🧠 Neuroanesthesia Monitoring: Invasive vs Non-Invasive

🔹 Introduction

Goal to preserve cerebral perfusion, oxygenation, and neuronal integrity. Neuroanesthesia monitoring ensures:

• Detection of cerebral ischemia or elevated ICP
• Guidance for anesthesia depth and ventilation
• Improved neurological outcomes

Monitoring modalities are broadly divided into:

🔹 Invasive vs Non-Invasive Monitoring: 

🔹 Non-Invasive Monitoring

✅ 1. Standard ASA Monitoring

• ECG
• Non-invasive blood pressure (NIBP)
• Pulse oximetry
• End-tidal CO₂ (EtCO₂)
• Temperature
• Neuromuscular monitoring (especially for motor-evoked potential cases)

These are essential for all anesthesia cases, including neurosurgery.

✅ 2. Neurological Examination & Level of Consciousness

• Glasgow Coma Scale (GCS) in awake craniotomy or TBI
• Pupil size/reactivity monitoring
• Useful in conscious sedation, intra-op neuro exams

✅ 3. Cerebral Oximetry (NIRS – Near-Infrared Spectroscopy)

• Non-invasive monitoring of regional cerebral oxygen saturation (rSO₂) from the frontal cortex
• Detects regional hypoxia before systemic changes occur
• Especially useful in:
• Carotid endarterectomy
• Cardiac surgery
• Sitting craniotomy

📌 Normal rSO₂: ~60–75%. A >20% drop from baseline suggests cerebral ischemia.

✅ 4. Transcranial Doppler (TCD)

• Ultrasound-based assessment of cerebral blood flow velocity in major intracranial vessels
• Detects:
• Vasospasm (e.g., post-SAH)
• Embolic events
• Cerebral autoregulation
• Operator-dependent; not for continuous monitoring

✅ 5. Evoked Potentials

• Somatosensory Evoked Potentials (SSEP): Monitor dorsal column-medial lemniscus pathway
• Motor Evoked Potentials (MEP): Monitor corticospinal tract integrity
• Visual (VEP) and Brainstem Auditory (BAEP) for special cases

Used in spine, brainstem, tumor, or AVM surgeries.
Need TIVA (Total IV Anesthesia) for optimal signal quality.

✅ 6. Bispectral Index (BIS) / Processed EEG

• Index of depth of anesthesia
• BIS value of 40–60 targeted for general anesthesia
• Helps avoid awareness and optimize anesthetic dose

🔹 Invasive Monitoring

✅ 1. Invasive Blood Pressure Monitoring (IBP)

• Arterial line (commonly radial) provides beat-to-beat BP
• Essential for:
• High-risk neurosurgery (e.g., aneurysm, AVM)
• Surgeries requiring deliberate hypotension or hypertension
• Intracranial hypertension

Allows ABG sampling and calculation of CPP = MAP – ICP

✅ 2. Central Venous Pressure (CVP) Monitoring

• Assesses volume status and guides fluid therapy
• Useful in:
• Major intracranial or spinal tumor resections
• Sitting position (risk of venous air embolism)
• TBI or status epilepticus needing aggressive fluid/vasopressor therapy

✅ 3. Jugular Venous Oximetry (SjvO₂)

• Measures global cerebral oxygen extraction
• Catheter inserted retrograde into jugular bulb
• Normal value: 55–75%
• Used in:
• Traumatic brain injury
• Poor-grade SAH
• Cardiac surgery with circulatory arrest

✅ 4. Intracranial Pressure (ICP) Monitoring

• Gold standard: Intraventricular catheter (EVD)
• Other methods: Intraparenchymal probes, subdural bolts
• Normal ICP: <15 mmHg
• Indications:
• Severe TBI (GCS ≤ 8)
• Hydrocephalus
• Refractory brain edema

Allows CSF drainage and CPP-guided therapy.

✅ 5. Brain Tissue Oxygen Monitoring (PbtO₂)

• Measures local partial pressure of brain tissue oxygen
• Inserted into penumbra region of injured brain
• Normal PbtO₂: 20–35 mmHg
• Valuable in guiding multimodal TBI management

✅ 6. Microdialysis and Cerebral Metabolic Monitoring

• Measures lactate, pyruvate, glutamate, glucose in interstitial brain fluid
• Research and advanced ICU setting; not routine

✅ 7. Electrocorticography (ECoG)

• Direct measurement of cortical electrical activity
• Used intraoperatively in epilepsy surgery to localize seizure foci

✅ 8. Ventriculostomy (EVD)

• Simultaneously monitors ICP and allows CSF drainage
• Used in:
• Hydrocephalus
• TBI with raised ICP
• SAH with hydrocephalus

🔹 Choosing the Right Monitoring

🔍 Suggested References

1. Miller’s Anesthesia, 9th Edition – Chapter on Neuroanesthesia Monitoring
2. Cottrell and Young’s Neuroanesthesia – Comprehensive discussions on cerebral monitoring
3. StatPearls – Neuroanesthesia and Monitoring Modalities
4. British Journal of Anaesthesia (BJA) – Review articles on neuromonitoring in surgery
5. WFSA – Educational Resources on intraoperative neurophysiologic monitoring

📝 Viva Corner (Sample Q&A)

1. Q: What is the gold standard for ICP monitoring?
   A: Intraventricular catheter (external ventricular drain, EVD)
2. Q: What is the normal range of SjvO₂?
   A: 55–75%
3. Q: What is the most sensitive monitor for detecting cerebral ischemia during CEA?
   A: NIRS and EEG (with SSEP)
4. Q: Which monitoring modality helps detect cerebral emboli?
   A: Transcranial Doppler (TCD)
5. Q: What BIS range corresponds to adequate depth of anesthesia?
   A: 40–60