The values shown on optical technology-based patient monitors must be cautiously interpreted: intravenous dye-induced errors

dyes such as indocyanine green (ICG), indigo carmine, and methylene blue are commonly used during various surgical procedures. The intraoperative administration of intravenous dyes can result in misreadings of the actual values measured by optical technology-based patient monitors (i.e., cerebral oximeters and pulse oximeters) because they interfere with the amount of the specific infrared or red wavelength detected by each device. Therefore, clinicians would benefit from a narrow review of the effects of intravenously administered dyes on regional cerebral tissue oxygen saturation and percutaneous oxygen saturation readings. The pulse oximeter was developed based on the fact that oxygenated and reduced hemoglobin have different rates of absorption of red and infrared light (Lambert-Beer law). Oxyhe-moglobin absorbs more infrared light (990 nm), whereas deoxy-hemoglobin absorbs more red light (660 nm). The device detects the difference in the extent of absorption of two wavelengths of light (660 and 925 nm) in arterial blood. The value is automatically calculated as an absorption ratio (extent of light detected at 660 nm/extent of light detected at 925 nm) and is represented as the arterial oxygen saturation (SaO 2). Similar to pulse oximetry, cerebral near-infrared spectrometry measures the absorption ratio of two different wavelengths of near-infrared light (730 and 805 nm) in venous, capillary, and arterial blood and represents the value as the regional cerebral tissue oxygen saturation (SctO 2). In clinical practice, intraoperative ICG is commonly used during neurosurgery to confirm the patency of the anastomosed vessels after middle cerebral artery-superficial temporal artery bypass surgery or carotid endarterectomy and the completeness of the secured aneurysm after cerebral aneurysmal clipping. The peak absorption wavelength of ICG is 805 nm [1]. A clinical study published in this issue of the Korean Journal of Anesthesiology demonstrates that the intravenous injection of ICG can produce false increases in SctO 2 readings in a dose-dependent fashion [2]. This finding can be partly explained by an increase in the absorption ratio of two different wavelengths of light: 730 and 805 nm. In other words, ICG absorbs light with a wavelength of 805 nm, falsely decreasing the amount of 805 nm light detected by the cerebral oximeter; this light is displaced as an increased SctO 2 reading. Another recent study showed that intravenous ICG administration caused a false decrease in SaO 2 readings [2,3]. ICG acts similarly to deoxyhemoglobin in arterial blood; this falsely decreases the absorption ratio of red and infrared wavelengths …