General Anesthetics Flashcards
What is a General Anesthetic ?
Why is it used ?
drug that brings about a reversible loss of consciousness
to facilitate surgery.
What are the ideal characteristics of a general anesthetic ?
Rapid, smooth loss of consciousness
Be rapidly reversible upon discontinuation
Possess a wide margin of safety
What are the Five primary effects of GA ?
Unconsciousness
Amnesia
Analgesia
Inhibition of autonomic reflexes
Skeletal muscle relaxation
What are the four stages of GA ?
Induction
Excitement
Surgical Anesthesia
Medullary Paralysis
What are the two classes of GA ?
- Intravenous : induction agents given via injections
- Inhalations : usually halogenated gases and vapors
Examples of Intravenous Induction Agents
Propofol
Etomidate
Ketamine
Barbiturates (Thiopental sodium)
Benzodiazepines
Opioids
How do Intravenous induction agents work ?
GABA binds to GABA receptors, opening chloride channels.
Cl ions enter causing hyperpolarization.
This inhibits neuron activity.
Propofol enhances GABA receptor activity, increasing chloride ion influx, leading to greater hyperpolarization.
This inhibits neuronal firing and induces sedation
Propofol
Use
additional benefit
duration of action
onset of action
induce, maintain GA
Etomidate
Ketamine
Thiopental sodium
Examples of Inhalation Anesthetics.
- Nitrous Oxide
- Halogenated anesthetics
Halothane
Isoflurane
Desflurane
Enflurane
Sevoflurane
Inhalation anesthetics are liquid at room temperature. Why ?
Have low vapor pressures and thus high boiling points
What is the Mechanism of Action of inhaled anesthetics ?
Increase GABA A activity
via 2 pore domain K channels
via NMDA receptors
What is MAC ?
minimum alveolar conc. necessary to prevent responding in 50% of pop.
is a measure of potency
What does the pharmacokinetics of inhalation anesthetics refer to
uptake, distribution and elimination
Uptake of inhalation anesthetics via gas exchange depends on alveolar conc.
What two factors affect alveolar conc.
Increasing
1. inspired concentration
2. alveolar ventilation
enhance delivery of anesthetics to alveolar
How are inhaled anesthetics absorbed into the bloodstream?
Inhaled anesthetics are absorbed through gas exchange in the alveoli.
What does the ratio FA/FI represent in the context of inhaled anesthetics?
The ratio FA/FI represents the alveolar concentration (
FA) divided by the inspired concentration (FI), which reflects how quickly the partial pressure of the anesthetic in the alveoli approaches equilibrium.
What does it mean when the FA/FI ratio approaches 1?
When FA/FI approaches 1, it indicates that equilibrium has been reached between the alveolar and inspired concentrations, leading to faster induction of anesthesia.
What are the factors controlling uptake of inhalation anesthetics
- solubilty
- cardiac output
- alveolar-venous partial pressure difference
What does the blood:gas partition coefficient indicate for an inhaled anesthetic?
The blood:gas partition coefficient defines the relative affinity of an anesthetic for the blood compared to the inspired gas.
What is the significance of a low blood:gas partition coefficient, as seen in desflurane and nitrous oxide?
A low blood:gas partition coefficient indicates that the anesthetic is relatively insoluble in blood, leading to faster induction and recovery times.
How does an increase in cardiac output affect the uptake of inhaled anesthetics?
An increase in cardiac output increases the uptake of the anesthetic into the bloodstream but distributes it across all tissues, not just the CNS.
Why does increased cardiac output not significantly enhance the delivery of anesthetics to the brain?
Cerebral blood flow is well-regulated, so increased cardiac output primarily delivers anesthetic to other tissues rather than significantly increasing delivery to the brain.
What determines the alveolar-venous partial pressure difference of an inhaled anesthetic?
The alveolar-venous partial pressure difference is primarily dependent on the uptake of the anesthetic by tissues, including non-neural tissues.
How does a greater alveolar-venous partial pressure difference affect the induction of anesthesia?
A greater alveolar-venous partial pressure difference means it will take more time to achieve equilibrium with brain tissue, delaying induction of anesthesia.
What factors influence the rate of recovery (reverse of induction) from inhaled anesthetics?
- blood:gas partition coefficient of the anesthetic agent.
- Alveolar ventilation (controlled by the anesthesiologist)
- Metabolism of the anesthetic
Which anesthetic agents have a rapid rate of recovery, and why?
Nitrous oxide, desflurane, and sevoflurane have a rapid rate of recovery due to their low blood:gas partition coefficients, making them relatively insoluble in blood.
How are modern inhaled anesthetics primarily eliminated, and why is their metabolism significant?
Modern inhaled anesthetics are primarily eliminated through ventilation, with only minimal metabolism. However, their metabolism is significant due to its implications for potential toxicity.
What is the rank order of inhaled anesthetics based on the extent of hepatic metabolism?
Halothane > Enflurane > Sevoflurane > Isoflurane > Desflurane > Nitrous Oxide
What are the cerebral effects of inhaled anesthetics?
- decreases cerebral metabolic rate and reduces blood flow within the brain
which can lead to implications in long surgeries - increases cerebral blood flow which is undesirable in patients who have increased intracranial pressure because of brain tumor, intracranial hemorrhage, or head injury.
What are the cardiovascular effects of inhaled anesthetics?
remember these are important for patients with heart failure
- depress normal cardiac contractily, especially halothane and enflurane. Resulting in reduced arterial pressure and little change in vascular resistance
- isoflurane, desflurane, and sevoflurane produce greater vasodilation with minimal effect on cardiac output. This decreases bp
What are the respiratory effects of inhaled anesthetics?
except nitrous oxide, all cause a dose-dependent
decrease in tidal volume- leading to increased CO2 tension
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increase in respiratory rate (rapid shallow breathing pattern).
What are the renal effects of inhaled anesthetics?
Inhaled anesthetics tend to decrease glomerular filtration rate (GFR) and urine flow.
What are the Effects of inhalation anesthetics on Uterine Smooth Muscle
Nitrous oxide little effect.
halogenated anesthetics are potent uterine muscle relaxants. important for pregnant women
Acute Toxicity of inhaled Anesthetics
Nephrotoxicity
Hematotoxicity
Malignant hyperthermia
Hepatotoxicity
Nephrotoxicity
Metabolism of enflurane and sevoflurane may generate compounds that are potentially nephrotoxic. (liberate fluoride ions
Hematotoxicity
Prolonged exposure to nitrous oxide decreases methionine synthase activity, which could cause megaloblastic anemia
Malignant hyperthermia
is a heritable genetic disorder of skeletal muscle that occurs in susceptible individuals exposed to volatile anesthetics while undergoing general anesthesia. ( Halothane
Hepatotoxicity
a small subset of individuals previously exposed to halothane has developed fulminant hepatic failure.
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Cases of hepatitis of others have rarely been reported
Important features of Nitrous oxide
completely eliminated by the lungs.
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Non toxic to liver , kidney and brain. Not much adverse effects on CVS , RS.
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Probably the safest with 30% oxygen
Adverse effects of Nitrous Dioxide
Adverse effects
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Pneumothorax.
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Megaloblastic anemia
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Peripheral neuropathy
Important features of Halothane
Induction is relatively slow.
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Fat soluble- accumulate during prolonged administration.
Uses : Maintenance anesthesia in adults
Adverse Effects of halothane
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Shivering during recovery
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Malignant hyperthermia
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Fulminant hepatic necrosis
Important features of Isoflurane
Induction with isoflurane and recovery from isoflurane are faster than with halothane.
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It is typically used for maintenance of anesthesia after induction.
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Halothane and Isoflurane may sensitize the myocardium to the arrhythmogenic effects of catecholamines
Important features of Enflurane
Induction of anesthesia and recovery from enflurane are relatively slow.
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Used for maintenance rather than induction of anesthesia.
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Enflurane provokes seizure attacks in susceptible patients.
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Enflurane produces significant skeletal muscle relaxation
Important features of Sevoflurane
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Effects on CVSand RSis modest.
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It is well-suited for inhalation induction of anesthesia (particularly in children)
most used
First examples of Simple Combinations of anesthetics
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Morphine
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Propofol
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N2O
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Sevoflurane
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Relaxant of choice
second examples of Simple Combinations of anesthetics
Fentanyl
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Thiopental sodium
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N2O
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Halothane
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Relaxant of choice
