Prolonged Decrease of Regional Cerebral Saturation Readings with the INVOS® Device After Continuous Intravenous Methylene Blue Administration

Abstract

In Response: Dr. Mittnacht points out an important limitation of cerebral near-infrared spectroscopy (NIRS) with the INVOS System (Somanetics Corp., Troy, MI) during infusions of methylene blue. Because of its absorption peak at 668 nm, methylene blue has been shown to interfere with pulse oximetry1 and could potentially interfere with the measurement of regional cerebral oxygen saturation index (rSO2) using the INVOS System due to the proximity of the 730 nm wavelength emitted by the sensor to this absorption peak. Administration of methylene blue would result in falsely low rSO2 readings due to the increase in absorption in the visible region of the spectrum. In earlier experiences in which methylene blue was administered as a bolus IV injection, the resulting effect on rSO2 was usually minor and transient. As mentioned by Dr. Mittnacht, the only other reference before his letter to the editor2 described a continuous infusion of methylene blue in a 10-yr-old girl with no apparent effect on INVOS System readings.3 Interference with the rSO2 measurement appears to be highly variable across patients and by dosage and administration method of methylene blue. We agree with Dr. Mittnacht that because the methylene blue infusion typically used for refractory vasoplegia is continued throughout the end of the case, the rSO2 readings remain falsely low and do not return to pre-dye administration levels despite increases in systemic perfusion pressure. The consistent experience of Mittnacht et al. has prompted Somanetics to add a precaution to the INVOS System labeling that methylene blue administration may cause interference with the rSO2 measurement. Ronald A. Widman Michael D. Wider, PhD Somanetics Corporation 1653 East Maple Rd. Troy, Michigan [email protected]