Relationships between the qNOX, qCON, burst suppression ratio, and muscle activity index of the CONOX monitor during total intravenous anesthesia: a pilot study

Background: Processed electroencephalographic (EEG) indices can help to navigate general anesthesia. The CONOX (Fresenius Kabi) calculates two indices, the qCON (hypnotic level) and the qNOX (nociception). The CONOX also calculates indices for electromyographic (EMG) activity and EEG burst suppression (BSR). Because all EEG parameters seem to influence each other, our goal was a detailed description of parameter relationships. Methods: We used qCON, qNOX, EMG, and BSR information from 14 patients receiving propofol anesthesia. We described index relationships with linear models, heat maps, and box plot representations. We also evaluated associations between qCON/qNOX and propofol/remifentanil effect site concentrations (ceP/ceR). Results: qNOX and qCON (qCON = 0.79*qNOX + 5.8; p < 0.001; R² = 0.84) had a strong linear association. We further confirmed the strong relationship between qCON/qNOX and BSR for qCON/qNOX < 25: qCON=-0.19*BSR + 25.6 (p < 0.001; R² = 0.72); qNOX=-0.20*BSR + 26.2 (p < 0.001; R² = 0.72). The relationship between qCON and EMG was strong at higher indices: qCON = 0.55*EMG + 33.0 (p < 0.001; R² = 0.68). There was no qCON > 80 without EMG > 0. The relationship between ceP and qCON was qCON=-3.8*ceP + 70.6 (p < 0.001; R² = 0.11). The heat maps also suggest that the qCON and qNOX can at least partially separate the hypnotic and analgetic components of anesthesia. Conclusion: We could describe relationships between qCON, qNOX, EMG, BSR, ceP, and ceR, which may help the anaesthesiologist better interpret the information provided. One major finding is the dependence of qCON > 80 on EMG activity. This may limit the possibility of detecting wakefulness in the absence of EMG. Further, qNOX seems generally higher than qCON, but high opioid doses may lead to higher qCON than qNOX indices.