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Lectures > IV anesthetic agents > Flashcards

IV anesthetic agents Flashcards

1
Q

Anesthetic options

A

Local anesthesia with or without sedation

Regional anesthesia with or without sedation

  • Spinal
  • Epidural
  • Nerve blocks

General Anesthesia
- May be combined with any of the above

Plan for anesthesia is based on

  • The age and medical condition of the patient
  • Type and duration of surgery
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2
Q

Balanced anesthetic technique

A
  • amnesia
  • analgeisa
  • muscle relaxation

surgery = extremely stressful

  • psychological
  • physiological
  • blood loss –> cardiovascular stress
  • fluid shifts –> respiratory stress
  • temp changes –> organ insult/removal

anesthesia = goal is to maintain physiological stability and maintain end organ homeostasis

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3
Q

Drugs used for anesthesia

A
  • Intravenous anesthetic agents
  • Inhalation anesthetic agents: e.g. isoflurane, desflurane, sevoflurane
  • Sedatives: e.g. midazolam, diazepam
  • Narcotics/Opioids: e.g. morphine, fentanyl
  • Local anesthetics: e.g.lidocaine, bupivacaiane
  • Muscle relaxants: e.g. succinylcholine, rocuronium
  • Others: antiemetics, anticholinergic, reversal agents
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4
Q

Classification of IV anesthetic agents

A

ISOPROPYLPHENOL
Propofol

PHENCYCLIDINE
Ketamine

CABOXYLATED IMIDAZOLE
Etomidate

ALPHA2 ADRENERGIC AGONIST
Dexmedetomidine

BARBITURATES
Thiopental, Methohexital

BENZODIAZEPINES
Diazepam, Midazolam, Lorazepam

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5
Q

Propofol

  • physicochemical properties
  • mechanism of action
A

Physicochemical properties
- Milky white solution, PH 7, Concentration 10mg/ml
- Contains10% soyabean oil, 2.25% glycerol and 1.2% lecithin = derived from eggs
- Available formulations support bacterial growth despite the addition of retardants
–> Sterile technique is important
–> Must be used within 6 hours of being
opened
–> Caution in patients with egg allergy

Mechanism

  • potentiation of cl- current mediated through the GABA type A receptor complex
  • GABA A receptors are present on the postsynaptic neurons. They are ligand –gated. Binding of GABA molecules on their binding sites on the receptor triggers opening of a chloride ion selective pore. The increased chloride conductance inhibits the firing of new action potentials. Propfol acts on The GABA A receptor and potentiates the effect of GABA)
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6
Q

Propofol - pharmacokinetics

A
  • Rapidly metabolized in the liver
  • Inactive, water soluble metabolites are excreted by the kidneys
  • Wake up after an induction bolus
  • –> Occurs in 8 t0 10 minutes
  • –> Due to the redistribution from highly perfused (brain) to poorly perfused (skeletal muscles) organs

Tissues are grouped into hypothetical compartments based on perfusion.
An important implication of different compartments and perfusion rates is the concept of redistribution. After a given amount of drug is administered, it reaches the highly perfused compartments first where it can equilibrate and rapidly exert its effects. With time, compartments with lower perfusion rates receive the drug and equilibrium is achieved between the blood and these tissues. As the tissues with lower perfusion, absorb the drug, drug transfers from the highly perfused compartment into the blood stream in order to maintain equilibrium throughout the body. This lowering of drug concentration in one compartment by delivery into another compartment is called redistribution

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7
Q

Propofol - effects

A

Central Nervous system:

  • Hypnotic with no analgesic properties
  • Decreases the cerebral blood flow and cerebral metabolic rate
  • Decreases intracranial pressure
  • Anticonvulsant effect
  • Neuroprotective during focal brain ischemia

Cardiovascular system:

  • Profound vasodilatation, reduction in preload and afterload
  • Decrease in systemic blood pressure
  • More pronounced with elderly patients, hypovolemic patients and rapid injection
  • Inhibits the normal baroreceptor reflex

Respiratory system

  • Potent respiratory depressant, produces apnea after an induction dose
  • Suppresses upper airway reflexes, well suited for instrumentation of the airway

Pain on injection

Antiemetic effect –> mechanism unclear

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8
Q

Propofol infusion syndrome

A
  • Lactic acidosis after infusions > 75mcg/kg/min for more than 24 hours
  • Mechanism unclear, may reflect poisoning of the electron transport system and impaired oxidation of long chain fatty acids (cytopathic hypoxia)
  • Reversible in early stages by prompt discontinuation of propofol infusion, may result in cardiogenic shock and death
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9
Q

Propofol - clinical uses

A

Induction of anesthesia:
- Bolus of 1 to 4 mg/kg IV

Maintenance of Anesthesia:
- As a infusion, in combination with volatile anesthetics and opioids (balanced anesthesia regimen) or with opioids and benzodiazepines (total intravenous anesthesia technique)

Sedation:
- Repeated boluses and/or intravenous infusion for procedure like endoscopy, MRI, dental extractions etc

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10
Q

Propofol - advantages and disadvantages

A

Advantages

  • Amnesia
  • Rapid onset of action and recovery
  • Antiemetic
  • Neuroprotective properties

Disadvantages

  • Pain on injection
  • Not analgesic
  • Cardiovascular and respiratory depression
  • Caution in patients with egg allergy
  • Propofol infusion syndrome
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11
Q

Ketamine

  • physicochemical properties
  • mechanism of action
A

Physicochemical properties:

  • Water soluble phencyclidine derivative
  • Two stereo isomers exist, only the racemic mixture is available the US
  • Available concentrations of 10, 50, 100mg/ml

Mechanism –> antagonism of NMDA receptor
- NMDA receptors are activated by glycine and glutamate which open channels on the receptor allowing the influx of Ca and NA into the cell and K out of the cell. Ketamine by antagonizing these actions produces a state of dissociative anesthesia

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12
Q

Ketamine - pharmacokinetics

A
  • Highly lipid soluble, low protein binding
  • Rapid onset, relatively short duration of action
  • Peak plasma concentration of ketamine occur within 1 minute after IV and 5 minutes after IM injection
  • Effect of a single bolus is terminated by redistribution
  • Metabolized in the liver to Nor ketamine (less potent)
  • Nor ketamine is conjugated to inactive water soluble metabolites and then excreted in the urine
  • Elimination half-time is 2 to 3 hours
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13
Q

Ketamine - effects

A

Central Nervous system:

  • Produces “dissociative anesthesia”: a cataleptic state where the eyes remain open with a slow nystagmic gaze
  • Potent analgesic
  • Cerebral vasodilation and increase in CBF
  • Increase in CMRO2
  • Can precipitate myoclonic and seizure like activity in normal patients, does not alter seizure threshold in epileptics, considered to possess anticonvulsant activity

Cardiovascular system:

  • Centrally mediated sympathetic stimulation −> Î BP, HR, CO and myocardial O2 consumption
  • In critically ill patients, with limited ability to increase their sympathetic nervous system activity, can cause myocardial depression

Respiratory system:

  • No significant respiratory depressant effect
  • Apnea can occur if administered rapidly IV
  • Upper airway skeletal muscle tone as well as airway reflexes are relatively well maintained
  • Bronchodilatation
  • Salivary and tracheobronchial secretions (decreased when combined with glycopyrrolate)

Emergence Reaction:

  • Nightmares, hallucinations, distorted visual, tactile and auditory sensitivity
  • Can be decreased by combining with a benzodiazepine (midazolam)
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14
Q

Ketamine - clinical uses

A

Induction of anesthesia:
- 1 to 2mg/kg IV or 2 to 5 mg/kg IM

Maintenance of anesthesia:
- Infusion 15 to 75 mcg/kg/min in combination with other drugs

Sedation:
- For short, painful procedures, e.g. dressing changes, suturing

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15
Q

Ketamine - advantages and disadvantages

A

Advantages:

  • Analgesia
  • Minimal respiratory depression
  • Can be given via IV, IM, oral, rectal epidural routes

Disadvantages:

  • Emergence delirium
  • Increased secretions
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16
Q

Etomidate

  • physicochemical properties
  • mechanism
A

Physicochemical properties:

  • Carboxylated imidazole derivative
  • D-(+) isomer is active, hypnotic properties
  • Available as a 2mg/ml solution in 35% propylene glycol, PH 6.9

Mechanism of action:
- Binds directly to specific sites on the GABAA receptors and enhances the affinity of the inhibitory neurotransmitter (GABA) for these receptors

17
Q

Etomidate - pharmacokinetics

A
  • Weak base, 99% ionized at physiologic PH
  • Penetrates the Brain rapidly, peak levels reached within 1 minute of IV injection
  • Recovery depends on redistribution
  • 76% bound to albumin
  • Metabolism is primarily by ester hydrolysis to inactive metabolites that are excreted in the urine and bile
  • Elimination half life of 2 to 5 hours
18
Q

Etomidate - effects

A

Cardiovascular system:

  • Cardiovascular stability after bolus injection
  • Minimal change in BP, HR and SVR
  • Minimal depression of myocardial contractility

Respiratory System:

  • Minimal depressant effect on ventilation
  • Apnea may occasionally occur after rapid IV injection

Central Nervous System:

  • Potent cerebral vasoconstrictor, decreases cerebral blood flow and metabolic rate
  • Myoclonic activity in 50% patients, may be associated with seizure like activity on the EEG

Endocrine System:

  • Adrenocortical suppression
  • Dose dependent inhibition of enzyme 11-beta-hydroxylase which converts cholesterol to cortisol
  • Enzyme inhibition lasts for 4 hours after an induction dose
19
Q

Etomidate - clinical use

A

Induction of anesthesia

  • especially in patients with an unstable CV system
  • induction dose is 0.2-0.4 mg/kg
20
Q

Etomidate - advantages and disadvantages

A

Advantages:

  • Cardiovascular stability
  • Minimal respiratory depression

Disadvantages:

  • Pain on injection
  • Myoclonic movements
  • Adrenocortical suppression
21
Q

DEXMEDETOMIDINE

  • physicochemical properties
  • mechanism of action
A

Physicochemical properties:

  • Active derivative of medetomidine, an imidazole derivative
  • Water soluble
  • Available as a parenteral formulation

Mechanism of action:

  • Stimulation of Alpha2 receptors in locus coeruleus region of the brain
  • Inhibition of the release of norepinephrine resulting in inhibition of neuronal firing in the spinal cord and brain and termination of propagation of nerve signals

The locus coeruleus (means dark blue spot— azure appearance in unstained brain tissue)) is a nucleus in the pons (part of the brainstem). It is the principal site for brain synthesis of nor epinephrine

22
Q

DEXMEDETOMIDINE - effects

A

Central nervous system:

  • Sedation
  • Analgesia
  • No significant effect on cerebral metabolic rate and intracranial pressure

Cardiovascular system:

  • Cardiovascular depression
  • Bradycardia
  • Hypotension

Respiratory system:

  • Minimal depressant effect on respiration
  • No effect on respiratory rate
  • Small to moderate decrease in tidal volume
  • Ventilatory response to carbon dioxide is unchanged
23
Q

DEXMEDETOMIDINE - clinical uses

A

Sedation:

  • Short-term sedation of intubated and ventilated patients in the ICU
  • Radiological procedures, MRI and interventional radiology

General Anesthesia:
- 0.5 to 1mcg/kg loading dose followed by an infusion of 0.5 to 0.7 mcg/kg/hr, decreases the dose requirement for inhaled anesthetics and analgesics

24
Q

DEXMEDETOMIDINE - advantages and disadvantages

A

Advantages:
- Sedation and analgesia without respiratory depression

Disadvantages:

  • Hypotension and bradycardia especially with bolus dose
  • Longer onset and offset times than propofol
25
Q

Barbiturates

  • physicochemical properties
  • mechanism of action
A

Physicochemical Properties

  • Derived from barbituric acid
  • –> Thiobarbiturates: Thiopental
  • –> Oxybarbiturate: Methohexita
  • Available as sodium salts, reconstituted with water:
  • –> 2.5% solution: Thiopental
  • –> 1% solution: Methohexital
  • Alkaline solutions (PH > 10)

Mechanism of action
- Binds directly to specific sites on the GABAA receptors and enhances the affinity of the inhibitory neurotransmitter (GABA) for these receptors

26
Q

Barbiturates - pharmacokinetics

A

Pharmacokinetics

  • Metabolized in the liver by
  • –> Oxidation
  • –> Desulfuration
  • –> N-dealkylation
  • Inactive metabolites are excreted in the urine
  • Drugs that cause enzyme induction enhance barbiturate metabolism
  • Methohexital is cleared more rapidly than thiopental (shorter elimination half-time)
27
Q

Barbiturates - effects

A

Central nervous system:

  • Sedation, anesthesia
  • No analgesic properties, may reduce the pain threshold (antianalgesic)
  • Cerebral vasoconstriction, ↓CBF, ↓ICP,
  • ↓CMRO2; dose dependent up to a maximum dose at which the EEG becomes a flat line
  • Methohexital activates epileptic foci (exception)

Cardiovascular system:

  • Peripheral vasodilatation with modest ↓ in BP
  • Exaggerated in patients with hypovolemia or cardiac disease who are less able to compensate for the effects of peripheral vasodilatation

Respiratory system:

  • Respiratory depressant effect: ↓minute ventilation, tidal volume and respiratory rate
  • ↓ventilatory responses to hypercapnia and hypoxia
  • Inadequate suppression of airway and laryngeal reflexes

Precipitation when mixed with acidic drugs in an IV line

Pain and tissue injury on accidental infiltration into paravenous tissue

Pain, intense vasoconstriction and gangrene on intra-arterial injection

Exacerbation of acute intermittent porphyria;
- Stimulation of enzyme D- aminolevulinic acid synthetase causing an increased production of heme

28
Q

Acute intermittent porphyria

A

Autosomal dominant metabolic disorder affecting the production of heme, the oxygen binding group of hemoglobin). Patients lack the enzyme porphobilinogen deaminase essential for synthesis of heme. Thus there are high concentrations of porphobilinogen in the cytoplasm. Symptoms include: severe abdominal pain, dark urine, peripheral numbness, tachycardia, hypertension, agitation, hallucinations, delirium, and seizures.

29
Q

Barbiturates - clinical uses

A

Induction of anesthesia:

  • Thiopental 3-5mg/kg
  • Methohexital 1-1.5mg/kg

Neuroprotection:

  • Treatment of raised intracranial pressure
  • Treatment of focal cerebral ischemia
30
Q

Benzodiazepenes

  • physicochemical properties
  • mechanism of action
A

Midazolam, Diazepam, Lorazepam

Physicochemical properties

  • Midazolam available as a parenteral solution (1mg/ml, 5mg/ml)
  • Diazepam available as oral and parenteral solutions (5mg/ml)
  • Highly protein bound to albumin
  • Highly lipid soluble

Mechanism of Action
- Enhance the affinity of the GABA receptor for GABA → increased Chloride conductance → hyperpolarization of post synaptic cell membrane → postsynaptic neurons more resistant to excitation

31
Q

Benzodiazepenes - pharmacokinetics

A

Pharmacokinetics

  • Rapid onset of action (lipid solubility and penetration of blood brain barrier)
  • Termination of effects by redistribution
  • Hepatic metabolism:
  • –> Midazolam to1-hydroxymidazolam (inactive)
  • –> Diazepam to desmethyldiazepam and oxazepam (both active)
  • The elimination half-life of diazepam greatly exceeds that of midazolam, resulting in a longer duration of action
32
Q

Benzodiazepenes - effects

A

Central nervous system:

  • Sedation, hypnosis, anterograde amnesia
  • Anticonvulsant
  • Decrease CMRO2 and CBF
  • No neuroprotective properties

Respiratory system:
- Minimal respiratory depression

Cardiovascular system:

  • Decrease in systemic vascular resistance and blood pressure (midazolam > diazepam)
  • Cardiac output not affected
  • Pain on injection with diazepam
33
Q

Benzodiazepenes - clinical uses

A 
Preoperative medication:
- Midazolam 
•	IV:  0.1mg/kg, maximum 2mg 
•	PO:  0.5 – 0.7 mg/kg
•	Rectally: 0.7 – 1mg/kg
•	Intranasal (not commonly used) 
- Diazepam
•	PO 5 to 10 mg

Sedation for short procedures
- Midazolam produces more rapid onset, greater amnesia and less postoperative sedation than diazepam

Suppression of seizure activity

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