General Anesthetics

WIDHARTO PHPHARMACOLOGY & THERAPY

FK UGM

IntroductionThe physiologic state of general anesthesia typically

includes: • analgesia, • amnesia, • loss of consciousness, • inhibition of sensory and autonomic reflexes, • skeletal muscle relaxation No single anesthetic agent is capable of achieving all of

these desirable effects without some dis-advantages when used alone

The use of combinations of intravenous and inhaled drugs

e

For more extensive surgical procedures, anesthesia frequently includes the use of:

a)preoperative: benzodiazepines, b) induction of anesthesia: i.v. thiopental or

propofol, and c) Maintenance of anesthesia: a combination of

inhaled and intravenous anesthetic drugs.

Types of General Anesthetics

Intravenous AnestheticsSeveral drugs are used intravenously, alone or in combination

with other drugs, to achieve an anesthetic state (as components of balanced anesthesia)

These drugs include the following: (1)barbiturates: (thiopental, methohexital); (2)benzodiazepines (midazolam, diazepam); (3)opioid analgesics (morphine, fentanyl, sufentanil,

alfentanil, remifentanil); (4) propofol; (5) ketamine; and (6) miscellaneous drugs (droperidol, etomidate, dexmed-

etomidine).

INTRAVENOUS ANESTHESIA

Signs & Stages of AnesthesiaThe traditional description of the signs and stages of anesthesia (Guedel's signs) were derived from

observations of the effects of diethyl ether

State of GA divided into four stages:

I. Stage of Analgesia: The patient initially experiences analgesia without amnesia. Later in stage I, both analgesia and amnesia are produced.

II. Stage of Excitement: During this stage, the patient often appears to be delirious and excited but definitely is amnesic Respiration is irregular both in volume and rate, and retching and vomiting may occur. The patient may struggle and is sometimes incontinent.

III. Stage of Surgical Anesthesia: This stage begins with the recurrence of regular respiration and extends to complete cessation of spontaneous respiration. Four planes of stage III (described in terms of changes in ocular movements, eye reflexes, and pupil size, which under specified conditions may represent signs of increasing depth of anesthesia.

IV. Stage of Medullary Depression: This stage of anesthesia includes severe depression of the vasomotor center in the medulla as well as the respiratory center. Without full circulatory and respiratory support, death rapidly ensues.

The adequacy of depth of anesthesia for the specific surgical requirements is assessed by changes inrespiratory and cardiovascular responses with surgical stimulation.

Properties of Inhaled Anesthetics.

Anesthetic NO Halothanea). Blood:Gas Partition Coefficient1 0.47 2.30b).Brain:Blood Partition Coefficient1 1.1 2.9c).Minimal Alveolar Conc (MAC) (%)2 > 100 0.75d) Metabolism none > 40% • e) Comments Incomplete

anesthetic; Rapid onset and medium

rate ofrecovery onset &

recover

Characteristics of Intravenous Anesthetics

• Etomidate, Rapid onset and moderately fast recovery

• Ketamine, Moderately rapid onset and recovery

• Midazolam, Slow onset and recovery; flumazenil reversal available

• Propofol, Rapid onset and rapid recovery

• Thiopental, Rapid onset and rapid recovery (bolus dose)—slow recovery following infusion

• Fentanyl, Slow onset and recovery; naloxone reversal available