New Monitoring Technique Detects Harmful Hemoglobin

Researchers at the University of California San Francisco (UCSF; USA) examined 12 healthy adult volunteer subjects who were fitted with sensors on the middle finger of each hand, while a radial arterial catheter was placed for blood sampling. Intravenous (IV) administration of 300 mg of sodium nitrite elevated the subjects' methemoglobin levels to a 7% - 11% target level, and hypoxia was induced to different levels of arterial functional oxygen saturation (Sao(2) by varying fractional inspired oxygen. Pulse CO-oximeter methemoglobin-reading performance of the selected device--the Masimo (Irvine, CA, USA) Rainbow SET Radical-7--was compared with arterial blood values measured with a multiwavelength oximeter, and the bias was analyzed to determine the incidence of meaningful reading errors and the predictive value at the various hypoxia levels.

The researchers found that an analysis of 307 blood draws and 602 values from the two oximeters showed that the Rainbow's methemoglobin readings are acceptably accurate over an oxygen saturation range of 74% - 100%, and a methemoglobin range of 0% - 14%. The researchers also pointed out that the new technique provides an important advantage over conventional pulse oximetry, which cannot detect methemoglobin. The study was published in the November 2010 issue of Anesthesia & Analgesia.

"The use of this technology to monitor methemoglobin also extends out of the operating room, and could be of value in the ICU, emergency room, general hospital wards, and dentist's office, as well as in patients undergoing GI procedures or bronchoscopy, since drugs that can cause methemoglobinema can be used in all these locations,” said coauthor John Feiner, M.D., of the department of anesthesia and perioperative care.

"Numerous drugs, including the common topical anesthetic benzocaine, are capable of forming high concentrations of methemoglobin. When this occurs, it places the patient at risk of severe hypoxia due to inadequate oxygen delivery,” added Steven L. Shafer, M.D., editor-in-chief of Anesthesia & Analgesia. "We now have the ability to directly measure methemoglobin in the operating room; prior to the development of this technology methemoglobinemia was nearly invisible in the operating room.”

Methemoglobin is a form of hemoglobin, in which the iron in the heme group is in the ferric state, not the ferrous state of normal hemoglobin, and is thus not capable of carrying oxygen. Normally 1% - 2% of people's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and can cause health problems known as methemoglobinemia. A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85%, regardless of the true level of oxygen saturation.

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