Abbreviations: Depth of anesthesia (DOA) PSI- Patient safety index, Bispectral Index (BIS) , Density spectral array (DSA), electromyography (EMG), SEFL – Spectral edge frequency on the Left , SEFR – Spectral edge frequency on the  Right, Suppression ratio (SR)

Introduction

Optimal “depth of anesthesia” (DOA) is crucial to ensure patient safety and improve surgical outcomes. Deep levels increase the risks of vasomotor depression, significant metabolic suppression culminating in burst suppression, delayed emergence, and postoperative complications, including cognitive dysfunction and postoperative delirium (POD), especially in high-risk patients (1–4). Conversely, inadequate depth raises the risk of a sympathetic state, movement to surgical stimulus and intraoperative awareness, leading to long-term psychological distress (1,2).

DOA monitoring has been shown to reduce anesthetic requirements, facilitate faster emergence, and lower the incidence of POD. A meta-analysis of 13 studies revealed a reduced risk of POD when DOA monitoring was employed (5). Standard DOA monitors such as the Patient State Index (PSI) and Bispectral Index (BIS) rely on frontal sensor placement to capture cortical electrical activity.

However, certain neurosurgical procedures, such as frontal craniotomies, make frontal sensor placement impractical as it overlaps with the surgical field. This limitation has driven the exploration of alternative sensor placements, including the infraorbital (malar) region (Fig. 1). While multiple studies have investigated alternative placements, the findings remain inconclusive due to differences in study designs, methodologies, and parameters evaluated (6–11).

Moreover, many studies focused solely on numerical indices, such as PSI or BIS, without analyzing the underlying EEG waveforms. These indices, while useful, have limitations, including patients at the extremes of age, specific anesthetic agents (e.g., ketamine, nitrous oxide, dexmedetomidine), and artifacts like EMG or eye movements. Only two studies have evaluated the SedLine® (Masimo) sensor using both numerical indices and full-spectrum EEG parameters at alternative locations, highlighting its potential as a viable option (Fig. 1) (10,11).

Figure 1. Alternate electrode configurations of the Masimo Sedline® monitor. With corresponding approximate electrodes in the classical 10-20 EEG recording system. Infraorbital captures the medial (Nb1 and Nb2) and lateral (Nb3 and Nb4) nasal bridge (10,11).

We present two surgical cases performed under balanced sevoflurane anesthesia, where the SedLine® sensor was placed in infraorbital and standard frontal configurations for educational purposes.

Case Presentation

Patient Background:

Patient 1: 40-year-old male with a history of end stage renal disease of unknown etiology undergoing living donor kidney transplant under balanced general anesthesia with sevoflurane and rocuronium. Analgesia was provided with fentanyl and hydromorphone.

Patient 2: 55-year-old male with a history of hypertension and adenocarcinoma of the rectum undergoing partial hepatectomy and anterior perineal resection under balanced general anesthesia with sevoflurane and rocuronium. Analgesia was provided with fentanyl and continuous epidural infusion of local anesthetics.

Methodology

DOA was monitored using SedLine® sensors in the standard frontal position and an alternative infraorbital (malar) position. Data were collected once the patient reached the stable state of anesthesia following intubation. EEG parameters assessed included:

• • Patient State Index (PSI): Numerical indicator of anesthetic depth (derived through a proprietary algorithm).
  • Spectral Edge Frequency (SEF-95): Frequency below which 95% of the EEG power resides. White horizontal line in the DSA.
  • Processed EEG parameters: Alpha, delta, theta and beta power distributions and waveform morphology.
  • Density Spectral Array (DSA): Two-dimensional display of the spectrogram, showing EEG power through color scheme of frequency over time.

Observations:

1. 1. PSI and SEF-95 Consistency (Fig. 2)
      PSI and SEF-95 values were closely aligned across frontal and infraorbital placements.

Figure 2. Comparison of PSI and SEF (right and left) for patient 1 (A) and patient 2 (B) at the standard and infraorbital locations.

1. 1. Artifact Susceptibility
      We didn’t encounter significant artifact during the maintenance phase. However, movement and EMG artifact were prevalent at both sites during induction and emergence.
   2. Alpha Power and Frequency Analysis (Fig. 3 and 4)
      Infraorbital sensors recorded weaker power on Density spectral array (DSA) across all frequencies but more pronounced in the alpha bands.
   3. Waveform Morphology (Fig. 3 and 4)
      EEG waveforms from both configurations showed similar patterns of cerebral activity, with synchronous channel activity and comparable frequency distributions. However, signals from the infraorbital placement had reduced amplitude, resulting from the increased distance and tissue (i.e. sinuses) encountered with lower facial sensor placement.

Figure 3. A comparison of simultaneous screenshots for Patient 1. Panel A – Screenshot from standard frontal sensor location; Panel B – Screenshot from the infraorbital location; Panel C – Infraorbital location with power bar setting changed from 10, -40 to 0, -40 to accentuate the red intensity. The reduced amplitude is evidence in visually comparing panel A to panels B and C.

Figure 4. A comparison of simultaneous screenshots for Patient 2. Panels A and C show the EEG using standard frontal location during the maintenance phase of anesthesia. Panels B and D demonstrate simultaneous EEG at the infraorbital location during maintenance phase. The power scale was changed to 5, -40 to accentuate red intensity. Infraorbital screenshots demonstrate a low amplitude EEG, with a reduction in power more pronounced in the alpha range.

Discussion

These two cases highlight the potential utility of infraorbital placement of SedLine® sensors as a practical alternative to standard frontal placement for depth of anesthesia (DOA) monitoring when frontal placement is not feasible. While amplitude differences were noted, the morphology and relative power of waveforms were preserved, ensuring that critical information about anesthetic depth could still be accurately interpreted.

Clinical Evidence Supporting Infraorbital Placement for Sedline® Sensor.

Tafegdzic et al. (10) conducted a prospective study involving 40 patients to validate nasal placement (infraorbital) of the SedLine® sensor. Their findings demonstrated strong alignment of PSI values between nasal and frontal placements, with an 83% match within 5 PSI units across all phases of anesthesia and a 95.7% match during the steady-state phase. Additionally, EEG tracings showed an 88% inter-rater agreement among experts, reinforcing the reliability of nasal placement for capturing meaningful EEG data (10).

Similarly, Isik et al. (11) examined five alternative electrode configurations, including infraorbital placement, in sedated ICU patients. Their results showed minor reductions in EEG power across all frequencies for alternative placements. Compared with the standard configuration, infraorbital placement demonstrated increased relative delta power and decreased relative alpha power, likely attributable to the electrode’s distance from the frontal cortex and interference from anatomical structures like air sinuses, which act as barriers to electrical conduction.  Despite these differences, the overall spectral distributions remained consistent, with changes unlikely to impact clinical interpretations (11).

Infraorbital Placement: Practical Implications

Standard SedLine® sensors are designed to capture signals from the frontal and prefrontal cortex, using four inputs (L1, R1, L2, R2). When placed infraorbitally, the lateral electrodes (L2/R2) correspond approximately to T1/T2 or FT9/FT10 in the 10-10 EEG system, recording activity from the orbitofrontal and temporal regions (10, 11). While this positioning provides an alternative means of monitoring cortical activity, it is associated with specific limitations:

1. 1. Susceptibility to Artifacts:
      Infraorbital sensors are more prone to movement artifacts, especially during induction and emergence phases, as these sensors are closer to facial muscles. Eye movement artifacts, such as saccades and flutter, are also more prominent, given the proximity of the sensors to the eyes. These artifacts can confound the interpretation of EEG data, particularly during periods of heightened patient activity.
   2. Reduced Signal Amplitude:
      Signals recorded from infraorbital electrodes tend to exhibit lower amplitude compared to frontal sensors. Although not demonstrated by Tafegdzic et al, lower signal amplitudes carry a risk of affecting the accuracy of processed indices such as PSI and burst-suppression ratio (10).
   3. Changes in Frequency Distribution:
      Infraorbital placement is associated with a shift in power distribution, favoring low-frequency delta waves due to the anatomical location of the electrodes. The reduction in alpha power, while minimal, could potentially impact the interpretation of certain anesthetic states.

Research Gaps and Future Direction

Despite promising findings, evidence supporting alternative sensor placements is limited to a small number of patients from only 2 published studies. Key areas for further investigation include:

• • Algorithm Refinement: Proprietary algorithms for calculating indices like PSI may need optimization to account for the unique signal characteristics of infraorbital placement.
  • Sensor Design Improvements: Redesigning sensors to minimize artifacts and enhance signal quality at alternative placements could further improve their clinical utility.
  • Validation Across Diverse Populations: Larger studies with varied patient populations and surgical procedures are needed to confirm the reliability and generalizability of infraorbital placement.
  • Compatibility with Other EEG Systems: Different DOA monitoring systems may employ distinct algorithms and hardware configurations. Comparative studies assessing compatibility with alternative placements across various manufacturers are warranted.

Clinical Recommendations

Given a need for monitoring DOA during total intravenous anesthesia (TIVA), which is commonly needed during craniotomy, the importance of finding an accurate alternate EEG sensor configuration is an underappreciated challenge. While the off-label use of infraorbital placement offers a practical solution when standard frontal placement is not feasible, clinicians must remain aware of its limitations. Increased artifact susceptibility and reduced signal amplitude necessitate careful interpretation of derived indices. Visual analysis of waveform morphology and frequency distribution remains critical to ensuring accurate assessment of anesthetic depth. Manipulation of a monitors’ settings, such as amplitude resolution and the power bar (if made available by the monitors’ manufacturer), may help augment the raw EEG amplitude and the color intensity on DSA to facilitate the clinician’s interpretation of the anesthetic state.

Conclusion

These case reports demonstrate the potential utility of infraorbital placement for SedLine® sensors in DOA monitoring, expanding its application in scenarios where standard frontal placement is impractical. Consistent PSI values and preserved waveform morphology suggest that this alternative configuration can provide reliable clinical information. However, the findings are based on limited evidence, emphasizing the need for further research to validate and refine this approach.

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