Keywords

DSA = Density Spectral Array

EMG = electromyographic activity

SEF = Spectral Edge Frequency 

Introduction/Background

Anesthesiologists commonly balance individual needs of patients, procedures, and surgeons. The ability to directly monitor the brain through EEG data allows real-time physiologic information from the target organ of many of our anesthetic medications, which can facilitate personalized anesthetic titration.^1-3 Patient diversity, including age, pathology, co-morbidities, and frailty can impact the EEG and anesthetic care.³ We will review two patients with differing hypnotic anesthetic titrations under general anesthesia.

Case Presentations

Patient 1:

A 40-year-old critically ill patient presented to the hospital with abdominal pain. He has no history of alcohol or drug use. His hospital course was complicated by hemorrhagic shock. After transfusion and improvement in vasopressor requirements, he presented for additional intervention. He was sedated on infusions of propofol and dexmedetomidine. He was also receiving a norepinephrine infusion. Propofol and dexmedetomidine infusions were turned off, and sevoflurane I/E 0.5/0.5% was titrated for maintenance of general anesthesia. Figure 1 reveals the intraoperative record, and figure 2 shows the EEG monitor screenshots. After the procedure was completed, sevoflurane was discontinued and neuromuscular blockade was reversed. The patient was awake and alert, following commands and meeting extubation criteria. He remained on low-dose vasopressor. He was successfully extubated and transferred to the ICU. Burst suppression was avoided in this case. Despite lower-than-average age-adjusted MAC, there is no EEG evidence of higher frequency beta waves.

Figure 1. Intraprocedure chart for patient one, showing vital signs and medications.

Figure 2. Three consecutive screenshot images of intraoperative EEG data, A-C, in chronological order. Background anesthetic signatures may be referenced.⁴

1. Initial screen shot reveals high power (red) of slow delta oscillations frequency 0-4 Hz evident on the DSA (large red arrow) and raw EEG (small red arrow). The patient has a low alpha power depicted with a green color (power bar) on the DSA and represented by the green arrows. It is possible that the patient’s encephalopathy contributed to the dominance of low frequencies on EEG. At this time, propofol and dexmedetomidine infusions were discontinued, and low dose of sevoflurane was initiated for maintenance of general anesthesia.
2. Shows maintenance with sevoflurane less than 1 MAC. Again, high power in the delta frequency band is clearly depicted in the raw EEG and DSA. There is reverberation artifact present on the right spectogram, shown as repeating boxy lines around 25 and 12 Hz most apparent in the yellow oval.
3. Just prior to emergence, brief increase in SEF and higher power (green color) of higher frequencies in the green oval. EEG was removed with the plan to remain intubated. However, the patient emerged meeting extubation criteria within 5 minutes of EEG removal.

Patient 2:

A 60-year-old male with a history of GERD and HTN planned for endoscopy. The patient reports severe symptomatic GERD and a history of awareness under general anesthesia. He reports alcohol and Marijuana use. General anesthesia was induced with a rapid sequence intubation. After induction, sevoflurane was titrated to I/E 3.9/2.8%. Propofol infusion at 100 mcg/kg/min was initiated. An infusion of vasopressor was not required. Figure 3 reveals the anesthetic record. Figure 4 shows the EEG screen shot images. The patient was extubated without complications. He denied any awareness under general anesthesia.

Figure 3. Intraprocedure chart for patient two, showing vital signs and medications.

Figure 4. Four consecutive screenshot images of intraoperative EEG data, A-D, in chronological order.

1. Following intubation and prior to procedure start the patient has greater than 1.5 age-adjusted MAC with SEF ~16 bilaterally depicted in the blue oval. Moderate yellow-green power is seen in the theta, alpha, and beta range on the DSA.
2. Maintenance with 1.5 age-adjusted MAC and propofol infusion with SEF remaining ~16. Burst suppression is apparent in the bilateral DSA with black vertical lines shown in the red box
3. & D. Emergence is apparent with characteristic “unzipping” on DSA, depicted within the yellow ovals. There is a transition from delta dominant slow wave anesthesia (DDSWA) to spindle dominant slow wave anesthesia (sDSWA) to non-slow wave anesthesia (NSWA).⁵ The faster frequencies are visible in the raw EEG shown with the yellow arrow. EMG is increasing visible in the EMG parameter 14 shown in the yellow triangle.

Discussion

EEG allows for individualization of our anesthetics. Here we see two patients receiving variable sevoflurane concentrations during maintenance of general anesthesia. The interplay between patient history and EEG data can provide valuable information to tailor anesthetic management.

Pertinent history for patient 1 includes lack of substance use, ICU sedation following intubation, and hemorrhagic shock necessitating optimal hemodynamic control. It is likely that without EEG, this patient would have received higher concentrations of sevoflurane and higher doses of vasopressors. Careful titration of hypnosis in this patient facilitated successful extubation. Without the ability to titrate sevoflurane to a lower concentration, the patient may have remained intubated.

Our second patient has a history of awareness under general anesthesia, alcohol use, and marijuana use. Risk factors for awareness are incompletely understood and may include inability to safely deliver appropriate anesthesia (patient instability), underappreciated or underdosed patient needs, human error, equipment failure, difficult airway, genetic predisposition, total intravenous anesthetic, and neuromuscular blockade.⁶

Both cases show examples of how EEG can help guide individualized hypnotic dosing. Balancing the dose of anesthetics is essential to avoid untoward effects of underdosing (awareness, movement) with overdosing (hypotension, vasopressors, medication side effects, and postoperative neurocognitive disorders including delirium). The anesthesia patient safety foundation (APSF) has recommended EEG monitoring in patients receiving less than 0.7 MAC, such as our first patient.⁷ EEG may allow for patient or pharmacologic individualization. Vulnerable patients who are most likely to be misinterpreted with a one size fits all approach may be protected by information obtained in the EEG. EEG data can be proficiently interpreted and applied to personalized anesthetic management.

References

1. https://snacc.org/eeg-signatures-anesthetic-titration/
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7. The APSF Committee on Technology. APSF-endorsed statement on revising recommendations for patient monitoring during anesthesia. APSF Newsletter. 2022;37(1):7–8.