb the light or allow it to pass through. The infrared light is adsorbed in different amounts by the different parts of the finger. While a great deal of the IR is adsorbed, some of it also passes through the finger to the detector on the other side. So if we know that our fingernail, each layer of skin, our bone, our venous blood, and our arterial blood adsorb some light, how can it measure only the oxygenated blood? Because our pulse extends all the way to our fingers, the amount of arterial blood in our capillaries varies regularly with each heart beat. The light adsorbed by the finger nail, skin, bone, and veins doesn’t change because their volume or density isn’t changing. Because these values remain constant, once the pulse-ox measures a base line reading it can ignore their input and just read the oxygenated hemoglobin level. Also, because of the regular fluctuation of arterial blood, the pulse-ox also reads the pulse based on the adsorption of light. Another important thing to note is that IR adsorption detects the amount of hemoglobin in the blood while red light adsorption measures the amount of oxygenated hemoglobin. To actually calculate the percentage of oxygen saturation, the device compares a ratio of the red to infrared light detected.

I hope that my explanation makes sense and that if any of you ever wondered what that finger thing was doing, now you understand.

2 comments:

Ernest Kim said...

It is very valuable information. I have been measured by pulse-ox few times but have not questioned myself how this device works. It is interesting that such a small device (size of one finger) can detect light adsorbance because other light detecting device such as spectrophotometer is much larger in size.

Laura said...

I'm so glad you explained this. I had also been wondering how the pulse-ox worked. I remember they used one on me when I had knee surgery a few years ago. If you wiggle your finger while it's in a pulse-ox it causes an alarm to go off, which is very funny when you are drugged for surgery. However, the nurses did not think it was funny. It's always interesting when you see chemistry applied in the "real" world and in situations where you don't expect it. I think you should explain this concept to our EMT class as well.