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Analgesia refers specifically to
loss of sensibility to pain.
anesthesia refers not only to loss of pain but also to
loss of all other sensations (eg, touch, temperature, taste).
General anesthetics have no
during general anesthesia, all
sensation is lost, and consciousness is lost as well.
General anesthetics are used to facilitate many other procedures, including
- urologic procedures,
- radiation therapy,
- electroconvulsive therapy,
- transbronchial biopsy,
- various cardiologic procedures.
balanced anesthesia refers to
the use of several drugs to ensure that induction of anesthesia is smooth and rapid and that analgesia and muscle relaxation are adequate.
The agents used most commonly to achieve balanced ansthesia goals are
- (1) short-acting barbiturates (for induction of anesthesia),
- (2) neuromuscular blocking agents (for muscle relaxation),
- (3) opioids and nitrous oxide (for analgesia).
The primary benefit of combining drugs to achieve surgical anesthesia is
accomplishing full general anesthesia at doses of the inhalation anesthetic that are lower (safer) than those that would be required if surgical anesthesia were attempted using an inhalation anesthetic alone.
Except for nitrous oxide, all the agents used today
enhance activation of receptors for gamma-aminobutyric acid (GABA),
As a result, these drugs promote generalized inhibition of CNS function.
the principal inhibitory transmitter in the central nervous system (CNS)
The minimum alveolar concentration
The minimum alveolar concentration (MAC) of an inhalation anesthetic is defined as
the minimum concentration of drug in alveolar air that will produce immobility in 50% of patients exposed to a painful stimulus.
A low MAC indicates high anesthetic potency!
To produce therapeutic effects, an inhalation anesthetic must reach a
CNS concentration sufficient to suppress neuronal excitability.
The principal determinants of anesthetic concentration are
- (1) uptake from the lungs
- (2) distribution to the CNS and other tissues.
Inhalation anesthetics are eliminated by
almost entirely in the expired air.
As a rule, they undergo minimal hepatic metabolism.
The principal adverse effects of general anesthetics are
depression of respiration and cardiac performance.
What is a rare, genetically determined, life-threatening reaction to general anesthetics
Coadministration of succinylcholine, a neuromuscular blocker, increases the risk of the reaction of What??
By enhancing analgesia, opioids
reduce the required dosage of general anesthetic.
By enhancing muscle relaxation, neuromuscular blockers
reduce the required dosage of general anesthetic.
concurrent use of CNS stimulants (amphetamines, cocaine) increases the
required dose of anesthetic.
Preanesthetic medications are administered for three main purposes:
- (1) reducing anxiety,
- (2) producing perioperative amnesia,
- (3) relieving preoperative and postoperative pain.
preanesthetic medications may be used to suppress certain adverse responses:
excessive salivation, excessive bronchial secretion, coughing, bradycardia, and vomiting
Intravenous midazolam (Versed) is used most often to
produce conscious sedation.
Opioids (eg, morphine, fentanyl) are administered to relieve
preoperative and postoperative pain.
These drugs may also help by suppressing cough. Opioids can delay awakening after surgery.
Anticholinergic drugs (eg, atropine) may be given to
decrease the risk of bradycardia during surgery.
Analgesics are needed to control
If pain is severe, opioids are indicated.
For mild pain, aspirin-like drugs may suffice.
Patients recovering from anesthesia often experience
nausea and vomiting
Among the most effective is ondansetron (Zofran), a drug developed to
suppress nausea and vomiting in patients undergoing cancer chemotherapy.
Halothane is a
high-potency anesthetic and hence has a low MAC (0.75%), indicating that unconsciousness can be produced when the drug’s concentration in alveolar air is only 0.75%.
Isoflurane (Forane) is used widely. The drug is
potent (MAC = 1.15%) and has properties much like those of halothane. Induction of anesthesia is smooth and rapid, depth of anesthesia can be adjusted with speed and ease, and patients emerge from anesthesia rapidly.
Nitrous oxide differs from other general anesthetics in two important ways:
(1) it has a very high MAC and therefore cannot be used alone to produce general anesthesia;
(2) it has high analgesic potency and therefore is frequently combined with other general anesthetics to supplement their analgesic effects.
Induction of anesthesia is usually accomplished with a
short-acting barbiturate, such as thiopental.
Stage 1 anesthesia
Analgesia--patient experiences analgesia, but is conscious
Stage 2 anesthesia
Excitement--systolic pressure rises,excitation/restlessness, increased respirations—often circumvented by a short-acting IV barbiturate
Stage 3 anesthesia
Stage 4 anesthesia
- Medullary Depression—respirations cease; this
- is an emergency!