Anesthetics

Chapter 8 Anesthetics

I. General Anesthetics

A. Goals of balanced anesthesia

• Elimination of perception and reaction to pain

• Loss of memory, which is not essential but desirable during most surgical procedures

3. Loss of consciousness

• Essential for many surgical procedures (e.g., cardiac, orthopedic)

4. Muscle relaxation

a. Occurs in varying degrees

b. Usually has to be further supplied by neuromuscular relaxants (See Chapter 6)

5. Suppression of autonomic and sensory reflexes

• Requires use of additional medications to suppress the enhanced autonomic and sensory reactions that occur during surgical procedures

Multiple drugs are required for balanced anesthesia.

B. Inhalation anesthetics (Box 8-1 and Table 8-1)

1. General considerations

a. Depth of anesthesia directly relates to the partial pressure of the anesthetic in the brain.

TABLE 8-1 Properties of Inhalation Anesthetics

Depth of anesthesia relates to partial pressure of the anesthetic in brain.

b. Anesthetic potency is expressed as the minimum alveolar concentration (MAC).

(1) 1 MAC is the concentration in inspired air at which 50% of patients have no response to a skin incision.

MAC is concentration in inspired air where 50% of patients have no response to skin incision; thus, surgery must be performed above 1 MAC.

(2) The higher the lipid solubility

(a) The greater the potency

(b) The lower the MAC (See Table 8-1).

(3) Inhalational anesthetics exert additive synergism

• 0.5 MAC from two anesthetics together will produce 1 MAC anesthesia.

MACs are additive.

c. Speed of induction is influenced by several factors.

(1) Higher inspired concentration equals more rapid induction.

(2) Lower solubility in blood equals more rapid induction.

(3) Higher ventilation rate equals more rapid induction.

(4) Lower pulmonary blood flow equals more rapid induction.

d. The lower the blood-gas partition coefficient, the more rapid is the onset and recovery from anesthesia (See Table 8-1).

Low partition coefficient for nitrous oxide (0.47) and desflurane (0.42) leads to rapid onset and recovery

e. Mechanism of action

(1) Usually described as nonspecific interaction with lipid bilayer of neuronal membrane (Meyer-Overton principle)

(2) Recent evidence suggest that Cl⁻ and K⁺ fluxes are increased (hyperpolarization)

2. Nitrous oxide (N₂O)

• Only gaseous anesthetic used

N₂O only gaseous anesthetic used

a. Characteristics

(1) Cannot produce surgical anesthesia by itself

A mixture of O₂ and N₂O often used as a carrier gas for halogenated anesthetics.

(2) To produce unconsciousness, N₂O must be used with other anesthetics.

(3) Significantly reduces the required concentration of halogenated anesthetics when used as an adjunct to anesthesia

N₂O has excellent analgesic properties.

b. Pharmacokinetics

(1) Extremely fast absorption and elimination

(2) Resulting in rapid:

(b) Recovery from anesthesia

(1) Has good analgesic properties

(2) Therefore, used to decrease pain in:

(b) Procedure that does not require unconsciousness

Dental procedures

d. Contraindications

(1) Head injury

(2) Preexisting increased intracranial pressure

(3) Brain tumors.

• N₂O can raise intracranial pressure.

N₂O contraindicated with head injuries.

3. Volatile halogenated hydrocarbons (see Box 8-1 and Table 8-1)

a. These agents are of variable potency (MAC) and blood solubility.

• Part of balanced anesthesia

c. Adverse effects

(1) They may sensitize the heart to the arrhythmogenic effects of catecholamines.

(2) Varied affects on cardiovascular system (See Table 8-1)

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Tags: Rapid Review Pharmacology

Apr 8, 2017 | Posted by admin in PHARMACY | Comments Off

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