Benzocaine-induced methemoglobinemia: Discussion (Part 4)

The other classic clinical finding is chocolate brown arterial blood, again seen in both patients. The blood fails to change colour when exposed to supplemental O2 or when a drop is placed on filter paper . This bedside maneuver is a quick and accurate test to differentiate cyanosis due to other causes than from MHg formation, because exposure of blood to a stream of O2 will turn the arterial blood bright red in cases of cyanosis not associated with methemoglobinemia. The determination of O2 saturation (SpO2) by pulse oximetry is based on the fact that oxyhemoglobin absorbs more light at 940 nm and reduced hemoglobin absorbs more light at 660 nm. Because MHg absorbs light equally at both these wavelengths, SpO2 by pulse oximetry can be misleading in these patients. Due to these alterations in normal light absorption, as MHg levels reach 30% to 35%, SpO2 measured by pulse oximetry becomes stable at 82% to 86% independent of the actual MHg level . Co-oximetry that measures light absorbance at four different wavelengths is an accurate method of determining MHg levels and is the method used in most hospital laboratories. Arterial blood gas analysis is also misleading in these patients. The way in which the partial pressure of O2 is calculated does not directly assess the number of O2 molecules bound to hemoglobin but measures the dissolved O2 in the sample and, thus, patients with life-threatening MHg can have normal or elevated SpO2 levels.