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PPNP 3 > LA & GA Drugs > Flashcards

LA & GA Drugs Flashcards

1
Q

MOA of LA

A

Stop axonal conduction by BLOCKING SODIUM CHANNELS in the axonal membrane when applied LOCALLY in APPROPRIATE
CONCENTRATION –> prevent sodium ion entry –> slow down or bring conduction to a halt

NON-SELECTIVE modifiers of neuronal function, ie. they will BLOCK ACTION POTENTIALS in all neurons to which they have access

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2
Q

Use-Dependency concept of LA (MOA)

A

Depth of LA nerve block increases with action potential frequency because LA
molecules:

  • gain access to the channel more readily when channel is open (LA works better when you are in more pain)
  • have higher affinity for the inactivated than for the resting (closed) channels
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3
Q

How to achieve selectivity for LAs?

A

By delivering LA to a limited area (topical application, injection into a limited area)

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4
Q

Factors affecting LA action

A
  1. More lipid soluble drugs are more potent and act longer
    - more hydrophobic: more potent, acts longer
  2. Act on all nerves
    Size: Smaller nerve > bigger nerve

Frequency of firing: high (sensory) > low (motor)

Position: circumferential > deep (large nerve trunk)
e.g. More superficial neurons (easier for LA to get to) >deeper neurons

Myelination: myelinated > nonmyelinated

  1. pH dependency
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5
Q

What kinds of axons do LAs have the greatest effect on?

A

Small myelinated axons

Small myelinated axons > small non-myelinated axons >
large myelinated axons

Note:
1. Size if axons most important factor
2. Myelination (2nd most impt factor)

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6
Q

Type A neuronal fibers

A

Function: Proprioception, motor

Size: Large
Least sensitive to block –> least sensitive to LA

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7
Q

Type C neuronal fibers

A

Anatomically located at dorsal root

Function: Pain
Size: very small
Most sensitive to block –> most sensitive to LA

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8
Q

Which fiber type will LA be more potent on? Type A fiber or Type C?

A

Type C Pain fibers

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9
Q

Factors affecting LA action - pH dependency

A

LA molecules are weak bases

Potency is strongly pH-dependent:
- Alkaline pH –> increase LA activity
- Acidic pH –> decrease LA activity

LA will not be very effective if tissues are already inflamed (acidic pH) –> best to receive LA before inflammation

Plays critical role in LA penetrate nerve sheath and axon membrane to reach the inner end of the sodium channel (LA binding site)

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10
Q

Main drug classes of LAs

A

Esters (-COO)
- Cocaine (medium duration)
- Tetracaine (long duration)
- Procaine

Amides (-CONH)
- Lidocaine (medium duration)
- Mepivacaine (medium duration)
- Bupivacaine (long duration)

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11
Q

Method of metabolism (how is it broken down?) of ester-type LAs

A

Plasma/tissue non-specific esterases

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12
Q

Method of metabolism (how is it broken down?) of amide-type LAs

A

Hepatic enzymes

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13
Q

What type of LA should a patient with chronic liver disease be prescribed with?

A

Ester-type LA

NOT AMIDE-TYPE (to prevent stressing hepatic enzymes when the liver is already diseased)

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14
Q

What determines the onset of LA

A

Anaesthetics that penetrate the axon most rapidly have the fastest onset

small size, high lipid solubility,
low ionization (@ tissue pH) –> faster onset

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15
Q

What leads to LA toxicity?

A
  1. Unintended large dose of LA if accidentally injected IV / intra-arterial –> systemic toxicity
  2. excessive (overdose) LA injected locally & subsequently leads to high (& toxic) blood level following absorption –> systemic toxicity (however onset of toxic S&S appear late)
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16
Q

Why is LA used in combination with epinephrine?

A

Epinephrine: reduces vessel diameter, causes vasocontriction

Prevent LA systemic distribution from site of action –> lowers rate of absorption into systemic circulation

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17
Q

Which organs will be most affected by LA toxicity?

A
  1. Brain (CNS)
  2. Heart (CVS)
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18
Q

How will the brain (CNS) be affected by LA toxicity?

A

sleepiness – visual and auditory – restlessness – nystagmus – shivering – convulsion – stoppage of vital functions – death

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19
Q

How will the heart (CVS) be affected by LA toxicity?

A

cardiac contraction – arteriolar dilation – hypotension – cardiovascular collapse

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20
Q

A patient has a heart disease. Which LA should NOT be prescribed to him?

A

Bupivacaine.

Bupivacaine is more cardiotoxic than most other LAs.

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21
Q

Which LA blocks NA (noadrenaline/ norepinephrine) reuptake? (causing vasoconstriction and hypertension)

A

cocaine

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22
Q

Which LA (ester/amide type) can be hydrolysed to PABA derivatives and what are the consequences?

A

Ester LAs

Ester LAs are hydrolysed to p-aminobenzoic acid (PABA) derivatives, which cause allergic reactions in a small percentage of the population (skin rash/anaphylactic shock)

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23
Q

A patient has a known allergy to PABA. Which type of LA should be prescribed?

A

Amide type LA

NO ESTER LA as ester LAs will be hydrolysed to PABA derivatives

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24
Q

Ester type vs Amide type LAs

A

Ester type:
- good for people with liver disease

Amide type:
- good for people with PABA allergies

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25
Q

Dangers of prilocaine

A

Prilocaine can be metabolised to O-toluidine which causes methaemoglobin (dysfunctional haemoglobin –> blood loses red colour, turns bluish)

How to treat methaemoglobin:
- iv methyleneblue/ascorbic acid
(which converts methaemoglobin to haemoglobin)

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26
Q

Clinical applications of LA – Topical (surface)

A
  1. Skin (minor burn/wound)
  2. Eye (remove foreign objects)
  3. Dental (applied on gum)
  4. Otorhinolaryngology (insertion of endoscope for GU scope)
  5. Gynaecology (episiotomy cuts, lidocaine)
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27
Q

Clinical applications of LA – Injected

A
  1. Epidural anaesthetics
    (lidocaine, bupivacaine)
  2. Dental anaesthesia
    Lidocaine (short term)
    Bupivacaine (long term, note cardiotoxicity!)
    combine with epinephrine –> for vasocontriction –> reduces rate of absorption into systemic circulation
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28
Q

What affects the choice of LA drug use?

A

Based on duration of action

Surface anaesthesia requires rapid penetration of the skin (mucosa) and limited tendency to diffuse away.

Note: Cocaine gives good penetration and vasoconstriction, thus most often used for ENT procedures, but clinical use in SG is very limited

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29
Q

What is GA used for?

A

To produce unconsciousness and a lack of responsiveness to all painful stimuli (inhibition of sensory and autonomic reflexes)

ie. triad of hypnosis, amnesia, analgesia

provide conditions for interventions - surgery, skeletal muscle relaxation

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30
Q

What should be the endpoint of GAs?

A

Keep patients safe and alive upon GA reversal

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31
Q

What is the additional consideration for GA?

A

Control of physiology

32
Q

What are the stages for GA?

A

Pre-assessment
Induction of anaesthesia
Airway management
Maintenance of anaesthesia
Reversal/Emergency
Post-operative Care

33
Q

What constitute an ideal GA? (What properties should an ideal anesthetic drug have?

A

Unconciousness
Analgesia
Muscle relaxation
Amnesia
Brief and pleasant
Depth of anaesthesia can be raised or lowered with ease
Minimal adverse effects
Margin of safety - large

NO SINGLE AGENT HAS ALL OF THESE PROPERTIES

34
Q

What are the 3 properties of balanced anaesthesia?

A

Pain relief
Unconsciousness
Inhibition of reflex

35
Q

What is the purpose of balanced anesthesia?

A

To ensure that induction is smooth and rapid, and that analgesia and muscle relaxation are adequate

36
Q

What are the drugs used in combination in GA?

A

Inhalation anaesthetics + IV anaesthetics

37
Q

What is the most commonly used drug for induction of anaesthesia?

A

Short-acting barbiturates

38
Q

What is the most commonly used drug for muscle relaxation?

A

Neuromuscular blocking agents

39
Q

What are the most commonly used drugs for analgesia?

A

Opioids and nitrous oxide

40
Q

Inhalant GAs - How does the blood solubility affect the onset?

A

The higher the blood solubility, the slower the onset

As blood solubility increases, the longer the anaesthetic stays in the blood instead of going to the brain.

41
Q

Does nitrous oxide have a faster or slower onset than halothane?

A

Nitrous oxide has a faster onset because its blood solubility is lower.

42
Q

Classify the following inhalation aneasthetics into volatile liquids and gases:

Halothane
Enflurane
Desflurane
Isoflurane
Sevoflurane
Nitrous oxide

A

Volatile liquids: -ane
Halothane
Enflurane
Desflurane
Isoflurane
Sevoflurane
These are adminstered using an agent specific-vaporizer

Gas:
Nitrous oxide

43
Q

What is the proposed MOA of inhalation anesthetics?

A
  1. Enhance neurotransmission at inhibitory synapses via allosterically increasing GABA receptor sensitivity to action by GABA itself (positive allosteric modulator)
  2. Depressing neurotransmission at excitatory synapses via blocking glutamate neurotransmitter acting on NMDA receptor thus preventing NMDA receptor activation (negative allosteric modulator)

oh god idk what this all means

44
Q

What is minimum alveolar concentration (MAC)?

A

An index of inhalation anasthetic potency (low MAC = high anasthetic potency)

Defined as the minimum concentration of drug in the alveolar air that will produce immobility n 50% of patients exposed to a painful stimulus.

MAC values alter with age, condition, concomitant administration of other drugs etc.

45
Q

What are the MACs of nitrous oxide, isoflurane, sevoflurane, & desflurane?

A

Nitrous oxide: 105% (not even 100% of NO will make 50% of patients immobilised)
Isoflurane: 1.2%
Sevoflurane: 2.2%
Desflurane: 6.3%

46
Q

What the factors that affect the absorption of inhalation GAs into the blood?

A
  • Concentration of anasthetic in inspired air
  • Solubility of GA
  • Blood flow through lungs

Increase in any of these factors -> increase rate of GA uptake into blood

47
Q

What affects the distribution of GAs?

A

Determined by regional blood flow - which tissues receive GA

Anaesthetic levels in highly perfused organs (brain, liver, lungs, heart) equilibrate with those in blood quickly after administration

48
Q

How are GAs eliminated?

A
  1. Expired breath
    - inhalation anaesthetics are eliminated almost entirely via the lungs
    - minimal hepatic metabolism
    - factors that determine uptake also determine elimination
  2. Metabolism
    (some metabolites can be toxic)
49
Q

What are the important properties of halothane?

A
  • Volatile liquid, non-flammable and non-irritating
  • LITTLE OR NO ANALGESIA until unconsciousness supervenes
  • Decreases BP due to depression of cardiac output, bradycardia & arrythmia may also occur leading to hypotension and dysrythmia
  • May lead to halothane-associated hepatitis
50
Q

What are the important properties of isoflurane?

A
  • POTENT (MAC 1.4%)
  • Medium rate of onset and recovery
  • Decreases BP due mainly to decrease in systemic vascular resistance
51
Q

What are the important properties of sevoflurane?

A
  • Metabolized in the liver to release inorganic fluoride (nephrotoxic)
52
Q

Which metabolites of which inhalant GAs are toxic?

A

Inorganic fluorides of isoflurane and enflurane are nephrotoxic; halothane is hepatotoxic

53
Q

What are the important properties of nitrous oxide?

A
  • Non-flammable
  • Rapid onset but lacks potency (MAC 105%)
  • Nitrous oxide alone gives analgesia and amnesia but not complete unconsciousness or surgical anaesthesia
  • No effect on BP and respiration
  • When used alone - analgesic agent (dentistry, during delivery)
54
Q

What is the main concern with nitrous oxide?

A

Postoperative nausea & vomiting

55
Q

Which inhalant GAs are compatible with epinephrine?

A

Isoflurane, Enflurane, Sevoflurane, Nitrous oxide

NOT Halothane

56
Q

What are the common IV GAs and what are their dosages?

A

Thiopentone 4-7 mg/kg
Etomidate 0.2-0.3 mg/kg
Propofol 2-4 mg/kg
Ketamine 1.5mg/kg
Midazolam 0.02 mg/kg

57
Q

What are the properties of IV GAs?

A
  • Induction agent (induces unconsciousness)
  • Does not keep patient asleep for very long
  • May be used alone or to supplement the effects of inhalation GAs
  • Depress respiration - need to take over ventilation of patients
58
Q

What the 2 advantages of using inhalation + IV GAs?

IMPORTANT

A
  1. Permit dosage of the inhalation agent to be reduced
  2. Produce effects that cannot be achieved with inhalation alone
59
Q

What are the important properties of thiopentone (sodium thiopental)?

A
  • Enters the brain easily and rapidly - rapid onset of action (unconsciousness occurs 10-20s after IV)
  • Ultra-short duration of action -> If used alone, patient will wake up in ~10 min
  • Multiple doses/infusions: duration of action depends on CLEARANCE
  • Extensively bound to plasma protein - small amount of free drug can be excreted by glomerular filtration + reabsorption in tubules
60
Q

What is the MOA of thiopentone?

A

Causes CNS depression by potentiating the action of the neurotransmitter GABA on the GABAa (alpha) receptor-gated chloride ion channels

61
Q

What are the important properties of propofol?

A
  • Induction rate is similar to thiopentone, recovery is more rapid (patients move sooner and feel better)
  • Rapid onset (unconsciousness develops within 60s)
  • Extensively used in “day surgery” - needs continuous, low-dose infusion for extended effects
  • SIGNIFICANT CVS EFFECT during induction (decrease BP and negative inotropic) -> hypotension -> to be used with caution in elderly pts, pts with compromised cardiac function, hypovolemic pts
62
Q

What are the important properties of ketamine?

A
  • Produces a state known as dissociative anaesthesia (pt feels dissociated from environment)
  • Rapid induction
  • Large Vd rapid clearance -> suitable for continuous infusion without the lengthening in duration of action
  • Psychologic adverse reactions (hallucination, disturbing dreams, delirium) during recovery -> risks may be reduced with premedication of diazepam or midazolam
63
Q

What types of drugs can augment GAs?

A

Sedation
Amnesia
Analgesia

64
Q

Why do we augment GAs?

A

Lower GA doses used -> reduce GA side effects

65
Q

What are the classes of drugs that are used as anaesthetic adjuncts?

A

Benzodiazepines (anxiolytics, amnesia, sedation prior to induction of anaesthesia)

a2 (alpha-2) Adrenergic Agonists (sedation prior to and/or during procedures in non-intubated pts)

Analgesics

Neuromuscular Blocking Agents (induction of anaesthesia to relax muscles (jaw, neck, airway) to facilitate laryngoscopy and endotracheal intubation)

66
Q

What are the important properties of IV Midazolam (benzodiazepine)?

A
  • Used for sedation during procedures not requiring GA
  • Metabolized in liver (elderly tend to be more sensitive, slower recovery)
  • Side-effects are compounded by concurrent usage of other agents
67
Q

What are the important properties of α2 (alpha-2) adrenergics - IV dexmedetomidine?

A
  • Short term sedation (<24hrs)
  • Little respiratory depression
68
Q

What are the important properties of analgesics (NSAIDs)?

A
  • Opioids (fentanyl, morphine) - perioperative period
  • Relative potency to morphine [duration of action]
    Sufentanil (1000x) (intermediate ~15min)
    Remidentanil (300x) (ultra short ~10min)
    Fentanyl (80x) (intermediate ~30min)
    Alfentanil (15x) (intermediate ~20min)
  • Metabolized in liver (except remidentanil is hydrolyzed by tissue & plasma esterases)
69
Q

What are the important properties of neuromuscular blockers?

A
  • Non-depolarizing (eg. vecuronium)
  • Aids many surgical procedures and provide additional insurance of immobility
70
Q

(Key Points) GA produces _________

A

unconsciousness and insensivity to painful stimuli

71
Q

(Key Points) What is balance anaesthesia?

A

3 properties:
Pain relief
Unconsciousness
Inhibition of reflex

To ensure that induction is smooth and rapid, and that analgesia and muscle relaxation are adequate

72
Q

(Key Points) How is MAC related to potency?

A

The lower the MAC, the higher the potency

73
Q

(Key Points) How are inhalation GAs eliminated?

A

Mostly through expired air

74
Q

(Key Points) What are the principal adverse effects of GA?

A

Depression of respiratory and cardiac performance

75
Q

(Key Points) How does nitrous oxide differ from other inhalation GAs?

A
  1. Very high MAC (cannot be used alone to produce GA
  2. High analgesic potency -> frequently combines with other inhalation GAs to supplement their analgesic effects
76
Q

(Key Points) How is induction of anaesthesia accomplished?

A

With a short-acting barbiturate (thiopentone)

77
Q

(Key Points) IV Ketamine causes what type of state?

A

Dissociative anaesthesia

PPNP 3 (19 decks)

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