Case 23 - BIS

SECTION 1 — CASE PRESENTATION AND CLINICAL CONTEXT A 38-year-old healthy female (BMI 21) presented for a laparoscopic left donor nephrectomy. The anesthetic plan included multimodal general anesthesia with opioid-sparing strategy and regional blockade. Anesthetic Regimen Premedication: Glycopyrrolate 0.2 mg Sedation: Midazolam 1 mg Analgesia: Fentanyl 200 µg Steroid: Dexamethasone 8 mg Induction: Propofol 100 mg Neuromuscular blockade: Atracurium 40 mg + infusion (30 mg/h) Adjuncts: Dexmedetomidine 30 µg, Magnesium sulfate 1 g, Paracetamol 1 g Maintenance gases: Oxygen, nitrous oxide, sevoflurane (MAC 0.8–1.4) Regional technique: Erector spinae plane (ESP) block after induction Pre-incision bolus: Propofol 40 mg for controlled hypotension The case produced four distinct BIS and EEG physiological states, each driven by pharmacologic and surgical events: BIS 36 — 10 minutes post-induction BIS 15 — Following 40 mg propofol bolus BIS 28 — Approximately 4 minutes after pneumoperitoneum BIS 32 — At 45 minutes, during MAC ~1.4 volatile anesthesia These phases reflect the evolutionary trajectory of cortical physiology under balanced anesthesia. The chapter uses these phases as an organizing framework to explore EEG neurobiology, pharmacology, anesthetic depth assessment, and clinical decision-making. Why This Case is Ideal for Teaching BIS Interpretation This case avoids many confounders (elderly age, hypothermia, shock, metabolic derangements) and includes: A young, healthy brain with intact thalamocortical connectivity Full neuromuscular blockade (eliminating EMG artifact) Highly standardized anesthetic regimen ESP block (stable analgesic background) Clear pharmacologic transitions Laparoscopy with predictable sympathetic surges Thus, it provides a classic model to demonstrate how EEG and BIS evolve with: GABAergic sedation α2-adrenergic modulation Opioid-induced hyperpolarization NMDA inhibition Volatile anesthetic effects Sympathetic activation Propofol redistribution kinetics This allows an unusually clean, high-fidelity demonstration of cortical electrophysiology under anesthesia. References Brown EN, Purdon PL. The Neuroscience of General Anesthesia. N Engl J Med. 2013;369:1015–1025. Mashour GA, Hudetz AG. Neural Correlates of Unconsciousness in Anesthesia. Trends Neurosci. 2018;41:150–159. Akeju O, Brown EN. Neural Oscillations Underlying General Anesthesia and Sleep. Curr Opin Anaesthesiol. 2017;30:441–451. SECTION 2 — FOUNDATIONS OF EEG UNDER ANESTHESIA: MOLECULAR & CIRCUIT-LEVEL MECHANISMS Understanding BIS requires understanding how anesthetics alter: Thalamocortical oscillators Inhibitory and excitatory synaptic currents Ion channel behavior Brainstem arousal systems 2.1 Thalamocortical Circuit Physiology General anesthesia primarily acts on the thalamus, cerebral cortex, and brainstem arousal nuclei, especially: Thalamic relay nuclei (generate alpha + sleep spindles) Thalamic reticular nucleus (TRN) (inhibitory gating) Corticothalamic pyramidal neurons Locus coeruleus (LC) (noradrenergic arousal) Ventrolateral preoptic nucleus (VLPO) (GABAergic sleep promotion) Brainstem reticular activating system (RAS) Alpha (8–12 Hz) Generated by hyperpolarized thalamic relay nuclei oscillating within the alpha resonance range. Enhanced by propofol and sevoflurane. Theta (4–7 Hz) Driven by LC suppression → corticothalamic spindle-like oscillations. Prominent under dexmedetomidine, opioids, N₂O. Delta (0.5–3 Hz) Represents deep cortical hyperpolarization. Occurs with high doses of GABAergic agents. Suppression Occurs when thalamocortical membranes fall below firing threshold due to overwhelming inhibitory currents. Burst Suppression A more extreme state reflecting alternation between periods of neuronal silence and synchronized bursts, often from: Very high anesthetic doses Hypothermia Brain injury Metabolic suppression Our patient never reached burst-suppression. 2.2 How Each Anesthetic Class Interacts with Ion Channels Propofol Potentiates GABA_A receptor activity (β2/β3 subunit) Inhibits HCN1 channels → prolongs hyperpolarization Produces alpha → delta → suppression depending on dose Rapid effect-site rise → suppression (seen in BIS 15 phase) Sevoflurane Enhances GABA_A Activates K2P channels (TREK-1, TASK-3) → leak K⁺ hyperpolarization Partial NMDA inhibition At MAC >1.2, causes paradoxical beta enhancement Dexmedetomidine α2A-agonist → LC inhibition → NOR ↓ Mimics stage N2 sleep physiology (spindles, theta dominance) Lowers BIS independently of cortical inhibition intensity Opioids (fentanyl) μ-receptor activation → GIRK K⁺ channels open → hyperpolarization Decreased glutamate and GABA release Increased delta oscillations Minimal hypnosis alone but potent synergist Magnesium sulfate NMDA receptor blockade Reduced excitatory neurotransmission Enhances slow-wave activity Nitrous oxide NMDA antagonism Early beta → stabilizes theta under volatiles Neuromuscular blockade No cortical effect But eliminates EMG (50–120 Hz) that would falsely elevate BIS 2.3 PK/PD Timeline in This Case 0–10 minutes: Propofol redistributing Sevoflurane equilibrating Dexmedetomidine onset ESP block suppressing nociceptive input 10–20 minutes: Propofol washout Stable alpha–theta rhythm 20 minutes: Propofol 40 mg bolus → Ce spike → rapid suppression 24–30 minutes: Pneumoperitoneum → sympathetic activation → EEG reactivation 45+ minutes: MAC 1.4 → strong alpha–theta with beta cap Steady-state anesthesia achieved References Ching S, Cimenser A, Purdon PL, et al. Thalamocortical Model for Propofol-Induced Unconsciousness. J Neurosci. 2010;30:5171–5182. Hemmings HC, Egan TD. Pharmacology and Physiology for Anesthesia. 2nd ed. Elsevier; 2019. Akeju O, Pavone KJ, Westover MB, et al. Effects of Dexmedetomidine on Neural Oscillations. Anesthesiology. 2014;121:1028–1037. Schneider G, Kochs E. EEG Changes with Volatile Agents. Br J Anaesth. 2002;89:323–330. Ishizawa Y. Mechanisms of Anesthetic-Induced Unconsciousness. J Anesth. 2011;25:319–327. Purdon PL, Pierce ET, Mukamel EA. EEG Signatures of Loss and Recovery of Consciousness Under Propofol. PNAS. 2013;110:E1142–E1151. SECTION 3 — PHASE 1 (BIS 36): BALANCED EARLY ANESTHESIA Timepoint: ~10 minutes post-induction Anesthetic state: Propofol redistribution + early sevoflurane equilibrium Adjuncts: Dexmedetomidine onset, fentanyl active, magnesium and ESP block contributing to analgesic stability This is the most stable phase of general anesthesia and produces a characteristic EEG. 3.1 Raw EEG Interpretation Moderate amplitude oscillations Alpha (8–12 Hz) — propofol + sevo synergy Theta (4–7 Hz) — dexmedetomidine + opioid support Minimal beta Minimal delta No suppression or discontinuity No EMG contamination (full paralysis) This reflects synchronized, rhythmic thalamocortical oscillations. 3.2 DSA Interpretation Dense red alpha band Red-orange theta band Very little activity above 15 Hz Smooth, stable power distribution No vertical blue “suppression” bands This demonstrates deep but structured unconsciousness. 3.3 SEF & MF SEF ≈ 12 Hz → strong alpha power MF ≈ 4–6 Hz → theta-weighted anesthesia 3.4 Clinical Meaning Adequate hypnosis Robust analgesic background Very low risk of awareness Ideal depth before surgical incision Balanced hypnosis, analgesia, and immobility This is the canonical early-maintenance EEG. References Purdon PL, Sampson A, Pavone KJ. Clinical Electroencephalography for Anesthesiologists. Anesthesiology. 2015;123:937–960. Akeju O, Brown EN. Neural Oscillations in Anesthesia. Curr Opin Anaesthesiol. 2017;30:441–451. Rampil IJ. A Primer for EEG Under Anesthesia. Anesthesiology. 1998;89:980–1002. Liley DTJ. EEG Interpretation in Anesthesia. Br J Anaesth Educ. 2020;20:164–172. SECTION 4 — PHASE 2 (BIS 15): PROPOFOL BOLUS–INDUCED SUPPRESSION A propofol 40 mg bolus was given to produce controlled hypotension for renal hilar dissection. This caused an abrupt effect-site concentration rise. 4.1 Mechanism of Suppression The synergistic combination of: High propofol Ce Dexmedetomidine suppressing LC Opioid GIRK-mediated hyperpolarization Magnesium NMDA blockade Sevoflurane GABA/K2P enhancement ESP block preventing nociceptive arousal Full NMB eliminating EMG …creates the perfect scenario for pure cortical suppression. 4.2 Raw EEG Low amplitude Slow (<3 Hz) baseline drift No isoelectric line No burst patterns No EMG contamination Represents functional, pharmacologic silence 4.3 DSA Uniform dark blue spectrum Loss of alpha and theta Narrow delta band No vertical suppression ratio bands No burst suppression striped pattern This is pure suppression, not burst suppression. 4.4 SEF & MF SEF < 4 Hz MF < 3 Hz 4.5 Safety Interpretation Low BIS <20 is benign when: BP normal HR normal EtCO₂ normal No suppression ratio >10% Patient young and healthy No hypothermia No hypoperfusion The patient remained hemodynamically stable. 4.6 Why This Is Desired Ensures profound hypnosis during controlled hypotension Prevents sympathetic surges Maintains surgical stillness Zero risk of awareness Avoids burst suppression This is exactly what anesthesiologists expect when using propofol boluses in multimodal anesthesia. References Purdon PL, Pierce ET, Mukamel EA. EEG and Consciousness Transitions Under Propofol. PNAS. 2013;110:E1142–E1151. Pilge S, Zanner R, Schneider G. BIS and EEG Suppression. Anaesthesist. 2014;63:207–219. Akeju O, Pavone KJ. Opioid-Induced EEG Dynamics. Anesthesiology. 2014;121:1013–1023. Sanders RD, Maze M. Alpha-2 Agonists and EEG. Handb Exp Pharmacol. 2011:89–107. SECTION 5 — PHASE 3 (BIS 28): REACTIVATION AFTER PNEUMOPERITONEUM Timepoint: Approximately 4 minutes after pneumoperitoneum Mechanistic drivers: CO₂ absorption, sympathetic activation, thalamic depolarization, propofol redistribution, stable volatile MAC During pneumoperitoneum, intra-abdominal pressure rises to ~12–14 mmHg, triggering: ↑ PaCO₂ ↑ Catecholamine release ↑ Sympathetic outflow ↑ Thalamic excitability ↑ Cerebral blood flow (CBF) ↑ SEF and BIS Simultaneously, the propofol bolus from Phase 2 is rapidly redistributing, reducing its suppressive thalamocortical effect. The resulting EEG transition is classic and expected. 5.1 Raw EEG Interpretation Alpha oscillations return (8–12 Hz) Theta prominence (4–7 Hz) from dexmedetomidine and opioids Mild beta appearance from sympathetic stimulation Increased amplitude compared with suppression No discontinuity No burst suppression This pattern represents the cortical “reawakening” from pharmacologic suppression but within deep anesthesia. 5.2 DSA Interpretation Reappearance of alpha (red/yellow) Strong theta (red/orange) Mild green/yellow beta cap No blue suppression band Increased spectral power density This is a hallmark of the interplay between volatile anesthesia and sympathetic activation. 5.3 SEF & MF SEF: 10–12 Hz — consistent with partial reactivation and mild beta MF: 4–6 Hz — theta-weighted, reflecting balanced anesthesia 5.4 Mechanistic Physiology Propofol Redistribution Ce falls rapidly → neurons depolarize toward oscillatory threshold. Sympathetic Activation Catecholamines (NE, E) act on: Thalamic relay cells → depolarizing Cortical pyramidal neurons → increased excitability Leading to mid-frequency oscillatory return (alpha–beta). CO₂ Effects Hypercapnia (even mild) increases: CBF Neuronal metabolic rate Cortical activity Sevoflurane MAC 0.9 Maintains deep hypnosis, stabilizes alpha–theta bands, prevents true arousal. 5.5 Clinical Meaning Safe, deep anesthesia Balanced hypnotic–analgesic state No risk of awareness Normal physiologic EEG response to pneumoperitoneum No need to treat BIS rise unless paired with tachycardia, hypertension, or EMG This phase confirms correct anesthetic titration and appropriate thalamocortical recovery. References Schneider G, Kochs EF, et al. EEG Patterns During Pneumoperitoneum. Br J Anaesth. 2002;89:323–330. Akeju O, Pavone KJ. Sympathetic Modulation of EEG in Anesthesia. Anesth Analg. 2017;125:365–372. Hemmings HC, Egan TD. Physiology for Anesthesia. 2nd ed. Elsevier; 2019. Rampil IJ. EEG and CO₂ Effects. Anesthesiology. 1998;89:980–1002. Ching S, Brown EN. Models of Thalamocortical Rhythms. J Neurosci. 2010;30:5171–5182. SECTION 6 — PHASE 4 (BIS 32): STABLE MAINTENANCE AT MAC 1.4 Timepoint: ~45 minutes after pneumoperitoneum State: Deep inhalational anesthesia with high MAC + surgical traction + dexmedetomidine background By this stage, several equilibria are reached: Sevoflurane MAC = 1.4 Propofol Ce normalized Dexmedetomidine steady-state Opioid plasma concentration stable ESP block fully active Surgical sympathetic stimulation constant This produces the “classic inhalational plateau pattern.” 6.1 Raw EEG Interpretation Strong alpha Pronounced theta Mild beta (“beta cap”) Stable amplitude No delta dominance No suppression This indicates deep, rhythmically structured unconsciousness. 6.2 DSA Interpretation Thick alpha band (8–12 Hz) Strong theta (4–7 Hz) Beta cap (13–20 Hz) reflecting sympathetic stimulation + high MAC No discontinuity High absolute power This DSA is typical of volatile-based deep anesthesia. 6.3 SEF & MF SEF: 9–10 Hz (Strong alpha power with mild beta superimposition) MF: 4–5 Hz (Theta-weighted state) These numerical metrics confirm a deep but nonsuppressed anesthetic...