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Anaesthesiology > Inhalation Agents > Flashcards

Inhalation Agents Flashcards

1
Q

Which inhalation agents can be used for induction (2)

A

Halothane and sevoflurane (preferred)
Because not irritating to airways, smell pleasant

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2
Q

Why would inhalation induction be used? (4)

A

• Children without iv access
• difficult iv access
• fear of needles
• difficult airway (spontaneous breathing maintained)

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3
Q

Classify the 2 groups of inhalation agents and their examples (11)

A

• Gases: nitrous oxide (technically a vapour below room temperature), xenon
• vapours:
-halogenated hydrocarbons (halothane),
-ethers ( diethylether, isoflurane, sevoflurane, desflurane)

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4
Q

What determines brain concentration of inhalation agents?

A

Partial pressure of agent in alveolus

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5
Q

What influences the rate of delivery of inhalation agents to the alveoli? (3)

A
  1. Alveolar ventilation (increased = faster onset)
  2. Functional residual capacity (smaller = faster)
  3. Inspiratory concentration of gas (higher-faster)
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6
Q

What influences the rate of removal of inhalation agents from the alveoli? (4)

A
  1. Partial pressure difference (alveolar pressure > blood = faster)
  2. Solubility of agent ( low BGPC -faster )
  3. Cardiac output
  4. Metabolism of agent
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7
Q

What is the BGPC halothane

A

2,43
Highest- most soluble, slower action, slower recovery

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8
Q

What is the BGPC isoflurane

A

1,4
Second most soluble and second slowest

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9
Q

What is the BGPC sevoflurane

A

0,64.

Middle

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10
Q

What is the BGPC N20

A

0,47
Second least soluble and second fastest

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11
Q

What is the BGPC desflurane

A

0,42 - least soluble and fastest onset

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12
Q

Define MAC

A

The minimum alveolar concentration necessary or required to prevent movement on surgical incision of the skin in 50% of patients

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13
Q

In lay terms, what is MAC?

A

Potency (fat solubility)
Lower = better

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14
Q

What is the MAC of halothane

A

0,77 (most potent )

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15
Q

What is the MAC of isoflurane

A

1,15
Second most potent

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16
Q

What is the MAC of sevoflurane

A

2,05
Middle

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17
Q

What is the MAC of desflurane

A

6.
Second least potent

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18
Q

What is the MAC of N20

A

105 (least potent)

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19
Q

Which patients may need higher MAC (7)?

A

HD CHIP

• Alcoholism chronic
• hyperthyroid
• hyperthermia
• infants and puberty
• Drug dependency
• narcotic misuse
• increased cardiac output

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20
Q

Which patients may need lower MAC (10)?

A

How’s His ALPHA HEAD

• Hypothermia
•Hypothyroid
• elderly
• pregnancy
• hypoxemia
• anaemia
• hypotension, low cardiac output
• drugs: analgesics, sedatives, opioids, antihypertensives, barbiturates
• acutely intoxicated with alcohol
• low cardiac output
Acidosis

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21
Q

Name the CNS effects of inhalation agents (4)

A

• Decrease CMRO2
• cerebral vasodilation! - increased cerebral blood flow (unlike iv) - limited use in increased ICP (usually only if go over 1 mac with the esters. But halothane very bad vasodilation, contraindicated tbi)
• decrease set point of thermoregulation (post-op shivering)
• stimulate chemo-emetic trigger zone - nausea and vomiting!

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22
Q

Name the CVS effects of inhalation agents with specific example (4)

A

• mostly halothane = Direct cardiovascular depressant (CCB): decrease inotropy, chronotropy, dromotropy, lusitropy, afterload, preload. Slows conduction, sensitise to catecholamines (don’t use halothane with adrenaline!), directly decrease contractility. This leads to hypotension and dysrhythmias
• hypotension ethers due to decrease SVR and vasodilation

• Coronary vasodilation
• dysrhythmogenic - bradycardia, SVT ,Re-entry phenomena esp halothane, not newer agents like isoflurane..
. Sensitise myocardium for catecholamines (especially halothane)
• isoflurane: mild tachycardia,

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23
Q

Name the respiratory effects of inhalation agents (4)

A

• Upper air way irritation (stinky) except halothane and sevoflurane; (therefore can not use ISO or des for induction)
• bronchodilation
• decrease ciliary function
• hypo-ventilation (depress muscle function) (not apnea)
• decreased response to hypercarbia and hypoxia

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24
Q

What colour is halothane?

A

Red

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25
Q

What colour is isoflurane

A

Purple

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26
Q

What colour is sevoflurane

A

Yellow

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27
Q

What is different and notable of sevoflurane? (2)

A

• Emergence delirium

• breakdown in desicated sodalime into compound A and changed by B lyase enzyme into nephrotoxic substance - not used if decreased renal function

Cardiotoxic - decrease SVR and contractility, thus decreasing after load and CO.

28
Q

What colour is desflurane?

A

Blue

29
Q

What is different and notable of desflurane? (7)

A

Airway irritation- careful in asthma /smokers
Very rapid recovery - ideal for obese and OSA.
Useful in liver/renal failure (less than 1%. metabolised)
High vapour pressure so need electronic vaporizer
Lowest bgpc
Produce Carbon monoxide in desiccated sodalime so risk co poisoning after weekend for eg
Sympathetic stimulation on increase of concentration- tachycardia few minutes

30
Q

Why is n20 sometimes used in combination with other volatile agents? (2)

A

• Increases potency (it has lowest)
• faster onset of action (second fastest- bgpc 0,47 ) - second gas effect (need to give additional O2 for 20 min after N2O stopped bc cause diffusion hypoxia)

31
Q

Name 7 side effects of nitrous oxide

A
  1. Nausea and vomiting!
  2. Bone marrow depression (prolonged exposure)
  3. Diffuses into gas filled spaces and increases the volume eg lung bullae, pneumothorax, distended bowel loops, gas used in eye or ear surgery, et tube cuff inflation (pressure necrosis)…
  4. cardiac suppression. If underlying CV disease
  5. Teratogenic first trimester!- block methionine synthase enzyme.
    6 Diffusion hypoxia (give oxygen for 20 min after n20 stopped)
  6. Increase pulmonary vascular resistance: pulmonary vasoconstriction, rv failure with PHT
32
Q

Which opioid has the highest potency?

A

Sufentanil (then remifentanil )

33
Q

Which opioid has the lowest potency?

A

Morphine (then alfentanil)

34
Q

What colour is enflurane vaporizer?

A

Orange

35
Q

Name 4 factors that influence the rate of uptake in blood of inhalation agents.

A

• Solubility in blood (BGPC )
• cardiac output (tachycardia slow rate of rise of alveolar concentration)
• concentration administered (fi)
• alveolar ventilation (FA) (hyperventilation slow rate of rise of alveolar conc)

36
Q

How does critical temperature influence phase change?

A

• A substance in gaseous phase below critical temperature of that substance can be forced into liquid phase by increasing pressure
. But if above critical temperature, no amount of pressure increase will force a phase change and it will remain a gas.

37
Q

Define vapour

A

Gaseous phase below critical temperature

38
Q

Define gas

A

Gaseous phase above critical temperature

39
Q

How is quantity of a gas measured?

A

By pressure gauge

40
Q

How is quantity of a vapour measured?

A

By weight

41
Q

Define vapour pressure

A

Partial pressure of gas particles that have escaped from a liquid by evaporation in a closed rigid container

42
Q

What is boiling point?

A

When vapour pressure = barometric pressure

43
Q

Barometric pressure in Gauteng?

A

650 mm Hg

44
Q

Why does desflurane need special electronic vaporiser to be administered

A

Vapour pressure desflurane= 681 mm Hg
Barometric pressure Gauteng =650
Therefore boils at room temperature

45
Q

What is bgpc?

A

Blood gas partition coefficient
Solubility of vapour in blood
Higher = more soluble, slower action, slower recovery
Low BGPC is better (faster)

46
Q

Moa inhalation agents?

A

All GABA agonists
Except n20: NMDA antagonist. Very good analgesic.

47
Q

Where is halothane metabolised and what is complication of this?

A

Liver 20%
Halothane hepatitis: immunologic reaction, toxic reductive metabolites, ischaemia
High mortality

48
Q

Name 6 predisposing factors to halothane hepatitis

A

• Female
• middle age
• obesity
• multiple administrations
• enzyme induction drug use
• decreased glutathione (fasting)

49
Q

Which agent has highest bgpc?

A

Halothane (2,3)

50
Q

Which inhalation agent is best for neuro anaesthesia and why (4)

A

Isoflurane
• maintain auto-regulation up to 1 MAC
• non-epileptogenic
• decrease CSF production and increase absorption therefore least increase in cerebral blood flow and ICP of all inhalation agents

Also sevoflurane

51
Q

Inhalation drug of choice liver disease patients? (2)

A

Desflurane best then

Isoflurane ( added benefit of increasing arterial blood flow to liver)
Only 0.2% metabolised by liver and maintains hepatic arterial blood supply

52
Q

Which agent has lowest bgpc?

A

Desflurane
Fastest onset and offset
0,45

53
Q

Inhalation drug of choice for short cases?

A

Desflurane
Lowest bgpc

54
Q

How calculate MAC pt?

A

End tidal volume of agent ÷ MAC of agent

55
Q

Which inhalational agent contraindicated in epilepsy?

A

Enflurane

56
Q

Which inhalational agent can cause coronary steal?

A

Halothane

57
Q

Name 4 factors that increase rate of induction of inhalation agents

A

• High inspiratory concentration
. Low blood gas solubility coefficient bGpC
• increased ventilation
• low cardiac output

58
Q

Define Henry’s law

A

Concentration of gas directly proportional to partial pressure of the gas

59
Q

Define Graham’s law

A

Rate of diffusion of gas inversely proportional to molecular weight of the gas

60
Q

Define Ostwald’s solubility coefficient

A

Uptake of gas is proportional to the solubility of the gas

61
Q

Name 2 major disadvantages of halothane

A

• Patients take very long to wake up (slow onset , offset and washout because highest bgpc therefore highest solubility)
• high incidence dysrhythmias (ventricular extrasystoles)

62
Q

Name 3 advantages nitrous oxide

A

• Great analgesic
• good to use with induction due to second gas effect
• good sedative

63
Q

Describe the second gas effect of nitrous oxide

A

• Low bgpc 0,47 therefore less soluble
• diffuses rapidly into blood
• reduced alveolar volume
• increased concentration of volatile agent

64
Q

Name 4 anatomical differences in the neurological system in paeds

A

• Poorly developed blood brain barrier (increased sensitivity to drugs that cross bbb)
• preterm- cerebral vessels thin walled and fragile
. Spinal cord: end l2-L3 premature, l3 newborns
• lack lumbar lordosis

65
Q

Name 6 physiological differences in the neurological system in paeds

A

• Immature myelinisation of nerves
• higher blood supply to spinal cord
• total CSF volume higher
• developing brain
• separation anxiety
• communication
• prone to intraventricular haemorrhages

Anaesthesiology (24 decks)

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