Neurophysiology

Quick Answer

Neurophysiology encompasses the electrical and chemical processes underlying neuronal function, cerebral blood flow regulation, brain metabolism, and intracranial pressure dynamics. Action potentials depend on voltage-gated sodium and potassium channels governed by the Nernst and Goldman-Hodgkin-Katz equations. Synaptic transmission involves neurotransmitter release, receptor binding, and postsynaptic potentials (EPSPs and IPSPs). The blood-brain barrier maintains brain homeostasis through tight junctions and selective transport. Cerebral blood flow is tightly autoregulated between MAP 60-160 mmHg via myogenic, metabolic, and neurogenic mechanisms, though this curve shifts rightward in hypertension and leftward in chronic hypotension. Cerebral perfusion pressure (CPP = MAP - ICP) must be maintained greater than 50-60 mmHg to prevent ischaemia. The Monro-Kellie doctrine states that total intracranial volume (brain 80%, blood 10%, CSF 10%) is fixed; increase in one component requires compensatory decrease in others. Brain metabolism is predominantly aerobic, consuming glucose at 20% of total body oxygen consumption despite being 2% of body weight. Anaerobic metabolism is limited due to poor glycogen stores. Sedative agents act primarily through GABAergic enhancement (propofol, benzodiazepines) or NMDA receptor antagonism (ketamine), with effects on cerebral metabolism, blood flow, and ICP that are clinically relevant in neurocritical care.

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CICM Exam Focus

Written Exam High-Yield Topics:

• Action potential phases: resting membrane potential, depolarization, repolarization, afterhyperpolarization
• Nernst equation and Goldman-Hodgkin-Katz equation applications
• Synaptic transmission: excitatory vs inhibitory postsynaptic potentials, neurotransmitter types
• Blood-brain barrier: structure, function, drug penetration, disruption in pathology
• Cerebral blood flow autoregulation: range (60-160 mmHg), mechanisms, clinical implications
• Cerebral perfusion pressure calculation and targets (CPP > 50-60 mmHg)
• Monro-Kellie doctrine: components, compensatory mechanisms, decompensation
• Brain metabolism: oxygen consumption, glucose utilization, coupling of flow and metabolism
• Sedation mechanisms: propofol (GABA), benzodiazepines (GABA), ketamine (NMDA), effects on CBF and ICP
• ICP waveform analysis: P1, P2, P3 components, pathologic patterns

Viva Voce Themes:

• Explain action potential generation and propagation
• Describe synaptic transmission and neurotransmitter systems
• Discuss blood-brain barrier structure and clinical implications
• Outline cerebral blood flow autoregulation and disorders
• Explain ICP dynamics and management principles
• Compare effects of different sedative agents on cerebral physiology
• Apply Monro-Kellie doctrine to clinical scenarios of raised ICP

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Key Points

• Resting membrane potential: -60 to -70 mV, maintained by Na+/K+-ATPase (3 Na+ out, 2 K+ in) and potassium leak channels
• Nernst equation: calculates equilibrium potential for individual ions (E = RT/zF × ln([ion]out/[ion]in))
• Goldman-Hodgkin-Katz equation: calculates resting membrane potential considering multiple ion permeabilities
• Action potential phases: resting potential → threshold (-55 mV) → rapid depolarization (Na+ influx) → peak (+30 mV) → repolarization (K+ efflux) → afterhyperpolarization (K+ slow closure)
• Absolute refractory period: Na+ channels inactivated, cannot generate new action potential
• Relative refractory period: membrane hyperpolarized, requires stronger stimulus
• Voltage-gated sodium channels: Nav1.1, Nav1.2, Nav1.6 predominant in CNS; rapid activation and inactivation
• Voltage-gated potassium channels: Kv1.1, Kv3.1, Kv3.2; delayed rectifiers responsible for repolarization
• Synaptic transmission: calcium entry → vesicle fusion → neurotransmitter release → receptor binding → postsynaptic potential
• Excitatory neurotransmitters: glutamate (main CNS), acetylcholine (neuromuscular junction, CNS), norepinephrine
• Inhibitory neurotransmitters: GABA (main CNS), glycine (spinal cord, brainstem)
• EPSP (Excitatory Postsynaptic Potential): depolarization, brings membrane closer to threshold
• IPSP (Inhibitory Postsynaptic Potential): hyperpolarization, moves membrane away from threshold
• Temporal summation: multiple EPSPs from same presynaptic neuron arriving in rapid succession
• Spatial summation: multiple EPSPs from different presynaptic neurons arriving simultaneously
• Blood-brain barrier: endothelial cells with tight junctions (zonulae occludentes), basal lamina, astrocytic endfeet
• BBB transport: passive diffusion (lipid-soluble), carrier-mediated (GLUT1 for glucose), active transport, receptor-mediated transcytosis
• Drugs crossing BBB: lipid-soluble (propofol, benzodiazepines), small molecules (below 400-500 Da), low protein binding
• BBB disruption: hypertension, infection, inflammation, trauma, tumours, ischemia
• Cerebral blood flow: 50-60 mL/100g/min, 15% of cardiac output, 20% of total body oxygen consumption
• CBF autoregulation: maintains constant flow across MAP 60-160 mmHg in normotensive adults
• Autoregulation mechanisms: myogenic (vascular smooth muscle response to stretch), metabolic (CO2, H+, adenosine), neurogenic (sympathetic innervation)
• CO2 reactivity: CBF changes 1-2 mL/100g/min per 1 mmHg PaCO2 change (PaCO2 20-80 mmHg range)
• O2 reactivity: minimal until PaO2 < 50 mmHg, then significant vasodilation
• Autoregulation shift: rightward in chronic hypertension (LLA may be greater than 80 mmHg), leftward in prematurity
• Impaired autoregulation: TBI, SAH, severe sepsis, post-cardiac arrest, malignant hypertension
• Cerebral perfusion pressure: CPP = MAP - ICP (or CVP if ICP not measurable)
• CPP targets: greater than 50-60 mmHg generally; individualized based on autoregulation status
• Optimal CPP (CPPopt): CPP range where autoregulation is most intact; associated with better outcomes
• Monro-Kellie doctrine: fixed intracranial volume (brain 80%, blood 10%, CSF 10%); increase in one requires decrease in others
• ICP compensation: CSF displacement to spinal sac, decreased CSF production, increased venous drainage
• ICP decompensation: when compensatory mechanisms exhausted; exponential ICP rise
• ICP waveform: P1 (percussion wave, arterial pulse), P2 (tidal wave, brain compliance), P3 (dicrotic wave, aortic valve closure)
• Pathologic ICP patterns: Lundberg A waves (plateau waves, 50-100 mmHg, 5-20 min), B waves (rhythmic oscillations), C waves (small amplitude)
• Brain metabolism: aerobic, glucose-dependent, oxygen consumption 3.5 mL/100g/min
• CMRO2 (Cerebral Metabolic Rate of Oxygen): 20% of total body O2 consumption
• CMRglc (Cerebral Metabolic Rate of Glucose): 5.5 mg/100g/min (approximately 5% of total glucose utilization)
• Coupling of CBF and CMR: neurovascular coupling ensures adequate blood flow to metabolic demand
• Anaerobic capacity: limited due to poor glycogen stores (below 1 μmol/g); rapid depletion leads to ischemia
• Propofol: GABA-A receptor positive allosteric modulator; reduces CMRO2 30-50%, CBF 30-50%, ICP
• Benzodiazepines: GABA-A receptor positive allosteric modulator; reduces CMRO2, CBF, ICP
• Ketamine: NMDA receptor antagonist; increases CMRO2 and CBF (controversial), maintains respiratory drive
• Dexmedetomidine: alpha-2 agonist; minimal effect on CBF and ICP, preserves neurovascular coupling
• Opioids: minimal effect on CBF and CMRO2 at sedative doses
• Barbiturates: GABA-A receptor positive allosteric modulator; profound CMRO2 and CBF reduction, burst suppression for refractory ICP

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Clinical Overview

Neuronal Membrane Physiology

Resting Membrane Potential (RMP)

The resting membrane potential of neurons is typically -60 to -70 mV, maintained by the electrochemical gradients of ions across the cell membrane and selective ion channel permeability.

Ion Concentration Gradients:

Nernst Equation:

E = (RT/zF) × ln([ion]out/[ion]in)

Where:
E = equilibrium potential (mV)
R = gas constant (8.314 J/mol·K)
T = absolute temperature (K)
z = ion valence
F = Faraday's constant (96,485 C/mol)

At 37°C (310K), this simplifies to:

E = (61/z) × log10([ion]out/[ion]in)

Goldman-Hodgkin-Katz (GHK) Equation:

The GHK equation calculates the resting membrane potential considering multiple ion permeabilities:

V = (RT/F) × ln((P[K][K]out + P[Na][Na]out + P[Cl][Cl]in) / (P[K][K]in + P[Na][Na]in + P[Cl][Cl]out))

Where P[X] is the membrane permeability to ion X.

The resting membrane potential is closest to the potassium equilibrium potential (-94 mV) because the membrane is predominantly permeable to K+ at rest due to leak channels (K2P channels: TASK, TREK, TWIK families).

Na+/K+-ATPase:

• Actively transports 3 Na+ out and 2 K+ in per ATP hydrolyzed
• Contributes approximately -4 mV to RMP (electrogenic contribution)
• Essential for maintaining ion concentration gradients
• Inhibition by cardiac glycosides (digoxin, ouabain) leads to depolarization

Exam Detail: K2P Channels (Two-Pore Domain Potassium Channels):

• KCNK family (K2P1-K2P18)
• Constitutively open "leak" channels
• Major determinant of resting membrane potential
• Modulated by pH, temperature, mechanical stretch, neurotransmitters
• TASK channels (TWIK-related acid-sensitive K+ channels): inhibited by extracellular acidosis
• TREK channels: activated by mechanical stretch, heat, polyunsaturated fatty acids
• Pharmacological targets: potential antidepressants, anaesthetics

Action Potential Phases

An action potential is a rapid, transient depolarization of the neuronal membrane that propagates along the axon.

Phase 1: Resting Potential (-60 to -70 mV)

• Voltage-gated Na+ and K+ channels closed
• Leak K+ channels maintain negative potential
• Na+/K+-ATPase maintains gradients

Phase 2: Threshold Depolarization (Threshold: -55 mV)

• Stimulus depolarizes membrane to threshold
• Voltage-gated Na+ channels (Nav) open rapidly
• Positive feedback: Na+ influx causes further depolarization, opening more Na+ channels

Phase 3: Rapid Depolarization (Peak: +30 to +40 mV)

• Massive Na+ influx through Nav channels
• Membrane potential approaches Na+ equilibrium potential (+61 mV)
• Depolarization spreads to adjacent membrane regions (propagation)

Phase 4: Repolarization

• Voltage-gated K+ channels (Kv) open with slight delay (delayed rectifiers)
• K+ efflux repolarizes membrane
• Nav channels inactivate (ball-and-chain mechanism)

Phase 5: Afterhyperpolarization (AHP)

• Membrane potential briefly becomes more negative than RMP (-80 mV)
• Due to slow closure of Kv channels
• Constitutes relative refractory period (stronger stimulus required)

Phase 6: Return to Resting

• Kv channels close
• Na+/K+-ATPase restores ion gradients
• Membrane returns to resting potential

Exam Detail: Voltage-Gated Sodium Channels (Nav):

• Structure: α-subunit (24 transmembrane segments, 4 domains) + β-subunits
• Nav subtypes: Nav1.1, Nav1.2, Nav1.3, Nav1.6 (CNS predominant)
• Activation: rapid opening at threshold (-55 mV)
• Inactivation: IFM motif (Ile-Phe-Met) acts as "ball" to block pore (fast inactivation, below 1 ms)
• Recovery: requires repolarization to -60 to -70 mV (slow inactivation recovery, 10-100 ms)
• Pharmacology:
  • "Local anaesthetics: bind to inner pore, block Na+ current (use-dependent blockade)"
  • "Anti-epileptics: phenytoin, carbamazepine (use-dependent Na+ channel blockade)"
  • "Toxins: tetrodotoxin (TTX) - binds to site 1, completely blocks; batrachotoxin - prevents inactivation"

Voltage-Gated Potassium Channels (Kv):

• Structure: 6 transmembrane segments, tetrameric assembly
• Kv subtypes: Kv1.1, Kv1.2 (delayed rectifiers); Kv3.1, Kv3.2 (fast-spiking interneurons)
• Delayed rectifiers: open slowly, inactivate slowly, responsible for repolarization
• A-type currents: rapidly inactivating (Kv4.2, Kv4.3), regulate firing frequency
• Pharmacology: 4-aminopyridine (blocks Kv, increases neurotransmitter release, used in MS)

Action Potential Propagation

Saltatory Conduction (Myelinated Axons):

• Myelin sheath produced by oligodendrocytes (CNS) or Schwann cells (PNS)
• Nodes of Ranvier: unmyelinated gaps with high Na+ channel density
• Action potential "jumps" from node to node
• Velocity: 50-100 m/s (fast)

Continuous Conduction (Unmyelinated Axons):

• Depolarization spreads continuously along membrane
• Slower: 0.5-2 m/s

Factors Affecting Conduction Velocity:

1. Myelination: myelinated > unmyelinated (saltatory conduction)
2. Axon diameter: larger diameter > smaller diameter (lower resistance)
3. Temperature: higher temperature > lower temperature (increased channel kinetics)

Exam Detail: Demyelinating Diseases:

• Multiple sclerosis: loss of oligodendrocyte myelin in CNS, slowed conduction, conduction block
• Guillain-Barré syndrome: loss of Schwann cell myelin in PNS, slowed conduction, areflexia
• Conduction block: action potential fails to propagate, causes neurological deficits
• Pathophysiology: exposed K+ channels cause current leak, decreased membrane capacitance disrupts saltatory conduction

Synaptic Transmission

Structure of the Synapse

Presynaptic Terminal:

• Synaptic vesicles containing neurotransmitter
• Voltage-gated calcium channels (Cav2.1, Cav2.2)
• Active zone: site of vesicle fusion
• Mitochondria for ATP production

Synaptic Cleft:

• 20-40 nm wide
• Enzymes for neurotransmitter degradation (acetylcholinesterase)
• Extracellular matrix proteins

Postsynaptic Membrane:

• Receptor proteins (ionotropic or metabotropic)
• Postsynaptic density (scaffold proteins, signalling molecules)
• Ion channels

Exam Detail: Synaptic Vesicle Cycle:

1. Loading: neurotransmitter actively transported into vesicles (VGLUT for glutamate, VGAT for GABA)
2. Docking: vesicles dock at active zone (SNARE complex: synaptobrevin, syntaxin, SNAP-25)
3. Priming: vesicles become release-competent
4. Fusion: Ca2+ entry triggers fusion (synaptotagmin as Ca2+ sensor)
5. Endocytosis: vesicle membrane recycled (clathrin-mediated or kiss-and-run)

Neurotransmitter Release

Sequence:

1. Action potential arrives at presynaptic terminal
2. Depolarization opens voltage-gated calcium channels (Cav2.1 P/Q-type, Cav2.2 N-type)
3. Calcium influx (local [Ca2+] increases greater than 100 μM)
4. Calcium binds to synaptotagmin (vesicle protein)
5. Synaptotagmin triggers SNARE complex-mediated vesicle fusion
6. Neurotransmitter released into synaptic cleft (quantal release: ~5000 molecules per vesicle)

Calcium Dependence:

• Extracellular [Ca2+]: 1.2 mM
• Intracellular resting [Ca2+]: ~100 nM
• Vesicle release requires greater than 10 μM local [Ca2+]
• Fourth-power relationship: release ∝ [Ca2+]^4 (steep calcium dependence)

Quantal Release:

• Miniature endplate potentials (MEPPs): spontaneous single vesicle release
• Endplate potentials (EPPs): evoked release of multiple vesicles
• Quantal content: number of vesicles released per action potential (typically 100-200 at NMJ)

Exam Detail: SNARE Complex:

• Synaptobrevin (VAMP): vesicle-associated membrane protein
• Syntaxin: plasma membrane protein
• SNAP-25: plasma membrane protein (palmitoylated)
• Formation brings vesicle and plasma membrane together
• Cleaved by botulinum toxin:
  • "BoNT/A: SNAP-25"
  • "BoNT/B: synaptobrevin"
  • "BoNT/C: syntaxin"
• Tetanus toxin: cleaves synaptobrevin (inhibitory synapses preferentially)

Postsynaptic Receptors

Ionotropic Receptors (Ligand-Gated Ion Channels):

• Directly open ion channel upon neurotransmitter binding
• Fast synaptic transmission (milliseconds)
• Examples:
  • "Glutamate: NMDA, AMPA, kainate receptors (Na+, Ca2+ influx, depolarization)"
  • "GABA-A: Cl- influx, hyperpolarization"
  • "Glycine: Cl- influx, hyperpolarization (spinal cord)"
  • "Nicotinic ACh: Na+, K+, Ca2+ influx, depolarization (neuromuscular junction, autonomic ganglia)"

Metabotropic Receptors (G-Protein Coupled Receptors):

• Activate second messenger systems via G-proteins
• Slower, longer-lasting effects (seconds to minutes)
• Examples:
  • "GABA-B: Gi/o, decreased cAMP, K+ channel opening, Ca2+ channel closure"
  • "Muscarinic ACh: M1, M3, M5 (Gq, IP3/DAG pathway), M2, M4 (Gi/o)"
  • "Glutamate: mGluR (Groups I, II, III)"
  • "Dopamine: D1-like (Gs), D2-like (Gi/o)"
  • "Adrenergic: α1 (Gq), α2 (Gi/o), β (Gs)"

Second Messenger Systems:

• cAMP pathway: Gs → adenylate cyclase → cAMP → PKA
• IP3/DAG pathway: Gq → PLC → IP3 + DAG → Ca2+ release + PKC activation
• Direct channel modulation: Gβγ subunits directly open K+ channels (GIRK)

Postsynaptic Potentials

Excitatory Postsynaptic Potential (EPSP):

• Depolarizing postsynaptic potential
• Caused by excitatory neurotransmitters (glutamate, ACh, norepinephrine)
• Na+ influx (AMPA receptors) or Na+ + Ca2+ influx (NMDA receptors)
• Brings membrane potential closer to threshold
• Typical amplitude: 0.5-5 mV, duration: 10-20 ms
• Spatial summation: multiple EPSPs from different presynaptic neurons
• Temporal summation: multiple EPSPs from same presynaptic neuron in rapid succession

Inhibitory Postsynaptic Potential (IPSP):

• Hyperpolarizing postsynaptic potential
• Caused by inhibitory neurotransmitters (GABA, glycine)
• Cl- influx (GABA-A, glycine receptors) or K+ efflux (GABA-B)
• Moves membrane potential away from threshold
• Typical amplitude: 0.5-5 mV, duration: 10-200 ms (longer for GABA-B)
• Shunting inhibition: increased membrane conductance reduces effectiveness of excitatory inputs

Exam Detail: NMDA Receptor Properties:

• Coincidence detector: requires both glutamate binding and postsynaptic depolarization (Mg2+ block removal at -30 mV)
• Calcium-permeable: Ca2+ influx triggers intracellular signalling cascades
• Long-term potentiation (LTP): basis of learning and memory
• Antagonists: ketamine, PCP, memantine (Alzheimer's disease)
• Excitotoxicity: excessive Ca2+ influx causes neuronal death (stroke, TBI)

AMPA Receptor Properties:

• Fast EPSPs (peak at below 1 ms)
• Sodium-permeable (some subunits also permeable to Ca2+)
• Desensitization during prolonged agonist exposure
• Trafficking: insertion into membrane increases synaptic strength (LTP)

Major Neurotransmitter Systems

Glutamate (Excitatory):

• Main excitatory neurotransmitter in CNS
• Synthesized from α-ketoglutarate (TCA cycle)
• Vesicular transporter: VGLUT
• Receptors: NMDA, AMPA, kainate (ionotropic); mGluR (metabotropic)
• Clearance: EAAT transporters (astrocytes)
• Metabolism: glutamine synthetase (astrocytes) → glutamine → glutamate (neurons) (glutamine-glutamate cycle)
• Excitotoxicity: excessive activation causes neuronal death (stroke, TBI, seizures)
• Antagonists: NMDA antagonists (ketamine, memantine), AMPA antagonists (perampanel)

GABA (Inhibitory):

• Main inhibitory neurotransmitter in CNS
• Synthesized from glutamate by glutamic acid decarboxylase (GAD)
• Vesicular transporter: VGAT
• Receptors: GABA-A (ionotropic), GABA-B (metabotropic)
• Clearance: GAT transporters
• Metabolism: GABA transaminase → succinic semialdehyde → succinate (TCA cycle)
• Pharmacology:
  • "Benzodiazepines: positive allosteric modulators of GABA-A"
  • "Barbiturates: positive allosteric modulators (prolong channel opening)"
  • "Propofol: positive allosteric modulator of GABA-A"
  • "Etomidate: positive allosteric modulator of GABA-A"
• Antagonists: flumazenil (benzodiazepine competitive antagonist), bicuculline (GABA-A competitive antagonist)

Glycine (Inhibitory):

• Main inhibitory neurotransmitter in spinal cord and brainstem
• Synthesized from serine by serine hydroxymethyltransferase
• Receptor: glycine receptor (ionotropic, Cl- channel)
• Co-agonist with glutamate at NMDA receptor (required for activation)
• Clearance: glycine transporter (GlyT)
• Antagonist: strychnine (competitive antagonist, causes seizures, muscle spasms)

Acetylcholine (Excitatory/Modulatory):

• Synthesized from choline and acetyl-CoA by choline acetyltransferase
• Vesicular transporter: VAChT
• Receptors:
  • "Nicotinic (nAChR): ionotropic (neuromuscular junction, autonomic ganglia, CNS)"
  • "Muscarinic (mAChR): metabotropic (CNS, parasympathetic target organs)"
• Clearance: acetylcholinesterase (AChE) at synapse; butyrylcholinesterase in plasma
• Pharmacology:
  • "Agonists: nicotine, muscarine"
  • "Antagonists: curare (neuromuscular junction), atropine (muscarinic)"
  • "Inhibitors: organophosphates (AChE inhibitors, cause cholinergic crisis), neostigmine (reversible, myasthenia gravis)"

Dopamine (Modulatory):

• Synthesized from tyrosine → L-DOPA → dopamine (tyrosine hydroxylase, aromatic L-amino acid decarboxylase)
• Vesicular transporter: VMAT2
• Receptors:
  • "D1-like (D1, D5): Gs, increase cAMP (direct pathway, motor facilitation)"
  • "D2-like (D2, D3, D4): Gi/o, decrease cAMP (indirect pathway, motor inhibition)"
• Clearance: dopamine transporter (DAT)
• Metabolism: monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT)
• Pathways:
  • "Nigrostriatal: substantia nigra → striatum (motor control, degenerates in Parkinson's)"
  • "Mesolimbic: ventral tegmental area → nucleus accumbens (reward, addiction)"
  • "Mesocortical: ventral tegmental area → prefrontal cortex (executive function)"
• Pharmacology: L-DOPA (Parkinson's), antipsychotics (D2 antagonists), amphetamines (DA release)

Norepinephrine (Modulatory):

• Synthesized from dopamine (dopamine β-hydroxylase)
• Vesicular transporter: VMAT2
• Receptors: α1 (Gq), α2 (Gi/o), β (Gs)
• Clearance: norepinephrine transporter (NET)
• Metabolism: MAO and COMT
• Pathways: locus coeruleus → widespread cortical and subcortical projections (arousal, attention, stress response)
• Pharmacology: α-agonists (phenylephrine), β-agonists (isoproterenol), β-blockers (propranolol), SNRIs (NET inhibition)

Serotonin (Modulatory):

• Synthesized from tryptophan → 5-HTP → serotonin (tryptophan hydroxylase, aromatic L-amino acid decarboxylase)
• Vesicular transporter: VMAT2
• Receptors: 5-HT1 (Gi/o), 5-HT2 (Gq), 5-HT3 (ionotropic), 5-HT4 (Gs), 5-HT5-7 (various)
• Clearance: serotonin transporter (SERT)
• Metabolism: MAO to 5-HIAA
• Origins: raphe nuclei → widespread projections (mood, sleep, appetite, pain perception)
• Pharmacology: SSRIs (SERT inhibition), atypical antipsychotics (5-HT2A antagonism), ondansetron (5-HT3 antagonist)

Histamine (Modulatory):

• Synthesized from histidine by histidine decarboxylase
• Receptors: H1 (Gq), H2 (Gs), H3 (Gi/o, presynaptic autoreceptor), H4 (Gi/o)
• Origins: tuberomammillary nucleus of hypothalamus (arousal, wakefulness)
• Pharmacology: H1 antagonists (diphenhydramine, sedating), H2 antagonists (ranitidine, PUD)

Exam Detail: Receptor Desensitization and Downregulation:

• Desensitization: rapid (seconds to minutes), phosphorylation by kinases (GRKs, PKC, PKA)
• Internalization: receptor endocytosis via clathrin-coated pits
• Downregulation: prolonged exposure (hours to days), decreased receptor synthesis
• Clinical relevance: tolerance to opioids, benzodiazepines; upregulation of receptors after withdrawal (rebound, withdrawal symptoms)

Long-Term Potentiation (LTP):

• Persistent strengthening of synapses
• Induction: high-frequency stimulation, NMDA receptor activation, Ca2+ influx
• Maintenance: insertion of AMPA receptors, structural changes (new dendritic spines)
• Molecular mechanisms: CaMKII, PKC, PKA, CREB transcription factor
• Role: learning and memory

Blood-Brain Barrier

Structure

Cellular Components:

1. Endothelial Cells:

   • Tight junctions (zonulae occludentes): occludin, claudins, junctional adhesion molecules (JAMs)
   • Adherens junctions: VE-cadherin
   • Lack of fenestrations
   • Reduced pinocytotic vesicles
   • High mitochondrial content (active transport)

2. Basal Lamina:

   • Extracellular matrix
   • Type IV collagen, laminin, fibronectin
   • Provides structural support

3. Astrocytic Endfeet:

   • Ensheath cerebral capillaries
   • Secrete factors that induce BBB properties (TGF-β, GDNF)
   • Aquaporin-4 channels (water homeostasis)
   • Endfeet swelling contributes to cerebral edema

4. Pericytes:

   • Embedded in basal lamina
   • Regulate endothelial function
   • Contribute to BBB formation and maintenance

Tight Junction Proteins:

Functions

1. Protection: prevents entry of toxins, pathogens, and xenobiotics
2. Homeostasis: maintains stable ionic environment for neuronal function
3. Regulation: controls transport of nutrients, waste products, and signalling molecules
4. Selective Permeability: allows passage of essential molecules while excluding others

Transport Mechanisms

1. Paracellular Transport (Restricted):

• Tight junctions limit water-soluble molecule passage
• Small molecules (below 400 Da) with minimal protein binding may cross slowly

2. Transcellular Transport:

A. Passive Diffusion:

• Lipid-soluble, small molecules (below 400-500 Da)
• Examples: oxygen, carbon dioxide, ethanol, caffeine
• Facilitated by high lipid solubility and lack of charge

B. Carrier-Mediated Transport:

• Saturable, selective, energy-dependent
• Examples:
  • "GLUT1: glucose transport (Km ~5 mM)"
  • "LAT1: large neutral amino acids (phenylalanine, leucine, tyrosine)"
  • "MCT1: monocarboxylates (lactate, pyruvate)"
  • "CAT1: cationic amino acids (arginine, lysine)"
• Competitive inhibition: drug interactions (e.g., L-DOPA competes with large neutral amino acids)

C. Active Transport (ATP-dependent):

• P-glycoprotein (MDR1, ABCB1): efflux pump, limits CNS penetration of many drugs
• Other ABC transporters: MRP1, BCRP
• Substrates: many drugs (e.g., digoxin, cyclosporine, vincristine), toxins
• Inhibition: verapamil, quinidine (increase CNS penetration of P-gp substrates)

D. Receptor-Mediated Transcytosis:

• Vesicular transport after receptor binding
• Examples:
  • "Transferrin receptor: iron transport"
  • "Insulin receptor: insulin transport"
  • "LDL receptor: lipoprotein transport"
• Drug delivery strategy: antibody-drug conjugates targeting BBB receptors

E. Adsorptive-Mediated Transcytosis:

• Cationic molecules interact with negative surface charge
• Example: cationic albumin, cell-penetrating peptides

Drug Penetration

Factors Affecting BBB Penetration:

1. Lipophilicity: more lipid-soluble = better penetration (log P 2-5 optimal)
2. Molecular weight: below 400-500 Da preferred
3. Ionization: unionized at physiological pH crosses more readily
4. Protein binding: only unbound fraction available for diffusion
5. P-glycoprotein substrate: effluxed, limited penetration

Examples:

BBB Disruption

Causes:

1. Hypertension: acute severe hypertension (greater than 180 mmHg systolic) causes forced opening of tight junctions (vasogenic edema)
2. Infection: meningitis, encephalitis (cytokine-mediated disruption, leukocyte migration)
3. Inflammation: multiple sclerosis, autoimmune encephalitis
4. Trauma: traumatic brain injury (mechanical disruption, BBB breakdown)
5. Ischemia: stroke (energy failure, tight junction disruption)
6. Tumours: glioblastoma, metastases (angiogenic vessels have incomplete BBB)
7. Radiation: radiation necrosis
8. Hyperosmolar agents: mannitol (therapeutic BBB disruption for chemotherapy)
9. Seizures: prolonged seizures increase BBB permeability

Consequences:

• Vasogenic edema (fluid extravasation)
• Increased intracranial pressure
• Entry of neurotoxic substances
• Leukocyte infiltration (neuroinflammation)
• Enhanced drug delivery (therapeutic opportunity in tumours)

Exam Detail: Neurovascular Unit:

• Functional unit comprising endothelial cells, pericytes, astrocytes, neurons, microglia
• Coordinates CBF regulation and BBB function
• Neuronal activity triggers astrocyte-mediated vasodilation (neurovascular coupling)
• Dysfunction contributes to neurodegenerative diseases (Alzheimer's, vascular dementia)

BBB and Disease States:

• Multiple sclerosis: T-cell migration across disrupted BBB, demyelination
• Alzheimer's disease: BBB dysfunction, reduced P-glycoprotein, impaired Aβ clearance
• Epilepsy: BBB disruption lowers seizure threshold, promotes epileptogenesis
• Brain tumours: heterogeneous BBB, angiogenic vessels leaky, enabling chemotherapy penetration
• HIV: Trojan horse mechanism: infected monocytes cross BBB, establish CNS reservoir

Cerebral Blood Flow

Physiology

Normal Values:

• Cerebral blood flow (CBF): 50-60 mL/100g/min
• Total CBF: 750-900 mL/min (15% of cardiac output)
• Gray matter: 70-80 mL/100g/min
• White matter: 20-30 mL/100g/min

Cerebral Metabolic Requirements:

• Oxygen consumption: 3.5 mL/100g/min (20% of total body O2 consumption)
• Glucose consumption: 5.5 mg/100g/min (5% of total glucose utilization)
• Brain weight: 2% of body weight
• Disproportionately high metabolic demand

Determinants of CBF:

CBF = CPP / CVR

Where:

• CPP = Cerebral Perfusion Pressure = MAP - ICP (or CVP if ICP not measurable)
• CVR = Cerebral Vascular Resistance

Neurovascular Coupling:

• Local neuronal activity increases regional CBF
• Mediated by:
  • "Astrocyte endfeet: release vasoactive metabolites (K+, prostaglandins, nitric oxide)"
  • "Neuronal release: glutamate, nitric oxide, adenosine"
  • "Direct innervation: cholinergic, noradrenergic, serotonergic fibers"
• Functional MRI (BOLD signal) relies on neurovascular coupling

Autoregulation

Definition:

• Maintenance of constant CBF across a range of MAP
• Prevents cerebral hypoperfusion (ischemia) at low pressures
• Prevents cerebral hyperperfusion (edema, hemorrhage) at high pressures

Normal Autoregulation Curve:

• Lower limit: 60 mmHg MAP (below this, CBF decreases linearly with MAP)
• Upper limit: 160 mmHg MAP (above this, CBF increases linearly with MAP)
• Plateau: 60-160 mmHg, constant CBF (autoregulatory range)

Mechanisms:

1. Myogenic Mechanism:

   • Vascular smooth muscle responds to transmural pressure
   • Increased pressure → stretch → vasoconstriction (Bayliss effect)
   • Decreased pressure → decreased stretch → vasodilation
   • Rapid response (seconds)

2. Metabolic Mechanism:

   • Local metabolites cause vasodilation in response to increased metabolic demand
   • Vasodilators: CO2 (most potent), H+, adenosine, K+, lactate
   • Vasoconstrictors: decreased CO2, decreased H+, increased O2
   • Slower response (minutes)

3. Neurogenic Mechanism:

   • Sympathetic innervation: protects against hyperperfusion at high pressures
   • Parasympathetic: minor role
   • Sensory nerves: release calcitonin gene-related peptide (CGRP, vasodilator)

CO2 Reactivity:

• CBF changes 1-2 mL/100g/min per 1 mmHg PaCO2 change
• Hypercapnia (PaCO2 > 45 mmHg): vasodilation, increased CBF, increased ICP
• Hypocapnia (PaCO2 < 35 mmHg): vasoconstriction, decreased CBF, decreased ICP
• Therapeutic hyperventilation for acute ICP elevation (PaCO2 30-35 mmHg)
• Limited duration: renal compensation for respiratory alkalosis decreases CSF HCO3-, reducing CO2 reactivity after 6-12 hours

O2 Reactivity:

• Minimal CBF change with PaO2 above 50 mmHg
• Hypoxemia (PaO2 < 50 mmHg): marked vasodilation, increased CBF
• Severe hypoxemia: maximal vasodilation, flow becomes pressure-passive

Clinical States Affecting Autoregulation:

Monitoring Autoregulation:

1. Transcranial Doppler (TCD):

   • Measures cerebral blood flow velocity (middle cerebral artery)
   • Calculates autoregulation indices (Mx, Sx)
   • Pressure-reactivity index (PRx): correlation between MAP and ICP
   • PRx < 0: intact autoregulation
   • PRx > 0: impaired autoregulation (flow becomes pressure-passive)

2. Near-Infrared Spectroscopy (NIRS):

   • Measures regional oxygen saturation (rSO2)
   • Calculates cerebral oximetry index (COx): correlation between MAP and rSO2
   • Non-invasive, continuous bedside monitoring
   • COx > 0.3: impaired autoregulation

3. Optimal CPP (CPPopt):

   • CPP range where autoregulation is most intact (lowest PRx or COx)
   • Individualized CPP targets based on autoregulation monitoring
   • Evidence: better outcomes when CPP maintained near CPPopt

Exam Detail: Pressure-Passive Flow:

• When autoregulation impaired, CBF varies directly with MAP
• Risk of cerebral hypoperfusion at low MAP
• Risk of cerebral hyperperfusion, edema, hemorrhage at high MAP
• Requires careful MAP management to maintain CBF within acceptable range

Cerebral Venous Outflow:

• Venous drainage: superficial veins → superior sagittal sinus → transverse sinuses → internal jugular veins
• Deep veins: vein of Galen → straight sinus → transverse sinuses
• Venous pressure: 5-10 mmHg
• Venous congestion increases ICP, impairs arterial inflow
• Head elevation (15-30°) improves venous drainage, reduces ICP

Clinical Implications

MAP Targets in Neurocritical Care:

• Patients with intact autoregulation: MAP > 80 mmHg (maintain CPP > 60 mmHg)
• Patients with impaired autoregulation: individualized based on PRx/COx monitoring
• Chronic hypertensive patients: higher MAP targets (greater than 90-100 mmHg) to avoid hypoperfusion
• Post-cardiac arrest: higher MAP targets (85-100 mmHg) may improve neurological outcomes

Sepsis-Associated Encephalopathy:

• Impaired autoregulation in greater than 50% of septic shock patients
• Endothelial dysfunction, BBB disruption, neuroinflammation
• Associated with delirium and long-term cognitive impairment
• Higher MAP targets may be beneficial

Traumatic Brain Injury:

• Autoregulation impairment correlates with worse outcomes
• PRx monitoring guides CPP optimization
• CPPopt varies between patients and over time
• Guidelines: CPP 60-70 mmHg (avoid below 50 mmHg or greater than 70 mmHg unless autoregulation monitoring)

Subarachnoid Hemorrhage:

• Vasospasm: delayed cerebral ischemia 3-14 days post-bleed
• Impaired autoregulation precedes clinical vasospasm
• TCD monitoring for vasospasm (flow velocity > 120 cm/s suggests vasospasm)
• Triple-H therapy (hypertension, hypervolemia, hemodilution) historically used, now evolving to induced hypertension alone

Intracranial Pressure Dynamics

Monro-Kellie Doctrine

Principle:

• Total intracranial volume is fixed
• Brain: 80% (1400 mL)
• Blood: 10% (150 mL)
• CSF: 10% (150 mL)

Compensatory Mechanisms:

• Displacement of CSF to spinal sac
• Decreased CSF production
• Increased CSF absorption
• Decreased cerebral blood volume (venous compression)

Pressure-Volume Relationship:

• Initial volume increase: minimal ICP change (compensation)
• Critical volume: compensatory mechanisms exhausted
• Exponential ICP rise: small volume increase causes large ICP increase (decompensation)

Normal ICP

Values:

• Supine: 5-15 mmHg (7-20 cmH2O)
• Elevated: greater than 20 mmHg
• Severe: greater than 30 mmHg
• Herniation risk: greater than 40-50 mmHg

Measurement:

• External ventricular drain (EVD): gold standard
• Intraparenchymal monitor: fiberoptic or strain gauge
• Subdural bolt: less accurate
• Lumbar puncture: contraindicated if mass lesion (risk of herniation)

ICP Waveform

Normal Waveform (Three Peaks):

Pathologic Waveforms:

1. Lundberg A Waves (Plateau Waves):

   • Sudden ICP rise to 50-100 mmHg
   • Duration: 5-20 minutes
   • Return to baseline
   • Indicates impaired compensation, high risk of herniation
   • Precursor to fatal herniation

2. Lundberg B Waves:

   • Rhythmic oscillations
   • Amplitude: 20-30 mmHg
   • Frequency: 0.5-2 per minute
   • Indicates unstable compensation, early sign of decompensation

3. Lundberg C Waves:

   • Small amplitude (5-10 mmHg)
   • Frequency: 4-8 per minute
   • Related to respiratory cycle (Traube-Hering-Mayer waves)
   • Less clinically significant

Raised ICP Management

Causes:

• Mass lesions: tumours, hematomas (epidural, subdural, intracerebral)
• Cerebral edema: cytotoxic (cell swelling), vasogenic (BBB disruption), interstitial (hydrocephalus)
• Increased CSF: hydrocephalus (obstructive, communicating)
• Increased cerebral blood volume: hyperemia, venous congestion
• Idiopathic intracranial hypertension (pseudotumor cerebri)

Manifestations:

• Headache (worse in morning, Valsalva)
• Nausea, vomiting (projectile)
• Papilledema (optic disc swelling)
• Visual changes (blurred vision, diplopia, transient obscurations)
• Altered mental status (confusion, decreased GCS)
• Cushing's triad (late sign): hypertension, bradycardia, irregular respirations
• Herniation syndromes (see below)

Herniation Syndromes:

1. Uncal (Transtentorial) Herniation:

   • Uncus of temporal lobe herniates through tentorial notch
   • Compresses cranial nerve III: ipsilateral pupillary dilation (blown pupil)
   • Compresses cerebral peduncle: contralateral hemiparesis (Kernohan's notch phenomenon: ipsilateral weakness from contralateral peduncle compression)
   • Compresses posterior cerebral artery: occipital infarction (cortical blindness)
   • Life-threatening emergency

2. Central (Transtentorial) Herniation:

   • Diencephalon herniates through tentorial notch
   • Early: small, reactive pupils
   • Progression: bilateral fixed pupils, abnormal posturing (decorticate → decerebrate)
   • Compression of brainstem: respiratory pattern changes

3. Tonsillar Herniation:

   • Cerebellar tonsils herniate through foramen magnum
   • Compression of medulla: respiratory arrest, cardiovascular collapse
   • Neck stiffness, opisthotonos
   • Often fatal

4. Upward Herniation:

   • Posterior fossa mass pushes cerebellum upward through tentorial notch
   • Compresses midbrain, quadrigeminal cistern
   • Upward gaze palsy (Parinaud's syndrome)

Management:

General Measures:

• Head elevation: 15-30° (improves venous drainage)
• Neck neutral position (avoid jugular compression)
• Adequate sedation and analgesia (reduce agitation, metabolic demand)
• Avoid endotracheal tube obstruction, suctioning (brief, limit duration)
• Optimize MAP (maintain CPP > 60 mmHg)
• Treat seizures (propofol, levetiracetam)
• Maintain normothermia (fever increases CBF, ICP)

First-Line:

• Hyperventilation: PaCO2 30-35 mmHg (transient effect, 6-12 hours)
• Osmotic therapy:
  • "Mannitol: 0.25-1 g/kg IV bolus, repeat q4-6h, maintain serum osmolality < 320 mOsm/kg"
  • "Hypertonic saline: 3%, 7.5%, 23.4%; 2-5 mL/kg 3% or 250 mL 7.5% bolus"
• Elevate head of bed
• Sedation

Second-Line (Refractory):

• Barbiturate coma: pentobarbital/thiopentone to burst suppression (reduces CMRO2, CBF, ICP)
• Hypothermia: 32-35°C (reduces CMRO2 6-7% per 1°C)
• Decompressive craniectomy: remove skull flap to allow brain expansion
• EVD placement: CSF drainage

Specific Interventions:

• Surgical evacuation: mass lesions (hematomas)
• Ventriculostomy: hydrocephalus
• Corticosteroids: vasogenic edema (tumours, abscesses)
• Optimize oxygenation: SpO2 > 94%
• Avoid hypotension: MAP > 80-90 mmHg (or higher if chronic hypertension)

Brain Metabolism

Energy Substrates

Glucose:

• Primary energy source
• Transported across BBB via GLUT1 (endothelial cells) and GLUT3 (neurons)
• Aerobic glycolysis: glucose → pyruvate → acetyl-CoA → TCA cycle → 36-38 ATP per glucose
• Anaerobic glycolysis: glucose → lactate (2 ATP per glucose, inefficient)
• CMRglc: 5.5 mg/100g/min
• Blood glucose range: 3.9-10.0 mmol/L (70-180 mg/dL)
• Hypoglycemia (below 2.2 mmol/L or below 40 mg/dL): neuronal injury, seizures, coma
• Hyperglycemia (greater than 10 mmol/L or greater than 180 mg/dL): worse outcomes after ischemic stroke, TBI

Lactate:

• Can be used as alternative fuel (astrocyte-neuron lactate shuttle)
• Produced by astrocytes during intense neuronal activity
• Transported via MCT1 (astrocytes) and MCT2 (neurons)
• Elevated CSF lactate: marker of cerebral ischemia, mitochondrial dysfunction

Ketone Bodies:

• Used during fasting, ketogenic diet
• β-hydroxybutyrate, acetoacetate
• Transported via MCT1
• Provide efficient fuel during metabolic stress

Oxygen:

• Final electron acceptor in oxidative phosphorylation
• Cerebral O2 consumption: 3.5 mL/100g/min (20% of total body)
• PaO2 < 50 mmHg: anaerobic metabolism, lactate production, neuronal injury
• Hyperoxia: minimal additional benefit (hemoglobin already saturated at normal PaO2)

Cerebral Metabolic Rate (CMR)

CMRO2 (Cerebral Metabolic Rate of Oxygen):

• Normal: 3.5 mL/100g/min
• Reduced by:
  • Hypothermia (6-7% reduction per 1°C)
  • "Anesthetic agents (propofol: 30-50% reduction; barbiturates: greater than 50% reduction)"
  • Sedation
  • Coma
• Increased by:
  • Seizures (up to 200-300% increase)
  • Fever (10-13% increase per 1°C)
  • Pain, agitation

CMRglc (Cerebral Metabolic Rate of Glucose):

• Normal: 5.5 mg/100g/min
• Coupled to CMRO2 (approximately 6:1 glucose:O2 molar ratio)
• Increased in tumors (Warburg effect: aerobic glycolysis)
• Decreased by anesthetic agents, hypothermia

Coupling of CBF and CMR:

• Neurovascular coupling ensures adequate blood flow to metabolic demand
• Flow-metabolism coupling maintained by:
  • CO2 production (vasodilation)
  • Adenosine release
  • Nitric oxide
  • K+ efflux
• Disrupted in:
  • Ischemia
  • Traumatic brain injury
  • Sepsis
  • Anesthetics (propofol preserves coupling better than volatile agents)

Cerebral Ischemia

Thresholds:

• CBF < 20 mL/100g/min: electrical silence (EEG flattening)
• CBF < 10-12 mL/100g/min: ion pump failure, membrane depolarization
• CBF < 6-8 mL/100g/min: irreversible neuronal injury

Ischemic Cascade:

1. Decreased CBF → energy failure
2. ATP depletion → Na+/K+-ATPase failure → membrane depolarization
3. Glutamate release → excitotoxicity
4. Ca2+ influx → activation of proteases, lipases, endonucleases
5. Free radical production → oxidative stress
6. Mitochondrial dysfunction → apoptosis, necrosis

Penumbra:

• Ischemic but potentially salvageable tissue
• CBF: 10-20 mL/100g/min
• Reduced metabolic activity but not yet infarcted
• Target for reperfusion therapies (thrombolysis, thrombectomy)

Reperfusion Injury:

• Oxidative stress
• Inflammation
• Blood-brain barrier disruption
• Cerebral edema
• Hemorrhagic transformation

Sedation Mechanisms in Neurocritical Care

GABAergic Agents

Propofol:

• Mechanism: positive allosteric modulator of GABA-A receptors
• Potentiates GABA-induced Cl- influx, hyperpolarization, decreased neuronal excitability
• Effects on cerebral physiology:
  • "Reduces CMRO2: 30-50%"
  • "Reduces CBF: 30-50%"
  • Reduces ICP
  • Preserves neurovascular coupling
  • Reduces cerebral blood volume
• Dose: 0.5-4 mg/kg/hr infusion
• Advantages: rapid onset (15-30 sec), rapid offset (5-10 min), antiemetic, anticonvulsant
• Disadvantages: hypotension, pain on injection, propofol infusion syndrome (PIS) with high-dose long-term infusion
• PIS: metabolic acidosis, rhabdomyolysis, renal failure, cardiac failure; risk increased with dose > 5 mg/kg/hr for > 48h

Benzodiazepines (Midazolam, Lorazepam):

• Mechanism: positive allosteric modulator of GABA-A receptors (bind to α subunit)
• Effects on cerebral physiology:
  • Reduces CMRO2
  • Reduces CBF
  • Reduces ICP
• Midazolam: rapid onset (2-3 min), intermediate-acting (1-4 h)
• Lorazepam: slower onset (5-15 min), longer-acting (10-20 h), less accumulation
• Disadvantages: context-sensitive half-life (prolonged with prolonged infusion), tolerance, withdrawal, delirium risk
• Antagonist: flumazenil

Barbiturates (Pentobarbital, Thiopentone):

• Mechanism: positive allosteric modulator of GABA-A receptors (prolongs channel opening)
• Effects on cerebral physiology:
  • "Profoundly reduces CMRO2: greater than 50%"
  • "Profoundly reduces CBF: greater than 50%"
  • Reduces ICP
• Indications: refractory intracranial hypertension, status epilepticus
• Target: EEG burst suppression
• Disadvantages: profound hypotension (requires vasopressors), prolonged elimination (days to weeks), immunosuppression
• Not first-line due to significant side effects

NMDA Antagonist

Ketamine:

• Mechanism: non-competitive NMDA receptor antagonist
• Effects on cerebral physiology:
  • Historically thought to increase ICP (controversial)
  • "Recent evidence: minimal effect on ICP in ventilated, sedated patients"
  • Increases CMRO2 (by upregulating neuronal activity) but also increases CBF proportionally
  • Preserves neurovascular coupling
  • Maintains respiratory drive (useful for procedures)
• Advantages: bronchodilation, analgesia, minimal respiratory depression, hemodynamic stability (sympathomimetic)
• Disadvantages: emergence reactions (hallucinations), increased salivation, nystagmus
• Contraindication (historical): raised ICP (relative, now debated)
• Dose: 0.1-0.5 mg/kg/hr infusion (analgesia/sedation)
• Used in: TBI with refractory pain/ agitation, asthma, status epilepticus (adjunct)

Alpha-2 Agonist

Dexmedetomidine:

• Mechanism: alpha-2 adrenergic receptor agonist (presynaptic inhibition of norepinephrine release, postsynaptic activation in locus coeruleus)
• Effects on cerebral physiology:
  • Minimal effect on CBF (5-10% reduction)
  • Minimal effect on CMRO2
  • Minimal effect on ICP
  • Preserves neurovascular coupling
  • No respiratory depression
  • Analgesic and sedative properties
• Advantages: cooperative sedation (patient can be aroused), reduced delirium, sympathetic attenuation
• Disadvantages: bradycardia, hypotension, withdrawal with prolonged infusion (agitation, tachycardia, hypertension)
• Dose: 0.2-1.0 mcg/kg/hr
• Used in: delirium prevention, weaning from mechanical ventilation, neurosurgical procedures

Opioids

Fentanyl, Morphine, Remifentanil:

• Mechanism: mu-opioid receptor agonists
• Effects on cerebral physiology:
  • Minimal effect on CBF at sedative doses
  • Minimal effect on CMRO2 at sedative doses
  • Minimal effect on ICP
• Advantages: potent analgesia, synergistic with other sedatives
• Disadvantages: respiratory depression (dose-dependent), chest wall rigidity (fentanyl rapid bolus), nausea/vomiting, constipation, tolerance, dependence
• Specific agents:
  • "Fentanyl: high potency, short-acting (30-60 min)"
  • "Morphine: longer-acting (2-4 h), histamine release (hypotension, pruritus)"
  • "Remifentanil: ultra-short-acting (5-10 min), rapid offset, context-insensitive half-life"
• Antagonist: naloxone

Volatile Anesthetic Agents (ICU: Rarely Used)

Isoflurane, Sevoflurane, Desflurane:

• Mechanism: GABA-A potentiation, NMDA antagonism, glycine potentiation, 2-pore domain K+ channel activation
• Effects on cerebral physiology:
  • Dose-dependent CBF increase (cerebral vasodilation)
  • Dose-dependent CMRO2 reduction
  • Increased ICP (due to vasodilation)
  • Disrupt neurovascular coupling at high doses
• Generally avoided in neurocritical care due to ICP concerns
• Used in: operating room for neurosurgical procedures (with hyperventilation to counteract vasodilation)

Comparison of Sedative Effects

Clinical Applications

Raised ICP:

• First-line: propofol (reduces CBF, CMRO2, ICP)
• Refractory: barbiturate coma (burst suppression)
• Alternative: dexmedetomidine (if hemodynamically stable, minimal ICP effect)
• Avoid: ketamine (historically contraindicated, now debated but not first-line)

Delirium Prevention:

• Dexmedetomidine (reduces delirium incidence)
• Avoid benzodiazepines (increase delirium risk)

Status Epilepticus:

• Propofol, midazolam, pentobarbital (reduce neuronal excitability, EEG burst suppression)

Analgesia:

• Opioids (fentanyl, morphine, remifentanil)
• Ketamine (adjuvant for neuropathic pain, opioid-sparing)

Hemodynamic Instability:

• Dexmedetomidine (less hypotension than propofol/benzodiazepines)
• Ketamine (sympathomimetic, increases MAP)
• Reduce dose of other agents if hypotension

Weaning from Mechanical Ventilation:

• Dexmedetomidine (preserves respiratory drive, facilitates spontaneous breathing trials)
• Avoid long-acting agents (lorazepam) due to delayed awakening

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Clinical Applications

Neurocritical Care Monitoring

Multimodal Monitoring:

• ICP monitoring (EVD, intraparenchymal)
• CPP calculation (CPP = MAP - ICP)
• Autoregulation monitoring (PRx, COx)
• CBF monitoring (TCD, NIRS)
• EEG monitoring (seizure detection, burst suppression)
• Brain tissue oxygenation (PbO2 probe)
• Cerebral microdialysis (glucose, lactate, pyruvate, glutamate)

PRx-Guided CPP Management:

• Calculate PRx (correlation between MAP and ICP)
• Identify CPPopt (CPP with lowest PRx)
• Target CPP near CPPopt
• Evidence: improved outcomes in TBI

NIRS Monitoring:

• Regional cerebral oxygen saturation (rSO2)
• Thresholds: below 50% desaturation, below 20% decrease from baseline concerning
• Used in: cardiac surgery, carotid endarterectomy, TBI, post-cardiac arrest

Therapeutic Hypothermia

Indications:

• Out-of-hospital cardiac arrest (comatose survivors)
• Refractory intracranial hypertension
• Severe TBI (controversial)

Temperature Targets:

• Targeted temperature management: 32-36°C for at least 24 hours
• Post-cardiac arrest: 32-36°C for 24 hours, then avoid fever for 72 hours
• Avoid hyperthermia (fever increases CMRO2, CBF, ICP)

Physiological Effects:

• CMRO2 reduction: 6-7% per 1°C
• CBF reduction: proportional to CMRO2
• ICP reduction
• Metabolic suppression
• Decreased inflammatory response

Complications:

• Coagulopathy
• Arrhythmias (below 30°C)
• Infection risk
• Electrolyte shifts (hypophosphatemia, hypomagnesemia)
• Insulin resistance

Seizure Management

Status Epilepticus:

• Metabolic demand ↑↑↑ (CMRO2, CBF)
• ICP ↑
• Ischemia risk
• Treatment: benzodiazepines, propofol, pentobarbital (EEG burst suppression)

Non-Convulsive Seizures:

• More common in ICU than recognized
• Requires continuous EEG monitoring
• Common in: TBI, SAH, post-cardiac arrest, sepsis
• Treatment: levetiracetam, propofol, midazolam

Post-Cardiac Arrest Care

Pathophysiology:

• Global cerebral ischemia-reperfusion injury
• Impaired autoregulation
• BBB disruption
• Neuroinflammation
• Excitotoxicity

Targeted Temperature Management:

• 32-36°C for 24 hours
• Prevent hyperthermia (fever) for 72 hours

Hemodynamic Optimization:

• MAP 85-100 mmHg (higher than usual targets)
• Aim: maintain CBF in impaired autoregulation state
• Norepinephrine first-line vasopressor

Seizure Prophylaxis:

• Levetiracetam or phenytoin
• Continuous EEG monitoring

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Assessment

SAQ Practice Questions

SAQ 1: Cerebral Blood Flow Autoregulation (15 marks)

A 65-year-old male with a history of hypertension presents to ICU with a severe traumatic brain injury (GCS 6). His ICP is 25 mmHg and blood pressure is 110/70 mmHg.

a) Calculate the patient's cerebral perfusion pressure (CPP). (2 marks)

CPP = MAP - ICP MAP = SBP + (2 × DBP) / 3 = 110 + (2 × 70) / 3 = 110 + 140 / 3 = 83.3 mmHg CPP = 83.3 - 25 = 58.3 mmHg

b) Explain the physiology of cerebral blood flow autoregulation, including the normal range and mechanisms. (5 marks)

Cerebral blood flow autoregulation maintains constant CBF across a range of MAP, preventing ischemia at low pressures and hyperperfusion/edema at high pressures.

Normal autoregulation range:

• Lower limit: 60 mmHg MAP
• Upper limit: 160 mmHg MAP
• Within this range, CBF remains constant at 50-60 mL/100g/min

Mechanisms:

1. Myogenic: Vascular smooth muscle responds to transmural pressure (stretch causes vasoconstriction)
2. Metabolic: Local metabolites (CO2, H+, adenosine, K+) cause vasodilation with increased metabolic demand
3. Neurogenic: Sympathetic innervation protects against hyperperfusion at high pressures

CO2 reactivity: CBF changes 1-2 mL/100g/min per 1 mmHg PaCO2 change. Hypocapnia (PaCO2 < 35 mmHg) causes vasoconstriction, reducing CBF and ICP. Hypercapnia (PaCO2 > 45 mmHg) causes vasodilation, increasing CBF and ICP.

c) Discuss how autoregulation is altered in chronic hypertension and severe traumatic brain injury. (4 marks)

Chronic hypertension:

• Rightward shift of autoregulation curve
• Lower limit may be greater than 80-100 mmHg
• Upper limit may be greater than 180 mmHg
• Higher MAP targets required to avoid cerebral hypoperfusion
• If normotensive MAP targets used, risk of watershed ischemia

Severe TBI:

• Autoregulation often impaired or absent
• CBF becomes pressure-passive (follows MAP directly)
• Heterogeneous impairment (some regions impaired, others intact)
• Risk of hypoperfusion at low MAP and hyperperfusion at high MAP
• Requires careful MAP management based on autoregulation monitoring (PRx, COx)

d) Describe methods for monitoring autoregulation in ICU and how this can guide management. (4 marks)

Monitoring methods:

1. Transcranial Doppler (TCD):

   • Measures cerebral blood flow velocity in middle cerebral artery
   • Calculates pressure-reactivity index (PRx): correlation between MAP and ICP
   • PRx < 0: intact autoregulation; PRx > 0: impaired autoregulation (pressure-passive flow)

2. Near-Infrared Spectroscopy (NIRS):

   • Measures regional cerebral oxygen saturation (rSO2)
   • Calculates cerebral oximetry index (COx): correlation between MAP and rSO2
   • COx > 0.3: impaired autoregulation

3. Optimal CPP (CPPopt):

   • CPP range where autoregulation is most intact (lowest PRx or COx)
   • Individualized CPP targets based on monitoring
   • Maintaining CPP near CPPopt improves outcomes in TBI

Clinical application:

• In this patient, PRx monitoring would identify CPPopt
• If PRx < 0 at CPP 60-70 mmHg, current CPP 58 mmHg is adequate
• If PRx > 0, consider increasing MAP (with norepinephrine) to achieve CPP in optimal range
• In chronic hypertensive patients, higher MAP targets (90-100 mmHg) may be needed

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SAQ 2: Raised Intracranial Pressure Management (15 marks)

A 28-year-old female presents to ICU after a motor vehicle accident. CT brain shows a large right subdural hematoma with 8 mm midline shift. ICP monitor reads 45 mmHg. She is intubated, sedated with propofol.

a) Describe the Monro-Kellie doctrine and explain how it applies to this patient. (4 marks)

Monro-Kellie doctrine states that the total intracranial volume is fixed within the rigid skull. Components:

• Brain: 80% (1400 mL)
• Blood: 10% (150 mL)
• CSF: 10% (150 mL)

Principle: an increase in one component requires a compensatory decrease in others to maintain normal ICP.

Application to this patient:

• Subdural hematoma (mass) increases intracranial volume
• Compensatory mechanisms: CSF displacement to spinal sac, decreased CSF production, increased CSF absorption, decreased cerebral blood volume (venous compression)
• Critical volume: compensatory mechanisms eventually exhausted
• Exponential ICP rise: small additional volume causes large ICP increase (decompensation)
• ICP 45 mmHg indicates decompensation, high risk of herniation

b) List and explain the causes of the ICP waveform peaks. (3 marks)

Normal ICP waveform has three peaks:

P1 (Percussion wave):

• Origin: Arterial pulse (systole)
• Sharp, prominent peak (highest amplitude in normal waveform)

P2 (Tidal wave):

• Origin: Brain compliance
• Rounded, smaller than P1 in normal waveform
• Becomes prominent when compliance decreased

P3 (Dicrotic wave):

• Origin: Aortic valve closure
• Small dicrotic notch

In raised ICP (decreased compliance):

• P2 becomes equal to or larger than P1
• Pathologic patterns: Lundberg A waves (plateau waves), B waves (rhythmic oscillations)

c) Describe the management of raised ICP, including first-line and second-line interventions. (8 marks)

First-line interventions:

1. Head elevation: 15-30° (improves venous drainage, reduces ICP)
2. Neck neutral position (avoid jugular compression)
3. Optimize MAP (maintain CPP > 60 mmHg)
4. Adequate sedation and analgesia (reduce agitation, metabolic demand)
5. Hyperventilation: PaCO2 30-35 mmHg (causes vasoconstriction, reduces CBF and ICP; transient effect 6-12h)
6. Osmotic therapy:
   • Mannitol: 0.25-1 g/kg IV bolus, repeat q4-6h, maintain serum osmolality < 320 mOsm/kg (creates osmotic gradient, draws water from brain)
   • Hypertonic saline: 3%, 7.5%, 23.4%; 2-5 mL/kg 3% or 250 mL 7.5% bolus

Second-line interventions (refractory ICP):

1. Barbiturate coma: pentobarbital/thiopentone to EEG burst suppression (profoundly reduces CMRO2, CBF, ICP)
2. Hypothermia: 32-35°C (reduces CMRO2 6-7% per 1°C)
3. Decompressive craniectomy: remove skull flap to allow brain expansion
4. EVD placement: CSF drainage

Specific for this patient:

• Emergency neurosurgical consultation for hematoma evacuation (mass lesion requiring surgical decompression)
• Mannitol or hypertonic saline while awaiting surgery
• Hyperventilation (transient measure)
• Optimize sedation

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Viva Practice Questions

Viva 1: Cerebral Blood Flow and Autoregulation (20 marks)

Examiner: Can you explain cerebral blood flow autoregulation to me?

Candidate: Cerebral blood flow autoregulation is the brain's ability to maintain constant blood flow across a range of mean arterial pressures. In a normotensive adult, CBF remains constant at 50-60 mL/100g/min across a MAP range of 60-160 mmHg. Below 60 mmHg, CBF decreases linearly with MAP, risking cerebral ischemia. Above 160 mmHg, CBF increases linearly with MAP, risking cerebral hyperperfusion, edema, and hemorrhage.

The mechanisms involve myogenic, metabolic, and neurogenic components. The myogenic mechanism involves vascular smooth muscle responding to transmural pressure stretch with vasoconstriction. The metabolic mechanism uses local metabolites like CO2, H+, adenosine, and K+ to cause vasodilation in response to increased metabolic demand. CO2 is particularly potent, with CBF changing 1-2 mL/100g/min per 1 mmHg PaCO2 change.

Examiner: How is autoregulation altered in chronic hypertension?

Candidate: In chronic hypertension, the autoregulation curve shifts to the right. The lower limit increases to 80-100 mmHg or higher, and the upper limit may exceed 180 mmHg. This is due to vascular remodeling and hypertrophy of cerebral vessels.

This has important clinical implications. If we use standard MAP targets (e.g., 65 mmHg) in a chronically hypertensive patient, their brain may be in a hypoperfused state, risking watershed ischemia. Therefore, higher MAP targets (90-100 mmHg or higher) are often required to maintain adequate cerebral perfusion in these patients.

Examiner: How would you monitor autoregulation in an ICU patient with traumatic brain injury?

Candidate: Autoregulation monitoring can be performed using several methods:

1. Transcranial Doppler (TCD): Measures cerebral blood flow velocity in the middle cerebral artery. We calculate the pressure-reactivity index (PRx), which is the correlation between MAP and ICP. A PRx < 0 indicates intact autoregulation, while PRx > 0 indicates impaired autoregulation where flow becomes pressure-passive.

2. Near-Infrared Spectroscopy (NIRS): Measures regional cerebral oxygen saturation (rSO2). The cerebral oximetry index (COx) is calculated as the correlation between MAP and rSO2. A COx > 0.3 indicates impaired autoregulation.

These indices allow us to identify the optimal CPP (CPPopt) for an individual patient - the CPP range where autoregulation is most intact (lowest PRx or COx). Maintaining CPP near CPPopt has been shown to improve outcomes in traumatic brain injury.

Examiner: What are the effects of common ICU sedatives on cerebral blood flow?

Candidate:

Propofol: GABA-A receptor positive allosteric modulator. Reduces CMRO2 by 30-50%, reduces CBF by 30-50%, and reduces ICP. Preserves neurovascular coupling. First-line for raised ICP.

Midazolam: GABA-A receptor positive allosteric modulator. Reduces CMRO2 and CBF, reduces ICP. Can cause context-sensitive half-life with prolonged infusions and increase delirium risk.

Ketamine: NMDA receptor antagonist. Historically contraindicated in raised ICP due to concerns about increasing ICP, but recent evidence shows minimal effect on ICP in ventilated, sedated patients. Increases CMRO2 and CBF but preserves neurovascular coupling. Sympathomimetic effects help maintain hemodynamics.

Dexmedetomidine: Alpha-2 agonist. Minimal effect on CBF, CMRO2, and ICP. Preserves neurovascular coupling. No respiratory depression. Advantages include reduced delirium risk and cooperative sedation.

Opioids (fentanyl, morphine): Minimal effect on CBF and CMRO2 at sedative doses. Primarily analgesic.

Barbiturates: Profound reduction in CMRO2 (greater than 50%) and CBF. Used for refractory intracranial hypertension (barbiturate coma) but cause significant hypotension requiring vasopressors.

Examiner: How would you manage a patient with sepsis-associated encephalopathy?

Candidate: Sepsis-associated encephalopathy involves impaired cerebral autoregulation, blood-brain barrier disruption, and neuroinflammation. Over 50% of septic shock patients exhibit impaired autoregulation within the first 48 hours, which is associated with delirium and long-term cognitive dysfunction.

Management involves:

• Treating the underlying sepsis (source control, antibiotics)
• Hemodynamic optimization: higher MAP targets (85-100 mmHg) may be beneficial given impaired autoregulation
• Avoiding hypotension
• Delirium prevention: dexmedetomidine preferred over benzodiazepines
• Minimizing sedatives to allow neurological assessment
• Consider autoregulation monitoring (if available) to guide MAP targets
• Supportive care: adequate oxygenation, normoglycemia, avoiding hyperthermia

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Viva 2: Neurotransmission and Synaptic Function (20 marks)

Examiner: Can you describe the sequence of events in synaptic transmission?

Candidate: Synaptic transmission begins when an action potential arrives at the presynaptic terminal. The depolarization opens voltage-gated calcium channels (Cav2.1 P/Q-type, Cav2.2 N-type), allowing calcium influx. This local increase in intracellular calcium triggers synaptic vesicle fusion with the presynaptic membrane.

Vesicle fusion is mediated by the SNARE complex, comprising synaptobrevin (on the vesicle), syntaxin, and SNAP-25 (on the plasma membrane). Calcium binds to synaptotagmin, which acts as the calcium sensor and triggers SNARE complex formation.

Neurotransmitter is released into the synaptic cleft in quantal packets (approximately 5000 molecules per vesicle). The neurotransmitter diffuses across the 20-40 nm cleft and binds to postsynaptic receptors.

Binding to ionotropic receptors causes direct opening of ion channels (fast transmission, milliseconds), while binding to metabotropic receptors activates G-proteins and second messenger systems (slower transmission, seconds to minutes).

The postsynaptic response is either an excitatory postsynaptic potential (EPSP, depolarization) or inhibitory postsynaptic potential (IPSP, hyperpolarization). Multiple EPSPs can summate (spatially from different presynaptic neurons, or temporally from the same neuron) to reach threshold and generate an action potential.

Neurotransmitter is cleared from the cleft by reuptake transporters, enzymatic degradation, or diffusion, terminating the signal.

Examiner: What are the major excitatory and inhibitory neurotransmitters in the CNS?

Candidate:

Excitatory neurotransmitters:

• Glutamate: Main excitatory neurotransmitter in CNS. Binds to NMDA, AMPA, and kainate receptors (ionotropic) and mGluR (metabotropic). Causes Na+ and Ca2+ influx, depolarization. Excessive glutamate causes excitotoxicity (stroke, TBI, seizures).
• Acetylcholine: Excitatory at neuromuscular junction and autonomic ganglia (nicotinic receptors). Also has modulatory functions in CNS (muscarinic receptors).
• Norepinephrine: Modulatory, involved in arousal and attention.

Inhibitory neurotransmitters:

• GABA: Main inhibitory neurotransmitter in CNS. GABA-A receptors are ionotropic Cl- channels, GABA-B receptors are metabotropic (Gi/o). Cl- influx causes hyperpolarization. Benzodiazepines, propofol, and barbiturates act as positive allosteric modulators of GABA-A receptors.
• Glycine: Main inhibitory neurotransmitter in spinal cord and brainstem. Glycine receptor is an ionotropic Cl- channel. Also acts as a co-agonist at NMDA receptors.

Examiner: How do excitotoxicity and neuronal injury occur in cerebral ischemia?

Candidate: In cerebral ischemia, decreased cerebral blood flow causes energy failure. ATP depletion leads to failure of the Na+/K+-ATPase, resulting in membrane depolarization. Depolarization causes excessive glutamate release from presynaptic terminals.

Glutamate binds to postsynaptic NMDA and AMPA receptors, causing excessive Ca2+ and Na+ influx. The massive calcium influx activates intracellular enzymes including proteases, lipases, and endonucleases, which damage cellular structures.

Mitochondrial dysfunction occurs, leading to free radical production (oxidative stress) and energy depletion. This triggers apoptotic and necrotic cell death pathways.

The penumbra refers to ischemic but potentially salvageable tissue with CBF of 10-20 mL/100g/min. This tissue has reduced metabolic activity but is not yet infarcted and is the target for reperfusion therapies (thrombolysis, thrombectomy).

Examiner: What is the blood-brain barrier and how does it affect drug delivery to the CNS?

Candidate: The blood-brain barrier is a specialized structure that maintains brain homeostasis and protects the CNS from toxins and pathogens. It consists of endothelial cells with tight junctions (occludin, claudins, JAMs), a basal lamina, astrocytic endfeet, and pericytes.

The tight junctions severely restrict paracellular transport. Molecules can cross via:

1. Passive diffusion: lipid-soluble, small molecules (below 400-500 Da) such as propofol, midazolam, caffeine
2. Carrier-mediated transport: saturable transporters like GLUT1 (glucose), LAT1 (large neutral amino acids)
3. Active transport: ATP-dependent efflux pumps like P-glycoprotein, which pumps many drugs out of the brain
4. Receptor-mediated transcytosis: vesicular transport after receptor binding (e.g., transferrin receptor)

Factors affecting BBB penetration include lipophilicity, molecular weight, ionization state, protein binding, and whether the drug is a P-glycoprotein substrate.

Drugs with excellent BBB penetration include propofol, midazolam, fentanyl, and remifentanil. Drugs with poor penetration include gentamicin, vancomycin, and dopamine. L-DOPA crosses via carrier-mediated transport (LAT1), which is why it's effective for Parkinson's disease.

The BBB can be disrupted in various pathologies including hypertension, infection, inflammation, trauma, ischemia, and tumours. Therapeutic disruption (e.g., with mannitol) can be used to enhance chemotherapy delivery to brain tumours.

Examiner: How do different sedative agents used in ICU affect neurotransmission?

Candidate:

Propofol: Positive allosteric modulator of GABA-A receptors. Binds to a site distinct from benzodiazepines, enhancing GABA-induced Cl- influx and hyperpolarization. Also modulates glycine receptors and inhibits NMDA receptors at high concentrations. Rapid onset and offset, reduces cerebral metabolism and blood flow.

Midazolam: Positive allosteric modulator of GABA-A receptors, binding to the α subunit benzodiazepine site. Potentiates GABA effects. Causes context-sensitive half-life with prolonged infusion.

Ketamine: Non-competitive NMDA receptor antagonist, binding to the phencyclidine site within the ion channel. Blocks Ca2+ influx through NMDA receptors. Also affects opioid receptors and monoamine reuptake. Dissociative anesthesia properties, sympathomimetic effects.

Dexmedetomidine: Alpha-2 adrenergic receptor agonist. Presynaptic alpha-2 activation inhibits norepinephrine release. Postsynaptic alpha-2 activation in the locus coeruleus produces sedation and anxiolysis. Minimal respiratory depression.

Barbiturates: Positive allosteric modulators of GABA-A receptors. Prolong channel opening duration more than increasing opening frequency. At high concentrations, also directly activate GABA-A receptors. Profound cerebral metabolic suppression, used for refractory intracranial hypertension.

Benzodiazepine antagonist: Flumazenil competes for the benzodiazepine binding site on GABA-A receptors, reversing benzodiazepine effects. Opioid antagonist: Naloxone competitively blocks mu-opioid receptors, reversing opioid effects.

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