A 37-year-old male patient with a giant cell tumor of the left femur was scheduled to undergo curettage of the femur using an intraoperative computed tomography three-dimensional imaging system (O-arm™) and Stealth Station™ navigation system. Throughout the procedure, the following variables were continuously monitored: electrocardiogram, peripheral oxygen saturation (SpO2) using a TL-273 SpO2 Probe™ (NIHON KOHDEN Corporation, Tokyo), end-tidal concentration of carbon dioxide (CO2), naso-esophageal temperature, and bispectral index. A small incision was made to expose the left femur. The patient reference frame was attached to the diaphysis of the femur to allow for optimal camera positioning and line of site. The SpO2 value, which was maintained between 97 and 99% until the time, disappeared suddenly with abnormal pulse wave (Fig. 1a). At first, the cause of interference was not recognized because the infrared (IR) light was not visible to the human eye. However, a distortion in the SpO2 value was reproduced by repeated movement of cameras on the head of the Stealth Station™ navigation system. Other possible causes for interference in SpO2, such as body movements, a disconnection, and an inadequate attachment of the pulse oximetry probe to the patient, were excluded. We recognized that the interference signal was coming from the Stealth Station™ navigation system. To eliminate the interference from the IR light, the pulse oximetry probe was covered with a black plastic shield (Fig. 1b) and the interference was completely eliminated.
Discussion
We found out two important clinical issues. First, the Stealth Station™ navigation system can interfere with the SpO2 value. It is because pulse oximetry measurement is based on the red (R) and IR light absorption characteristics of the oxygenated and deoxygenated hemoglobin. After the R and IR signals pass through the measuring site and are received at the photo-detector, the R/IR ratio is calculated. The ratio converts to a SpO2 value. The Stealth Station™ navigation system emits a strong IR light, and the light from the navigation system interferes with the detection of IR light signals emitted from the pulse oximetry probe.
Second, a black plastic shield was successful in eliminating the interfering signal. To eliminate the IR light interference, shielding the pulse oximetry probe with aluminum was recommended [2, 3]. However, a black plastic shield, one of the accessories of Masimo Radical-7, is indicated for use with Masimo adhesive, re-usable and responsive sensor, to reduce ambient light interference. The black plastic shield could completely eliminate the strong IR light from influencing the navigation system. However, only for eliminating the IR light, this black plastic shield may be expensive. When aluminum foil cannot be used immediately or when repositioning the probe to other sites where the IR light does not reach pulse oximetry probe is difficult, this black plastic shield can be an alternative method to solve the IR light interference.
Not only the pulse oximetry but the mainstream capnograph also incorporates an IR sensor which measures the absorbance of IR light due to the presence of CO2. The electromagnetic interference from the Stealth Station™ navigation system might have interfered with the measurement in the capnograph leading to a misreading. Therefore, knowledge about the IR light-induced artifactual changes in intraoperative monitoring is important to prevent both diagnostic confusion and unnecessary interventions.
