Complementary Use of Effect Site-Target Controlled Infusion and SmartPilot View for Anesthetic Management in Semi-awake Craniotomy Near BIS 85
Awake craniotomy allows intraoperative functional monitoring for disorders of cortical nerve fibers, but may not always be useful for monitoring deep white matter.
A neurosurgeon requested intraoperative neurophysiological monitoring for a semiawake patient for 2 reasons. First, awake craniotomy is difficult in cases with existing aphasia. Second, the possibility of nerve-fiber damage is higher in deep white matter than in the cortex. As anesthesiologists, we therefore planned to maintain BIS within the range of 80 to 90 (≈85) by using propofol (P) effect site (ES)-target controlled infusion (TCI) and remifetanil (R) ES-TCI.
The male patient in his 60s (height, 162 cm; weight, 52 kg) was scheduled for tumor resection located in the left hippocampus. We administered fentanyl at 1 μg/kg in P-ES-TCI (modified Marsh procedure) for induction with a laryngeal mask airway before connection to a ventilator. Infiltration anesthesia and 6 nerve blocks (supratrochlear nerve, supraorbital nerve, zygomaticotemporal nerve, auriculotemporal nerve, greater occipital nerve, lesser occipital nerve) were performed using 0.375% ropivacaine before attaching a head frame to the patient. For anesthetic maintenance, we used P-ES-TCI and R-ES-TCI according to the Minto model. During intraoperative nerve monitoring, we administered P-ES concentration (ESC) and R-ESC to maintain BIS within 80 to 90. Preferred nerve monitoring was demonstrated in the ESC of: P, 0.74 to 1.60 μg/mL; and R equivalent, 1.80 to 2.59 ng/mL. The anesthetic effect displayed on SmartPilot View (SPV) (Drägerwerk AG, Lübeck, Germany) was around tolerance of shake and shout (TOSS) 50 (Fig.1).
The patient had no memory of the surgery and no complaints of pain in postoperative interview. The level of sedation was comparable with “moderate” in the “Practice Guidelines for Sedation and Analgesia by Non-Anesthesiologists”1.
In this case, we deliberately decreased anesthesia effect without returning to full wakefulness to maintain BIS at 80 to 90, which we defined as a “semiawake” state (BIS≈85). This level of anesthesia is sometimes difficult to maintain, but the display of the level of anesthesia on the SPV facilitates stable maintenance of semiawake anesthesia, and thus coordination of the level of anesthesia desired by the neurosurgeon.
This case provided valuable experience, and we conclude that SPV and TCI during intraoperative neurophysiological monitoring of cortical nerve fibers in a semiawake state can be useful.