Description

Major lumbar spinal procedures such as microlumbar discectomy at L4–5 demand careful integration of physiology, pharmacology, and neuromonitoring. When the patient has morbid obesity (BMI 46) and uncontrolled type 2 diabetes (HbA1c 9.5%), virtually every anesthetic drug, every physiologic system, and every electroencephalographic output becomes altered. Depth-of-anesthesia monitoring using the Bispectral Index (BIS) becomes not only helpful but essential.

In routine practice, BIS provides a surrogate estimate of hypnotic depth based on:

• Cortical EEG power

• Phase coupling

• Synchronicity

• Spectral distribution

• Burst suppression

• High-frequency contamination (usually EMG)

• Artifact filtering (SQI)

However, in obesity and diabetes, BIS must be interpreted differently.
Obesity pushes BIS upward (higher baseline, more EMG, sympathetic overactivity).
Diabetes pushes BIS downward (lower cortical power, microvascular dysfunction, volatile sensitivity).

This chapter provides the most detailed integrated analysis of how:

• Morbid obesity

• Uncontrolled diabetes

• Induction agents

• Opioids

• Muscle relaxants

• Volatile anesthetics

• Dexmedetomidine

• Non-narcotic adjuncts

• N₂O

• Magnesium

• NSAIDs

• Basic physiology

interact with every BIS-derived parameter:

The entire anesthetic regimen:

• Glycopyrrolate 0.2 mg

• Midazolam 1 mg

• Fentanyl 200 µg

• Dexona 8 mg

• Propofol 150 mg

• Atracurium 40 mg + infusion 30 mg/hr

• Dexmedetomidine 30 µg

• Magnesium sulfate 1 g

• Paracetamol 1 g

• Diclofenac 100 mg suppository

• Morphine 5 mg IM at end

was applied over a 3-hour surgical timeline, during which BIS progressed through the pattern you documented:

This chapter explains — step by step — why these BIS values occurred, how obesity and diabetes altered each parameter, how each drug contributed, and what these findings teach us about future anesthetics.

Pathophysiology of Morbid Obesity & Its Influence on BIS, SEF, SR, TP, EMG, and SQI

Morbid obesity alters nearly every physiologic system affecting EEG generation, drug distribution, and neuromuscular activity. The following subsections describe, in detail, how obesity modifies each of the BIS-derived parameters.

HOW OBESITY AFFECTS BIS 

1) Higher Baseline BIS

Morbid obesity increases:

• Sympathetic activity

• Anxiety

• Resting EMG tone

• Beta frequency EEG activity

Thus pre-induction BIS is often 96–99, even when the patient appears calm.

2) Larger Volume of Distribution

Highly lipophilic drugs (propofol, fentanyl, midazolam, dexmedetomidine) accumulate in adipose tissue.

Result:

• Slower offset

• More gradual BIS rise during emergence

• Delayed cortical reactivation

3) Increased Cardiac Output

Obese patients maintain higher resting CO.

Effect:

• Faster brain delivery of induction drugs

• Rapid BIS drop after propofol or...