General anaesthetics Flashcards by John Bamanshaw

What are GAs used for?

To produce unconsciousness and a lack of response to all stimuli through inhibition of sensory and autonomic reflexes

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What is the triad of anaesthetics?

Analgesia, hypnosis and amnesia

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What are the two modes of administration of GAs?

Inhalation and intravenous

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What are the most commonly used classes of GAs?`

Short acting barbiturates for anaesthesia

Neuromuscular blocking agents for muscle relaxation

Opioids and nitrous oxide for analgesia

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Describe how solubility of the drug in blood affects its onset

The greater the solubility, the slower the onset – as the drug will stay in the blood longer.

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What are the two proposed mechanisms of action of GAs?

By allosterically increasing GABA receptor sensitivity to GABA, which is the main inhibitory neurotransmitter. The action of GABA then leads to an influx of anions into the cell –> hyperpolarization –> inability to depolarize –> no action potential

By blocking the action of glutamate (which is the main stimulatory neurotransmitter) at NMDA receptors.

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What is the minimum alveolar concentration (MAC)?

The index of inhalation anaesthetic potency

It is defined as the minimum concentration of the drug in the alveolar air that will produce immobility in 50% of patients exposed to painful stimuli

Lower MAC = higher potency as less drug needed to produce an affect

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What non-drug related factors can MAC be affected by?

Age, comorbidities, other drugs

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What is absorption of inhalation GAs determined by?

The concentration of the anesthetic in the inspired air

The solubility of the GA

The blood flow through the lungs

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What is the distribution of inhalation GAs determined by?

The blood flow through the region

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Describe the elimination of inhalation GAs

It is mainly eliminated by the lungs. Has minimal hepatic metabolism.

Factors that affect absorption also affect elimination

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What is halothane?

The first inhaled anaesthetic, still used as the standard today

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What are some major inhalant anesthetics?

Halothane, isoflurane, sevoflurane and nitrous oxide

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Describe the potency, onset and recovery of Halothane

It is potent with an MAC of 0.75%.

It has medium onset and recovery

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Describe the analgesic properties of halothane

Little to no analgesia

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What are the possible effects of halothane?

Respiratory depression - dose dependent

Decreases cardiac output – hypotension

Bradycardia and arrhythmia

Relaxes skeletal muscle and potentiates skeletal muscle relaxants

May lead to halothane-associated hepatitis

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Describe the potency, onset and recovery of isoflurane

High potency with an MAC of 1.4%. Medium onset and recovery

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What are the effects of isoflurane?

Similar to halothane with less hypotension and arrhythmia

Decreases in BP is largely due to decrease in systemic vascular resistance

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Describe the potency, onset and recovery of sevoflurane

High potency with an MAC of 2%. Fast onset and recovery (within minutes)

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What are the issues with using sevoflurane?

Metabolized in the liver to release inorganic fluoride which is nephrotoxic

Unstable when exposed to the carbon dioxide absorbents in anaesthetic machines – degrades to a compound that is also nephrotoxic

Monitor the kidneys when using sevoflurane!!

Describe the potency, onset and recovery of Nitrous oxide

Low potency with an MAC of 110%.. but rapid onset and recovery.

Describe nitrous oxides’ effects with regards to the anaesthetic triad

Provides analgesia and amnesia but not unconsciousness or surgical anaesthesia

What are the uses of nitrous oxides?

Given to patients undergoing GA to supplement the analgesic effects of the primary anaesthetic

Used in dentistry

Used in delivery

What is a major concern of nitrous oxide use?

Postoperative nausea and vomiting

What are some IV GAs?

Thiopentone, propofol, ketamine and midazolam

What are IV GAs usually used for and why?

As induction agents due to their rapid onset

What are the advantages of using IV GAs together with inhalants?

Permits the dosage of the inhalant to be reduced Achieves effects the inhalant cannot achieve on its own

What is the effect of IV GAs on respiration?

Tends to depress respiration -- need to take over ventilation

What is thiopentone?

A barbiturate -- causes anaesthesia

Describe the onset, duration of action, distribution and elimination of thiopentone

Rapid onset -- within 10-20 secs Ultra-short duration of action -- dependent on clearance It has a large volume of distribution -- easily distributed to tissues around the body It is metabolized by the liver to an active metabolite (can cause liver cirrhosis, prolonged action) Is eliminated very slowly. Extensively bound to plasma protein

What is the mechanism of action of thiopentone?

Potentiates the action of the neuroinhibitory molecule GABA, enhancing the binding of GABA to its receptor --> causes influx of anions --> hyperpolarization --> inability to depolarize

What is propofol?

The most common IV anaesthetic used in singapore

Describe the induction rate and recovery of propofol

Induction rate is similar to that of thiopentone (fast.) but has more rapid recovery

Describe the onset and duration of action of propofol

Rapid onset of action (within 60 secs) but short duration of action (3-5 mins per injection) since it is rapidly distributed from the brain to other tissues Continuous, low-dose infusion needed

What is a benefit of using Propofol?

It helps reduce post-operation vomiting (emesis) -- can be used in conjunction with NO

What is an adverse effect of propofol? Who should it be used with caution in?

Cardiac effects during induction -- decreased BP and contractility -- can lead to hypotension Should be used with caution in the elderly, patients with compromised cardiac function or hypovolemic patients

What is unusual about ketamine as an anaesthetic?

It produces a dissociative anaesthesia rather than unconsciousness

What are the possible routes of administration of ketamine?

Intramuscular, oral, rectal, IV

What are the effects of ketamine?

Sedation, immobility, analgesia and amnesia

Describe the induction, metabolism and excretion of ketamine

It has a rapid induction Metabolized in the liver. However metabolite is still active just less active. Excreted in the urine and bile

Describe the distribution and clearance of ketamine and its implications

Has a high Vd i.e is distributed widely easily. Has a rapid clearance. Hence suitable for continuous infusion without the lengthening in duration of action

What are the adverse effects of ketamine? How can these be prevented?

Psychological reactions like hallucinations, disturbing dreams or delirium This can be alleviated by premedicating with benzodiazepines like diazepam and midazolam

Ketamine is the only IV anaesthetic that possesses ....... properties

analgesic

What are some adjuncts for GA and their effects?

Benzodiazepines to induce anxiolysis, amnesia and sedation prior to induction of GA alpha 2 agonists to induce sedation Analgesics to reduce dose of anaesthetic required Neuromuscular blocking agent to relax muscles of the jaw, neck and airway.

Describe the onset and metabolism of midazolam

rapid onset, metabolized in the liver (elderly may have slower recovery due to reduced liver function)

Describe the therapeutic index of midazolam and its adverse effects

Has a high therapeutic index Has less cvs and respi depression compared to other IV anaesthetics

What are the effects of alpha 2 adrenergic agonists?

Sedation and analgesia (no anaesthesia)

What are the side effects of alpha 2 agonists?

Little respiratory depression Tolerable decrease in blood pressure and heart rate nausea, dry mouth, hypotension, bradycardia