Anesthesia Flashcards
Definition of general anesthesia:
State of CNS depression
- patient has complete absence of sensations and is unconscious
- controlled AND reversible
Five goals of general anesthesia:
- loss of awareness or consciousness
- amnesia
- analgesia
- blunting of the autonomic nervous system (BANS)
- skeletal muscle relaxation
Two types of anesthetics:
Inhalation-gaseous
Injection
Dalton’s law as it applies to inhaled anesthetics:
P(total)=P(anesthetic) + P(oxygen)
**need oxygen pressure of 20.9% to survive so most you can have for the other gases is ~79% (usually need waaayyy less)
Henry’s Law as it applies to inhaled anesthetics:
Drug dissolved in fluid (blood) does NOT raise the partial pressure of the drug in that fluid
**clinical effect of drug is related to the amount of UNDISSOLVED drug in the blood
***analogous to the relationship between free and protein bound drugs in the blood
More anesthetic dissolved in blood means….
**takes longer to attain equilibrium (when undissolved drug can begin to have desired clinical effect)
**greater concentration of anesthetic at equilibrium
Compare drug partial pressure in delivered gas, inspired air, alveoli, arteries, and brain during:
Induction
At anesthesia
Recovery
Induction:
delivered gas > inspired air > alveoli > arteries > brain
At anesthesia:
inspired air = alveolar = arterial = brain
Recovery:
delivered gas < EXpired air < alveoli < VENOUS < brain
Describe drug solubility with respect to speed of onset for nitrous oxide, desflurane, sevoflurane, isoflurane, halothane:
Lease soluble Fastest onset
Nitrous oxide
desflurane
sevoflurane
isoflurane
halothane most soluble slowest onset
Where in the body does anesthesia take place?
brain
All inhaled anesthetics share which characteristic (could be called only known mechanism of action)?
they all cause changes in membrane fluidity
**no know specific site of action
What is the dose of anesthetic (vol %) that produces surgical anesthesia in 50% of patients?
Minimum Alveolar Concentration (MAC)
**surgical anesthesia usually achieved around 1.3-1.5 MACs
deep anesthesia ensues around 2 MACs
Lowest MAC = ?
most potent
4 factors affecting MACs:
age
pre-existing disease
effects of other CNS depressants
ambient temp (ORs are COLD)
Amount of drug necessary to produce effect of specified intensity:
Potency
What does lipid solubility have to do with potency?
more lipid soluble = greater potency
**expressed as oil:gas partition coefficient
***don’t confuse with water (blood) solubility which deals with speed of onset (blood:gas partition coefficient)
Most potent inhaled anesthetic?
Halothane (oil:gas partition coefficient = 224)
Fastest onset inhaled anesthetic?
Nitrous oxide (blood:gas partition coefficient = 0.5)
Type of anesthetics that leave body most slowly?
fat soluble
Three routes of elimination (inhaled anesthetics):
Lungs (primary)
skin and mucous membranes
can enter atmosphere (OR needs good ventilation!!)
Only toxicologically important aspect of clearing inhaled anesthetics:
liberation of chemically reactive halides (bromide, chloride, and fluoride) can harm kidneys, liver, and reproductive organs
CNS side effects of inhaled anesthetics:
decreased brain metabolic rate
increased cerebral blood flow
increased intracranial pressure
CV side effects of inhaled anesthetics:
decreased myocardial contractility and stroke volume –> lower arterial pressure
sensitizes myocardium to catecholamines–>increased automaticity (adding epi could cause fatal arrhythmia!!)
NON CNS or CV side effects of inhaled anesthetics?
respiratory depression (isoflurane, desflurane, sevoflurane > halothane > nitrous oxide)
muscle relaxation at high doses
Malignant hyperthermia (except nitrous), most common with halothane
how do you treat malignant hyperthermia
pharmacologically?
physically?
dantrolene to block Ca+ release from sarcoplasmic reticulum
cool patient with ice bags/cold water
Most potent inhaled anesthetic (MAC 0.7-0.9)
least expensive
no laryngeal irritation
higher risk for malignant hyperthermia
inadequate analgesia and muscle relaxation
Halothane
Potent inhaled anesthetic
DOES not sensitize myocardium to catecholamines
Less hepatotoxicity
rare arrhythmias
pungent odor
potential for malignant hyperthermia
Isoflurane
New inhaled anesthetic approved in 1996
high potency
low blood solubility (rapid onset, rapid recovery, almost perfect anesthetic)
Sevoflurane
Only inhaled anesthetic that is actually a gas
low blood solubility–> rapid onset
little effect on CV function
mild to moderate analgesia
MAC = 104%, can’t use alone!!!!
no muscle relaxing effect
diffusion hypoxia if rapidly discontinued
Nitrous oxide
Three advantages of injectable anesthetics:
act faster
best suited for INDUCTION of anesthesia
useful for short operations
disadvantage of injectable anesthetics
muscle relaxation after IV is poor
-unsuitable as a single drug anesthetic
5 injectable anesthetics:
barbiturates
benzodiazepines
propofol
ketamine
opiods
Facilitates GABA induced Cl- entry into neurons leading to hyperpolerization and CNS depression
rapid onset (very lipid soluble)
short action (quick recovery)
Danger: toxicity, anesthetic dose is between 50-70% of LD50
Barbiturates
Thiopental and Methohexital
Facilitates GABA induced Cl- entry into neurons leading to CNS depression
less CV suppression than barbiturates
most important for AMNESTIC action
insufficient for anesthesia when given alone
used as induction agent prior to anesthesia
benzodiazepines
Midazolam (Versed) and Diazepam (Valium)
Rapid induction and recovery from anesthesia
May be given alone to maintain anesthesia or used for induction as part of balanced anesthesia technique
Must be given as emulsion patients generally awaken from anesthesia feeling more “clear headed” and are not nauseous (anti-emetic action)
Most significant respiratory effect is apnea (22-45%)
May result in injection site pain
Propofol (Diprivan)
**abused among providers “cat nap” deaths
related to PCP
has dissociative effect
anesthetic, analgesic, amnestic, and sedative
airway reflexes and respiration maintained
may actually stimulate CV sys
drawback: may induce hallucination
abused at raves
possible depression tx
ketamine
high potency, short acting
analgesia
anesthesia
hemodynamic stability- good for pts w compromised myocardial function
respiration must be maintained artificially
usually supplemented with inhaled anesthetic, benzos, or propofol
opiods
- fentanyl
- sufentanyl
- alfentanil
