General Anaesthesia Flashcards
What is balanced anesthesia?
Pain relief, unconsciousness and inhibition of reflex
For inhalant GAs, how does solubility in blood relate to onset of action?
The higher the blood solubility, the slower the onset.
What are the volatile liquid inhalant GAs?
Halothane (classical), enflurane, desflurane, isoflurane and sevoflurane.
What is the only gas inhalant GA?
Nitrous oxide
What is the proposed MOA for inhalant GAs?
- enhance neurotransmission at inhibitory synapses by allosterically increasing GABA receptor sensitivity to action by GABA
- depress neurotransmission at excitatory synapses by blocking glutamate neurotransmitter acting on NMDA receptor, preventing NMDA receptor activation.
What is MAC?
MAC is minimum alveolar concentration.
Defined as minimum conc of drug in alveolar air that will produce immobility in 50% of patients exposed to a painful stimulus.
It is an index of potency.
Low MAC -> High potency
What are the MACs like for the inhalant GAs?
Nitrous oxide has vv high MAC -> extremely low potency despite fast onset of action
Isoflurane, sevoflurane and desflurane have high potency.
Can MAC values change?
Yes, MAC can change with age, medical conditions, concomitant administration of other drugs etc.
What affects the absorption of inhalant GA into the blood?
- conc of anaesthetic in inspired air
- solubility of GA
- blood flow through the lungs
if there is increase in any of these factors -> increased rate of GA uptake into the blood
What is the distribution of inhalant GAs dependent on?
Determined by regional blood flow. If organ is highly perfused, GA will get into these organs more quickly.
How are inhalant GAs eliminated from the body?
Inhalant GA are eliminated almost entirely via the lungs
There is minimal hepatic metabolism.
How are inhalant GAs metabolized?
Some metabolites can be toxic.
Inorganic fluorides of isoflurane and enflurane are nephrotoxic.
Halothane is hepatotoxic.
What is halothane?
Adverse effects of halothane?
How is halothane useful?
- first modern inhaled anesthetic
- volatile liquid, non-flammable and non-irritating
- potent
- medium rate of onset and recovery
- little or no analgesia until unconsciousness supervenes
- causes respiratory depression dose-dependently
- decreases BP due to depression of cardiac output ; bradycardia and arrhythmia may also occur leading to hypotension and dysrhythmia.
- relaxes skeletal muscle and potentiates skeletal muscle relaxants
- may lead to halothane-associated hepatitis (hepatotoxic)
What is isoflurane?
- pungent smell
- potent
- medium rate of onset and recovery
- similar to halothane with less hypotension and arrhythmia
- decreases BP due to decrease in systemic vascular resistance
What is sevoflurane?
How is sevoflurane metabolized?
What is something to note about sevoflurane?
- potent
- more rapid rate of onset and recovery
- metabolized in the liver to release inorganic fluoride, also nephrotoxic
- unstable when exposed to carbon dioxide absorbents in anesthetic machines, degrading to a compound that is potentially nephrotoxic.
What is nitrous oxide?
- odorless gas
- non flammable
- rapid onset and recovery but v low potency
- nitrous oxide alone gives ANALGESIA and AMNESIA but not complete unconsciousness or surgical anesthesia
How is nitrous oxide often used in clinical practice?
Patients undergoing GA receive nitrous oxide to supplement the analgesic effects of the primary anesthetic.
Can be used alone as analgesic agent in dentistry or during delivery.
What is the major concern of nitrous oxide?
Post-operative nausea and vomiting
What are some examples of IV GA?
Thiopentone, etomidate, propofol, ketamine and midazolam.
What are the advantages of using a combination of inhaled and intravenous anaesthetic?
- permit dosage of inhalation agent to be reduced
- produce effects that cannot be achieved with an inhalation alone
What is thiopentone?
- a barbiturate with extremely high lipid solubility -> enters brain easily and rapidly -> rapid onset of action: unconscious 10-20secs after IV
- for a single dose, it has ultra-short duration of action-> if given alone, wake up in 10 min
- liver cirrhosis can lead to prolongation of clinical action
What is the MOA of thiopentone?
It is a barbiturate, so it causes CNS depression by potentiating the action of GABA on the GABA receptor-gated chloride channel
What is propofol?
Adverse effect of propofol?
- most common IV anaesthetic used in SG
- readily made in injectable form, don’t need to re constitute (unlike thiopentone)
- induction rate is similar to thiopentone, and recovery is more rapid (patients move sooner and feel better)
- used for both induction and maintenance
- rapid onset (unconscious within 60 sec)
- short duration of action bc of rapid redistribution from brain to other tissues
- extensively used in day surgery (simple surgery)
- needs continuous, low dose infusion for extended effects.
- reduced postoperative vomiting
- significant cardiovascular effect during induction -> hypotension -> use with caution in elderly patients, patients with compromised cardiac function and hypovolemic patients
What is ketamine?
- used in IM, oral and rectal routes
- produces state called dissociative anaesthesia (pt feels disconnected from environment)
- can cause sedation, immobility, analgesia and amnesia
- rapid induction; responsiveness to pain is lost
What are the adverse reactions to ketamine?
- unpleasant psychologic reactions (hallucinations, disturbing dreams, delirium) may occur during recovery from ketamine
- risk of psychologic reactions may be reduced with premedication of diazepam or midazolam
Why is ketamine popular in 3rd world countries?
It is the only IV anaesthetic that possesses analgesic property.
What are the 4 anesthetic adjuvants?
Benzodiazepines, alpha 2 adrenergic agonists, analgesics and neuromuscular blocking agents.
How are benzodiazepines used as an adjuvant?
IV midazolam is used as an adjuvant.
used for anxiolysis, amnesia and sedation prior to induction of anaesthesia OR
used for sedation during procedures not requiring GA
has a rapid onset when used for induction.
adverse effects can be minimized by injecting midazolam slowly (over 2 or more minutes) and by waiting another 2 or more minutes for full effects to develop before dosing again.
How are alpha 2 adrenergics used as an adjuvant?
IV dexmedetomidine is used as an alpha 2 agonist.
Sedation and analgesic effects
How are analgesics used as adjuvants?
For minor surgical procedures, COX-2 inhibitors and paracetamol are used.
Opioids (fentanyl and morphine) are used during perioperative period.
Remifentanil is an opioid. - hydrolysed by tissue and plasma esterases
How are neuromuscular blockers used as adjuvants?
Succinylcholine.
Administered during induction of anaesthesia to relax muscles -> facilitate laryngoscopy and endotracheal intubation.
Aids surgical procedures
What is an important thing to note about neuromuscular blocking agents?
Barbiturates will precipitate when mixed with muscle relaxants. should be allowed to clear from the IV line prior to injection of muscle relaxant.
