Monitoring: Equipment and Computers
Background and Goal of Study: Due to its peak absorption at 609 and 668 nm, methylene blue (MB) is known to cause transient, but artifactual low readings (1) of the functional oxygen saturation (SpO2). This study was designed in order to determine whether or not this spurious decrease in SpO2 is associated with a concurrent alteration of the arterial pO2 after MB injection.
Materials and Methods: After institutional approval and informed consent, SpO2 data of 16 patients (58-92 yrs) undergoing transurethral vesical procedures and receiving 50 mg of MB intraoperatively to visualize the urethral orifices, were assessed by means of four fourth generation pulse oximeters. An arterial line was placed into the contralateral radial artery and arterial blood gas samples were taken to determine the pO2 (Radiometer ABL700) prior to the IV injection of MB (t1: pO2 (1)), at minimum SpO2 (t2: pO2 (2)), and after SpO2 having returned to its baseline (t3: pO2 (3)). The differences between pO2 (1) and pO2 (2) (ΔpO2 (1-2)) and pO2 (2) and pO2 (3) (ΔpO2 (3-2)), respectively, were calculated for each patient. Additionally, the interval between t1 and t2 (Ät1) and t2 and t3 (Ät2) was determined.
Results and Discussions: Data (mean ± SD) are shown in the table.
Average pO2 appears significantly reduced by 15 mmHg approximately, although SpO2 has already returned to its initial level.
Conclusion: The IV injection of low-dose MB causes a significant drop in pO2, therefore further research is required to investigate the underlying pathophysiology.