Subject: Re: How do they measure blood with light?

Based on your description, it sounds like you had a pulse-ox, or pulse oximeter connected to your finger. This machine uses a red light to measure the percentage of oxygenated hemoglobin in your blood. Hemoglobin is the protein in red blood cells that transports oxygen, acquired in the lungs, to other sites around your body. Each hemoglobin contains four heme groups, each of which has an iron atom at its center. The iron gives blood its red color. When it binds oxygen it becomes oxidized and assumes a "bright red" color. When it gives up its oxygen the color becomes more purple, but not blue!!. The saying that deoxygenated blood is blue is completely wrong. The red light in the pulse-ox detects this color change and can calculate the percentage of blood that is oxygenated. In healthy individuals this percentage should be 98-100%. It becomes concerning when the levels drop below 90-93%.

Doctors use the pulse-ox because it is easy to do and it is not invasive (no needles!). However, it can only provide so much information. While it tells the percentage of oxygenated blood, it does not indicate how much blood or hemoglobin you have in your circulation. Doctors measure the hemoglobin concentration and the hematocrit to ascertain this value. For instance, if you recently lost a lot of blood, your %saturation might be 100%, but you still may not be getting enough oxygen to your tissues due to the decreased amount of hemoglobin. Venous blood can be drawn to check these values.

A pulse-ox also does not give any indication as to the amount of oxygen carried by the blood, and the pH of the blood. Both of these values are important to know in sick individuals. If tissues are not being adequately oxygenated due to lung problems, a person may be given oxygen to boost the levels in the lungs so more can make it into the blood.

The pH (concentration of acid) is also an important value to know. Normal blood pH ranges from 7.35-7.45, slightly on the alkaline side. The most common source of acid is a buildup of carbon dioxide - CO2. CO2 reacts with water to produce carbonc acid (see link for details). Delerium, damage to tissues, and ultimately death can occur if the blood pH get too much "out of whack." Pneumonia, emphysema and other lung diseases can produce a respiratory acidosis whereby the pH of the blood drops due to an accumulation of CO2 in the blood.

Doctors measure the pH and concerntration of O2 and CO2 by taking an arterial blood gas, or "ABG." Arterial blood is drawn, most commonly from the radial artery in the wrist. The stick hurts, more so than a venous stick. Arteries are thicker than veins, and they lie beneath more tissue, so the needle has to go deeper. I suspect this is what you had in addition to the pulse-ox.

Hope this explains everything.. glad to hear you're better.

-Lynn Bry, "Let's get the med student to do the ABG"..
, St. Louis MO..