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Anaesthesia > Inhalational anaesthetics > Flashcards

Inhalational anaesthetics Flashcards

1
Q

What is minimum alveolar concentration (MAC)?

A

alveolar concentration of inhaled anaesthetic agent, when administered in 100% oxygen at 1 atmospheric pressure to unpremedicated patients, will result in lack of movement to surgical stimulus in 50% of the patients

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2
Q

what are the patient factors that increases MAC requirements?

A
  • Age: young children
  • Hypermetabolic state e.g. sepsis, thyrotoxicosis
  • Hyperthermia
  • Hypernatremia
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3
Q

What are the patient factors that decreases MAC requirements?

A
  • Age: preterm infants and neonates, elderly
  • Hypothyroidism
  • Hypothermia
  • Hyponatremia
  • Pregnancy
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4
Q

What are the pharmacological factors that increase MAC requirements?

A
  • Chronic alcohol ingestion
  • Chronic opioid use
  • Increased catecholamines (e.g. cocaine use, ephedrine)
  • Tricyclic antidepressants (TCA)
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5
Q

What are the pharmacological factors that decrease MAC requirements?

A
  • Use of other depressant agents such as opioids, local anaesthetics, clonidine
  • Acute alcohol intoxication
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6
Q

What are the factors that affect the rate of onset on inhalational anaesthetics (lungs)?

A
  1. inspired partial pressure
  2. minute ventilation
  3. type of breathing circuit
  4. rate of gas flow
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7
Q

What are the factors that affect the rate of onset on inhalational anaesthetics (arterial)?

A
  1. solubility of anaesthetic agent- blood: gas partition coefficient
  2. cardiac output
  3. alveolar venous partial pressure difference
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8
Q

What are the factors that affect the rate of onset on inhalational anaesthetics (brain)?

A
  1. solubility of anaesthetic agent- brain: blood partition coefficient
  2. cerebral blood flow
  3. arterial venous partial pressure difference
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9
Q

what is the partition coefficient?

A

distribution ratio describing how inhaled agent distributes itself between 2 phases at equilibrium

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10
Q

does a high blood: gas partition coefficient lead to a faster/ slower induction time?

A

SLOWER!!

  • HIGH blood:gas partition coefficient = highly soluble gas –> higher uptake of gas into blood component –> longer time to equilibration with the Pbrain –> slower induction time
  • LOW blood:gas partition coefficient = insoluble gas –> minimal amount of inhaled agent dissolved
    before equilibrium achieved –> faster induction
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11
Q

what are the commonly used inhalational anaesthetics?

A
  1. Sevoflurane
  2. Desflurane
  3. Isoflurane
  4. Nitrous oxide
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12
Q

What are the CNS clinical effects on inhaled anaesthetic agents?

A
  • Provide state of hypnosis under general anaesthesia
  • Commonly used in maintenance phase
  • Can be used for induction as well (inhalational induction), but only with specific agents (nitrous oxide, sevoflurane) due to problems of smell, airway irritation, and rate of onset
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13
Q

What are the CVS clinical effects on inhaled anaesthetic agents?

A
  • Cause vasodilation and myocardial depression

- Hence leading to hypotension

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14
Q

What are the respiratory clinical effects on inhaled anaesthetic agents?

A
  • Bronchodilatation

- Respiratory depression

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15
Q

What are other clinical effects on inhaled anaesthetic agents?

A
  • Tocolytic
  • Can induce post-operative nausea and vomiting
  • All (except nitrous oxide) have potential to precipitate malignant hyperthermia
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16
Q

Which inhalational agent is the cheapest?

A

isoflurane (cheaper than sevoflurane/ desflurane)

17
Q

Rank the agents (isoflurane, sevoflurane, desflurane, nitrous oxide) in increasing order of MAC

A

isoflurane (1.1%) < sevoflurane (2%), desflurane (6%), Nitrous oxide (104%)

18
Q

Rank the agents (isoflurane, sevoflurane, desflurane, nitrous oxide) in increasing order of solubility (blood gas partition coefficient).

A

Nitrous oxide < desoflurane < sevoflurane < isoflurane

19
Q

Which anaesthethic reacts w strong bases in soda lime to form ‘Compound A’ – an ether metabolite w/ nephrotoxicity shown in rats (but concs produced are lower in humans)?

A

sevoflurane

20
Q

how does isoflurane smell?

A

Pungent, irritating to airwa

21
Q

how does sevoflurane smell?

A

Sweet smelling, less irritating to airway 🡪 suitable for induction

22
Q

how does desflurane smell?

A

Pungent, irritating to airway

23
Q

How does nitrous oxide smell?

A

Odourless, colourless 🡪 suitable for inductio

24
Q

Why is nitrous oxide not used alone? What scenarios is it commonly used in?

A

Low potency (MAC 104%), cannot be used as sole anaesthetic agent 🡪 often combined with a more potent inhalational anaesthetic, to achieve second gas effect (presence of N2O increases the speed at which the alveolar concentration of the other inhalational anaesthetics increases, which leads to a faster onset of action)

Analgesic property: can be given w/ O2 in 50:50 ratio as “Entonox”, commonly in labour analgesia (unlike other inhalational agents)

25
Q

What are the side effects of desflurane?

A

Sympathetic stimulation w/ sudden increases in conc delivered (HTN, tachycardia)

26
Q

What are the side effects of nitrous oxide?

A

Diffusion hypoxia upon recovery

Can diffuse into gas-filled body compartments and cause expansion of gas present there 🡪 potential damage 🡪 avoid in middle ear, pneumothorax, intestinal obstruction (IO)

High incidence of PONV 🡪 give prophylactic anti-emetic, or use IV agents

BM suppression w/ prolonged exposure –> N2O oxidises the cobalt atom in vitamin B12

27
Q

What kind of procedures of sevoflurane preferred over desflurane?

A

Sevoflurane is longer acting than desflurane, therefore sevoflurane is preferred in:

1) Longer procedures/surgeries

2) Procedures that do not require ETT (i.e. using LMA instead)
- ETT must be removed in OT upon reversal of GA, before sending the patient to PACU – this requires the anaesthetic used to have a short hangover time (not a property of sevoflurane; desflurane is preferred)
- In contrast, LMAs can be removed safely in PACU – so the airway and breathing can remain secured over the extended hangover time of sevoflurane

28
Q

What is malignant hyperthemia and how does it present?

A

Malignant hyperthermia (MH) manifests clinically as a hypermetabolic crisis when an MH-susceptible (MHS) individual is exposed to a volatile anaesthetic (e.g. isoflurane, sevoflurane, desflurane), or succinylcholine.

Hypercarbia: The most consistent presenting manifestation of MH is hypercarbia, signalled by an increase in end-tidal carbon dioxide (ETCO2), that is resistant to increases in minute ventilation, and that is not caused by hypoventilation or CO2 absorption (e.g. during laparoscopy)

Generalised muscle rigidity: Generalized muscle rigidity (i.e. sustained contracture) in the presence of neuromuscular blockade is considered pathognomonic for MH, if other signs of hypermetabolism are also present

Arrhythmias: Patients with MH frequently develop sinus tachycardia. They can also exhibit peaked T waves and arrhythmias (i.e. ventricular ectopy, tachycardia or fibrillation) due to sudden acute hyperkalaemia)

Hyperthermia: Although it may occur at any point in the clinical course of MH, hyperthermia is often a later sign of MH and is often absent when the diagnosis is initially suspected. Accurate temperature monitoring may allow earlier diagnosis of MH and earlier treatment

Myoglobinuria: Brownish-, cola-, or tea-coloured urine indicates the presence of myoglobinuria, which is due to rhabdomyolysis, and peaks about 14 hours after an acute MH episode

Anaesthesia (17 decks)

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