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Anaesthesia > Introduction to Anaesthesia and CNS physiology + Intravenous anaesthetics > Flashcards

Introduction to Anaesthesia and CNS physiology + Intravenous anaesthetics Flashcards

1
Q

What is general anesthesia?

A

Drug-induced, reversible depression of the central nervous system (CNS), resulting in the loss of
response to and perception of all external stimuli

Clinical state of

  • Unconsciousness
  • Amnesia
  • Analgesia
  • Immobility
  • Attenuation of autonomic responses to noxious stimulation
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2
Q

[Physiology of normal sleep ]

Control of sleep/wake switch mechanism in the hypothalamus

  • Promote wakefulness: ____________ in the posterior hypothalamus
  • Promote sleep: _______________ in the anterior hypothalamus
  • These 2 set of nuclei are mutually inhibitory

Direction of the sleep/wake switch influenced by:
- Neurohumoral factors
o Somnogens (promoting sleep) e.g. __________ accumulates from degradation of ATP
during prolonged intervals of wakefulness
o Excitatory arousal substances e.g. _______, _______, ______, _______
- Circadian rhythm: Directly modulated by light and the ____________ in hypothalamus
- Brainstem arousal nuclei (part of reticular activating system)
- Sleep/wake switch and brainstem arousal nuclei act on higher order circuits in thalamus and cerebral cortex to maintain wakefulness and awareness

General anaesthetics appear to act on multiple components of the sleep/wake control system

A

Tuberomammillary nucleus (TMN);

Ventrolateral-Preoptic nucleus (VLPO);

Adenosine,

orexin, glutamate, acetylcholine, amines;

suprachiasmatic nucleus

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3
Q

what are the similarities between GA and natural sleep?

A
  • Behavioural: unconsciousness, awareness
  • EEG patterns are similar at certain stages of GA
  • Action of GA drugs on specific sleep nuclei
  • Distribution of brain metabolism as demonstrated on functional MRI studies
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4
Q

What are the differences between GA and natural sleep?

A
  • Unrousability
  • Ability to achieve EEG burst suppression or isoelectricity at deep stages of GA
  • Instability of cardiorespiratory, thermoregulatory systems
  • Circadian rhythm disturbance and disruption of sleep architecture
  • Side effects of anaesthetic drugs
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5
Q

Where in the CNS do anaesthetics works?

A

a) spinal cord: act to inhibit purposeful responses to noxious stimulation
b) Brainstem: reticular activating system, locus coeruleus, thalamus
c) cerebral cortex: Major site for integration, storage and retrieval of information, including complex functions like memory and awareness

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6
Q

How do anaesthetics interfere with electrophysiological function of the CNS?

A

Predominantly inhibitory actions on vital CNS functions

a) Depress pacemaker functions and firing of specific neurons e.g. respiratory generators in brainstem leading to depression of respiratory rate and apnea
b) Reduce neuronal excitability e.g. hyperpolarise neurons to inhibit initiation of action potentials
c) Reduce communication between neurons e.g. inhibition of neurotransmitter release, enhancement of inhibitory neurotransmitter effect

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7
Q

BBB: Barrier between capillary endothelial cells (Blood) and astrocytes (Brain)

What are the main components?

A

a) Tight junctions between capillary endothelial cells
b) Basement membrane
c) Astrocytic end-feet “glia limitans”

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8
Q

What are the factors affecting passive transmembrane diffusion across the BBB?

A
  1. Size: generally less than 400-600 Da
  2. Lipid solubility
  3. Electrical charge
  4. Concentration gradient
  5. Degree of protein-binding
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9
Q

What are the factors affecting passive carrier transporter- assisted diffusion across the BBB?

A
  1. Saturable transport mechanism

2. Concentration gradient

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10
Q

What are the factors affecting active receptor mediated transport across the BBB?

A
  1. Energy-dependent

2. Binding to specific receptors

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11
Q

what are other techniques to induce drugs in the CNS?

A
  • Direct access e.g. lumbar puncture and introduction of chemotherapy drugs (invasive)
  • Via circumventricular areas e.g. transnasal delivery
  • Advanced pharmaceutical technology e.g nano-particles
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12
Q

what are the ideal physical properties of anaesthetic agents?

A
  1. Stability in solution, easy storage
  2. Intravenous agents: Non-irritant to veins on injection, and minimal local tissue damage in event of extravasation
  3. Inhalational agents: Non-irritant to airway
  4. Cost
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13
Q

what are the ideal pharmacokinetic properties of anaesthetic agents?

A
  1. Rapid and smooth onset
  2. Rapid and smooth recovery
  3. Easy and rapid titratability to surgical stimuli
  4. Rapid metabolism to inactive metabolites
  5. Non-organ dependent metabolism
  6. Lack of tissue accumulation, especially with prolonged use
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14
Q

what are the ideal pharmacodynamic properties of anaesthetic agents?

A
  1. Minimal depressive effects on cardiovascular and respiratory systems
  2. Lack of excitatory or emergence phenomenon
  3. Minimal side effects and organ-toxicity
  4. Low potential for allergic reactions/anaphylaxis
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15
Q

[Propofol] in what patients is this beneficial and why?

A

CNS depression with lowering of cerebral blood flow, cerebral metabolism –> Advantageous in neurosurgical patients with high intracranial pressures

Anti-emetic properties –> Can be used as an infusion to maintain anaesthesia in patients with history of severe post-operative nausea and vomiting (PONV)

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16
Q

[propofol] in what patients should this be used cautiously and why?

A

Vasodilatation and myocardial depression, leading to significant hypotension –> Use with caution in patients with preexisting hypotension and hypovolemia that is not well-resuscitated

Respiratory depressant with apnea and depressed airway reflexes -> Use with caution in patients where one is not confident of supporting or securing airway upon induction

17
Q

What are the unique features of thiopentone (barbituate)?

A
  • Highly irritant agent with risks of skin necrosis upon extravasation, and arterial thrombosis upon accidental intra-arterial injection
  • Can be used to induce barbiturate coma in patients with refractory intracranial hypertension
  • Long elimination half-life and zero-order kinetics with infusions –> prolonged recovery time if used as infusion
18
Q

In what kind of patients are thiopentone (barbituate) contraindicated in?

A

Contraindication in patients with porphyria – induces ALA synthetase (enzyme part of pathway), leading to accumulation of metabolism which may ppt an attack

19
Q

In what aspects are ethomidate superior to propofol and thiopentone?

A

Relative cardiovascular stability with less hypotension

20
Q

In which patients should ethomidate be used in caution?

A

Has been shown to cause adreno-cortical suppression: caution in critically ill patients

21
Q

What is the mechanism of action of ketamine?

A
  • Unique agent with antagonistic action on NMDA receptors
  • Leads to state of dissociative anaesthesia: dissociation of the thalamocortical and limbic systems.
  • Uncoupling of sensory, motor, memory, integrative, emotional aspects of the brain
  • Sympathetic excitation with raised heart rate and blood pressure
  • Increased secretions, with airway reflexes usually maintained
  • Leads to raised intracranial pressures
22
Q

how does the patient present as they recover from kentamine?

A

Emergence delirium as patient recovers: hallucinations, confusion, hypertonia and involuntary movements

23
Q

in what scenarios is kentamine clinically useful?

A

a. hypotensive patients or situations where high HR is desired (e.g. cardiac tamponade)
b. short procedures in children e.g. emergency department, bone marrow aspiration

24
Q

in what patients should kentamine be used with caution?

A

patients with raised ICP, raised intraocular pressures, ischemic heart disease or unable to tolerate sympathetic surges

25
Q

What are the indications of TIVA?

A
  • Neurosurgical procedures (when reduction of ICP is important)
  • Surgical procedures requiring neurophysiological monitoring
  • Airway surgery
  • Patients at high risk of malignant hyperthermia (a/w inhalational agents)
  • Patients at high risk of PONV (a/w inhalational agents)
26
Q

What are the advantages of TIVA?

A
  • Reduced post-operative nausea and vomiting (PONV)
  • Reduced intra-cranial pressure (ICP)
  • Reduced atmospheric pollution
  • Improved quality of recovery

Anaesthesia (17 decks)

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