General anaesthetics Flashcards
What are GAs used for?
To produce unconsciousness and a lack of response to all stimuli through inhibition of sensory and autonomic reflexes
What is the triad of anaesthetics?
Analgesia, hypnosis and amnesia
What are the two modes of administration of GAs?
Inhalation and intravenous
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
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.
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.
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
What non-drug related factors can MAC be affected by?
Age, comorbidities, other drugs
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
What is the distribution of inhalation GAs determined by?
The blood flow through the region
Describe the elimination of inhalation GAs
It is mainly eliminated by the lungs. Has minimal hepatic metabolism.
Factors that affect absorption also affect elimination
What is halothane?
The first inhaled anaesthetic, still used as the standard today
What are some major inhalant anesthetics?
Halothane, isoflurane, sevoflurane and nitrous oxide
Describe the potency, onset and recovery of Halothane
It is potent with an MAC of 0.75%.
It has medium onset and recovery
Describe the analgesic properties of halothane
Little to no analgesia
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
Describe the potency, onset and recovery of isoflurane
High potency with an MAC of 1.4%. Medium onset and recovery
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
Describe the potency, onset and recovery of sevoflurane
High potency with an MAC of 2%. Fast onset and recovery (within minutes)
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
