General Anaesthesia Flashcards
General anaesthesia results in
Medically induced coma and loss of protective reflexes
Should general anaesthesia result in amnesia
Yes
What is monitored anaesthesia
Minor procedures where patient maintains patient airway and responds to commands
What are the 4 stages of general anaesthesia
- Analgesia (first without amnesia then with)
- Excitement (amnestic, delirious, irregular respiration)
- Surgical anaesthesia (loss of motor and autonomic response to noxious stimuli, loss of eye movements)
- Medullary depression
How is general anaesthesic thiopentone administered
Intravenously (inducing agent)
What is propofol
Intravenous inducing agent, general anaesthesia
How are inhaled anaesthetics distributed to the brain
Transferred from alveolar air to the blood, transfer from blood to brain
What are the 5 factors affecting rate of inhaled general anaesthesia reaching brain concentration
Solubility, concentration in inspired air, rate and depth of pulmonary ventilation, pulmonary blood flow, arteriovenous concentration gradient
How does solubility of general anaesthesia affect rate of onset
Low blood solubility, able to saturate in blood faster and reach high arterial tension more quickly. This allows for more rapid equilbration with the brain and faster onset
How does concentration in inspired air affect rate of onset of general anaesthesia
Increase concentration, increase rate of transfer into blood and brain
Higher concentration, higher onset
How does rate and depth of pulmonary ventilation affect onset of general anaesthesia
Increased pulmonary ventilation, increased onset rate
How does pulmonary blood flow affect rate of onset of inhaled general anaesthesia
Increase blood flow, decrease rate of rise of anesthetic tension between blood and brain (keep carrying to tissues)
How does arteriovenous concentration gradient affect inhaled general anaesthesia onset
High concentration gradient between artery and vein means increased uptake by tissues, more time needed to achieve equilibrium with brain
How is inhaled general anaesthesia eliminated
Mainly excreted through the lungs
Hepatic metabolism
Bacteria in gut can break down nitrous oxide
How does solubility affect metabolism of inhaled general anaesthesia
Low solubility, eliminated faster
More soluble, accumulate in muscle, skin, fat, more slowly eliminated
What is the mechanism of action of general anaesthesia
Modifies ion currents. Directly interact with ligand gated ion channels eg activates GABA receptor and glycine receptor, inhibits nicotinic receptor
It acts at multiple levels of CNS, affect sensitivity of specific neurons and pathways
How does inhaled general anaesthesia affect cardiovascular function
Decrease mean arterial pressure by reducing cardiac output, reducing systemic resistance, reducing myocardial function
How does inhaled general anaesthesia affect respiratory function
Decreased minute ventilation
Decreased response to hypercapnia, increased apneic threshold
Depress mucociliary function, bronchodilation
How do inhaled general anaesthesia affect the brain
Increases cerebral blood flow by decreasing cerebral vascular resistance. Undesirable in patients with increased intracranial pressure
Patient with high intracranial pressure. Which inhaled general anaesthesia should i use
Nitrous oxide as it is least likely to increase cerebral blood flow
How does inhaled general anaesthesia affect the kidney
Decreases renal blood flow
How do inhaled general anaesthesia affect the uterus
Halogenated general anaesthesia are uterine muscle relaxants
What is the side effect of repeated exposure to halothane (inhaled ga)
Hepatitis. Formation of reactive metabolites that directly damage liver or initiate immune mediated response
Which inhaled general anaesthesia can cause renal dysfunction and how
Methoxyfluorane via release of fluoride during metabolism
What is malignant hyperthermia in response to general anaesthesia
Autosomal dominant skeletal disease that results in inherited susceptibility for adverse reaction to GA. GA in susceptible individual triggers hypertension, tachycardia, severe muscle rigidity, hypothermia, acidosis. Increase in muscle cell calcium.
How do you treat malignant hyperthermia
general anaesthesia rxn.
Treat with dantrolene to reduce release of calcium
Compare rate of onset and recovery for halothane and nitrous oxide
Halothane has medium rate of onset and recovery while nitrous oxide has rapid onset and recovery
Is halothane volatile
Yes
Which is more potent — halothane or nitrous oxide
Halothane is more potent
Nitrous oxide is not potent, unable to induce complete unconsciousness and surgical anaesthesia
What is halothane used for
Induction and maintenance of anesthesia
Able to relax skeletal muscles and dose dependent respiratory depression
What is nitrous oxide used for
Labour pains. Good for analgesia as higher doses will not render the woman unconsciousness. Adjunct with other inhaled anesthetics
[stronger analgesic properties, analgesia sets in way before full anesthesia]
Why are intravenous general anaesthesics commonly used for induction
Onset of anesthetic action faster
Are intravenous general anaesthesics appropriate to use for short outpatient procedures
Yes, recovery is sufficiently rapid
Mechanism of action of barbiturate
Bind to GABA receptor to increase duration of GABA gated chloride channel opening
Bind to AMPA receptor to depress glutamate mediated excitation
What is thiopental
Thiopental is a barbiturate (GABA and AMPA receptor)
Used for induction of anesthesia because it rapidly crosses BBB. Has high lipid solubility hence rapidly distributes out of blood and brain to muscle and fat, allowing for rapid recovery
Is thiopental appropriate to use in patients with high intracranial pressure
Yes. It is desirable. It decreases cerebral metabolism, decreases O2 consumption and blood flow
Effect of Thiopental on cardiovascular system
Decrease cardiac output, decrease stroke volume, decrease arterial bp
What is the mechanism of benzodiazepine
intravenous general anaesthesics
Increase frequency of gaba gated chloride channel openings. Does not directly activate GABA, but increases efficiency of GABA. Potentiates GABAergic inhibition
Name some benzodiazepines
Diazepam, lorazepam, midazolam
What are benzodiazepines used for
Sedative, anxiolytic, amnestic properties
Use for pre anesthetic and adjuvants during procedures under LA
Can benzodiazepine be used for surgical anaesthesia?
No. Reaches plateau inadequate for surgical anesthesia. Slower onset of CNS depressant effects compared to barbiturate
What are the disadvantages of using benzodiazepine for deep sedation and how to overcome them
High dose is required. This will prolong post anesthetic recovery and may cause anterograde amnesia
Accelerate recovery with high doses by using antagonist flumazenil (req multiple doses, patient may lapse back into deeper sedation)
Can benzodiazepines be used in pregnancy
Avoid in first trimester due to increased risk of cleft palate formation
Mechanism of propofol
Slow channel closing time by potentiating GABA receptor activity. Also a sodium channel blocker
How is the rate of onset and recover of propofol
As rapid as barbiturates but recovery is more rapid ie patient can ambulated sooner and less nausea
What s propofol used for
Induction, maintenance of anesthesia
Sedation
Monitored anesthesia
Causes potent respiratory depression
How it propofol cleared
Rapidly metabolised by liver and excreted by kidney
What is the mechanism of action of ketamine
NMDA receptor antagonist
What are some effects of ketamine
Dissociative amnesia, catatonia, analgesia without loss of consciousness
Decrease respiratory rate
Stimulate cardiovascular system by stimulating sympathetic system and inhibiting noradrenaline uptake
Increase cerebral blood flow and oxygen consumption, increasing intracranial pressure
Is ketamine rapidly distributed to brain
Yes
What is unique about ketamine
Only iv anaesthetic with analgesic and anesthetic property
Do intravenous general anaesthesics have analgesic property
Only ketamine
Which intravenous general anaesthesic can result in post operative disorientation, illusions, dreams
Ketamine
What is balanced anesthesia
Combination of inhaled and IV anesthesia
IV used to induce and inhaled to maintain.
May also have muscle relaxants to facilitate traceable intubation, LA for pre/post op analgesia, cardiovascular drugs to control transient autonomic responses to noxious surgical stimuli
What are the steps to monitored anaesthesia care
Regional or local anesthesia supplemented by IV.
- Midazolam used as premeds for anxiolytic and mild sedation
- Titrated variable propofol infusion such that preserves spontaneous breathing and airway reflexes
- Opioid analgesics/ketamine
Patient is aware throughout but very relaxed
How to reverse opioid analgesics
Nalaxone
How does increased lipid solubility affect onset of intravenous general anaesthesics
Increased lipid solubility crosses BBB to go to brain more quickly (vs inhaled greater lipid solubility go to tissues greater av gradient onset slower)
How does general anaesthesia affect TURP
Positive pressure ventilation in general anaesthesia increases venous resistance, reducing absorption of irrigation fluid, decreasing risk of TURP syndrome
