Patent blue (4-[(4-diethylaminophenyl)-(4-diethylazaniumylidencyclohexa-2,5-dienyliden) methyl]-6-hydroxy-3-sulfo-benzolsulfonate, sodium salt) is a contrast dye used for the intraoperative detection of the primary lymphatic nodes draining the area of tumour infiltration. The dye is known to interact with pulse oximeter readings. However, the degree of alteration seems to be moderate and predictable when patent blue is injected into the perimammilar region during breast surgery.

Here we report severe interference with the anaesthetic monitoring when patent blue was injected into the cervix prior to laparoscopy-assisted radical vaginal hysterectomy for cervical cancer.

Injection of patent blue into the cervix induced a rapid (within 14 ± 9 min after the injection) and severe (from ⩾98% to 89 ± 2%) decrease in pulse oximeter readings, accompanied by positive methaemoglobin values of 7.3 ± 2.5% (arterial co-oximetry, Bayer Rapidlab 865 blood gas analyser; Bayer, Fernwald, Germany). Control of these values by a different device (Radiometer ABL co-oximeter blood gas analyser; Radiometer, Willich, Germany) yielded negative methaemoglobin results (<1.7%, mean 0.9 ± 0.6%). The arterial PO2 was normal in all patients throughout the procedure.

Injection of patent blue into the cervix uteri interferes dramatically with pulse oximeter readings. This situation is further complicated by device-dependent arterial co-oximetry methaemoglobin results. For the time being it is recommendable to monitor adequate oxygenation of the patient in the presence of patent blue by regular control of the arterial PO2. Clearly, the unresolved issue of reliable methaemoglobin determination in the presence of patent blue remains a matter of clinical concern for anaesthetists.