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Anaesthesia > Pharmacology of Volatile & Gaseous Agents > Flashcards

Pharmacology of Volatile & Gaseous Agents Flashcards

1
Q

Inhalant anaesthetics are mainly used for…

A

maintenance of anaesthesia

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2
Q

Advantages of inhalant anaesthetics?

A
  • Safe?
  • Some situations IV anaesthesia is safer
  • Can adjust the amount to the P quickly
  • Rapid action, elimination and recovery
  • Easy to prolong anaesthesia
  • Minimal metabolism (newer agents)
  • Reduced likelihood of arrhythmias
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3
Q

Disadvantages of inhalant anaesthetics?

A
  • Expensive: Drug itself! The anaesthetic machine, O2 source, tubings/connections all also req’d
  • Pollution → staff and environmental
  • Cardiovascular and respiratory depression
  • No analgesia – this must be provided
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4
Q

Molecular mechanisms of action

A
  • classical lock & key
  • efficacy of inhalant anaesthetic demonstrated humans, vertebrates, invertebrates, etc
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5
Q

Meyer-Overton Rule

A
  • Site of Action: Lipid cell membane
  • some correlation w/ potency & lipophilicty of drug
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6
Q

Inhalants depress the excitatory neurotransmission & enhance

A

inhbitory neurotransmission

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7
Q

Sites of Action of inhalant anaesthetics?

A
  • spinal cord: muscle relaxation
  • CNS: hypnosis
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8
Q

Volatile liquids

A

Halothane, isoflurane, sevoflurane, desflurane, enflurane, methoxyflurane

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9
Q

Compressed gases

A

Nitrous oxide
xenon

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10
Q

Saturated vapor pressure

A
  • pressure the vapor molecules exert when liquid & vapor phases are in equilibrium
  • measures volatility of the agent
  • temperature dependent
  • vaporisers are coordinated to specific agents, so one agent cannot go into the vaporiser for another agent
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11
Q

Solubility of inhalant anaesthetics

A
  • solubility in liquieds & solids is different
  • at some point, they are in equilibrium so they have the same partial pressures between all phases
  • consists of partition coefficients, blood/gas coefficient, oil/gas partition coefficient
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12
Q

What is the blood/gas partition coefficient?

A
  • lower solubility indicates speed of induction, recovery, & change of anaesthetic depth
  • if low = gases will esert high partial pressure so changes will be fast –> blood quickly saturated & moves to brain
  • if high = blood acts as sink so changes are slower
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13
Q

Oil/gas partition coefficient

A
  • describes the ratio of a concentration of anaesthetic in oil & gas phases at equilibrium
  • correlates w/ the potency of an anaesthetic drug
  • High = low inhalant conc to maintain anaesthesia
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14
Q

MAC

A

“Minimum alveolar concentration of an anaesthetic at 1 atmosphere that produces immobility in 50% of subjects exposed to a supramaximal noxious stimulus”

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15
Q

Factors affecting uptake of volatile & gaseous agents

A
  • you want to increase alveolar pressure –> equilibrium w/ CNS
  • increased alveolar delivery: increase in Fi of anaesthetic, increase in alveolar ventilation, decreased removal from the alveoli
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16
Q

Factors affecting speed of induction

A
  • to increase speed of onset you must increase PA of the agent, more to alveoli, more to brain
  • Increased alveolar delivery (input) of agent –> increased inspired anaesthetic concentration, increased alveolar ventilation
  • decreased removal from alveoli
17
Q

How do you increase alveolar delivery of an agent

A
  • increase vaporizer setting
  • increase FGF
  • decrease gas vol of breathing circuit on circle
  • acts in first phase
18
Q

How do you increase alveolar ventilation

A
  • increase minute ventilation
  • decrease fxnl residual conc
19
Q

How do you decrease removal from alveoli?

A
  • decreased blood:gas solubility
  • decreased CO
  • decreased tissue solubility
  • acts btw alveoli & blood
20
Q

What is the effect of CO on anaesthetic uptake?

A
  • greater the CO, greater the amount of blood passing through the lungs carrying away anaesthetic
  • takes longer for P to be deeper or achieve enough conc in the brain
21
Q

Elimination of anaesthetic

A

Recovery: reduced partial pressure in the CNS to allow return to consciousness
Metabolism-dependent

22
Q

How do you speed up recovery w/ circle breathing system?

A
  • increase FGF
  • increase Minute volume
  • empty reservoir bag
23
Q

How do you speed p recovery w/ non-rebreathing system?

A
  • switch vaporizer off b/c anaesthetic is not returning to the system
24
Q

Common characteristics of halogenates

A
  • cardiovascular depression
  • CO reduction
  • direct myocardial depression
  • peripheral vasodilation
  • impairment of tissues’ auto-regulation mechanims
  • CNS depression
  • respiratory depression: reduction of MV via reducing TV/RR
  • hepatic & renal blood flow
25
Q

Isoflurane

A
  • MAC: 1.2-1.7%
  • physico-chemical properties: fast onset & recovery, pungent odour
  • Metabolisation by liver, elimination by lungs
  • dose-dependent vasodilation, respiratory depression
  • CAUTION: CARBON MONOXIDE CAN FORM IF INTERACTS W/ DRY SODA LIME
26
Q

What are isoflurane effects on hepatic blood flow?

A
  • reduces flow on hepatic portal vein
  • increases flow thru hepatic artery
  • little cahnge in perfusion
  • may result in improved oxygenation
  • iso best for liver patients
27
Q

Sevoflurane

A
  • MAC: 2.3-2.6%
  • Physico-chemical properties: pleasant smell, non-irritant for mm, fast onset/recovery
  • Metabolization in the liver: 2-5%
  • Compound A forms if degradation by soda lime –> nephrotoxic
  • Preserves cerebral autoregulation compared to iso
28
Q

Why does sevoflurane come in plastic bottles?

A

Impurities in the glass –> hydrofluoric acid (toxic volatile acid) –> glass –> silicon fluoride (volatile, pungent, highly toxic)

29
Q

Nitrous oxide

A
  • USE W/ EXTREME CAUTION
  • gas at room temp
  • MAC 200% (not possible), must use w/ other meds
  • rapid onset
  • no metabolism
  • provides analgesia
  • non-irritant
  • need to give w/ O2 at the appropriate rate
  • augments ventilation
  • mild SNS stimulation
  • reisk of hypoxia, arrhythmias, & death
30
Q

What occurs when there is nitrous oxide that is diffused to air filled cavities?

A
  • Pneumothx
  • GDV
  • colic (horses)
  • ophthalmic procedures
  • air embolism
  • craniotome
  • laparoscopic procedures
31
Q

Environmental impact of gaseous agents

A
  • low compared to CO2, but significant contribution
  • volatile agents: potent greenhouse gases
  • N2O lesser short-term impact, but longer in atmosphere
  • contribution to global warming (des > sevo > iso) or switch to TIVA
32
Q

Health hazards of volatile gases

A
  • N2O: increase infertility/abortion risk
  • mutagenic, carcinogenic, teratogenic, foetal death, spontaneous abortion, birth defects, cancer if exposed

Anaesthesia (24 decks)

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