general anesthetics Flashcards
general anesthesia - definition
- generalized, reversible CNS depression; all sensory perception is removed
- only used to facilitate surgery/ surgical procedures; has no therapeutic or diagnostic purpose unto itself
5 progressive phases of anesthesia
- sedation: pre-operative anxiolysis to facilitate induction
- induction: sedation and rapid loss of consciousness
- paralysis: total relaxation and loss of control of musculature with marked immobility; requires intubation
- maintenance: with inhalation anesthetics
- recovery: patients regain senses and muscle control
mechanisms of action
- facilitate GABA: enhance the inhibitory effect of GABA binding to GABA-a; open Cl- channels, allows influx of Cl-; results in lowered membrane potential; makes it harder to depolarize and propagate an action potential
- inhibit glutamate: inhibits the excitatory effects of glutamate binding to NMDA/AMPA receptors; allows influx of Na+ and Ca++ and efflux of K+; results in higher resting membrane potential / facilitates propagation of action potentials
- facilitate 2-pore K+ channels: allows influx of Na+ and efflux of K+; results in overall lowered neuronal membrane potential; makes it harder to depolarize and propagate an action potential
4 stages of anesthesia: (list)
stage I: analgesia/ induction
stage II: excitement/ delirium
stage III: surgical anesthesia
stage IV: overdose/ medullary depression/ paralysis
4 stages of anesthesia: stage I
pt is conscious but drowsy. variable amnesia.
4 stages of anesthesia: stage II
dangerous stage due to risk of laryngospasm and vomiting; efforts made to limit this stage
- pt experiences delirium; may exhibit violent combative beh; hyper-reactive / uncontrolled mvmts; elevated/irregular bp and resp rate
4 stages of anesthesia: stage III
ideal stage for surgery; no spontaneous mvmt, no effect (stable) on heart rate and MAP; further CNS depression results in progressive loss of muscle tone and reflexes
4 stages of anesthesia: stage IV
loss of spontaneous respirations, profound drop in bp, death
IV anesthetics (list of 5)
- used as induction agents or for pre-procedural anxiolysis
benzodiazepines (GABA-a receptor) barbiturates (GABA-a receptor) etomidate (GABA-a receptor) propofol (GABA-a receptor) ketamine (NMDA receptor)
midazolam
- short-acting benzodiazepene
- for anesthesia induction or conscious sedation
lorazepam
- intermediate-acting benzodiazepene
- pre-op sedative
- amnestic
diazepam
- long-acting benzodiazepene
- pre-op sedative
- long half life due to active metabolites
flumazenil
- benzodiazepene antagonist used for reversal or overdose
thiopental
- most common barbiturate for induction
- no reversal agent
- short onset of action and duration
- facilitates GABA activity, also depresses brainstem activity
side-effects:
- severe post-op n/v (treat with serotonin antagonist)
- ** no increase in ICP / specific indication for head trauma
- profound respiratory depression
- dose-dependent decrease in MAP
etomidate
- induction agent favored for pts at risk for hypotension (elderly, other pts with limited cardiac reserve)
- short onset of action and duration
- increases affinity of GABA for its receptor
- ** no effect on BP
- minimal effect on heart and respiration
- greater margin of safety than barbiturates, greater post-op n/v than barbiturates
- pain on injection (use with lidocaine)
propofol
- most common induction agent
- facilitates action of GABA
- ** rapid onset 30 sec
- ** rapid recovery 4-8 min
- side-effects: ** anti-emetic effect, cardiac depression, hypotension, impaired baroreflex, pain on injection (use with lidocaine)
- fospropofol: inactive, pro-drug form of propofol, converts to propofol in vivo
ketamine
- NMDA receptor antagonist
- dissociative anesthetic best suited for asthma pts, children undergoing short and painful procedures
- pt may appear conscious with eyes opened but is unable to respond to sensory stimuli
- can be administered IV, IM, PO, PR
side-effects: effective analgesic; bronchodilation, **hypertension, increased HR and CO, increased cerebral blood flow and ICP; post-op hallucination, vivid dreams, excitation, NO pain with injection (no need for lidocaine)
inhalational anesthetics (list of 4)
- used for maintenance of anesthesia only
- excreted by expiration
- very dangerous / very small therapeutic index
- very steep dose-response curve
- NO antagonist
nitrous oxide (non-halogenated)
desflurane (halogenated)
sevoflurane (halogenated)
isoflurane (halogenated)
Minimum Alveolar Concentration (MAC)
- describes the potency of inhalational anesthetics
- MAC of 1 (vol %) = minimum concentration preventing muscular response to a standard surgical incision in 50% of unparalyzed patients
- low MAC = high potency
- high MAC = low potency
factors that increase MAC
- these factors make the pt LESS sensitive to inhalational anesthetics
hyperthermia (greater than 42C)
drugs that increase CNS activity (catecholamines)
chronic ethanol use
pregnancy
factors that decrease MAC
- these factors make the pt MORE sensitive to inhalational anesthetics
hypothermia
advanced age (30% fatality in pts > 80 yrs)
acute ethanol intoxication
second gas effect (when two inhaled anesthetics are mixed, both become more potent)
factors that do NOT effect MAC
gender height weight duration of anesthesia thyroid gland dysfxn
nitrous oxide (N2O)
- non-halogenated
- NMDA antagonist
- very high MAC 105% (even if given at 100%, half of pts will still not lose muscular response to surgical incision; would have to be given with hyperbaric technique to achieve no muscular response in 50% of patients to increase the concentration sufficiently. because inhaled anesthetics must be mixed with minimum 20% oxygen to support life, N2O can never be given alone, only used with other inhalational anesthetics to achieve potency via second gas effect)
- ** does not trigger malignant hyperthermia
- will increase BP, CO when mixed with halogenated inhalational anesthetics
- will decrease BP, CO when mixed with opioids
halogenated / “volatile” inhalational anesthetics (characteristics)
- MOA: activation of GABA-a and two-pore K+ channels
side-effects:
- dose-dependent drop in BP
- moderate bronchodilation & skeletal muscle relaxation
- respiratory depression
- can affect pregnancy/ crosses placenta; increased miscarriage rates in female OR workers
- ** can trigger hyperthermia (treat with DANTROLENE)
