GA 2 Flashcards
Signs of development of general anesthesia
- Loss of fine motor function/coordination
- Alteration of consciousness/analgesia
- Loss of temperature regulation
- Unconsciousness
- Changes in eye motion
- Loss of muscle tone
- Respiratory failure*
- CV failure*
- Coma and death*
- occur with overdose
Time course of surgical anesthesia
• Induction: time between initiation of administration of anesthetic and attainment
of surgical anesthesia, that is, until Stage III is reached
• Maintenance: time during which surgical anesthesia is in effect (surgery carried
out during this period)
• Recovery: time following termination of administration of anesthetic until
complete recovery of patient from anesthesia
Phase I: lung factors
The rate of increase in the partial pressure of anesthetic gas in the alveoli and pulmonary capillary blood is proportional to the rate of ventilation (respiratory minute ventilation). The rate of induction of anesthesia can be increased by over-ventilation. The depth of anesthesia is not affected by ventilation rate.
Recovery from anesthesia is also proportional to rate of ventilation. Thus respiratory
depression as a consequence of overly deep anesthesia can prolong the recovery
time
Solubility in blood
The blood:gas partition coefficient (λ) is a measure of the solubility of anesthetic gas in blood. The blood:gas partition coefficient is defined as the concentration of anesthetic in blood divided by anesthetic concentration in the
inspired gas mixture.
Important point: induction of anesthesia is ______
slower for a more soluble anesthetic gas.
Higher GA solubility in blood, means that ______ in blood
in order to reach stage III, surgical anesthesia:
more anesthetic must be dissolved
Descending depression
loss of fine motor function and
progressive loss of function from higher (e.g., cognition
and consciousness) to lower (e.g., respiratory control) levels within the central
nervous system.
impairment of coordination • alteration of consciousness and often analgesia • loss of temperature regulation • unconsciousness • effects on eye motion, pupil size and light reflex • loss of muscle tone • respiratory failure • cardiovascular failure • coma and death
The rate at which an effective concentration of anesthetic is reached in the brain
depends upon 5 factors:
(1) concentration of the anesthetic in inspired air,
(2) alveolar ventilation rate,
(3) pulmonary blood flow (cardiac output),
(4) blood:gas partition coefficient, and
(5) potency (oil:gas partition coefficient).
- A high-solubility GA (halothane) exhibits a slower approach to equilibrium because_________
- A low-solubility GA (nitrous oxide), exhibits a more rapid increase in its partial pressure in blood.
a larger amount must be dissolved in blood
• Combining the effects of blood solubility and pulmonary flow:
the rate of transfer of volatile anesthetic from alveoli to blood is inversely related to both pulmonary blood flow and the solubility of the anesthetic gas in blood.
Pharmacokinetics of GA uptake: Summary
Rate of induction depends on uptake, distribution, and potency of inhaled
anesthetic
• High solubility and large volume of distribution of GA in fatty tissue, and low
perfusion of fatty tissue, produces
slow uptake and elimination from fatty tissue
• Uptake proceeds through 3 compartments
vessel-rich including brain, muscle
group, fat group)
↓cardiac output
↑uptake of inhaled anesthetic
• ↑rate and depth of alveolar ventilation
↑rate of rise in arterial GA anesthetic
• ↓GA solubility in blood (blood:gas partition)
speeds induction of anesthesia
• ↑ inspired GA ↑ rate of induction by
↑rate of anesthetic entry into arterial blood
_____ is related to kinetics of GA uptake and elimination
• blood:gas partition coefficient
oil:gas partition coefficient is an index
of GA potency
• Anesthetic potency is related
to 1/MAC
Clearance by___ is major route of removal for the volatile anesthetics
lungs
- Low potency – have to breathe a lot
- excellent analgesic
- Useful as adjunctive agent (anxiolytic and analgesic properties). Great for suppressing pain (dentists can use).
- Rapid onset and recovery
- Odd PK: Concentration effect, diffusion hypoxia and second gas effect
NO
MAC is 105%–>can’t be used as sole anesthetic agent
CI: respiratory obstruction, COPD, pregnancy (since you have to breathe so much of this).
NO
(i) Concentration effect:
∼1 liter/min of N20 taken up, owing to high inspired % (maybe 75%)
Big anesthetic vol. taken out of lung into blood, sucks more N2O gas into lung
Thus, uptake rate is faster than predicted
NO
ii) Diffusion hypoxia
when anesthetic administration is terminated
Large N2O volume leaving blood expands lung and dilutes alveolar O2 - hypoxia
NO
iii) Second gas effect
Like concentration effect, but with 2 gaseous GAs (75% N2O, 1% halothane)
Huge volume uptake rate of N2O sucks more of both gases into lungs
Thus, uptake of halothane is increased over expected value alone
NO
MAC = 0.75%
Not a good analgesic
Hepatotoxic
Malignant hyperthermia
Halothane
Moderate to high potency
poor analgesic
Low blood:gas partition coefficient
Halothane
Somewhat more potent
Less incidence of hepatotoxicity, renal toxicity
Little seizure propensity
Isoflurane (Forane
pleasant odor – no coughing – so can be used for induction
• Drawback: chemically unstable, releases fluoride ions, which are toxic to kidneys
Sevoflurane
Drugs used for surgery
benzodiazepines
diazepam (Valium®), midazolam, lorazepam
- used preoperatively to ease induction or for sedative purposes
Drugs used for surgery
Induction agents:
rapid deep anesthesia (20 sec), avoiding excitation/delirium
– thiopental, propofol
Drugs used for surgery
Analgesics:
- opioids: morphine, fentanyl (latter has advantageously short duration of action)
Drugs used for surgery
Neuromuscular blockers:
relax skeletal muscle, particularly for abdominal surgeries
– D-tubocurarine, vecuronium, succinylcholine
Drugs used for surgery
Anticholinergic drugs:
reduce GA-induced hypotension, bradycardia, and excess
salivary secretions that can choke patient during anesthesia
– glycopyrrolate, atropine, scopolamine
Drugs used for surgery
Anti-emetics
reduce post-operative nausea and vomiting (common problem)
– ondansetron (5-HT3–type serotonin receptor antagonist)
