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MBCHB3: Nervous system module. > Lecture 29: Inhalation and IV anaesthetics > Flashcards

Lecture 29: Inhalation and IV anaesthetics Flashcards

1
Q

What is the triad of goals for an anaethetist?

A

Hypnosis = IV or Volatile agent
Immobility = Muscle relaxant
Autonomic Areflexia = Opioids

Modern approach is to find a balance of all of these, ether back in the day could achieve all three but not balanced.

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2
Q

How do volatile agents work?

A

Inhalation:

P(i) P(A)P(a)P(br)

Exhalation

The agents are not metabolised by the body, they are inhaled, do their job and are exhaled.

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3
Q

How do volatile anaesthetics act?

A

A ‘Unifying’ explanation based on a non-specific effect such as expansion of lipid bilayer and disruption of receptor and ion channel function

BUT not all lipophilic volatile agents produce anaesthetisa

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4
Q

What is the increasingly likely theory of how volatile anaesthetic agents work?

A

Increasingly likely it is through GABA modulation in the brain and glycine modulation in the spinal cord

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5
Q

What is MAC?

A

1 Mac =minimum alveolar concentration (%) producing immobility on standard surgical stimulus in 50% of patients.

I.e more potent agents have lower MAC

MAC is a means of describing dose and potency referenced to a standard clinical effect

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6
Q

Describe the dose response curve for MAC 6% desoflurane and what happens when other drugs are given?

A

The dose response curve is steep for desoflurane. i.e 6% for 50% probs of no movement. 8% des = 95% no chance.

If fentanyl is given with desoflurane it left shifts the curve.

BUT we dont give desoflurane to stop movement. we give to put to sleep

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7
Q

What increases MAC?

A
  • Young age
  • Hyperthermia
  • Hyperthyroid
  • Drugs i.e meth
  • Heavy alcohol
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8
Q

What decreases MAC?

A
  • Old age
  • Hypothermia
  • Hypothyroid
  • Drugs i.e opioids, depressants
  • Pregnancy
  • Low oxygen, high CO2
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9
Q

How is dosing titrated?

A

Can supply Fi at 6% but this is all thats inhaled so need to work out a % what means 6% will be the alveolar concentration (Desoflurane)

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10
Q

What can be controlled to achieve F(a) desired?

A

Fi and minute volume i.e RR. Titrated against end tidal gas.

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11
Q

Whats the difficulty with achieving a desired F(a)?

A

Constantly takes drugs away there need to increase RR and Fi % value to achieve F(a)

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12
Q

What are the pharmacodynamics of inhaled volatile agents on the CNS?

A
  • Hypnosis, immobility, amnesia
  • Decrease CMRO2
  • Dose dependent increase CBF and ICP (Care with use in neurosurgery)
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13
Q

What are the pharmacodynamics of inhaled volatile agents on the CVS?

A
  • Peripheral vasodilation, lower BP
  • HR unchanged
  • Modern agents do not affect SV greatly
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14
Q

What are the pharmacodynamics of the inhaled volatile agents on the RS?

A

Respiratory depressant

  • Impair ventilatory response to hypoxia
  • Impair ventilatory response to CO2

(I.e intubation needed to manage RR)

Bronchodilation

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15
Q

What are modern inhalation anaesthetics?

A

Methyl ethyl ethers

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16
Q

What is the benefit of methyl ethyl ethers?

A

Increasing fluoride substitution = Decrease solubility, increase stability, decrease flammability, decrease dehalogination

17
Q

Summarise the modern anaesthetic agents:

A

Isoflurane - Relative cardiovascular stability

Sevoflurane - Good for gas inductions (can react with CO2 absorber, nephrotoxic byproduct) NOT PUNGENT

Desflurane - Rapid onset and offset. Good for longcases. Greenhouse gas though PUNGENT

18
Q

What are the types of IV anaesthetic agents?

A
  • Barbiturates (Thiopentone)
  • Phenols (Propofol)
  • Imidazoles (Etomidate)
  • Phencyclidine derivatives (Ketamine)
  • Benzodiazepines (Midazolam)
19
Q

Describe the mechanism of action for IV anaesthetics:

A

Thiopentone, Propofol, Etomidate, Midazolam all work by enhancing GABA, prolong Cl- current hyperpolarisation

Ketamine: Bind to PCP receptor antagonise glutamate supress excitation
- Also has analgesic effects

20
Q

What is the pharmacokinetics of IV anaesthetics:

A
  • Highly lipid soluble and cross BBB
  • Drug from IV bolus taken up by VRG organs, then leave these organs as less perfused tissues take up drug and concentration in blood falls
  • Offset after single IV dose is therefore due primarily to redistribution (NOT METABOLISM)
  • > Pt will wake even though total drug in body not changed much
  • Actual metabolism is slower
21
Q

Write some notes on Thiopentone:

A
  • Very rapid onset (10s)
  • Rapid offset by redistribution
  • Slow clearance (will accumulate in multi-dosing or infusing)
  • Metabolised in liver, induces liver enzymes
  • Some decrease in PVR and BP (Enhanced in shock)
  • Resp depression and loss of airway reflexes
22
Q

Write some notes on propofol:

A
  • mod rapid onset (20sec)
  • Rapid offset by redistribution
  • Fast clearance (10x that of thio)
  • > Cleaner offset (No hangover)
  • > Minor accumulatioon in infusion
  • > Can be used as infusion for maintenance
  • Metabolised in liver
  • Significant decrease in PVR and BP (Enhanced in shock)
  • Resp. depression and loss of airway reflexes
23
Q

Compare and contrast propofol and thiopentone:

A
  • Propofol has replaced thiopentane as standard IV anaesthetic
  • > Emphasis on high turnover/day stay (propro wears off faster, less hangover)
  • > Can be used in infusion for maintenance (Good for neurosurgery; Dec. CBF and ICP
  • > Less enzyme induction

So why bother with anything else? - Both of these agents can cause CV instability

24
Q

Write some notes on etomidate:

A
  • Remarkable CV stability
  • Less resp. depression
  • Rapid clearance and good recovery profile

BUT

  • Adrenocortical inhibition
  • Myoclonus and epileptogenic
25
Q

Write some notes on ketamine:

A
  • Analgesic
  • CV stimulant (Good in shocked patient)
  • Preserves airway reflexes and resp. drive
  • Increases CMRO2, CBF and ICP (Not good for neurosurgery)
  • Dissociative state, emergence slower and complicated by dysphoria
26
Q

Write some notes on total IV anaesthesia:

A
  • Avoids inhalation route
  • Avoids complications of vapour
  • > Malignant hyperthermia (Rare)
  • > Post-op nausea and vomiting
  • > Intracranial hypertension

BUT expensive and no agent monitoring

MBCHB3: Nervous system module. (39 decks)

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